UMTS Air Interface
UMTS Air Interf ace
Contents 1 1.1 1.2 1.3 2 2.1 2.2 2.3 2.4 2.5 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4
UMTS Air interface technology Duplex methods UMTS Frequency Access method UMTS Air interface description Principle Data processing Codes Logical, transport and physical Channels Air interface protocol stack High Speed Downlink Packet Access HSDPA HSDPA performance HSDPA implementation : HSDPA channels : MAC Layer Split Adaptive Modulation and Coding (AMC) Scheme : Error Correction (HARQ) Fast packet scheduling Impact on the Iub Interface Handset Capabilities Exercises
3 4 7 8 11 12 13 21 28 35 51 52 53 55 57 58 59 61 63 64 65
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Solution
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TM51103EN03GLA2 © 2010 Nokia Siemens Networks
1
UMTS Air Interface
2
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
1
UMTS Air int erface technol ogy
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
3
UMTS Air Interface
1.1
Duplex methods
The duplex define the way how two communicating entities will communicate with each others. We define here three ways: •
Simplex: This is one way communication method used for broadcasting (TV, Radio)
•
Half-duplex: This is a two ways communication method; the two communicating entities cannot transmit and receive simultaneously.
Full-duplex: This method is the same as half duplex except that the two entities can communicate simultaneously. We define two means to achieve full or half duplex method: •
•
FDD: Frequency Division Duplex :
The frequency band is split into two sub-band one for the uplink and the other for the downlink. Then the receiver and the transmitter use two carriers at the same time. Advantages: Using this method we can avoid collision between uplink and downlink. •
•
4
Drawbacks: Frequency resources are wasted
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
Uplink
Do wnlink Downlink
Fig 1 Frequency division duplex
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TM51103EN03GLA2 © 2010 Nokia Siemens Networks
5
UMTS Air Interface
•
TDD: Time Division Duplex :
•
The two communicating entities use the same frequency band, but it doesn’t communicate simultaneously. It uses two different time period, one period for the uplink and the other one for the downlink. •
Advantages: The frequency resources are not wasted.
•
Drawbacks: Collision may occur during communication.
Time
Downlink Downlink Frequency
Uplink Uplink
TM51103EN03GLA01 1
© Nokia Siemens Net works
Fig 2 Time division duplex
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TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
1.2
UMTS Frequency
The IMT-2000 has allocated the band from 806 MHz - 960 MHz, 1710 MHz - 2025 MHz, 2110 MHz – 2200 MHz and finally 2500Mhz – 2690 MHz for a worldwide mobile communication implementation. The frequency band which is used for UMTS use is summarized in the following graph:
TM51103EN03GLA01 1
© Nokia Siemens Net works
Fig 3 IMT-2000 frequency allocation for different mobile system
1920-1980 and 2110-2170 MHz Frequency Division Duplex (FDD, W-CDMA) Paired uplink and downlink, channel spacing is 5 MHz. An Operator needs 3 - 4 channels (2x15 MHz or 2x20 MHz) to be able to build a high-speed, high-capacity network. 1900-1920 and 2010-2025 MHz Time Division Duplex (TDD, TD/CDMA) Unpaired, channel spacing is 5 MHz. Tx and Rx are not separated in frequency. 1980-2010 and 2170-2200 MHz Satellite uplink and downlink.
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
7
UMTS Air Interface
1.3
Access method
The goal of a network operator is to achieve a higher capacity with fewer resources. In order to do this different access methods are used following is defining these methods:
1.3.1
FDMA: Frequency divi sion m ult ipl e access
The used frequency band is divided into different carriers as shown below. The same number of carrier is used for both uplink and downlink. Each carrier is indexed with UARFCN (UTRA absolute radio frequency carrier number).
Uplink
UAR UAR UAR UAR UAR UAR UAR UAR UAR UAR UAR UAR FC FC FC FC FC FC FC FC FC FC FC FC
Downlink
UAR UAR UAR UAR UAR UAR UAR UAR UAR UAR UAR UAR FC FC FC FC FC FC FC FC FC FC FC FC
Frequency
Fig 4 Frequency division multiple access technique
The advantage of this technique is that the bandwidth is used more efficiently. It means that one operator can reuse its set of frequency according to a certain pattern called cluster.
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TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
1.3.2
TDMA: Time divis ion mu lti ple access
This is a time domain multiplexing technique. The principle is simple one carrier is divided into different timer period called timeslot.
F R
Timeslot
E Q U E N C Y
Time
TM51103EN03GLA01 1
© Nokia Siemens Networks
Fig 5 TDMA Time division multiple access
In order to increase the capacity of the network the two previously discussed techniques are used together.
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
9
UMTS Air Interface
1.3.3
CDMA: Code divis ion mu lti ple access
This is a technique which is using code division in the air interface. Let’s assume that there is a crowd of people speaking together, so if everyone will speak loudly nobody can listen to his talker. The principle introduced by CDMA is as simple as that: each one will speak with low level and with his own language so everybody can have a coherent discussion without disturbing his neighbor. So in CDMA system the subscribers share the same frequency and the same time but they got different codes.
Fig 6 CDMA Concept expressed in terms of power, frequency and time
The capacity of the cell is not anymore function of number of timeslots in the air interface but it’s expressed in function of power allowed within one cell, or to be more specific this capacity is expressed with allowed signal to interference ratio within one cell. Then when more subscribers acess the cell then they will add more interference level to the cell till the interference level reach a planed level or threshold.
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TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
2
UMTS Air int erface de scri pti on
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
11
UMTS Air Interface
2.1
Principle
The air interface is the interface located between the UE and the base station and in the standard it is referred as Uu interface. The transmission in the air interface is based on CDMA technology and it’s called W-CDMA (Wideband CDMA) because it’s using 3 times the bandwidth which is used by the CDMA and then for the WCDMA we allocate 3.84 MHz effective band. Adding the guard band the total bandwidth will reach 5 MHz.
TM51103EN03GLA01 1
© Nokia Siemens Networks
Fig 7 UMTS bandwidth
Different variants bandwidths are specified by the standard 5 MHz, 10 MHz and 20 MHz, the mostly used by operators is 5 Mhz.
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TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
2.2
Data processi ng
Before sending the data over the Uu interface data need to be processed in order to comply with the air interface requirement in term of bandwidth and QoS. This processing in the following steps:
Fig 8 Data process
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
13
UMTS Air Interface
2.2.1
Channel coding
Channel coding refers to a technique used to protect data against losses in the air interface. The technique used here is adding redundancy to the signal giving it more chance to be transmitted correctly over the Uu interface. For the channel coding in UTRA two options are supported for FDD and three options are supported for TDD: •
Convolutional coding.
•
Turbo coding.
•
No coding (only TDD).
•
Channel coding selection is indicated by higher layers. In order to randomize transmission errors, bit interleaving is performed further.
2.2.2
Rate Matching
After channel coding data needs to be put into radio frames and sometime the amount of data is less or exceed the size of these radio frames. So in order make a correct framing bits are added or by puncturing in a controlled way and this process is called rate matching. The following graph shows which are allowed data rate to be matched:
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TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
TM51103EN03GLA01
Fig 9 UMTS Rate matching
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TM51103EN03GLA2 © 2010 Nokia Siemens Networks
15
UMTS Air Interface
2.2.3
Radio framing , and Spreading
After rate matching the data from previous block comes with a tight bandwidth and a higher output power. So in order to reduce the power of the signal we multiply it by a code, channelization code, so that the signal will be spread all over the total bandwidth reducing then the power under the noise level.
