Shariq Yasin Senior RF Consultant
Long Term Evolution (LTE) is the latest step in moving forward from the cellular 3G services ( e.g. GSM to UMTS to HSPA to LTE or CDMA to LTE). LTE is based on standards developed by the 3rd Generation Partnership Project (3GPP). LTE may also be referred more formally as Evolved UMTS Terrestrial Radio Access (E-UTRA) and Evolved UMTS Terrestrial Radio Access Network (E-UTRAN The following are the main objectives for LTE. LTE . ◦ ◦ ◦ ◦ ◦
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Increased downlink and uplink peak data rates. Scalable bandwidth Improved spectral efficiency All IP network A standard’s based interface that can support a multitude of user types. LTE networks are intended to bridge the functional data exchange gap between very high data rate fixed wireless Local Area Networks (LAN) and very high mobility cellular networks.
Specialized agency of UN. Standardization of Telecommunication and allocation of Spectrum The Telecommunication Standardization Sector, ITU-T The Radiocommunication Sector, ITU-R The Telecommunication Development Sector, ITU-D
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Defined by ITU for the Global Standard for 3rdGeneration wireless communications communications in 1999 Includes six radio interfaces as a part of the ITU-R M.1457 M.1457 Recommendation: 1. 2. 3. 4. 5. 6.
WCDMA (UTRA-FDD) CDMA2000 TD-CDMA and TD-SCDMA EDGE DECT IP-OFDMA TDD WMAN (in 2007)
Created in Dec 1998, for the collaboration between telecommunication associations to develop globally acceptable 3G mobile phone system under the scope of IMT2000 Technical Specifications and Technical Reports for a 3G Mobile System based on evolved GSM core networks and the radio access technologies that they support (i.e., Universal Terrestrial Radio Access (UTRA) both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes).
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The Third Generation Partnership Project (3GPP) has approved the functional freeze of LTE (Long Term Evolution) as part of 3GPP Release 8 -11thDecember 2008 Orthogonal Frequency Division Multiplexing (OFDM) has been selected for the Downlink and Single Carrier-Frequency Division Multiple Access (SC-FDMA) for the Uplink. The Downlink will support data modulation schemes QPSK, 16QAM, and 64QAM and the Uplink will support BPSK, QPSK, 8PSK and 16QAM. Supports MIMO and SMART Antenna technology Increased downlink and uplink peak data rates. Scalable bandwidth Improved spectral efficiency All IP network A standard’s based interface that can support a multitude of user types. LTE networks are intended to bridge the functional data exchange gap between very high data rate fixed wireless Local Area Networks (LAN) and very high mobility cellular networks.
Increased peak data rate:100Mbps for DL with 20MHz (2 Rx Antenna at UE), 50Mbps for UL with 20MHz Improved spectral efficiency: 5bps/Hz for DL and 2.5bps/Hz for UL Improved cell edge performance (in terms of bit rate) Reduced latency
The key driving factors for LTE are: ◦ ◦ ◦ ◦ ◦ ◦
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Efficient spectrum utilization Flexible spectrum allocation Reduced cost for the operator Improved system capacity and coverage Higher data rate with reduced latency The E-UTRAN uses a simplified single node architecture consisting of the eNBs (E-UTRAN Node B). The eNB communicates with the Evolved Packet Core (EPC) using the S1 interface; specifically with the MME (Mobility ManagementEntity) and the UPE (User Plane Entity) identified as S-GW (Serving Gateway) using S1-C and S1-U for control plane and user plane respectively. The MME and the UPE are preferably implemented as separate network nodes so as to facilitate independent scaling of the control and user plane. Also the eNB communicates with other eNB using the X2 interface (X2-C and X2-U for control and user plane respectively).
