NANDU’S ROUGH GUIDES
Rough Guide to 3G and HSPA (Radio Part) An Effort to Understand the Complexities of 3G/HSPA Nandakumar Nair 10/24/2009
[email protected]
This rough guide is an effort to help RF engineers who are interested in learning the basic principles of 3G/HSPA and apply in their daily work.
Contents 1.
INTRODUCTION
5
2.
GENERAL CONCEPTS OF 3G/UMTS/WCDMA
5
2.1
3G : General Information
5
2.2
UMTS Network
5
2.3
Why do we need 3G? Is 2G not enough?
6
2.4
What is the main difference between 3G and 2G?
7
2.5
Why does 3G have less coverage compared to GSM900?
7
2.6
Why is WCDMA called Wideband CDMA?
7
2.7
What are the frequency bands used in 3G?
7
2.8
QOS Classes in 3G
8
2.9
What are the main services available (and used) in 3G/HSPA as of October 2009?
8
2.10
Main difference in performance between R99 Packet and HSDPA
8
3.
TECHNICAL CONCEPTS OF 3G/UMTS/WCDMA
9
3.1
Noise Floor
9
3.2
Pilot
9
3.3
Ec/No, RSCP
9
3.4
Codes
10
What is the difference between Scrambling, Spreading and Channelization Codes?
10
3.5
Scrambling Codes
10
3.6
Spreading Codes
11
3.7
Spreading Factor
12
3.8
Spreading and Processing Gain : What do they mean for us?
13
3.9
Soft & Softer Handover
14
2
Contents 1.
INTRODUCTION
5
2.
GENERAL CONCEPTS OF 3G/UMTS/WCDMA
5
2.1
3G : General Information
5
2.2
UMTS Network
5
2.3
Why do we need 3G? Is 2G not enough?
6
2.4
What is the main difference between 3G and 2G?
7
2.5
Why does 3G have less coverage compared to GSM900?
7
2.6
Why is WCDMA called Wideband CDMA?
7
2.7
What are the frequency bands used in 3G?
7
2.8
QOS Classes in 3G
8
2.9
What are the main services available (and used) in 3G/HSPA as of October 2009?
8
2.10
Main difference in performance between R99 Packet and HSDPA
8
3.
TECHNICAL CONCEPTS OF 3G/UMTS/WCDMA
9
3.1
Noise Floor
9
3.2
Pilot
9
3.3
Ec/No, RSCP
9
3.4
Codes
10
What is the difference between Scrambling, Spreading and Channelization Codes?
10
3.5
Scrambling Codes
10
3.6
Spreading Codes
11
3.7
Spreading Factor
12
3.8
Spreading and Processing Gain : What do they mean for us?
13
3.9
Soft & Softer Handover
14
2
3.10
Power Control
15
3.11
Achievable Speeds in 3G
17
3.12
What factors affect the data rates available to a user?
18
3.13
What are the main issues in a real 3G network?
18
3.14
What is the difference between RAB and RB?
20
4.
HSDPA
20
4.1
HSDPA – Techniques
21
4.2
HSDPA Channel Structure
21
4.3
Advantages of HSDPA over R99
22
4.4
What is the maximum possible speed in HSDPA?
23
4.5
Why is CQI important?
24
4.6
Limiting factors of HSDPA
25
5.
EUL
25
5.1
EUL – Techniques
25
5.2
EUL - Channels
26
5.3
Achievable Speeds in EUL
26
6.
HSDPA & EUL
27
6.1
Resource Utilization in HSDPA and EUL
27
6.2
Difference between HSDPA and EUL
28
7.
KPIS
28
8.
CAPACITY MANAGEMENT
28
9.
NETWORK ELEMENTS UTILIZATION
29
10. INTER-RAT & INTER-FREQUENCY HANDOVERS 3
31
10.1
Inter-RAT Handovers
31
10.2
Inter-Frequency Handovers
32
10.3
Compressed Mode
32
11.
WHAT NEXT AFTER HSPA?
11.1
HSPA+
33
11.2
MIMO
34
11.3
Dual Carrier HSPA
34
11.4
Continuous Packet Connectivity
35
12.
33
APPENDIX
12.1
UE Categories
37
12.2
Modulation Schemes
38
12.3
SIB List
39
12.4
UTRAN Protocols
39
Acknowledgements & References
40
4
Rough Guide to 3G and HSPA 1. Introduction This Rough Guide has been written with the objective of aiding those, who already have some experience with 3G. Prior knowledge will be helpful for deeper understanding of the material presented in this guide. Please note that only WCDMA is considered in this guide and for 2G, only GSM is considered. Most of the topics covered are Radio related. Core Network details are not explained.
2. General Concepts of 3G/UMTS/WCDMA 2.1
3G : General Information
UMTS – Universal Mobile Telecommunications System Provides mainly Speech, Video, R99 data and HS services 3GPP Releases Rel 99 Rel 5 Rel 6 Rel 7 Rel 8 Rel 9 Rel 10
2.2
3G UMTS HSDPA EUL HSPA + LTE, All IP network (SAE) SAES Enhancements, WiMax and LTE/UMTS Interoperability LTE advanced
UMTS Network
UMTS can be considered as an evolution of GSM. While UMTS has its own radio access network known as UTRAN ( UMTS Terrestrial Radio Access Network ), usually UMTS and GSM/EDGE have a shared Core Network. Generally UMTS networks are built up on existing GSM networks and both networks co-exist. UMTS networks in general have lesser coverage due to the fact that most of them operate at higher frequency bands. This is not a big issue as UMTS-GSM handover is possible.