TM51103EN03GLA01 1
© Nokia Siemens Networks
Fig 10 Spreading
By doing that the receiver transmits the signal with a lower level allowing then less interference in the air interface. The length of the code that the signal will be multiplied with is expressed as follow:
The chip is the smallest logical unit in a code it means a chip is a bit in the code. The code frequency is higher than the signal frequency so that we obtain spreading of the signal over the bandwidth. The chip rate used is 3.84 million chips per second (Mcps/s) and it is fixed. The characteristics of the spreading codes will be discussed later.
16
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
2.2.4
Modulation
There are several considerations that were taken into account when making the choice for the overall format for the UMTS WCDMA modulation formats. Some of the considerations were: •
It is necessary to ensure that the data is carried efficiently over the available spectrum, and therefore maximum use is made of the available spectrum, and hence the capacity of the system is maximized.
•
The modulation format should be chosen to avoid the audio interference caused to many nearby electronics equipment resulting from the pulsed transmission format used on many 2G systems such as GSM
•
As the uplink and downlink have different requirements, the exact format for the modulation format used on either direction is slightly different.
•
UMTS modulation schemes for both uplink and downlink, although somewhat different are both based around QPSK formats. This provides many advantages over other schemes that could be used in terms of spectral efficiency and other requirements.
Fig 11 UMTS Modulation
The OQPSK is the Offset QPSK the difference with QPSK is that there is no jump is permitted over the intermediate states.
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
17
UMTS Air Interface
18
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
2.2.5
Multipath pro pagation
Due to the environment of serving area the microwave can be reflected by different obstacles before it reach the BTS or the MS and this is the multipath propagation. So at the receiver side there will be a combination of different signals. In order to deal with such a propagation context the RAKE receiver is used.
TM51103EN03GLA01 1
© Nokia Siemens Networks
Fig 12 RAKE receiver block diagram
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
19
UMTS Air Interface
In a W-CDMA receiver the following steps take place (excluding the error correction coding): 1. Descrambling: Received signals are multiplied by the scrambling code and delayed versions of the scrambling code. The delays are determined by a path searcher prior to descrambling. Each delay corresponds to a separate multipath that will eventually be combined by the Rake receiver. 2. Despreading: The descrambled data of each path are dispread by simply multiplying the descrambled data by the spreading code. 3. Integration and dump: The dispread data is then integrated over one symbol period, giving one complex sample output per quadrature phase-shift keying (QPSK) symbol. This process is carried out for all the paths that will be combined by the RAKE receiver. 4. The same symbols obtained via different paths are then combined together using the corresponding channel information using a combining scheme like maximum ratio combing (MRC). 5. The combined outputs are then sent to a simple decision device to decide on the transmitted bits. 6. The objective of the channel estimation block is to estimate the channel phase and amplitude [denoted in Figure 1 as g(t, τi)] for each of the identified paths. Once this information is known, it can be used for combining each path of the received signal.
20
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
2.3
Codes
Previously we talked about spreading codes, spreading is done using a channelization codes and scrambling codes:
Fig 13 Spreading using channelization and scrambling codes
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
21
UMTS Air Interface
2.3.1
Channelization code
Channelization codes are used •
UL: to separate physical data and control data from same terminal
DL: to separate connection to different terminals in a same cell. For a good separation these code are orthogonal and then we use OVSF codes •
(orthogonal variable spreading factorfactor codes) these codes are also requirement called Walsh codes. It uses a different spreading according to bandwidth increasing then the data rate of the signal.
TM51103EN03GLA01 1 ©No kia Siemens Networks
Fig 14 Code tree
One important limitation of OVSF-WCDMA is that the system must maintain the orthogonality among the assigned codes. The maintenance of the orthogonality among the assigned OVSF codes causes the code blocking problem due to their tree structure. When users are using a higher data rate then they will use a shorter code this will lead to a blocking to the remaining tree branch and then limiting the access to the other users.
22
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
2.3.2
Scra mbling codes
Scrambling codes has a different use from the channelization code, they are used to distinguish in the UL between different users and downlink between different Node B. One scrambling code then is allocated by cell or by user. The scrambling codes have a lower orthogonality than the channelization codes. These codes are organized into 512 code sets. We define then 512 primary scrambling codes and in a lower hierarchical level we define from 1 to 15 secondary scrambling codes achieving then a total number of 8096 codes. The scrambling code is identified by first identifying its code set to significantly reduce the degree of code uncertainly.
Fig 15 Scrambling codes set
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
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UMTS Air Interface
2.3.3
Code management
Code management is devoted to managing the downlink OVSF (Orthogonal Variable Spreading Factor) code tree used to allocate physical channel orthogonality among different users. Clearly, the advantage of the OVSF codes used in the UTRAN downlink is perfect orthogonality. However, the drawback is the limited number of available codes. Therefore, it is important to be able to allocate/reallocate the channelization codes in the downlink with an efficient method, in order to prevent ‘code blocking’. ‘Code blocking’ indicates the situation where a new call could be accepted on the basis of interference analysis and also on the basis of the ‘spare capacity’ of the code tree but, due to an inefficient code assignment, this spare capacity is not available for the new call that must, therefore, be blocked. This situation is depicted in Figure 4.24, where two transmissions with SF ¼ 4 and two transmissions with SF ¼ 8 are assumed to have been assigned the corresponding code sequences Cch,4,2, Cch,4,3, Cch,8,1 and Cch,8,3, respectively, which prevent the use of the codes marked with a cross in Figure 4.24. It is worth noting that, with such OVSF code tree occupancy, the arrival of a new call requesting for SF ¼ 4 would experience code blocking, since no code at that layer is available. On the contrary, if the code allocation shown in below figure was used, it would allow the support of the two SF ¼ 4 users, the two SF ¼ 8 users and still would provide room to support a new SF ¼ 4 request with code Cch,4,1.
24
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
Fig 16 Example of code blocking
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TM51103EN03GLA2 © 2010 Nokia Siemens Networks
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UMTS Air Interface
Fig 17 :Example of code allocation preventing code blocking
In general terms, a code allocation strategy would aim at minimizing code tree fragmentation, preserving the maximum number of high rate codes and eliminating code blocking. Nevertheless, since the purpose of the code allocation/reallocation strategies is to prevent code blocking, this may require ‘code handover’, that is, a call using a given code is forced to use a different code belonging to the same layer.
26
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
2.3.4
Multius er detectio n in WC DMA sys tems
Before sending user data in the air interface it must be multiplied by a scrambling code C1. While sending over the air interface different signals of different users are combined. In order to extract the user data from the other signals we must multiply it by the same code again.
1111
1
0000
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1
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Userdata
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Code
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Code
Fig 18 Scrambling process
The characteristic of this scrambling code is that they are not orthogonal but they have very good orthogonality propriety and they are pseudo random. Fig. 1
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
27
UMTS Air Interface
2.4
Log ical, transpo rt and physi cal Channels
UTRA FDD radio interface has logical channels, which are mapped to transport channels, which are again mapped to physical channels. Logical to Transport channel conversion happens in Medium Access Control (MAC) layer, which is a lower sub-layer in Data Link Layer (Layer 2).