32 Commercial Networks
ITU has introduced a term IMT-Advanced for mobile systems beyond IMT-2000 scope. In June 2008, 3GPP held two workshops on “Requirements for Further Advancements for E-UTRA” IMT-Advanced 3GPP RAN working on LTEAdvanced
Evolved NodeB (eNB) Evolved Radio Access Network (eRAN) Serving Gateway (SGW) Mobility Management Entity (MME) Packet Data Network Gateway (PDN GW)
e-Node hosts the following functions:
Functions for Radio Resource Management: Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both uplink and downlink (scheduling);
IP header compression and encryption of user data stream;
Selection of an MME at UE attachment;
Routing of User Plane data towards t owards Serving Gateway;
Scheduling and transmission of paging and broadcast messages (originated from the MME);
Measurement Measurement and measurement reporting configuration for mobility and scheduling;
MME (Mobility Management Entity) hosts the following functions:
NAS signaling and security;
AS Security control;
Idle state mobility handling;
EPS (Evolved Packet System) bearer control;
Support paging, handover, roaming and authentication.
P-GW (PDN Gateway) hosts the following functions:
Per-user based packet filtering; UE IP address allocation; UL and DL service level charging, gating and rate enforcement;
S-GW (Serving Gateway) hosts the following functions:
Packet routing and forwarding; Local mobility anchor point for handover; Lawful interception; UL and DL charging per UE, PDN, and QCI; Accounting on user and QCI granularity for inter-operator charging.
OFDMA
Modulation
Adaptive Modulation and Coding refers to the ability of the network to determine the modulation type and the coding rate dynamically based on the current RF channel conditions reported by the UE in Measurement Reports. The RF digital modulation used use d to transport the information is QPSK, 16-QAM, and 64-QAM. The pictures below show the ideal constellations for each modulation where each dot represents a possible symbol. In the th e QPSK case, there are four possible symbol states and each symbol carries two bits of information. In 16-QAM, there are 16 symbol states. Each 16-QAM symbol carries 4 bits. In 64-QAM, there are 64 symbol states. Each 64-QAM symbol carries 6 bits. Higher-order modulation is more sensitive to poor channel conditions than the lower-order modulation because the detector in the receiver must resolve smaller differences as the constellations become more dense. Coding refers to an error-correction methodology that adds extra bits to the data stream that allow error correction. Specified as fractions, Code Rates specify the number of data bits in the numerator and the total number of bits in the denominator. Thus if the Code Rate is 1/3, protection bits are added so one bit of data is sent as three bits.
LTE takes advantage of OFDMA, a multi-carrier scheme that allocates radio resources to multiple users. OFDMA uses Orthogonal Frequency Division Multiplexing (OFDM). For LTE, OFDM splits the carrier frequency bandwidth into many small subcarriers spaced at 15 kHz, and then modulates each individual subcarrier using the QPSK, 16-QAM, or 64QAM digital modulation formats. OFDMA assigns each user the bandwidth needed for their transmission. Unassigned subcarriers are off, thus reducing power consumption and interference. OFDMA uses OFDM; however, it is the scheduling and assignment of resources that makes OFDMA distinctive. The OFDM diagram in Figure 2 below shows that the entire bandwidth belongs to a single user for a period. In the OFDMA diagram, multiple users are sharing the bandwidth at each point in time.
Space Frequency Block Coding (SFBC) or Cyclic Delay Diversity (CDD)
Maximal Ratio Combining (MRC)
TX and RX diversity, used independently or together; used to enhance throughput in the presence of adverse channel conditions.
All the following functions are assigned to eNodeB(s) in the E-UTRAN Radio bearer control Radio admission control Connection mobility management Dynamic resource allocation Inter cell interference coordination Load balancing Inter RAT RRM functions
eNB Inter Cell RRM RB Control Connection Mobility Cont. MME Radio Admission Control NAS Security eNB Measurement Configuration & Provision
Idle State Mobility Handling
Dynamic Resource Allocation (Scheduler)
EPS Bearer Control RRC PDCP S-GW
P-GW
RLC Mobility Anchoring
MAC
UE IP address allocation
S1 PHY
Packet Filtering internet
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Broadcast Control Channel (BCCH) Paging Control Channel (PCCH) Common Control Channel (CCCH) Multicast Control Channel (MCCH) Dedicated Control Channel (DCCH)
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Dedicated Traffic Channel (DTCH) Multicast Traffic Channel (MTCH)
Downlink Channels ◦
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Broadcast Channel (BCH) Downlink Shared Channel (DL-SCH) Paging Channel (PCH) Multicast Channel (MCH)
Uplink Channels ◦
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Uplink Shared Channel (UL-SCH) Random Access Channel (RACH)
Downlink Channels
Physical Broadcast Channel (PBCH (PBCH): ): Carries system information for cell search, such as cell ID.
