Lecture11 - LTE
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Transcript of Lecture11 - LTE
Lecture 11
Fundamentals of LTE
1ELEC 5360
Outline
• Introduction
• Downlink Processing
• Uplink Processing
• Advanced Techniques
Main reference:– A. Ghosh, J. Zhang, J. G. Andrews, and R. Muhamed, Fundamentals of
LTE, Prentice‐Hall, 2010.
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Cellular Evolution
Krnet 2006, H.-W. LEE 3ELEC 5360
Evolution of Cellular Systems
(From 3G American, “Transition to 4G”)
Standards bodies
3GPP (The 3rd Generation Partnership Project)
3GPP2
IEEE 802.16
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Global Mobile Market Shares 4Q 2011
• 64 commercial networks in 34 countries• 312 operators are investing in LTE in 98 countries
– GSA confirms LTE is the fastest developing mobile system technology ever.
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LTE Around You
• Mobile devices with LTE support
• LTE networks in Hong Kong– CSL (Nov. 2010)– China Mobile Hong Kong (Apr. 2012)– Hutchison 3 (May 2012)– PCCW (May 2012)
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What is LTE?• Separable network structure in 3GPP
– Radio Access Network (RAN) + Core Network (CN)
• LTE (Long‐Term Evolution): The project focusing on enhancing the 3G RAN, and optimizing 3GPP’s overall radio access architecture
• EPC (Evolved Packet Core): The project focusing on the CN evolution with a packet‐based architecture
• EPS (Evolved Packet System): The complete packet system consisting of LTE and EPC
73G Network LTE Network
Radio interface
RAN
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The Road to LTE‐‐ Performance Evolution of 3GPP Standards
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Motivations for LTE
Need to ensure the continuity of competitiveness of the 3G system for the future
User demand for higher data rates and quality of service
Packet Switch optimized system
Continued demand for cost reduction (CAPEX and OPEX)
Low complexity
Avoid unnecessary fragmentation of technologies for paired and unpaired band operation
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Key Features of LTE
High spectral efficiency OFDM in Downlink, frequency domain channel‐dependent scheduling DFTS‐OFDM(“Single‐Carrier FDMA”) in Uplink, Low PAPR, User
orthogonality in frequency domain Multi‐antenna (MIMO) transmission
Very low latency Short setup time & Short transfer delay Short handover latency and interruption time
Support of variable bandwidth 1.4, 3, 5, 10, 15 and 20 MHz
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Key Features of LTE
Simple protocol architecture Shared channel based PS (packet switch) mode only with VoIP capability
Simple Architecture eNodeB as the only E‐UTRAN node Smaller number of RAN interfaces
Compatibility and inter‐working with earlier 3GPP Releases Inter‐working with other systems, e.g. cdma2000 FDD and TDD within a single radio access technology Efficient Multicast/Broadcast
Single frequency network by OFDM
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Network Architecture of LTE
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UE: mobile terminaleNode‐B: base station
MME: Mobility management entityServing GW: Serving gatewayPDN GW: Packet data network gateway
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Radio Interface Protocols
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LTE Protocol StackThe Packet Flow in the User Plane
• As other communication standards, the LTE radio interface is designed based on a layered protocol stack
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Layered Protocol Stack
• Packet Data Convergence Protocol (PDCP)– IP packet header compression and decompression– Ciphering of data and signaling, integrity protection for signaling
• Radio Link Control (RLC)– Segmentation and concatenation of data units– Error correction through ARQ, in‐sequence delivery of packets
• Medium Access Control (MAC)– Error correction through Hybrid‐ARQ– Dynamic scheduling, transport format selection
• Physical Layer (PHY)– Actual transmission and reception, channel measurements– Signaling of H‐ARQ feedback, signaling scheduled allocations
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Radio Interface Protocol
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An IP packet flow with a defined QoS between
the PDN‐GW and the UE
What to transmit: defined based on the type of information it carries
How to transmit: characterized by how and with what characteristics
data is transferred
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Hierarchical Channel StructureLogical Channels: What to Transmit