Radio resources resources control contro l Control and measur ement
Logical channel
Medium Access Access Control Control
Transport channel
Phys Physical ical Layer Layer
Physical channel
TM51103EN03GLA01 1 ©No kia Siemens Networks
Fig 19 Protocol stack
Different channels transport channels can be mapped into one physical channel and different logical channel can be mapped to a transport channel. This channel organization allows signaling information to be transfer to the concerned and appropriate protocol level in a network element.
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TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
TM51103EN03GLA01 1 ©No kia SiemensNe tworks
Fig 20 Channels
Fig 21 Channels 2
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
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UMTS Air Interface
2.4.1
Physical channels
A physical channel is basically defined by a frequency and a spreading code. The physical channel uses a cosine or sine waveform as a signal carrier. We can distinguish between two kinds of physical channels: Dedicated physical channel. o o
Common physical channel.
A dedicated physical channel is allocated only for one connection but common channels are used simultaneously or alternatively by different connections. The physical layers map under control of the MAC the transport channels to the physical channel according to their physical requirement. o Dedicates Physical Data Channel DPDCH: Used in uplink direction to transmit signaling and user data from higher layer. o Dedicated physical control channel DPCCH: This channel is used to control the data transmission over the air interface. The information included in this channel are power control commands, pilot bits … o Dedicated physical channel DPCH: The DPDCH and the DPCCH are implemented on DPCH. o Physical Random Access Channel PRACH: This physical channel is used during the initial access procedure or call setup. The information contained on this channel is RACH. o
o
o
30
Physical common packetthrough controlthe channel PCPCH: Packet data of the CPCH is sent via PCPCH use of CSMA/CD technique. Common Pilot Channel CPICH: CPICH is an important channel used for cell phase and time reference as well as channel estimation. This channel will help the UE to identify the primary scrambling code by sending a bit pattern at a fixed data rate at 30 kb/s and with a known 256 spreading factor. The same channel code is always used by the CPICH. Common control physical channel CCPCH: This is a downlink channel which is used to carry broadcast information and synchronization to the mobile station. We have two CCPCH:
•
P-CCPCH: Primary common control channel which used to broadcast BCH cell info for different users within a cell serving area.
•
S-CCPCH: Secondary Common control physical channel which is used to carry the FACH Forward Access channel and PCH paging channel.
•
Synchronization channel SCH: Physical channel is used to for cell search and frame synchronization. We can distinguish two SCH:
•
Primary SCH: The 10ms radio frames of the SCH are divided into 15 slots, each of length 2560chips. The Primary SCH consists of a modulated code PSC (Primary synchronization code) of length 256chips, and is transmitted once every slot.
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
TM51103EN03GLA01 1 ©No kia Siemens Networks
Fig 22 WCDMA Frame
•
Secondary SCH: consists of repeatedly transmitting a length 15 sequence of modulated codes of length 256 chips, the Secondary Synchronization Codes (SSC), transmitted in parallel with the Primary SCH. The SSC is denoted csi,k , where i = 0, 1, …, 63 is the number of the scrambling code group, and k = 0, 1, …, 14 is the slot number. Each SSC is chosen from a set of 16 different codes of length 256. This sequence on the Secondary SCH indicates which of the code groups the cell's downlink scrambling code belongs to. Physical downlink shared channel: This channel is used to carry data o over DSCH and different connections can share this channel. A DPCH is always allocated to the PDSCH.
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
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UMTS Air Interface
2.4.2
Transport channels
The output from, and input to, MAC is in the form of transport channels, which can be seen as service between MAC and Layer 1 (physical layer). Generally, transport channels map onto specific physical channels and have specific characteristics in terms of direction, data rate (including variation) and power control requirements. The configuration of a transport channel is related dynamically to QoS requirements. •
Random Access Channel RACH: This channel is mapped to PRACH and it is used to send a small amount of data for a connection setup or initial access in uplink direction. When it use the RACH the mobile send a first preamble and then wait for an indication from the network that a first preamble was received and then it send a second preamble.
•
Broadcast channel BCH: This downlink channel is used to transmit cell specific information to the mobile. This information is contained on the BCCH which is itself mapped to BCH.
•
Forward Access channel FACH: This channel is used to transfer a small amount of user data or signaling over the air interface and also to grant access to the mobile during initial access procedure after receiving second preamble of the RACH.
•
Dedicated channel DCH: This channel to carry user data traffic different logical channel can be mapped over this channel (DCCH or DTCH).
•
Data Shared Channel DSCH: In UMTS, the Downlink Shared Channel (DSCH) is used to transmit data packets from the Node B to the User Equipment (UE). Each DSCH is associated with a Dedicated Channel (DCH) which is used for power control, channel estimation and transmission of associated control information for the DSCH.
•
Common CPICH: channel is used in UMTS enable estimation.pilot The channel CPICH uses a preThis defined bit sequence. It has atofixed ratechannel of 30Kbps with a SF (Spreading Factor) of 256. This allows the UE (User Equipment) to equalize the channel in order to achieve a phase reference with the SCH (Synchronization Channel) and also allows estimations in terms of power control. The same channel code is always employed on the Primary CPICH.
32
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
2.4.3
Logical channe ls
Logical channel are used for different purpose depending on the information carried within these channels. This information could be paging information or BCCH or other signaling information. •
Broadcast Control Channel BCCH: This logical channel carry specific information and parameter about the cell.
•
Paging Control Channel (PCCH): A downlink channel that transfers paging information.
•
Dedicated Control Channel (DCCH): A point-to-point bidirectional channel that transmits dedicated control information between a UE and the RNC. This channel is established during the RRC connection establishment procedure.
Common Control Channel (CCCH): A bidirectional channel for transmitting control information between the network and UEs. This logical channel is always mapped onto RACH/FACH transport channels. A long UTRAN UE identity is required (U-RNTI, which includes SRNC address), so that the uplink messages can be routed to the correct serving RNC even if the RNC receiving the message is not the serving RNC of this UE. The Traffic Channels are: •
•
Dedicated Traffic Channel (DTCH): A Dedicated Traffic Channel (DTCH) is a point-to point channel, dedicated to one UE, for the transfer of user information. A DTCH can exist in both uplink and downlink.
•
Common Traffic Channel (CTCH): A point-to-multipoint downlink channel for transfer of dedicated user information for all, or a group of specified, UEs.