MCH
BCH
PCH
DL-SCH
Physical Downlink Control Channel (PDCCH ( PDCCH)) : Carries the resource allocation of PCH and DL-SCH, and Hybrid ARQ information. Physical Downlink Shared Channel (PDSCH ( PDSCH)) : Carries the downlink user data.
MAC Layer Physical Layer
PBCH
PMCH
PDSCH
PDCCH
Downlink Physical channels
Physical Control Format Indicator Channel (PCFICH (PCFICH)) : Carriers information of the OFDM symbols number used for the PDCCH. Physical Hybrid ARQ Indicator Channel (PHICH ( PHICH)) : Carries Hybrid ARQ ACK/NACK ACK/NACK in response to uplink transmissions. Physical Multicast Channel (PMCH (PMCH)) : Carries the multicast information.
UL-SCH
RACH
Uplink Transport channels
Uplink Channels
Downlink Transport channels
MAC Layer Physical Layer
Physical Random Access Channel (PRACH ( PRACH)) : Carries the random access preamble. Physical Uplink Shared Channel (PUSCH ( PUSCH)) : Carries the uplink user data. Physical Uplink Control Channel (PUCCH ( PUCCH)) : Carries the HARQ ACK/NACK, Scheduling Request Request (SR) and Channel Quality Indicator (CQI), etc.
PUSCH
PRACH
PUCCH
Uplink Physical channels
Multiple carrier modulation (MCM) helps in countering the frequency selective fading as the channel appears to have nearly flat frequency response for the narrow band subcarrier. The frequency range of the resource block and the number of resource blocks can be changed (or adapted to the channel condition) allowing flexible spectrum allocation. Higher peak data rates can be achieved by using multiple resource blocks and not by reducing the symbol duration or using still higher order modulation thereby reducing the receiver complexity. The multiple orthogonal subcarriers inherently provides higher spectral efficiency. The cyclic prefix (CP) is the partial repetition of the bit/symbol sequence from the end to the beginning. This makes the time domain input sequence to appear periodic over a duration so that the DFT representation is possible for any frequency domain processing. Also the duration if chosen larger than the channel delay spread, will help in reducing the inter-symbol interference.
The reference signal consists of known symbols transmitted at a well defined OFDM symbol position in the slot. This assists the receiver at the user terminal in estimating the channel impulse response so that channel distortion in the received signal can be compensated for. There is one reference signal transmitted per downlink antenna port and an exclusive symbol position is assigned for an antenna port (when one antenna port transmits a reference signal other ports are silent). Primary and secondary synchronization signals are transmitted at a fixed subframes (first and sixth) position in a frame and assists in the cell search and synchronization process at the user terminal. Each cell is assigned unique Primary sync signal.
The uplink transmission uses the SC-FDMA (Single Carrrier FDMA) scheme. The SC-FDMA scheme is realized as a two stage process where the first stage transforms the input signal to frequency domain (represented by DFT coefficients) and the second stage converts these DFT coefficients to an OFDM signal using the OFDM scheme. Because of this association with OFDM, the SC-FDMA is also called as DFT-Spread OFDM. The reasons (in addition to those applicable for OFDM for downlink) for this choice are given below: The two stage process allows selection of appropriate frequency range for the subcarriers while mapping the set of DFT coefficients to the Resource Blocks. Unique frequency can be allocated to different users at any given time so that there is no co-channel interference between users in the same cell. Also channels with significant co-channel interference can be avoided. The transformation is equivalent to shift in the center frequency of the single carrier input signal. The subcarriers do not combine in random phases to cause large variation in the instantaneous power of the modulated signal. This means lower PAPR (Peak to Average Power Ratio). The PAPR (Peak to Average Power Ratio) of SC-FDMA is lesser than that of the conventional OFDMA, so the RF power amplifier (PA) can be operated at a point nearer to recommended operating point. This increases the efficiency of a PA thereby reducing the power consumption at the user terminal.