5
Please note that the network below has a common core network for both 3G and 2G.
Fig 1: UMTS/GSM Network
2.3
Why do we need 3G? Is 2G not enough?
3G gives much higher data rates compared to 2G. 2G was mainly designed keeping in mind the requirements for Speech traffic. 3G has been developed mainly to cater to data services, in addition to Speech traffic. Multiplexing of services with different QOS requirements on a single connection is possible with 3G.
6
2.4
What is the main difference between 3G and 2G?
WCDMA 5MHz
Carrier Bandwidth Frequency Re-use Factor Frequency Diversity
Packet Data
1 Multipath diversity with rake receivers achieved with 5MHz bandwidth Load based Scheduling
Power Control Frequency
2.5
1500Hz
GSM 200kHz
1-18 Frequency Hopping
Time Slot based Scheduling with GPRS 2Hz or lower
Why does 3G have less coverage compared to GSM900?
GSM900 works at a lower frequency band than 3G, which usually works at the 2GHz band. Lower frequency signals are attenuated less, which gives them greater propagation capability.
2.6
Why is WCDMA called Wideband CDMA?
WCDMA has a higher bandwidth of 5 MHz compared to IS-95(cdmaOne), which has only 1.25MHz.
2.7
What are the frequency bands used in 3G?
FDD – Frequency Division Duplexing is mainly used for UMTS. Hence, for uplink and downlink, we have different frequency bands. UL – Uplink (mobile to base station)
1920-1980 MHz
DL - Downlink (base station to mobile) 2110-2170 MHz
Point to remember: Generally, operators are given 5MHz Carriers and can have one or more carriers depending on the operator requirements as well as frequency band availability.
7
2.8
QOS Classes in 3G
CSIB – Conversational, Streaming, Interactive, Background
Traffic Class
Conversational (Real Time)
Streaming (Real Time)
Interactive (Best Effort)
Background (Best Effort)
Basic Features
- Preserve time relation (variation) between information entities of the stream - Conversational pattern (stringent and low delay )
- Preserve time relation (variation) between information entities of the stream
- Request response pattern
-Destination is not expecting the data within a certain time
-Preserve payload content
-Preserve payload content
voice
streaming video
Example of the application
web browsing
emails
2.9 What are the main services available (and used) in 3G/HSPA as of October 2009?
Service CS12 CS64 PS64 PS128 PS384 HSDPA EUL –
– Speech Service with 12.2 kbps dedicated channel – Video Telephony with 64kbps dedicated channel Packet Switching with 64kbps dedicated channel – Packet Switching with 128kbps dedicated channel – Packet Switching with 384kbps dedicated channel High Speed Downlink Packet Access – shared channel Enhanced Uplink
2.10 Main difference in performance between R99 Packet and HSDPA
-
R99 Packet service requires dedicated channels whereas HSDPA users have a shared channel Speeds of HSDPA are much higher compared to 3G(R99). In real networks, an average HS subscriber gets around 5-8 times throughput, compared to an R99 data user. We can easily say that an average HS user can get between 1100kbps to 2000kbps..whereas an average R99 user can get around 250- 280kbps. 8
Ofcourse, all these values depend on the configuration of the network. For example speeds of about 6Mbps was reported during random field tests in one of the networks in Kuwait. Introduction of higher capacity UEs as well as higher modulation schemes will further increase the HS throughputs.
3. Technical Concepts of 3G/UMTS/WCDMA 3.1
Noise Floor
Main idea of WCDMA is to spread the User signal over the whole band, pushing the signal under the noise floor. Only a receiver with knowledge of the correct PN (pseudorandom noise) sequence can detect the signal. Any other receiver will see only the noise. Hence the security is high.
3.2
Pilot
Pilot coverage decides the coverage boundary for a particular site. Proper Pilot power planning is very important. Too-weak pilot will lead to coverage holes, whereas too-strong pilots will lead to overshooting and interference.
Point to remember : In real networks Pilot power normally varies between 27-33dBm. Usually in urban areas Pilots have values between 27-30dBm.
3.3
Ec/No, RSCP
Ec/No signifies the level difference be tween received pilot signal and the overall noise floor. No is the noise floor, which signifies all the signals (useful and interfering) present at the receiver side. For example: A value of Ec/No= -8dB tells us that t he spread signal is 8 dB below the noise floor
Higher the Ec/No value, the better it is…. Please note that the existing receivers have rake receiver functionality which enables them to decode multiple pilots and use them accordingly based on their strength. For example: If there are 3 pilots present….the mobile receiver will compare Ec1/No, Ec2/No and Ec3/No and decide which pilot will be the best server. More details are provided in Handover and Pilot Pollution Sections. RSCP : Received Signal Code Power is the received power on one code after despreading, defined on the pilot symbols.
Ec/No = RSCP/RSSI 9
3.4
Codes
What is the difference between Scrambling, Spreading and Channelization Codes?
Spreading Code = Channe lization Code
Usage
Length No: of Codes
Channelization Code/ Spreading Code DL – Separation of DL dedicated user channels UL – Separation of Data and Control channels from the same terminal Variable Depends on SF
Scr mbling Code DL – Separation of Cells (Sectors) UL – Separation of UEs
Fixed DL – 51 UL – Unlimited (Millions)
Fig 2: Usage of Scrambli g Codes and Channelization codes
3.5
Scrambling Co es
Downlink Scrambling C des
3 types of scrambling c des are available in DL: primary, seconda ry and alternative. Downlink primary scram ling codes are used for cell separation. One rimary scrambling code, is allocated for eac h cell. Secondary scrambling codes are no used. Alternative scrambling codes can be u sed in compressed mode. 10
How many Scrambling Codes are available in DL? – 512
Uplink Scrambling Codes
2 types of scrambling codes are available in UL : long and short. Only the long ones are used. Uplink scrambling codes are used for separating the different UEs in the same cell. RNC allocates the code.