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
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UMTS Air Interface
TM51103EN03GLA01 1 ©No kia Siemens Networks
Fig 23 Channels mapping
34
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
2.5
Air interface protocol stack
The protocol stack in the air interface is 3 level layered as shown below:
Control plane
User plane
Control and measurements
RRC RRC
User plane Radio Bearer
3
Control plane Radio Bearer PDCP PDCP Signaling Signaling BMC BMC
2
RLC RLC
Logical Logical Channels Channels
MAC MAC
Transport Transport Channels Channels
Physical Physical layer layer
1
PhysicalChannels PhysicalChannels
TM51103EN03GLA01 1 ©No kia Siemens Networks
Fig 24 Air interface protocol stack
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
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UMTS Air Interface
The physical layer offers services to the MAC layer via transport channels that were characterized by how and with what characteristics data is transferred. The MAC layer, in turn, offers services to the RLC layer by means of logical channels. The logical channels are characterized by what type of data is transmitted. The RLC layer offers services to higher layers via service access points (SAPs), which describe how the RLC layer handles the data packets and if, for example, the automatic repeat request (ARQ) function is used. On the control plane, the RLC services are used by the RRC layer for signaling transport. On the user plane, the RLC services are used either by the service-specific protocol layers PDCP or BMC or by other higher-layer u-plane functions (e.g. speech codec). The RLC services are called Signaling Radio Bearers in the control plane and Radio Bearers in the user plane for services not using the PDCP or BMC protocols. The RLC protocol can operate in three modes – transparent, unacknowledged and acknowledged mode. The Packet Data Convergence Protocol (PDCP) exists only for the PS domain services. Its main function is header compression. Services offered by PDCP are called Radio Bearers. The Broadcast Multicast Control protocol (BMC) is used to convey over the radio interface messages srcinating from the Cell Broadcast Centre. In Release ’99 of the 3GPP specifications, the only specified broadcasting service is the SMS Cell Broadcast service, which is derived from GSM. The service offered by BMC protocol is also called a Radio Bearer. The RRC layer offers services to higher layers (to the Non-Access Stratum) via service access points, which are used by the higher layer protocols in the UE side and by the Iu RANAP protocol in the UTRAN side. All higher layer signaling (mobility management, session management, and so on) is encapsulated into RRC messages for transmission over the radio interface. The control interfaces between the RRC and all the lower layer protocols are used by the RRC layer to configure characteristics of the lower layer protocol entities, including parameters for the physical, transport and logical channels. The same control interfaces are used by the RRC layer, for example to command the lower layers to perform certain types of measurement and by the lower layers to report measurement results and errors to the RRC.
36
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
2.5.1
Mediu m Acc ess Contr ol MAC
In the Medium Access Control (MAC) layer the logical channels are mapped to the transport channels. The MAC layer is also responsible for selecting an appropriate transport format for each transport channel depending on the instantaneous source rate of the logical channels. The transport format is selected with respect to the transport format combination set which is defined by the admission control for each connection. The functions of the MAC layer include: •
Mapping between logical channels and transport channels.
•
Selection of appropriate Transport Format (from the Transport Format Combination Set) for each Transport Channel, depending on the instantaneous source rate.
•
Priority handling between data flows of one UE. This is achieved by selecting ‘high bit rate’ and ‘low bit rate’ transport formats for different data flows.
•
Priority handling between UEs by means of dynamic scheduling. A dynamic scheduling function may be applied for common and shared downlink transport channels FACH and DSCH.
•
Identification of UEs on common transport channels. When a common transport channel (RACH, FACH or CPCH) carries data from dedicated-type logical channels
•
(DCCH, DTCH), the identification of the UE (Cell Radio Network Temporary Identity
•
(C-RNTI) or UTRAN Radio Network Temporary Identity (U-RNTI)) is included in the MAC header.
•
Multiplexing/demultiplexing of higher layer PDUs into/from transport blocks delivered to/from the physical layer on common transport channels. MAC handles service multiplexing for common transport channels (RACH/FACH/CPCH). This is necessary, since it cannot be done in the physical layer.
•
Multiplexing/ demultiplexing of higher layer PDUs into/from transport block sets delivered to/from the physical layer on dedicated transport channels. MAC allows service multiplexing also for dedicated transport channels. While the physical layer multiplexing makes it possible to multiplex any type of service, including services with different quality of service parameters, MAC multiplexing is possible only for services with the same QoS parameters.
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
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UMTS Air Interface
•
38
Traffic volume monitoring. MAC receives RLC PDUs together with status information on the amount of data in the RLC transmission buffer. MAC compares the amount of data corresponding to a transport channel with the thresholds set by RRC. If the amount of data is too high or too low, MAC sends a measurement report on traffic volume status to RRC. The RRC can also request MAC to send these measurements periodically. The RRC uses these reports for triggering reconfiguration of Radio Bearers and/or Transport Channels.
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
2.5.2
The Radio Link Control Protocol
The radio link control protocol provides segmentation and retransmission services for both user and control data. Each RLC instance is configured by RRC to operate in one of three modes: transparent mode (Tr), unacknowledged mode (UM) or acknowledged mode (AM). The service the RLC layer provides in the control plane is called Signaling Radio Bearer (SRB). In the user plane, the service provided by the RLC layer is called a Radio Bearer only if is theprovided PDCP and BMC protocols are not used by that service; otherwise the (RB) RB service by the PDCP or BMC. Each mode provides a different set of services defining the use of that mode by the higher layers. Transfer of user data is a service which is common to all three modes. Transparent mode is defined for “quick and dirty” data transfer across the radio interface, and is the only one of the three modes which does not involve the addition of any header information onto the data unit. Erroneous data units are discarded or marked as erroneous. Transparent mode is the mode normally used by both the PNFE and BCFE entities within RRC, for paging/notification and cell broadcast messaging. In Unacknowledged mode, as in transparent mode, no retransmission protocol is used, and so data delivery is not guaranteed. Received erroneous data can be either marked or discarded, depending on configuration. For both Transparent mode data transfer & unacknowledged mode data transfer, RLC provides a function for the segmentation of large data units into smaller ones (and re-assembly at the receive end). The segment lengths are defined when the channel is established. In unacknowledged mode, segment lengths are given by a length indicator which is within the header added to the data unit. Unacknowledged mode additionally provides a service whereby small packet data units can be concatenated together (again indicated within a header field), a ciphering service, and a sequence number check which allows the receiver to check whether or not data has been lost. The functions of the RLC layer are: •
Segmentation and reassembly. This function performs segmentation/reassembly of variable-length higher layer PDUs into/from smaller RLC Payload Units (PUs). One RLC PDU carries one PU. The RLC PDU size is set according to the smallest possible bit rate for the service using the RLC entity. Thus, for variable rate services, several RLC PDUs need to be transmitted during one transmission time interval when any bit rate higher than the lowest one is used.
•
Concatenation. If the contents of an RLC SDU do not fill an integral number of RLC PUs, the first segment of the next RLC SDU may be put into the RLC PU in concatenation with the last segment of the previous RLC SDU.
•
Padding. When concatenation is not applicable and the remaining data to be transmitted does not fill an entire RLC PDU of given size, the remainder of the data field is filled with padding bits.
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
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UMTS Air Interface
•
Transfer of user data. RLC supports acknowledged, unacknowledged and transparent data transfer. Transfer of user data is controlled by QoS setting.
•
Error correction. This function provides error correction by retransmission in the acknowledged data transfer mode.
•
In-sequence delivery of higher layer PDUs. This function preserves the order of higher layer PDUs that were submitted for transfer by RLC using the acknowledged data transfer service. If this function is not used, out-of-sequence delivery is provided.
•
Duplicate detection. This function detects duplicated received RLC PDUs and ensures that the resultant higher layer PDU is delivered only once to the upper layer.
•
Flow control. This function allows an RLC receiver to control the rate at which the peer RLC transmitting entity may send information.
•
Sequence number check (Unacknowledged data transfer mode). This function guarantees the integrity of reassembled PDUs and provides a means of detecting corrupted RLC SDUs through checking the sequence number in RLC PDUs when they are reassembled into an RLC SDU. A corrupted RLC SDU is discarded.