This signal send by the user terminal along with the uplink transmission, assists the network in estimating the channel impulse response for the uplink bursts so as to effectively demodulate the uplink channel. This signal is sent by the user terminal assists the network in estimating the overall channel conditions and to allocate appropriate frequency resources for uplink transmission.
Downlink Modulation Schemes Phy Ch
Modulation Scheme
Phy Ch
Modulation Scheme
PBCH
QPSK
PCFICH
QPSK
PDCCH
QPSK
PHICH
BPSK
PDSCH
QPSK, 16QAM, 64QAM
PMCH
QPSK, 16QAM, 64QAM
Uplink Modulation Schemes Phy Ch
Modulation Scheme
PUCCH
BPSK, QPSK
PUSCH
QPSK, 16QAM, 64QAM
PRACH
Zadoff-Chu
Local cell identity = 1 Cell name = DXBL32331 Sector No. = 1 Csg indicator = False
Cell FDD TDD indication = FDD Subframe assignment = NULL Special subframe patterns = NULL
Uplink cyclic prefix length = Normal
Cell specific offset(dB) = 0dB
Downlink cyclic prefix length = Normal
Intra frequency offset(dB) = 0dB
Frequency band = 3 Uplink earfcn indication = Not configure Uplink earfcn = NULL Downlink earfcn = 1850 Uplink bandwidth(MHz) = 20M Downlink bandwidth(MHz) = 20M Cell identity = 1 Physical cell identity = 66 Additional spectrum emission = 1 Cell active state = Active Cell admin state = Unblock Cell middle block timer(min) = NULL
Root sequence index = 728 High speed speed flag flag = Low speed speed cell flag flag Preamble format = 0 Cell radius(m) = 10000 Customization bandwidth configure indicator = Not configure Customization real uplink bandwith(0.1MHz) = NULL Customization real downlink bandwidth(0.1MHz) = NULL
Advantage:
Individual network planning for LTE:
No additional feeder and connector loss for LTE; No negative impact to 2G/3G network.
Convenience and accuracy network optimization for LTE:
Individual antenna adjustment
Disadvantage:
Require more tower installation space; Require higher tower load.
Separate antenna/feeder for LTE LTE 2G/3G
LTE
LTE
LTE
4 ports antenna RRU inst. near antenna
Risks: Additional loss by co-feeder will: Reduce 11~14% cell radius Increase 26~35% site quantity (2.6GHz, 30m 7/8’’ feeder)
4 ports antenna Co-feeder
LTE
2 ports antenna Co-feeder
Conclusion:
Select the Co-antenna/feeder solution based on the real situation Need to evaluate and balance the benefits and risks of the solution
In LTE there are only Two States: ◦
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Idle Mode Connected Mode
As for UMTS, there were five UE States: ◦
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Idle Cell_FACH Cell_DCH Cell_PCH URA_PCH
Basic Principle of Cell Search:
Cell search is the procedure of UE synchronizes with E-UTRAN in time-freq domain, and acquires the serving cell ID.
Two steps in cell search:
Step 1: Symbol synchronization and acquirement of ID within Cell Group by demodulating the Primary Synchronization Synchronization Signal; Step 2: Frame synchronization, acquirement of CP length and Cell Group ID by demodulating the Secondary Synchronization Synchronization Signal.