3.6
Spreading Codes
Downlink Spreading Codes (Channelization Codes)
DL spreading codes differentiate the dedicated user connections/channels within one cell. Ideally they are orthogonal to each other, though due to multipath propagation, some orthogonality might be lost. Channelization codes are managed with the help of a code-tree. Basic rule is that codes are orthogonal, if they do not descend from an already used code. If a code is used, then all the codes below and above on the same branch are unavailable for service. Resource manager keeps track of the codes allocated so that orthogonality of the code tree is preserved.
Fig 3: Code Tree for orthogonally spreading codes
Example : Code management with the help of the code tree
If code C2(0) in the Tree of orthogonal spreading codes (in the figure above) is allocated, then:
11
All codes below it in the same branch become unavailable, starting with C3(0) and C3(1), then, on the next level, C4(0), C4(1), C4(2) and C4(4), and so on. All codes above it in the same branch to root become unavailable, that is, C1(0) and C0(0) cannot be assigned to any user . Spreading codes of some channels (mainly Pilot and P-CCPCH) are fixed. Spreading codes for all other downlink physical channels are allocated by the resource manager.
3.7
Spreading Factor
Higher the bit rate of the data service, lesser the spreading factor. Service Half Rate – AMR Speech CS64
Spreading Factor 256 128 32
PS64 32 PS128 16 PS384 8 HSDPA 16 Table giving DL spreading factors for different services
Points to remember :
Usually UL spreading factor for a service is half the value of that in the DL (when the RAB bearer rates are the same in both UL and DL). For example: DL SF for speech(AMR12.2) service is 128, where as in UL, it has a SF of 64. Why should we avoid pulsed transmission in the UL?
During the silent periods, only information for link maintenance purposes are needed in UL direction. A typical example is Power Control commands at 1.5KHz which can interfere with the telephony voice frequency band. To avoid audible interference to audio devices in UL, data and control channels are not time multiplexed in WCDMA. Continuous transmission is achieved with I/Q code multiplexing or by using parallel control and data channels.
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3.8
Spreading and Processing Gain : What do they mean for us?
In WCDMA, the baseband signal is spread using a spreading code… By spreading, 1) Baseband signal is spread over the entire spectrum (3.84MHz), with help of a spreading code 2) Overall noise floor rises, but the baseband signal is hidden below the noise floor and hence difficult to detect 3) Effect of Narrow-band interference is reduced, since only a small part of the signal will be affected and data can be recovered with effective techniques 4) Effect of Multipath fading is also reduced 5) Higher the bit rate of the service, lower the SF (Speech SF = 128, PS384 SF= 8) and lower the processing gain Despreading is done at the RX side. By despreading 1) We get the baseband signal back and gain from the processing gain. Point to remember : Spreading and despreading can be considered as a process of pushing the actual baseband signal below the noise floor and then retrieving it.
Processing Gain = 10 log (chiprate / bit rate) To get a good service, the requirement is
Rx Sig Level + Processing Gain > Eb/No Eg: PG for speech = 10 log ( 3.48Mcps / 12.2Mbps) = 25dB Eb/No requirement for speech = 5dB (for good service) Rx sig level = 5 – 25 = -20dB (which implies that even if the received signal is 20 dB below the noise floor, the WCDMA receiver can detect the speech signal). In GSM, the C/I requirement is about 9-12dB. This directly gives an advantage of about 20-25 dB for WCDMA.
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3.9
Soft & Softer Handover
Soft handover is the condition in which the UE is connected to more than one NodeB at the same time. While in connected mode, UE continuously measures the neighbouring signals and compares the measurement results with specific handover thresholds set by the operator. When the threshold is exceeded, UE sends a measurement report to the RNC. RNC decides if the SHO should take place. Soft Handover is also called MEHO – Mobile Evaluated Handover Active Set : Set of cells which are in soft handover. There are 3 types of Soft Handover 1) Handover between sectors in the same site (Softer Handover) 2) Intra-RNC SHO 3) Inter-RNC SHO Majority of Soft handovers are usually Intra-RNC SHO.
Advantages of SHO: 1) Seamless handover without disconnection of RAB 2) Macro diversity gain..achieved in both UL and DL due to the combining of signals from different cells 3) Better performance in areas where a single cell is not strong enough Disadvantages of SHO 1) Increased consumption of radio resource as one UE in SHO, will use more than one radio link at a time Point to remember : SHO is kept in mind during the initial planning and ideally an overhead of 30-40% is assumed.
Events
Mobile sends Measurement Report to RNC, when certain thresholds are crossed. For SHO, it is important to know Event 1a, 1b, 1c and 1d. Event 1a : addition of a new cell to the Active Set
Event 1b: deletion of a cell from the Active Set 14
Event 1c: replacement of weaker cell in Active Set by another stronger cell (not in the Active Set) Event 1d : replacement of best cell in Active Set by a stronger cell (from Active Set, Monitored Set or Detected Set)
3.10 Power Control Main purpose of Power control mechanism is to 1) maintain the quality of service 2) minimize the transmitted power in both UL and DL
In WCDMA, downlink transmitted power determines the interference and hence the air interface capacity. So it is important to avoid excessive transmission in DL. A single UE can create problems with excessive transmission in the UL. Power control mechanism takes care of this. Power control is done on both common and dedicated channels. Power control in common channels ensure that sufficient coverage is available to setup UE-originating and UE-terminating calls as well as data transfer on RACH and FACH. Power control in dedicated channels ensure that connection quality is maintained in terms of BLER (Block Error Rate) There are mainly 3 types of power control. 1) Open loop power control 2) Closed loop power control (Fast Power Control) 3) Outer loop power control Open Loop Power Control – When the UE accesses the system it first sends a preamble
and waits for a response from the NodeB. If this expected response, AI (Acquisition Indicator), is not obtained, the UE transmits another preamble with slightly higher power. The process of ramping up preamble power continues till either a response is obtained from the NodeB or the allowed number of preamble steps are used. When the maximum number of steps in a preamble cycle is used, another preamble cycle is started, which in turn is limited by a maximum number of preamble cycles set by the operator.