•
Protocol error detection and recovery. This function detects and recovers from errors in the operation of the RLC protocol.
•
•
40
Ciphering is performed the RLCalgorithm layer for acknowledged and unacknowledged RLC modes. The same in ciphering is used as for MAC layer ciphering, the only difference being the time-varying input parameter (COUNT-C) for the algorithm, which for RLC is incremented together with the RLC PDU numbers. For retransmission, the same ciphering COUNT-C is used as for the srcinal transmission (resulting in the same ciphering mask); this would not be so if ciphering were on the MAC layer. An identical ciphering mask for retransmissions is essential from Release 5 onwards when the HSDPA feature with physical layer retransmission combining is used. The ciphering details are described in 3GPP specification TS 33.102 [4]. Suspend/resume function for data transfer. Suspension is needed during the security mode control procedure so that the same ciphering keys are always used by the peer entities. Suspensions and resumptions are local operations commanded by RRC via the control interface.
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
2.5.3
The Packet Data C onv ergence Prot oco l PDCP
The Packet Data Convergence Protocol (PDCP) [6] exists only in the user plane and only for services from the PS domain. The PDCP contains compression methods, which are needed to get better spectral efficiency for services requiring IP packets to be transmitted over the radio. For 3GPP Release ’99 standards, a header compression method is defined, for which several header compression algorithms can be used. As an example of why header compression is valuable, the size of the combined RTP/UDP/IP headers is at least 40 bytes for IPv4 and at least 60 bytes for IPv6, while the payload, for example for IP voice service, can be about 20 bytes or less. The main PDCP functions are: •
Compression of redundant protocol control information (e.g. TCP/IP and TP/UDP/IP headers) at the transmitting entity, and decompression at the receiving entity. The header compression method is specific to the particular network layer, transport layer or upper layer protocol combinations, for example TCP/IP and RTP/UDP/IP. The only compression method that is mentioned in the PDCP Release ’99 specification is RFC2507.
•
Transfer of user data. This means that the PDCP receives a PDCP SDU from the non access stratum and forwards it to the appropriate RLC entity and vice versa.
•
Support for lossless SRNS relocation. In practice this means that those PDCP entities which are configured to support lossless SRNS relocation have PDU sequence numbers, which together with unconfirmed PDCP packets are forwarded to the new SRNC during relocation. Only applicable when PDCP is using acknowledged mode RLC with in sequence delivery.
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
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UMTS Air Interface
2.5.4
The Bro adcast /Multicast Contr ol Proto col B MC
The Broadcast/Multicast Control (BMC) protocol exists also only in the user plane. This protocol is designed to adapt broadcast and multicast services, srcinating from the Broadcast domain, on the radio interface. In Release ’99 of the standard, the only service using this protocol is the SMS Cell Broadcast service. This service is directly taken from GSM. It uses UM RLC using the CTCH logical channel which is mapped into the FACHarea, transport channel. Each CB message is targeted to a geographical and RNC maps this SMS area into cells. The main functions of the BMC protocol are:
42
•
Storage of Cell Broadcast messages. The BMC in RNC stores the Cell Broadcast messages received over the CBC–RNC interface for scheduled transmission.
•
Traffic volume monitoring and radio resource request for CBS. On the UTRAN side, the BMC calculates the required transmission rate for the Cell Broadcast Service based on the messages received over the CBC–RNC interface, and requests appropriate CTCH/ FACH resources from RRC.
•
Scheduling of BMC messages. The BMC receives scheduling information together with each Cell Broadcast message over the CBC–RNC interface. Based on this scheduling information, on the UTRAN side the BMC generates schedule messages and schedules BMC message sequences accordingly. On the UE side, the BMC evaluates the schedule messages and indicates scheduling parameters to RRC, which are used by RRC to configure the lower layers for CBS discontinuous reception.
•
Transmission of BMC messages to UE. This function transmits the BMC messages (Scheduling and Cell Broadcast messages) according to the schedule.
•
Delivery of Cell Broadcast messages to the upper layer. This UE function delivers the received non-corrupted Cell Broadcast messages to the upper layer.
•
When sending SMS CB messages to a cell for the first time, appropriate capacity has to be allocated in the cell. The CTCH has to be configured and the transport channel used has to be indicated to all UEs via (RRC) system information broadcast on the BCH. The capacity allocated for SMS CB is cell-specific and may vary over time to allow efficient use of the radio resources.
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
2.5.5
The Radio Re sour ce Control Prot ocol RRC
The major part of the control signaling between UE and UTRAN is Radio Resource Control messages. RRC messages carry all parameters required to set up, modify and release Layer 2 and Layer 1 protocol entities. RRC messages carry in their payload also all higher layer signaling (MM, CM, SM, etc.). The mobility of user equipment in the connected mode is controlled by RRC signaling (measurements, handovers, cell updates, etc.).
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
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UMTS Air Interface
2.5.6
Radio Acc ess Beare r
A bearer is a data stream that spans some part of the system and has a specific quality of service (QoS). Figure below shows the most important bearers in UMTS.
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TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
TM51103EN03GLA01 1 ©No kia Siemens Networks
Fig 25 Bearers used in UMTS. (Adapted from 3GPP TS 23.107.)
When the mobile and the network agree to set up a data stream, the system first implements it using a UMTS bearer. This carries information such as voice or packet data between the mobile termination and the far end of the core network (MSC, GMSC or GGSN). If the MT and TE are implemented as two different devices, then another bearer transports information between them. However, this bearer lies outside the scope of UMTS, so we will not consider it further. The same applies to the bearer that lies beyond the far end of the core network. The UMTS bearer is associated with a number of QoS parameters. These describe the service that the user expects to receive, using parameters such as the required data rate, error rate and delay. The system cannot supply this quality of service right away, because the UMTS bearer spans different interfaces that use different transport protocols. It therefore breaks bearer into bearers that have access a smaller scope. A CN bearer handlesthe theUMTS path over thedown core network, while a radio bearer (RAB) handles the path between the mobile and its first point of contact there. In turn, the radio access bearer is broken down into an Iu bearer between the core network and the SRNC, and a radio bearer between the SRNC and the mobile. Each bearer is then implemented using the transport protocols that are appropriate for the corresponding interface, which provide the user with the quality of service expected. On the air interface, for example, the radio bearer is implemented using the RLC, MAC and physical layer protocols.