• Provide services to the RLC• Logical control channels
– Broadcast Control Channel (BCCH)– Multicast Control Channel (MCCH)– Paging Control Channel (PCCH)– Common Control Channel (CCCH)– Dedicated Control Channel (DCCH)
• Logical traffic channels– Dedicated Traffic Channel (DTCH)– Multicast Traffic Channel (MTCH)
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Hierarchical Channel StructureTransport Channels: How to Transmit
• Provide services to the MAC• Downlink transport channels
– Downlink shared channel (DL‐SCH)– Broadcast channel (BCH)– Multicast channel (MCH)– Paging channel (PCH)
• Uplink transport channels– Uplink shared channel (UL‐SCH)– Random access channel (RACH)
• Control information– Downlink control information (DCI)– Control format indicator (CFI)– H‐ARQ Indicator (HI)– Uplink Control Information (UCI)
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Hierarchical Channel StructurePhysical Channels: Actual Transmission
• Correspond to a set of resource elements• Downlink physical channels
– Physical downlink control channel (PDCCH)– Physical downlink shared channel (PDSCH)– Physical broadcast channel (PBCH)– Physical multicast channel (PMCH)– Physical hybrid‐ARQ indicator channel (PHICH)– Physical control format indicator channel (PCFICH)
• Uplink physical channels– Physical uplink control channel (PUCCH)– Physical uplink shared channel (PUSCH)– Physical random access channel (PRACH)
• Physical signals– Reference signal– Synchronization signal
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Hierarchical Channel Structure
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Downlink Processing
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Downlink OFDMA Radio Resource
• Why OFDM?– OFDM is efficient in combating the frequency‐selective fading channel,
which makes it a suitable technique for wireless broadband systems such as LTE
– It is possible to exploit frequency‐selective scheduling with OFDM‐based multiple access – OFDMA
– The transceiver structure of OFDM with FFT/IFFT enables scalable bandwidth operation with a low complexity
– OFDM makes it much easier to support multi‐antenna transmission– OFDM enables multicast/broadcast services on a synchronized single
frequency network
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Time Domain – Frame Structure
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Frame Structure Type 1 (For FDD)
CP (Cyclic Prefix)• Normal CP: for urban environment and high data rate applications• Extended CP: for multicell multicast/broadcast and very large cells
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Time Domain – Frame Structure
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Frame Structure Type 2 (For TDD)
Special fields• DwPTS (Downlink Pilot TimeSlot), GP (Guard Period), UpPTS (Uplink Pilot TimeSlot)• Provide large guard period to switch between transmission and reception
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Physical Resource Blocks for OFDMA
• Each resource block is 180 kHz in the frequency domain, and 0.5 ms in the time domain (one slot)
• The resource block is the basic element for radio resource allocation
• The minimum size of radio resource that can be allocated is one subframe of 1ms, corresponding to two resource blocks
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Typical Parameters for DL Transmission
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DL Transport Channel Processing
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Channel Coding
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Provide error detection
Prevent excessive complexity and memory requirement for decoding
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Channel Coding
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Rate 1/3 tail‐biting convolution encoder
Rate 1/3 turbo encoder
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Rate Matching
• Rate matching performs interleaving, repetition or puncturing to generate a transport block that fits the payload size
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Interlacing: guarantee equal number of parity
1 and parity 2 bits
The virtual circular buffer
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Modulation
• Scrambing before modulation
– To randomize inter‐cell interference
• Modulation schemes for different physical channels
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Coded sequence
Scrambingsequence
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For data transmission
MIMO Modes in DL