15
Point to remember : Three parameters are controlled by the operator in the case of Open loop power control ( preamble step, number of preamble steps in a preamble cycle and the number of preamble cycles).
Closed Loop Power Control (Fast Power Control) – setting of TX power based on SIR
target (in NodeB). Done with a frequency of 1500Hz. UE and BTS continuously compare the actual SIR of the received signal with a target SIR. Based on the comparison, BTS/UE tells the UE/BTS to either increase or decrease the transmission power. Outer Loop Power Control – setting of SIR target based on Frame quality (in RNC).
Outer loop power control aims to provide the required quality in both UL and DL, by monitoring the BLER of the received signal. Based on the BLER, the SIR target for the Fast Power Control is increased or decreased. For example: if the received BLER is not meeting the expected quality, then the SIR target is increased and if the received BLER is higher than the expected quality, then the SIR target is decreased.
Fig 4: Power Control Mechanism
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3.11 Achievable Speeds in 3G
How do we get the speed of 2Mbps for R99 ?
Data Rate = Chip Rate / Spreading Factor In R99, 3 codes with SF4 gives the max possible data rate. Data Rate for one SF4 code = (3.84Mcps / 4 ) = 960ksps ksps = kilo symbols per second Since R99 uses only QPSK, 1 symbol = 2 bits Hence, Data Rate = 480ksps = 960 * 2 bits = 1920kbps = 1.92Mbps For 3 SF4 codes, data rate = 3 * 1.92Mbps = 5.76 Mbps
BUT, Data Rate = Net User Data + Channel Code Redundancy + Control Data After taking out Channel Code Redundancy and Control data, Net User Data == 2Mbps (the above value is for one sector with one carrier)
Point to remember : The code rate used in R99 is 1/3
Why do we have 384kbps as the max possible data for a single R99 Packet user in 3G?
Currently PS384 is the highest RAB available in DL for R99 Packet users. SF for PS384 = 8 Data Rate for one SF4 code = (3.84Mcps / 8 ) = 480ksps ksps = kilo symbols per second Since R99 uses only QPSK, 1 symbol = 2 bits Hence, Data Rate = 480ksps = 480 * 2 bits = 960kbps
BUT, Data Rate = Net User Data + Channel Code Redundancy + Control Data After taking out Channel Code Redundancy and Control data, Net User Data == 384kbps (max possible)
17
How do you calculate maximum possible speed in HSDPA?
Using the formula (data rate = chiprate/spreading factor), 1 QPSK code at SF16 = 480kbps 1 16-QAM code at SF16 = 960kbps 1 64-QAM code at SF16 = 1440kbps
For HSDPA after applying ¾ coding rate 1QPSK Code = 360kbps 1 16-QAM Code = 720kbps 1 64-QAM Code = 1080kbps 10 codes with 16QAM = 720 * 10 = 7200 kbps = 7.2Mbps 15 codes with 16QAM = 720 *15 = 10.8Mbps (max per cell or sector) 15 codes with 64QAM = 1080 *15 = 16.2Mbps (max per cell or sector)
Theoretical max of HSDPA with one carrier = 15 Codes * 1440kbps = 21.6Mbps for a single carrier (assuming coding rate of 1, which is impossible in actual conditions)
3.12 What factors affect the data rates available to a user? -
User position in the cell Interference from other users and neighbouring cells Number of subscribers accessing the same cell Speed of the customer (if he is mobile)
3.13 What are the main issues in a real 3G network?
Pilot Pollution (Improper Pilot Power Planning)
Main objective of Pilot planning is to have a dominant signal at a given place. In practice, this is difficult to achieve. 2 to 3 strong signals are still ok, since Soft handover will manage the situation. But if you have more signals coming at the same place with more-or-less equal strength, then the UE gets confused and cannot correctly decode the
18
signals due to low Useful Signal-to-Interference ratio (Ec/No) and hence the call gets dropped. Points to remember:
-
Strive to have ONE dominant Pilot signal at a given place. In Ec/No, Ec is good (as long as there is no pilot pollution). No is interference.
Power, Tilt and Azimuths optimization mainly used to avoid pilot pollution. Missing Neighbour Definitions
This can be observed on Field with Tems or any monitoring tool (as Detected Set). When the UE is getting a strong signal which is not defined as a neighbour to the existing cells in the Active Set, the new signal adds to the interference. Soft handover does not take place and as a result Ec/No degrades. As a result the call drops when the new signal is about 15dB higher than the cells in the Active set. Improper UEs
Though not observed on a wide scale, this can be a problem. A malfunctioning UE can cause many problems like -
Demanding too much power from the base station In-efficient channel switching Excessive transmission of power in UL
IRAT HO Parameter Definition
-
Improper definitions can lead to un-necessary handover between 3G and 2g. This can be a problem especially for indoor customers using HSDPA or data services. Overall throughput of the data user will be affected due to unnecessary handovers/cell changes.