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UMTS Air Interface
Five special radio bearers carry signaling messages between the mobile and it’s serving RNC. They are known as signaling radio bearers (SRBs), and they are: •
RB0 – for all CCCH messages (RLC unacknowledged mode and RLC transparent mode)
•
RB1 – for DCCH signaling using RLC unacknowledged mode
•
RB2 – for DCCH signaling using RLC acknowledged mode (except those carrying NAS signaling)
•
RB3 – for DCCH signaling using RLC unacknowledged mode and carrying NAS signaling. (Optionally RB4 also)
RB5 RB31 – for DCCH signaling using RLC transparent mode. Each of them is implemented in a particular way that is appropriate for a particular type of message. RB 0 is used to set up signaling communications between the mobile and the network; the other signaling radio bearers handle all subsequent communications. RBs 1 and 2 carry RRC messages between the mobile and it’s serving RNC, the main difference between them being in the configuration of the RLC protocol. RBs 3 and 4 are used to forward non-access stratum messages that begin or end in the core network. RB 4 is optional, but if it is implemented, then RB 3 is used for high priority messages, and RB 4 is used for low priority ones. •
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TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
Controlplanesignalingradiobearer
UserplaneRadiobearer
3
RRC RRC
RB0
RB1
Control and measure ments
RB2
R B3
RB4
PDCP PDCP
BMC BMC
… RB31
RB5
Mapped to TRM SAP/UM SAP/AM SAP TRMSAP
UMSAP
AMSAP
2 RLC RLC
Logical channels
MAC MAC
Transport channels
Physical Physical layer layer
1
Physical channels
TM51103EN03GLA01 1 ©No kia Siemens Networks
Fig 26 Bearers
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TM51103EN03GLA2 © 2010 Nokia Siemens Networks
47
UMTS Air Interface
2.5.7
Access and non Access Stratum :
During the specification of the UMTS by 3GPP the stratified structure of UMTS network was introduced. This structure which is conformal to ISO-OSI model allows distinguishing between independent services in the UMTS network. Then the UMTS network is divided into two levels, the Access Stratum and Non Access stratum, these two levels correspond to a repartition of logical functions within the network.
48
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
TM51103EN03GLA01 1 ©No kia Siemens Networks
Fig 27 Access and non Access Stratum
The Access Stratum defines all the network function that are related to the Access network as an example the RRC and HO. As the UTRAN is defined as the Access network for UMTS then it is totally included in the Access Stratum. And then the Access stratum include a part of the CN which is the Iu and the and a part of the UE functions (RRM) The Access stratum support, by service provision, the Non access Stratum. As an example when a connection is established the Access stratum is responsible, after request from the non Access stratum, to establish a signaling link and radio bearer in the UTRAN according to a required QoS. This QoS is negotiated at the non Access stratum level between the network and the mobile station.
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
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50
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
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3
High Speed Down lin k Packet A ccess HSDPA
High speed downlink packet access (HSDPA) is an extension of the capabilities of UMTS included in 3GPP release 5, with the target of providing higher bit rates and capacity. It is also called 3.5G, with transmission rates up to 14.4 Mbps and 20 Mbps (for MIMO systems) over a 5MHz bandwidth. HSDPA can significantly enhance downlink speeds, with average realistic throughputs of 400–700 kbps and bursts at over 1 Mbps, even in the initial stage. This dramatically improves the user experience of different applications such as web browsing, streaming or Intranet access. Also, in combination with HSUPA, it can be the driver for advance services like VoIP. HSDPA shares the spectrum and codes from WCDMA and, most of the time, only requires a software upgrade of existing UMTS R99 base stations. HSDPA offers a lower cost per bit and is mainly intended for non-real-time (NRT) traffic, but potentially allows new application areas with higher data rates and lower delay variances. The maximum number of UEs on HSDPA does in theory depend on the number of available channellization codes for the associated DPCHs. The most critical parameter affecting HSDPA performance is the transmission power. Since the total power of the base station is shared with R99 DCH, a trade-off between HSDPA and R99 users needs to be considered. This trade-off is affected by the strategy chosen using HSDPA, depending on whether it is introduced as a high bit rate service for top users or as a way to improve efficiency and capacity of background NRT traffic.
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
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UMTS Air Interface
3.1
HSDPA perfo rm ance
In terms of performance, in a 5MHzchannel HSDPA can provide maximum peak rates of up to 14.4 Mbps with 15 spreading codes and with no channel coding. However, this would mean that one unique subscriber will have to use all available codes on the high speed downlink share channel (HSDSCH). This is not a realistic approach, especially for initial implementations, since typically the capacity is also shared withfor regular DCH channels. Codes allocated for HSDPA are fixed, and not usable DCH,UMTS so in practice implementations with 15 codes will require at least several 5MHz carriers to be available in the system. It is expected that HSDPA realizations will follow a progressive approach, starting with five codes, and evolving to 10 or 15 as higher capacity or resources to support it become available. Additionally, it is important to consider that not all UE classes will support 10 or 15 codes, so in order to get the full benefit from maximum throughputs, terminal availability needs to be considered. A realistic data rate will be about 600–800 kbps, taking real network conditions into account, while an estimated network round trip time (RTT) would be 80–100 ms. As a summary, the overall performance of an HSDPA network will depend on: •
The number of spreading codes (support of 5, 10 or 15 multicodes);
•
The modulation mode (QSPK (quadrature phase shift keying), 16-QAM (quadrature amplitude modulation)), where 16-QAM is optional for the network and also for the UE;
•
The error correction level;
Capabilities of end user devices. On the other hand, the impact of HSDPA on WCDMA R99 will be driven by sharing resources and the allocation of fixed codes and constant transmission power to HSDPA. HSDPA will cause a drop in downlink, since fixed codes are fully allocated. Additionally, HSDPA may lead to quality problems and lower data rates for WCDMA R99 connections if the network RF planning is not designed to tolerate the extra interference caused by lack of power control in the HSDPA transmission. In some cases, it may be a need to limit the amount of power for HSDPA in order to protect WCDMA R99. In such cases, the gain in HSDPA performance coming from increasing its transmission power should be closely checked with the degradation in WCDMA R99 performance. The type of modulation (QPSK or 16-QAM) can have an important impact in this case. •
52
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
3.2
HSDPA im plementati on :
Table below present basic features introduced in UMTS architecture and protocols in order to support HSDPA. In addition, other capabilities like the multiple-input multipleoutput (MIMO) receiver would be supported to provide further signal gain and higher throughputs. HSDPA introduces a new radio bearer in the UMTS system, the high speed downlink share (HSDSCH). allows several users to be time-multiplexed so thatchannel during silent periodsThis the channel resources are available to other users. The HSDSCH uses 2ms transmission time intervals (TTIs) and a fixed spreading factor of 16, which allows a maximum of 15 parallel codes for user traffic and signaling. Feature
HSDPA
MAC layer split
Functionality moved to Node B to improve efficiency of packet scheduler and retransmissions
Downlink frame size
2ms TTI (3 slots)
Channel feedback
Channel quality reported at 2ms rate (500 Hz) for CQI (channel quality indication), ACK (acknowledged)/ACK, TPC (transmission power control)
Adaptive modulation and coding (AMC)
QPSK and 16-QAM mandatory scheme
HARQ
Fast layer 1 retransmission (improves RTT); hase or incremental redundancy (IR)
Packet scheduling
Fast scheduling done in Node B with 2ms time basis; types: round-robin, proportional fair, fair throughput, etc.
Shared channel transmission
Dynamically shared in the time and code domains
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
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UMTS Air Interface
In addition to accelerating service access for users and improving data transfers, this reduced TTI allows the system to adapt itself faster to changing conditions. The uplink data transmission of the HSDPA user initially relies on release 99 DCH with different available rates (i.e. 64, 128 or 384 kbps). A new MAC-hs entity in added on the BTS to handle all these new features needed for HSDPA traffic, as shown in Figure B.1. Layers above MAC-hs (for the high speed downlink shared channels), such as MAC-d (for the dedicated transport channels) and RLC are similar to those in the release 99 networks. The adaptive modulation and coding (AMC) technique is used in order to compensate for variations in radio transmission conditions, while the transmission power remains constant. HSDPA-enabled user equipment sends channel quality reports to the base station at 2ms intervals, which are used to adapt the modulation or resources accordingly. At layer 1, the hybrid automatic repeat request (HARQ) with a Stop and Wait (SAW) Protocol is used as a retransmission mechanism. Unlike the UMTS R99, the HARQ is processed directly in Node B, which allows a faster response, instead of being handled by the RNC. The fast scheduling feature is also implemented in Node B, compared to UMTS R99, where the scheduler is located in the RNC. The scheduler determines to which terminal the transmission in HSDSCH will be directed and, depending on the AMC, at what data rate.