• 7 different transmission modes– Single‐antenna port (port 0): single antenna transmission– Transmit diversity– Open‐loop (OL) spatial multiplexing– Closed‐loop (CL) spatial multiplexing– Multiuser MIMO– Closed‐loop rank‐1 precoding– Single‐antenna port (port 5): beamforming
• Antenna configuration– Baseline: 2@Base, 2@UE– Higher order: up to 4@Base, 4@UE
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MIMO ModesCL MIMO Modes
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Layer Mapping and Precoding
• Layer mapping and precoding are for MIMO modes– The layer mapper maps Nc codewords to v spatial layers– The precoder maps the v layers to P antenna ports
• This two‐step approach allows the inclusion of processing of all the MIMO modes in a single framework
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Layer Mapping and Precoding
• Codeword– The output of each channel coding/rate matching stage associated
with a single transport block coming from the MAC layer– In LTE, limited to two codewords
• Layer– Correspond to a data stream of the spatial multiplexing channel– Each codeword is mapped into one or multiple layers, so v>=Nc
• Antenna port– A logical entity, may not correspond to an actual physical antenna– Antenna ports 0‐3 are cell specific, used for DL MIMO– Antenna port 4 is for multicast/broadcast services– Antenna port 5 is used for beamforming to a single UE
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Codeword‐to‐layer mapping for spatial multiplexing
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Supported MIMO Modes for Different Physical Channels
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OL Transmit Diversity
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For two antenna ports• Space Frequency Block Code (SFBC)• Similar to the Alamouti code, indexed in the frequency domain
For four antenna ports• SFBC combined with Frequency Switched Transmit Diversity (FSTD)• Diversity order is 2
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OL Spatial Multiplexing
• W(i): codebook‐based precoding matrix• Provide decorrelation between different spatial streams
• D(i): to support large‐delay cyclic delay diversity• U: a fixed unitary DFT‐based precoding matrix• D(i)U effectively makes sure that all layers undergo the same
channel quality– This reduces signaling overhead as only a single CQI needs to be fed
back– Also provides increased robustness against imperfect link adaptation
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OL Spatial Multiplexing
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CL MIMO Techniques
• CL MIMO requires explicit feedback– Rank indication (RI), precoder matrix index (PMI)
• CL Spatial Multiplexing– Each UE feeds back the index of the precoder (PMI)
• CL Rank‐1 Precoding (RI=1)– A special case of CL spatial multiplexing
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Codebook for CL MIMO
• For 2 antenna ports
• For 4 antenna ports– The codebook uses a Householder generating function
– the input vectors are given in the standard
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MU‐MIMO in DL
• In 3GPP Release 8, there is a limited support for MU‐MIMO in the downlink
• The precoder for MU‐MIMO is the same as that for the rank‐1 precoding
• The eNodeB transmits to two UEs simultaneously with a rank‐1 precoder for each UE
• Mutual interference can be suppressed during the scheduling process by selecting UEs with near‐orthogonal channel directions, or with advanced interference cancelation at the UE
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OFDMA Signal Generation
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Downlink Shared Channels
• Downlink Shared Channel (DL‐SCH) carriers both traffic and control data from logical channels– Modulation: QPSK, 16QAM, 64QAM– Encoder: rate 1/3 convolutional turbo code– MIMO modes: all 7 transmission modes
• Shared‐channel transmission– The PHY layer resources are treated as a common resource that can be
dynamically shared among different UEs– Enable dynamic scheduling
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Downlink Control Channels
• Downlink Control Information (DCI)– 10 different DCI formats (scheduling information, modulation and coding
scheme, etc.)– DCI format 0, for uplink transmission– DCI format 1/1A/1B/1C/1D, for downlink transmission with one codeword
without spatial multiplexing– DCI format 2 and 2A, for downlink transmission with spatial multiplxing– DCI format 3 and 3A carry transmit power control commands for the uplink– Coding: rate 1/3 convolution code– Modulation: QPSK– Carried on PDCCH, located in the first n OFDM symbols of each subframe