Cell Breathing
With more and more users coming into a cell, the actual power available for services is lesser than the power available in an empty cell. So the overall coverage of the cell shrinks. Cell breathing is more of a planning issue and has to be considered at the planning stage itself. Proper handover regions should be planned, to avoid any coverage gaps.
19
Prioritizing Neighbours
More efficient handover can be achieved by proper prioritization of neighbours. It is possible to give higher priority for some cells than to other cells, so as to make sure that chances for a handover is higher between certain cells. This parameter can also be used to avoid handover in certain locations between certain cells to some extent. Improper allocation of priority can lead to bad handover decisions. Low Sites
One major mistake RF planners did in the beginning was to install low sites for UMTS, thinking mistakenly that since interference is to be avoided in UMTS, it is better to have low sites with lower coverage areas. In actual practice, low sites are generally problematic as they overshoot and contribute to Pilot Pollution. Down-tilting of low sites can lead to coverage holes…(we should keep in mind that down-tilting is an efficient way of reducing overshooting).
3.14 What is the difference between RAB and RB? RAB – Radio Access Bearer – Link between UE and Core (Radio + Iub + Iu) RB – Radio Bearer – Link between UE and RNC (Radio + Iub)
4. HSDPA HSDPA has a fixed spreading factor of 16. Multiple codes can be reserved for HSDPA at this SF level and depending on the number of codes available, the speed varies. Details are given in the section What is the maximum possible speed in HSDPA? Generally operators reserve 5 or 10 codes per carrier (out of the 15 available) for HSDPA service, which implies that these codes are not available for other R99 services like Speech, CS64 and PS. There are different ways of code allocation for HSDPA, and this varies from vendor to vendor. When there is a shortage of codes, due to higher traffic, the operators can go for a second carrier. Operator can decide how to distribute HS and R99 traffic in different carriers. It is also possible to have a carrier fully allocated to HS, which implies that 15 codes will be available solely for HS and no other services will be possible in that carrier. Point to remember: Greater the number of codes you reserve for HS, lesser the resources available for R99 services.
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4.1
-
HSDPA – Techniques
Shared Channel Transmission (enabling one user to have more than one code) Shorter TTI (2ms) Higher Modulation Technique (16QAM ) Hybrid ARQ Retransmission Faster Scheduling based on Radio conditions Better Scheduling Techniques(code rate, modulation technique)
Fig 5: HSDPA Techniques
4.2
HSDPA Channel Structure
In addition to the new downlink shared channel HS-DSCH, some control channels are also required for HSDPA. Mainly they are HS-SCCH and HS-DPCCH.
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Channel HS – DSCH
Direction DL
HS-SCCH HS-DPCCH
DL UL
A-DCH
UL and DL
Content
User Data Control information to address UEs and information for decoding the transport block. UEs can see upto 4 HS-SCCH ACK/NAK, CQI SRB (Control signaling: RRC and NAS) in DL SRB and User data in UL
Table giving HSDPA Channels and related R99 Channels
Fig 6: HSDPA Channels
4.3
-
Advantages of HSDPA over R99
Faster Retransmission (due to control in NodeB), leading to much lower RTT
22
Fig 7: Retransmission methods in R99 and HSDPA
As seen in the picture above, in case of R99, retransmission decision is taken in the RNC (RLC layer), whereas in HSDPA, the retransmission decision is taken in NodeB (MAChs layer). This leads to a great reduction in overall RTT (Round Trip Time) -
More codes used by a single user, hence higher throughputs Shorter TTIs, hence better response time and RTT 16QAM is not used in R99 Soft Combining of re-transmission
Point to remember : There are mainly 2 types of scheduling in HSDPA – Round Robin and Proportional Fair. Round Robin scheduling, allocates resources to every user in a round robin manner regardless of the radio conditions, the users are in.
Proportional fair scheduling takes into account, the radio conditions also and tries to improve the overall cell throughput by giving slightly higher preference to users in better radio conditions. In actual testing conditions, not much difference in overall cell throughput was observed between the two scheduling techniques and since Round Robin scheduling came free of charge, with most vendors, it was the preferred scheduler.
4.4
What is the maximum possible speed in HSDPA?
Check section 3.11
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4.5
Why is CQI important?
CQI is the feedback which the system receives from the UE and it mainly indicates the radio condition of the UE. Depending on the CQI values, NodeB scheduler allocates resources to the UE. Higher the CQI, better the network. An average CQI value of about 22 and above, indicates a reasonably good network. CQI values less than 17, indicates a low quality network and optimization is required.
Fig 8: Overall picture of how radio conditions affect HS Throughput and Power Requirement
The figure above summarizes the tests conducted for a HS user in both bad and good radio conditions. In all the 3 graphs above, the left side represents a user in bad radio condition and the right side represents a user in a good radio condition.
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A user in a very bad radio condition reports an average CQI of 14, whereas the same user in excellent radio conditions reported an average CQI of 26. In bad radio conditions, the user consumed much more power, though he got almost the same throughput as the user in good radio condition. Points to remember :
-
-
4.6
It is very important to have a HS user in good radio conditions, since higher throughputs can be achieved with lesser transmitted power, leading to increased capacity for the system. For higher order modulations to work, CQI values should be high.
Limiting factors of HSDPA
Channelization Codes, Modulation Scheme, Channel Elements, Power, Simultaneous users, UE Category
5.
EUL
Main idea of EUL is to effectively use the interference headroom available in the uplink. Currently achievable peak individual user throughputs are around 1.4 to 2Mbps in EUL where as it is 384 kbps with R99. Overall, EUL should give higher throughputs and greater capacity than R99.