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TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
3.3
HSDPA c hannels :
There are five different physical channels that are used by HSDPA services. HSDPA data are carried on HSPDSCH, which is a shared channel for all HSDPA users in the cell. There are two physical control channels, one dedicated channel in uplink (HSDPCCH) and one shared channel in downlink (HSSCCH). In addition to these there are associated DPCHs for uplink and downlink. •
HSPDSCH (high speed physical downlink shared channel). This transfers actual HSDPA data of the transport HSDSCH and can use 1 to 15 code channels, with a spreading factor (SF) of 16. All these associated physical channels should be adjacent, QPSK or 16-QAM modulation is supported over 2 ms TTI slots. No power control is supported. In addition, the HSDSCH does not support soft handover due to the complexity of synchronizing the transmission and scheduling from different cells. Instead, cell reselection through a normal DCH would be implemented; i.e. the HSDPA user is given a DCH in the SHO area, and is then moved to the new cell, where it would get an HSDPA channel again after the procedure is completed.
•
HSSCCH (high speed shared control channel). This includes information to tell the UE how to decode the next HSPDSCH frame. It uses QPSK modulation and a fixed SF of 128. It shares the downlink power with the HSPDSCH, but may support power control in order to maximize the available power for the data channel. More than one HSSCCHs are required when code multiplexing is used, but a maximum of four is supported by the UE. Soft handover is not supported.
•
HSDPCCH (high speed dedicated physical control channel). This channel carries the ACK/NACK (not acknowledged) (repetition encoded) and channel quality indicator transmitted from the UE in the uplink direction, which is needed for L1 procedures. The primary modulation is BPSK (binary phase-shift keying) with an SF of 256 (15 kbps). The transmission power used is typically the same as that used for the uplink DPCH plus additional offset to provide higher protection. The HSDPCCH may be received by two different sectors in the same Node B, but in general soft handover is not supported.
•
Associated DPCH (dedicated physical channel). Two DPCHs are needed for each HSDPA UE, one in the downlink and another in the uplink. While the downlink DPCH is only used for signaling purposes, the uplink DPCH is the complementary data channel for the HSPDSCH, and may be allocated a data rate of 64, 128 or 384 kbps. The primary modulation is QPSK and the SF can be from 4 to 512. Soft handover is supported for both DPCHs.
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
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UMTS Air Interface
3.4
MAC Layer Split
As a result of new functionalities to be carried by Node B in HSDPA, the MAC layer is split into two entities. While MAC-d remains in the RNC in the same way as for R99, MAC-hs is located in Node B to allow rapid retransmission of NRT data.
Fig 28 UE and RNC
MAC-d is responsible for mapping between logical channels and transport channels, selection of and appropriate transport format and handling priorities. It also has to identify UEs in the common transport channels and multiplex/demultiplex upper layer PDUs and to measure the traffic volume. Ciphering for the transparent mode RLC is also managed by MAC-d. MAC-hs is responsible for packet scheduling, link adaptation and layer 1 error correction and retransmission (HARQ). Due to this split in the functionality of the MAC layer, the user data buffers, which used to be in the RNC, are moved to Node B. This makes the introduction necessary of a flow control mechanism in the Iub interface, in order to avoid the buffer overflow and throughput degradation due to buffers becoming empty. MAC-d schedules the number of RLC PDUs according to the credits granted by MAC-hs at each interval of 10 ms, and the aggregated rate of the HSDPA connections is controlled by the rate control implemented in MAC-hs. The MAC-d PDUs are framed into FP-HSDSCH frames, while a maximum number of 16 MAC-d flows per BTS are supported.
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
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3.5
Adaptive Modulation and Coding (AMC) Scheme :
Link adaptation (LA) is the key feature to the success of HSDPA, since there is no power control in HSPDSCH, and it is used to adapt HSPDSCH to different radio conditions. If LA does not work properly, cell capacity is lost and other techniques such as fast scheduling will not work. Link adaptation is done by changing the modulation and number of codes. The UE signals channel information to the network about the highest data rateblock it canerror accept under current conditions while still maintaining a controlled rate (i.e. the under 10 %). This CQI (channel quality indication) is signaled through the HSDPCCH. The network uses this in order to reconfigure the HSDSCH format for subsequent transmission to that UE. For example, if the CQI shows that the quality is degrading, the scheduler can choose a less aggressive coding/modulation format that will cope better with the poor conditions. Typically the link adaptation is divided into two phases, known as the inner loop and outer loop algorithms:
58
•
Inner loop algorithm. This takes the decision for the modulation and coding scheme to be used in the next TTI. This selection will be done only for new transmissions (i.e. not for retransmissions), and will be based on the received CQI, the available HSDSCH transmit power, the number of HSPDSCH codes, the RLC PDU size, input from the outer loop HSDSCH algorithm and the UE category. It is important that input parameters (CQI reports and DPCH power measurements) to the inner loop algorithm are subject to a minimum delay, because otherwise the LA would not be able to track fading in the radio channel properly.
•
Outer loop algorithm. The primary goal is to compensate any bias introduced by the inner loop algorithm. This bias might be introduced due to offsets in relative UE performance, due to improved receiver architecture, etc. Typically, the outer loop may be based on the BLER target obtained from RLC ACK/NACK information, but also the CQI may be used directly.
TM51103EN03GLA2 © 2010 Nokia Siemens Networks
UMTS Air Interface
3.6
Error Correction (HARQ)
Layer 1 retransmissions expand the system recovery capability from air transmission errors, and are subject to significantly shorter delays than RLC retransmissions, due to the closeness of UE and Node B. This result in lower delay jitter, which can be very beneficial for data services based on TCP or streaming applications. The use of HARQ adds increased robustness to the system and a spectral efficiency gain. Two retransmission strategies are supported: incremental Redundancy (IR) and chase combining. The basic idea of the chase combining scheme is to transmit an identical version of an erroneously detected data packet before the decoder combined the received copies weighted by the SNR prior to decoding. With the IR scheme, additional redundant information is incrementally transmitted if the decoding fails on the first attempt, by means of different puncturing schemes used in the coding of the retransmitted data packets. In the case of HSDPA, IR with a one-third punctured turbo code would typically be used for the retransmissions, although it has the drawback of requiring higher memory buffers in the UE than chase combining.
Fig 29 HARQ
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During the scheduling phase, the MAC-hs layer will give priority to retransmissions over new RLC packets, which will be transmitted with the same code as the srcinal transmission. HARQ can be used in the stop-and-wait mode or in the selective repeat mode. Stop-and-wait is simpler, but waiting for the receiver’s acknowledgement reduces efficiency; thus multiple stop-and-wait HARQ processes are often done in parallel in practice. When one HARQ process is waiting for an acknowledgement, another process can use the channel to send more data. There are a few aspects to consider for the HARQ mechanism: • If Node B receives an ACK from a UE, everything is fine.