Initial Cell Search:
The initial cell search is carried on after the UE power on. Usually, UE doesn’t know the network bandwidth and carrier frequency at the first time switch on. UE repeats the basic cell search, tries all the carrier frequency in the spectrum to demodulate the synchronization signals. This procedure takes time, but the time requirement are typically relatively relaxed. Some methods can reduce time, such as recording the former available network information as the prior search target. Once finish the cell search, which achieve synchronization synchronization of time-freq time- freq domain and acquirement of Cell ID, UE demodulates the PBCH and acquires for system information, such as bandwidth and Tx antenna number. After the procedure above, UE demodulates the PDCCH f or its paging period that allocated by system. UE wakes up from the IDLE state in the specified paging period, demodulates PDCCH for monitoring paging. If paging is detected, PDSCH resources will be demodulated to receive paging message.
Basic Principle of Random Access :
Random access is the procedure of uplink synchronization between UE and EUTRAN. Prior to random access, physical layer l ayer shall receive the following information from the higher layers:
Random access channel parameters: PRACH configuration, frequency position and preamble f ormat, etc. Parameters for determining the preamble root sequences and their cyclic shifts in the sequence set for the cell, in order to demodulate the random access preamble.
Two steps in physical layer random access:
UE transmission of random access preamble
Random access response from E-UTRAN
UE report CQI
Basic Principle of Power Control: DL Tx Power
Uplink power control determines the energy per DFT-SOFDM (also called SC-FDMA) symbol.
Downlink Power Control:
Downlink power control determines the EPRE (Energy per Resource Element);
The transmission power of downlink RS is usually constant. The transmission power of PDSCH is proportional with RS transmission power.
UL Tx Power
Downlink transmission power power will be adjusted by the comparison of UE report CQI and target CQI during theSystem adjust power control. parameters
Uplink Power Control:
Uplink power control consists of opened loop power and closed loop power control. A cell wide overload indicator (OI) is exchanged over X2 interface for integrated inter-cell power control, possible to enhance the system performance through power control. PUSCH, PUCCH, PRACH and Sounding RS can be controlled respectively by uplink power control. Take PUSCH power control for example:
PUSCH power control is the slow power control, to compensate the path loss and shadow fading and control inter-cell interference. The control principle is shown in above equation. The following factors impact PUSCH transmission power PPUSCH: UE maximum transmission power P MAX, UE allocated resource M PUSCH, initial transmission power PO_PUSCH, estimated path loss PL, modulation coding factor △TF and system adjustment factor f (not working during opened loop PC)
The uplink uses three physical channels: • Physical uplink shared channel (PUSCH): This carries the uplink shared channel. It can also carry the uplink control information, if a mobile needs to transmit data and control information at the same time. • Physical uplink control channel (PUCCH): This carries the uplink control information, if the mobile does not need to transmit data at the same time. • Physical random access channel (PRACH): This carries the random access channel. The downlink uses six: • Physical downlink shared channel chann el (PDSCH): This carries the downlink shared channel and the paging channel. • Physical multicast channel (PMCH): This carries the multicast channel. • Physical broadcast channel (PBCH): This carries the broadcast channel. • Physical control format indicator channel (PCFICH): This carries the control format indicators. • Physical downlink control channel (PDCCH): This carries the downlink control information. • Physical hybrid ARQ indicator channel (PHICH): This carries the hybrid ARQ indicators.
64QAM only used by Category 5 UE Assumption: Ideal channel conditions with optimum coding rate (approximately .98)
LTE UE testing includes a large number of test configurations. Configurations should include support for: 5, 10, 15 and 20 MHz bands ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦
UE categories 1-4 Up to 2x2 MIMO Periodic, aperiodic and closed loop CQI, PMI, RI Variety of handover scenarios UL sequence and frequency hopping All the required DCI formats DL distributed VRB (virtual RB) MU-MIMO Scheduling TTI bundling HARQ
The above are just examples. Total number of features and configurations is extremely large due to the considerable complexity of the LTE standards.
Reference signal received power (RSRP) identifies the signal level of the Reference Signal. It is defined ◦
as the linear average over the power contributions of the resource elements that carry cell-specific reference signals within the considered measurement frequency bandwidth.