For example: Assume that 4 users want to upload big amounts of data…(let us say, movies) If EUL is used, we need to have upto 32 channel elements. With R99, assuming that they are using 384 RAB in uplink, total CE requirement = 4 * 16 = 64 CEs, since each 384UL RAB requires 16 CEs. Hence in this case, 32 CEs are saved by using EUL.
5.1
EUL – Techniques
-
Hybrid ARQ with Soft Combining
-
Fast Channel Dependent Scheduling
-
Multi-code Transmission
25
-
Power Control
-
Soft Handover
5.2
EUL - Channels
Fig 9: EUL Channel Structure
5.3
Achievable Speeds in EUL
Case 1: Assuming that the UE category available can support only upto 2 SF4,
Data rate per channel = 3.84/4 = 0.96Msps 1symbol = 1bit since BPSK is used in EUL So, Data rate per channel = 0.96Mbps Since 2 channels (2 SF4) are possible, maximum rate = 0.96 * 2 = 1.92Mbps
26
After taking out all FEC, CRC, MAC-headers and L3 signaling, data rate at RLC level = 1.376Mbps Data rate at L1 (transport block level) = 1.46Mbps Point to remember: Above figure is the total bit rate achievable with EUL in one cell, when the maximum possible configuration is 2 * SF4 channels (and only BPSK is available). If we have SF2 available, we will be getting higher UL throughputs.
Case 2: Assuming that the maximum channel capacity of 2SF2 + 2SF4 is available,
Data rate per SF2 channel = 3.84/2 = 1.92Mbps Data rate per SF4 channel = 3.84/4 = 0.96Mbps Total data rate = (2*1.92) + (2*0.96) = 5.76Mbps Realistically with ¾ coding Max EUL Data Rate = 5.76 * ¾ = 4.32Mbps
Why is it NOT beneficial to have 16-QAM in EUL ?
Since UL is interference limited: -
It is better not to have power-inefficient higher-order modulation schemes
-
Cost effective design of UE power amplifier is possible with lower-order modulation schemes, since they have lesser PAR (Peak to Average Ratio) which in turn lead to lesser Electromagnetic Interference (EMI) generated by the UE.
6. HSDPA & EUL 6.1
Resource Utilization in HSDPA and EUL
In HSDPA, the shared resource is DL Transmission Power, Channelization Codes and Channel Elements In EUL, the shared resource is UL interference and Channel Elements
27
6.2
Difference between HSDPA and EUL
Spreading Factor Soft Handover Power Control Modulation Scheme Link Adaptation
HSDPA Fixed = 16 No (only A-DCH in SHO) No (Check RPA ) 16QAM & QPSK Rate Control
EUL Variable from 256-2 Yes Yes BPSK Rate & Power Control
7. KPIs Accessibility – both RRC and RAB phases considered Mobility – Soft/softer handover (30-40%), IRAT handover Retainability – Mainly Voice and HS drops. Currently the practice is to monitor Minutes/Drop Traffic – Erlangs for Speech/CS64 services, Data Volume for PS/HS services Integrity – CQI for HS, BLER for R99 (if needed)
8. Capacity Management
Main purpose of capacity management is to provide sufficient QOS and coverage for users. Admission Control and Congestion Control are the two main mechanisms used for capacity management. Admission Control ensures that a new user will be connected only if there are enough resources available for him. Congestion Control tries to keep the usage of the system within reasonable limits. For example, if there are 3 PS384 users in a cell and one of them moves into a bad signal area and requires more power to maintain the data rate, the system checks the used DL transmitted power. If it has crossed a threshold, the user is downgraded from PS384 to PS128 or to PS64, depending on the available power. By doing this, channel element 28
utilization is also reduced from 16 to 8(PS128) or 4(PS64), which effectively means that more speech users can be accommodated. Congestion control is based on 3 parameters -
Downlink overload (when the downlink transmitted power is exceeding some threshold for a set period of time) Uplink overload (when RTWP –received total wideband power exceeds a threshold for a set period of time) DL HSDPA Overload (when total power, which includes HS power exceeds a threshold for a set period of time)
Point to remember: Generally congestion control comes into play before admission control. Speech and video call users have higher priority over HS and PS users.
Admission for speech and video calls have strict criteria. Speech/video call users are connected only if dedicated resources are available for them. Data services have easier admission policies. EUL, especially has a very lenient admission policy, as connected users are allocated capacity based on availability and do not use other system resources.
Resources Monitored for Load Control:
Parameters monitored and used for capacity management are -
Downlink Transmitted Carrier Power Downlink Channelization Codes Uplink Received Total Wideband Power Interference No: of radio links in compressed mode No: of serving HS connections No: of serving connections No: of non-serving connections Node B Hardware Utilization (mainly Channel Elements)
9. Network Elements Utilization This section gives a rough idea of the parameters to be monitored to calculate the utilization of different network elements 29
-
RNC: Total Traffic, Simultaneous number of HS users, ATM connectivity, total number of NodeBs which can be connected to one RNC
-
NodeB: Channel Elements, Code Tree, DL Transmit Power
Channel Elements are one of the major hardware resource in NodeB to be planned and monitored carefully. Different services have different requirement of CEs. In most of the vendors, there is a fixed allocation of CEs for HS services. R99 services use CE when required. The tables below give sample CE requirements for different services. HS requirements are not included in these tables, as they are different for different vendors.