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If Node B receives a NACK from a UE, it means that the packet was received, but could not be detected properly. In this situation, Node B should retransmit using incremental redundancy.
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If Node B never receives any ACK/NACK, it should retransmit using another selfdecodable rate matching scheme.
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3.7
Fast packet scheduli ng
The objective of the packet scheduler is to optimize the cell capacity while delivering the minimum required service experience for all active users excluding an allowed outage target. Outage is defined from blocking, dropping and QoS requirements related to a given application.
Fig 30 Fast Packet Scheduling
The actual packet scheduling algorithm is not specified in 3GPP and there is a large degree of freedom available to manufacturers. However, with the definition of various QoS parameters such as discard timers and guaranteed bit rates, it is expected that the packet scheduler does its best to fulfill the requirements given for any user. This is especially significant in multivendor environments (e.g. RNC and node B from different where the QoS responsibility is distributed. packet scheduler needs to vendors) be flexible and adjustable by the parameters definedThe in 3GPP/release 5, including some indirect parameters such as complying with the power targets specified from the RNC. Different approaches have been proposed for the packet scheduler, the simplest approach is round-robin scheduling, or best effort, which performs a ‘blind’ allocation of resources without using quality information. It has low complexity and allows a fair distribution of power and code resources among users.
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On the other hand, algorithms like proportional fair scheduling, use information of user quality and fast fading behavior to select the most appropriate transmission turn for each user. This scheduler has a higher complexity, but can provide 20–60 % gains in throughput compared with round-robin scheduling. The gain depends on the number of HSDPA users in the cell, the radio conditions and transmitted power. UE capabilities also have an effect on scheduling. The UE’s ability to receive data depends on the UE category it supports. Category 1 and 2 UEs can receive data in every third TTI. Categories 3, 4 and 11 UEs can receive data in every second TTI. The remaining UE categories can receive in every TTI.
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Impact on the Iub Interface
With higher throughputs in HSDPA, there is a high probability of congestion between Node B and the RNC, which requires careful planning of the Iub interface. A careful flow control mechanisms needs to be introduced in order to avoid a strong reduction of data rates due to ATM discards, which would generate RLC retransmissions and TCP window reduction. The understanding in 3GPP is that Node B is in control of the flow, so it will send capacity allocation messages to the RNC. Node B knows the status of the buffers in the RNC from the capacity request, and uses this message to modify the capacity at any time, irrespective of the reported user buffer status. Two messages are defined for Iub flow control, as shown in Figure B.5. •
The HS-DSCH capacity request procedure allows the RNC to request HSDSCH capacity by indicating the user buffer size in the RNC for a given priority level.
The HS-DSCH capacity allocation is used by Node B to allocate resources for a given flow. It includes a number of parameters: the number of MAC-d PDUs that the RNC is allowed to transmit for the MAC-d flow (HSDSCH credits), the associated prioritythe level indicated, the maximum MAC-d PDU length, the time interval which HSDSCH credits granted may be transmitted (HSDSCH interval)during and the number of subsequent intervals that the HSDSCH credits granted may be transmitted (HSDSCH repetition period). There are different possible approaches for the flow control algorithm, but they need to be a trade-off between performance and implementation complexity. For instance, a simple flow control implementation may consist in sending periodic capacity allocations that either follow a round-robin approach among the UEs or is based on the MAC-hs or has RLC buffer status. However, this kind of implementation can have constraints regarding the guaranteed bit rate and number of simultaneous users. On the other hand, a more advanced flow control may take into account the buffer status in Node B, the guaranteed bit rate, scheduling priority, the buffer status in the RNC, the air interface bit rate and the discard timer for sending the capacity allocation messages.
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3.9
Handset Capabilities
HSDPA handsets are becoming more complex, due to the addition of several new features to existing 3G devices in order to achieve maximum capability, such as support of the 16-QAM modulation method, a roadmap for advanced receivers (equalizer, diversity), HARQ in layer 1, a faster turbo decoder, the need for increased and faster buffer memory, etc. The UE capabilities, presentedMAC-d in Table B.4, sent fromThey the serving (SRNC) to Node B when the HSDSCH flow is are established. includeRNC among others:
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The maximum number of bits a UE can receive within one TTI;
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The maximum number of HSDSCH codes the UE can receive simultaneously;
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The minimum inter-TTI arrival;
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The total buffer size minus the RLC AM buffer size;
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Five main parameters used to define the physical layer UE capability level (3GPP TS 25.306):
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The maximum number of HSDSCH multicodes that the UE can simultaneously receive; at least five multicodes must be supported in order to facilitate efficient multicode operation;
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The minimum inter-TTI interval, which defines the distance from the beginning of a TTI to the beginning of the next TTI that can be assigned to the same UE; e.g. if the allowed interval is 2 ms, this means that the UE can receive HSDSCH packets every 2 ms;
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The maximum number of HSDSCH transport channel bits that can be received within a single TTI
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The maximum number of soft channel bits over all the HARQ processes;
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If the UE supports 16-QAM (e.g. code efficiency limitation);
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Parameters are also available for specification of the L2 buffer capability (RLC+MAC). A UE with a low number of soft channel bits will not be able to support IR for the highest peak data rates and its performance will thus be slightly lower than for a UE supporting a larger number of soft channel bits.
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Exercises
1. List the different duplex methods and their advantages and drawbacks? FDD
2. Why we use stratification in UMTS?
3. Why cell are changing of boundaries ( breathing ) ?
4. What are the consequences of the cell breathing? 5. List different kinds of HO? 6. How we can make initial Access to the system?
7. Explain the HO mechanism
8. What is the difference between OQPSK and QPSK? 9. When the 16-QAM modulation is used ?
10. What is AMC and why we use it ?
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Solution
1. List the different duplex methods and their advantages and drawbacks? FDD •
Advantages: Using this method we can avoid collision between uplink and downlink.
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Drawbacks: Frequency resources are wasted
2. Why we use stratification in UMTS?
UMTS network is divided into two levels, the Access Stratum and Non Access stratum, these two levels correspond to a repartition of logical functions within the network. 3. Why cell are changing of boundaries ( breathing ) ? As there is incoming and outgoing HO then the interference within one cell will change and then the coverage of one cell will change 4. What are the consequences of the cell breathing? As the cell is changing boundaries due to cell breathing we have to reconsider the HO margin during planning for the mobile station that is near these boundaries 5. List different kinds of HO? Softer HO Soft HO Hard HO Intersystem HO Inter-frequency HO …
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6. How we can make initial Access to the system?
We send a RACH the mobile send a first preamble in that RACH and then wait for an indication AICH from the network that a first preamble was received and then it send a second preamble.
7. Explain the HO mechanism •
Measurement report
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Decision according to the threshold setting and algorithms for HO
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Execution and allocation of resources
8. What is the difference between OQPSK and QPSK? The difference is that there is no transition between intermediate states symbols when we have to transmit two opposite symbols
9. When the 16-QAM modulation is used ? HSDPA uses 16-QAM for transmitting 4 bit/ symbol and then achieving a higher data rates 10. What is AMC and why we use it ? AMC is the adaptative Modulation codec scheme we use it with HSDPA the principal is that the modulation is changed according to the air i nterface radio conditions allowing then the signal more robustness
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