Reference Signal Received Quality (RSRQ) identifies the quality of the Reference Signal. It is defined as ◦
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the ratio N×RSRP/(E-UTRA carrier RSSI), where N is the number of RB"s of the E-UTRA carrier RSSI measurement bandwidth. The measurements in the numerator and denominator shall be made over the same set of resource blocks. E-UTRA Carrier Received Signal Strength Indicator (RSSI), comprises the linear average of the total received power observed only in OFDM symbols containing reference symbols for antenna port 0, in the measurement bandwidth, over N number of resource blocks by the UE from all sources, including co-channel serving and nonserving cells, adjacent channel interference, thermal noise etc
Every EPS bearer ◦
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QoS class identifier (QCI) Allocation and retention priority (ARP)
Every GBR bearer ◦
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Guaranteed bit rate (GBR) Maximum bit rate (MBR) Non-GBR bearers, collectively Per APN aggregate maximum bit rate (APN-AMBR) Per UE aggregate maximum bit rate (UE-AMBR)
Every EPS bearer is associated with a number called the QoS class identifier (QCI). Network nodes use the QCI as a reference, so as to look l ook up the parameters that control the way in which packets from that data stream are forwarded. Example parameters include scheduling weights and queue management thresholds. Some QCI values have been standardised, and are associated with quality-of-service parameters that are listed in the table above. The parameters are as follows: • QCI: Standardised QoS class identifier. Other values can be defined by the network operator. • Bearer: Whether or not the bearer has a guaranteed bit rate. • Priority: This affects the scheduling at the network nodes. 1 is the highest priority. • Delay: Upper bound (with 98% confidence) for the delay that a packet can experience between the UE and the P-GW. • Packet error loss rate (PELR): Upper bound for the proportion of packets that are lost. (Non-GBR services can experience additional packet
Test Items Ite ms PCI (Planned) PCI (T ( Test) MIMO (2X2) Sector Swap (LTE) Sector Swap (3G) if Antenna is replaced Coverage (RSRP > -90dbm) -90dbm) Quality (RSRQ > -10db)
MME. The protocol over this reference point is eRANAP and it uses Stream Control Transmission Protocol (SCTP) as the transport protocol plane tunneling and inter-eNB path switching during handover. The transport protocol over this interface is GPRS Tunneling Protocol-User plane (GTP-U) trusted non-3GPP IP access and the Gateway. S2a is based on Proxy Mobile IP. To enable access via trusted non-3GPP IP accesses that do not support PMIP, S2a also supports Client Mobile IPv4 FA mode evolved Packet Data Gateway (ePDG) and the PDN GW. It is based on Proxy Mobile IP UE and the PDN GW. This reference point is implemented over trusted and/or untrusted non-3GPP Access and/or 3GPP access. This protocol is based on Client Mobile IP colocated mode information exchange for inter 3GPP access network mobility in idle and/or active state. It is based on Gn reference point as defined between SGSNs SGSN and the SGW and is based on Gn reference point as defined between SGSN and GGSN
PDN GW. It is used for SGW relocation due to UE mobility and if the t he SGW needs to connect conne ct to a non-collocated PDN GW for the required PDN connectivity. Two variants of this interface are being standardized depending on the protocol used, namely, GTP and the IETF based Proxy Mobile IP solution authorizing user access to the evolved system (AAA interface) int erface) between MME and HSS Rules Function (PCRF) to Policy and Charging Enforcement Function (PCEF) in the PDN GW. This interface is based on the Gx interface information transfer
Packet data network may be an operator-external public or private packet data network or an intra-operator packet data network, e.g. for provision of IMS services. This reference point corresponds to Gi for 2G/3G accesses PCRF in the 3GPP TS 23.203 and the ePDG. Traffic on this interface for a UE initiated tunnel tun nel has to be forced towards ePDG.
3GPP TS 36.201: "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer; General description". 3GPP TS 36.211-Evolved Universal Terrestrial Radio Access E-UTRA Physical Channels and Modulation 3GPP TS 36.133 (EUTRA Requirements and Radio Resource Management) http://en.wikipedia.org/wiki/3GPP_Long_Term_E volution UMTS –Global Standard for 3G Wireless Communication (NUST Lecture by Shariq Yasin) http://sites.google.com/site/lteencyclopedia/ho me