Spreading Factor
Bearer Data Rate (kbps)
Channel Element Requirement
128
AMR 12.2
1
32 32
64
2
16
128
4
8 384 Sample Table for DL Channel Element Requirement
8
Spreading Factor
Bearer Data Rate (kbps)
Channel Element Requirement
64
AMR 12.2
1
32
32
2
16
64
4
8
128
8
4 384 16 Sample Table for UL Channel Element Requirement Channelization Codes : With the introduction of HSPA, channelization codes have become a major limiting factor in terms of resource utilization. Since atleast 5 to 10 codes are reserved for HS, only the remaining codes are available for R99 services like Speech, CS64 and R99 Packet. Generally, vendors go for a second carrier in case of code congestion.
30
DL Transmit Power : In WCDMA, downlink is power limited , assuming that we have enough resources like CEs and channelization codes. Hence it is important to monitor the DL power consumption. We can say that Power == Capacity. We have to keep in mind that Packet users require more power compared to Speech users. Point to remember : Channel Element is a NodeB level resource. Channelization code is a cell level resource.
-
Iub: Proper planning should be done for VP/VC. Different methods are available. One of the main limitations if you have AAL2 switching is the number of CIDs available per VC. For example: If you have one STM1 link with 155Mbps, you can divide it into any number of VCs as you need.
Case 1: If you assign just one VC, you have a total of 248 CIDs available… Case 2: If you assign 10 VCs, you have 248 * 10 = 2480 CIDs available…. Assuming only voice users in the network, since each Voice user needs 2 CIDs, Total possible subscribers in case 1 = 248 / 2 = 124 speech users Total possible subscribers in case 2 = 2480 / 2 = 1240 speech users So in case1, even when there was more than enough capacity (155Mbps), we have a limitation of 128 speech users due to the definition of VC. In case2, with the same capacity available as in Case1, we have 10 times more speech users. Please keep in mind that the each HS user require 3 CIDs. Further, separate CIDs are needed for Control purpose also.
10. Inter-RAT & Inter-Frequency Handovers 10.1 Inter-RAT Handovers (event 3a) Required since 3G coverage is generally less compared to 2G. It is important to have proper parameters defined for Inter-RAT handovers (mainly UMTS-GSM). Event 2d occurs when the 3G measured quality is below a certain threshold for a certain period of time and this triggers measurement on IRAT or Inter-Frequency (depending on 31
vendor). Compressed mode measurements on 2G start after event 2d. Once event 2d is triggered, if the measured quality of 2G is above a certain threshold for a certain period of time, then event3a occurs. Actual 3G-2G handover is triggered by event 3a.
10.2 Inter-Frequency Handovers (event 2b) Required when 2 or more frequencies are implemented in a network. Event 2d occurs when the measured quality is below a certain threshold for a certain period of time and this triggers measurement on IRAT or Inter-Frequency (depending on nd vendor). Compressed mode measurements on the 2 frequency start after event 2d.Once nd event 2d is triggered, if the measured quality of the 2 frequency is above a certain threshold for a certain period of time, then event2b occurs. Actual IF HO is triggered by event 2b. Point to remember :
-
Event 3a : 3G-2G HO Event 2b : Inter-Frequency HO
Event 2f occurs when the measured quality is above a certain threshold for a certain period of time and this triggers the stopping of IRAT/Inter-Frequency measurements. Depending on the settings, when event 2d occurs, the system decides if IRAT or IF handover should take place . In some vendors both are possible. For example: In Ericsson you have to set either IRAT or IF HO, where as in Nokia it is possible to have IRAT and IF handovers from the same carrier. Event 6d occurs when the UL UE Tx power exceeds a certain threshold for a certain period of time. Event 3a (IRAT HO) or Event 2b(IF HO) follows. Event6b, occurs when the UL UE Tx power is below a certain threshold for a certain period of time. All ongoing HO attempts are aborted if DL Quality for both Ec/No and RSCP are good.
10.3 Compressed Mode Compressed mode mechanism enables the UE to carry out measurements on another frequency. Certain idle periods are created in radio frames during which the UE can perform measurements on other frequencies. No user data is lost as it is compressed in the time domain using one of the below 2 methods 32
-
Halving the spreading factor so that the same amount of data can be sent in half the time Higher layer scheduling in which layer2 restricts the high bit rate TFC (transport format combinations) so that the user throughput is reduced temporarily
Points to remember :
-
Currently, compressed mode is not used for HS-DSCH or EUL. It would be available soon. Compressed mode can be used for both UL and DL (depending on UE capability) The transmission/reception gap is always 7 slots (out of the total 15 slots in a frame)
Fig 10: Transmission Gaps created with Compressed Mode
11.
What next after HSPA?
11.1 HSPA+ HSPA+ is a natural evolution to HSPA and can be considered as an upgrade to the existing HSPA system. Many techniques are specified in HSPA+ for improved performance. They are -
MIMO Higher Order Modulation (64QAM) Multi-carrier HSPA Continuous Packet Connectivity 33
-
Enhanced Cell_FACH Voice Over HSPA
Below sections will give you a brief idea of some of these features.