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Downlink Control Channels
• Control Format Indicator (CFI)– Indicates how many OFDM symbols the DCI spans– The CFI takes values of 1, 2, or 3– CFI is mapped to PCFICH in the first OFDM symbol of the subframe– Encoder: rate 1/16 linear block code– Modulation: QPSK
• H‐ARQ Indicator (HI)– For H‐ARQ acknowledgement in response to uplink transmission– HI=1 for positive ack, HI=0 for negative ack– Encoder: rate 1/3 repetition code– Modulation: BPSK
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Broadcast Channels
• Carry system information such as downlink system bandwidth, antenna configuration, reference signal power
• Divided into two portions– Master Information Block (MIB) transmitted on PBCH– System Information Block (SIB) transmitted on PDSCH
• Encoder: rate 1/3 convolutional code• Modulation: QPSK• Resource mapping:
– Modulated symbols are mapped onto the 72 subcarriers centered around the DC subcarrier in slot 1 in subframe 0 during 4 consecutive radio frames
– Independent of bandwidth and duplex mode, allows UEs to decode without any prior knowledge
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Multicast Channels
• Multimedia Broadcast and Multicast Services (MBMS) supports multicast/broadcast services
• LTE provides enhanced support for the MBMS transmission– Enhanced MBMS (E‐MBMS)– Achieved through Single‐Frequency Network (SFN) operation– Multicell multicast/broadcast transmissions appear as a single
transmission over a multipath channel – OFDM is efficient in combating multipath channels
– Only single antenna port transmission is supported (antenna port 4)– The reduced subcarrier space of 7.5kHz is defined– Extended CP
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Downlink Physical Signals
• Downlink reference signals: – for channel estimation that enables coherent demodulation and for
channel quality measurement to assist user scheduling– Cell‐specific reference signals, associated with non MBSFN
transmission– MBSFN reference signals, associated with MBSFN transmission– UE‐specific reference signals, support single‐antenna port
transmission
• Synchronization Signals– Primary synchronization signal– Secondary synchronization signal– Enable acquisition of symbol timing and the precise frequency
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Cell‐Specific Reference Signal
• Cell‐specific reference signals are defined separately for antenna ports 0, 1, 2, and 3
• Reference signals on different antennas are orthogonal to each other
• Antenna ports 0 and 1 have twice as many reference symbols as ports 2 and 3
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Synchronization Signals
• Primary synchronization signal: carry the physical‐layer ID within the cell‐ID group
• Secondary synchronization signal: carry the physical‐layer cell‐ID group
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Uplink Processing
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Uplink SC‐FDMA Radio Resource
• Why SC‐FDMA in the uplink?– SC‐FDMA possesses most of the merits of OFDM– SC‐FDMA has a lower PAPR (peak‐average‐power‐ratio), which is highly desirable in the uplink as less expensive power amplifiers are needed at UEs
– An SC‐FDMA transceiver has a similar structure as OFDM, so the parametrization of radio resource in UL enjoys similarities to DL
• SC‐FDMA symbols, SC‐FDMA carriers
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Physical Resource Blocks for SC‐FDMA
• SC‐FDMA can be regarded as conventional OFDM with a DFT‐based precoder
• The resource grid is similar to DL
– SC‐FDMA symbols– SC‐FDMA carriers
• Only one subcarrier spacing: 15kHz
• No antenna port is defined, as only single antenna transmission is supported
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PHY Layer Processing
• Spatial multiplexing is not supported in the UL
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Coding and Modulation
• Channel Coding– Similar to DL– Uplink shared channel (UL‐SCH): rate 1/3 turbo code– Control information: block code or convolutional code
• Modulation– UL‐SCH supports QPSK, 16QAM, 64QAM– QPSK and 16QAM are mandatory, 64QAM is optional
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MIMO Modes
• Limited MIMO support in UL due to complexity/cost• Transmit Antenna Selection
– No antenna selection: UE shall transmit from antenna port 0– CL antenna selection: based on commands from eNode‐B (in DCI)– OL antenna selection: may select antenna based on H‐ARQ feedbacks
• MU‐MIMO in UL– Also referred to as “virtual” MIMO– Two UEs transmit simultaneously on the same radio resource– Provides spatial multiplexing gain, even with single‐antenna UEs– Orthogonal reference signals are assigned to each of 2 UEs