11.2 MIMO Multiple Input Multiple Output involves using multiple antennas at both transmit and receive side which leads to significant increase in achievable throughputs, without the necessity for additional bandwidth or transmit power. Point to remember:
HSPA+ Rel: 7 (MIMO) can theoretically support up to 28Mbps with a single 5MHz Carrier HSPA+ Rel: 8 (Higher Order Modulation + MIMO) can theoretically support up to 42Mbps with a single 5MHz carrier
11.3 Dual Carrier HSPA (also known as Dual Cell HSPA) DC- HSPA aims to increase the available user data rates by merging 2 carriers of 5MHz each, thus making available up to 10MHz carrier bandwidth for a user. Higher Bandwidth available to a user = = Higher Throughput for the user
Basic idea of DC-HSPA is to achieve better resource utilization by means of joint resource allocation and load balancing across the carriers. Some of the features for DC-HSPA are -
New MAC entity, MAC-ehs which supports HS-DSCH transmission/reception in more than one cell served by the same Node-B New UE categories required (Categories 21 to 24) Anchor Carrier : Carrier with all physical channels (as shown below) Supplementary Carrier: Carrier with just HS-PDSCH and HS-SCCH
34
Fig 11: DC-HSPA Channel Usage in the Multiplexed Carriers
Advantages of DC-HSPA are -
Higher data rates possible compared to the 5MHz single carrier, since a user can get all the code and power resources of both carriers in a single TTI Improved load sharing due to dynamic statistical multiplexing of users at connection management level Greater frequency selectivity and improved QOS due to joint scheduling. User can be assigned resources dynamically either on the anchor or on the supplementary carrier
Point to remember : Theoretical DL throughputs achievable with DC-HSPA without MIMO is around 43.2 Mbps
11.4 Continuous Packet Connectivity In future, data users are expected to stay connected for long times, even if they are not doing anything for a majority of the time they are connected. So it will be good to avoid unnecessary transmissions during these idle periods, so as to avoid interference and reduce system resource utilization. CPC consists of two main features UE DTX/DRX and HS-SCCH-less operation.
35
UE DTX (discontinuous transmission from UE) enables the UE to switch off continuous transmission of DPCCH (Dedicated Physical Control Channel) when there is no information to be transmitted in the uplink. This leads to
-
Reduced battery consumption Reduced interference, resulting in increased uplink capacity
UE DRX (discontinuous reception at UE) enables the UE to switch off their receivers, when there is no data to be received in downlink. This also leads to reduced battery consumption.
Services like VoIP, require transmission of lots of small packets in DL. This leads to significant overhead due to the HS-SCCH control channel. One solution to this problem is to remove HS-SCCH transmission completely for the first HARQ transmission. This involves blind decoding of up to 4 different formats of HS-DSCH, the DL data channel.
36
12.
Appendix
12.1 UE Categories Knowledge of different categories of UEs available is essential to understand the achievable throughputs. MIMO Dual Carrier
Code rate required to achieve max. data rate
Max. data rate [Mbit/s]
Category
Max. number of HS-DSCH codes
Modulation
1
5
QPSK and 16-QAM
0.76
1.2
2
5
QPSK and 16-QAM
0.76
1.2
3
5
QPSK and 16-QAM
0.76
1.8
4
5
QPSK and 16-QAM
0.76
1.8
5
5
QPSK and 16-QAM
0.76
3.6
6
5
QPSK and 16-QAM
0.76
3.6
7
10
QPSK and 16-QAM
0.75
7.2
8
10
QPSK and 16-QAM
0.76
7.2
9
15
QPSK and 16-QAM
0.7
10.1
10
15
QPSK and 16-QAM
0.97
14.4
11
5
QPSK only
0.76
0.9
12
5
0.76
1.8
13
15
0.82
17.6
14
15
QPSK only QPSK, 16-QAM and 64QAM QPSK, 16-QAM and 64QAM
15
15
QPSK, 16-QAM
MIMO
23.4
16
15
QPSK, 16-QAM
MIMO
27.9
19
15
QPSK, 16-QAM
MIMO
35.3
20
15
QPSK, 16-QAM, 64-QAM
MIMO
42.2
21
15
QPSK, 16-QAM
DC
23.4
22
15
QPSK, 16-QAM
DC
27.9
23
15
QPSK, 16-QAM, 64-QAM
DC
35.3
24
15
QPSK, 16-QAM, 64-QAM
DC
42.2
25
15
QPSK, 16-QAM
DC + MIMO
46.8
26
15
QPSK, 16-QAM
DC + MIMO
55.9
27
15
QPSK, 16-QAM, 64-QAM
DC + MIMO
70.6
28
15
QPSK, 16-QAM, 64-QAM
DC + MIMO
84.4
Table giving UE categories for HSDPA
37
0.98
21.1
Table giving UE categories for EUL
12.2 Modulation Schemes
Fig : Constellation diagrams of different modulation schemes 38
12.3 SIB List System information is broadcast regularly to the UE on the BCCH. It contains parameters related to Cell Selection, Reselection, Location and routing registration, Handover, Power Control etc. Any parameter change in the system information is notified to all UEs in the cell by a paging message or by a system information change indication message. The table below list the different SIB messages available.
System Information Blocks
Contents
MIB
PLMN identity for serving cell, SIB Scheduling Information
SB1 SIB1
SIB Scheduling Information Paging parameters, Timers and counters in Idle and Connected mode, LA and RA updating
SIB2
URA identity list
SIB3
Cell selection and reselection parameters
SIB4
Cell selection and reselection parameters. Connected mode only
SIB5 and SIB5bis
Paging parameters, Cell and common channel configuration
SIB7
Power control on common channel
SIB11
Measurement management, Cell selection and reselection parameters
SIB12
Measurement management
SIB18
PLMN identity for GSM neighbors listed in SIB11.
12.4 UTRAN Protocols
RRC :
-
Radio Resource Control Handles control plane signaling of Layer3 signaling between UEs and RNC
NBAP :
-
NodeB Application Protocol (Iub)
Signaling protocol responsible for the control of NodeB by RNC NBAP has two parts: C-NBAP and D-NBAP
C-NBAP (Common NBAP) controls the overall functionality of the NodeB 39