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SC‐FDMA Signal Generation
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Uplink Shared Channels
• Uplink Shared Channel (UL‐SCH) is the only transport channel carrying traffic data in the UL
• Encoder: 1/3 turbo encoder• Modulation: QPSK, 16QAM, 64QAM (optional)• MIMO Modes: antenna selection, MU‐MIMO
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Frequency Hopping
• Provide frequency diversity gain in UL• Intra‐subframe and inter‐subframe frequency hopping
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Uplink Control Information
• Uplink Control Information (UCI) provides following information– Downlink CQI (channel quality indicator): for adaptive modulation and
coding, channel‐dependent scheduling– H‐ARQ acknowledgement (H‐ARQ ACK): for downlink H‐ARQ process– Scheduling Request (SR): to request radio sources for UL transmission– PMI and RI: for DL MIMO transmission
• Without uplink resource allocation, UCI is transmitted on PUCCH
• With uplink resource allocation, UCI is frequency‐multiplexed with the UL‐SCH data on PUSCH
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Uplink Control Information‐‐ Channel Coding and Modulation
• Different types of control information are encoded differently– e.g., for UCI on PUCCH
• CQI/PMI are encoded with linear block code• H‐ARQ‐ACK, encoded as ‘1’ or ‘0’• CQI/PMI+H‐ARQ‐ACK, linear block code
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Uplink Control Information‐‐ Resource Mapping
• PUCCH is time‐division multiplexed with the PUSCH from the same UE
• PUCCH can be frequency‐division multiplexed with the PUSCH from other UEs
• PUCCH is transmitted at the bandwidth edge, to provide contiguous bandwidthfor data transmission
• The intra‐subframe frequency hoppingprovides frequency diversity
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Uplink Reference Signals
• LTE defines two types of reference signals in UL– Demodulation reference signals: for coherent demodulation, defined
for both PUSCH and PUCCH– Sounding reference signals: wideband reference signals for measuring
UL channel quality for uplink resource allocation– Why two types?
Demodulation reference signals in UL only transmitted on the subcarriers assigned to UEs, cannot provide sufficient wideband channel quality information
• Reference signals in UL cannot be transmitted at the same time with user data (Considering PAPR)
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Uplink Reference Signals‐‐ Resource Mapping
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Demodulation reference signals for PUSCH Sounding reference signals
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Random Access Channels
• Random access is for initial access or re‐establish UL synchronization
• Random access preamble:
• No intra‐cell interference from multiple random access attempts using different preambles (Zadoff‐Chu sequences)
• In frequency domain, random access occupies six consecutive resource blocks (72 subcarriers)
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Advanced Techniques
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H‐ARQ
• H‐ARQ (Hybrid Automatic Repeat reQuest): improve reliability– Combines forward error correction (FEC) and retransmission (ARQ)– Type I H‐ARQ (chase combining): retransmission is identical to the first
transmission– Type II H‐ARQ (incremental redundancy): retransmission provides
different redundancy– In LTE, rate 1/3 turbo coding is used as the FEC code while a CRC is
applied for error detection– At the transmitter: each (re)transmission, the same turbo‐coded data
is transmitted with different puncturing, i.e., different redundancy version
– At the receiver: decode the received code block combined with previously received versions
• No error detected, then feed back ACK• Error detected, feed back NACK
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H‐ARQ
• Time interval between two successive H‐ARQ transmission– Typically 8 msec in LTE
• To avoid waiting time, an N‐channel Stop‐and‐Wait protocol is used, consisting of N parallel H‐ARQ processes
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H‐ARQ
• DL: Asynchronous Adaptive H‐ARQ– H‐ARQ processes can be transmitted in any order without fixed timing– Provides higher throughput with re‐scheduling or changing
modulation/coding– Avoid potential collision of H‐ARQ retransmissions with high priority
scheduling transmissions, e.g., persistent scheduling– Requires more overhead, e.g., redundancy version, etc
• UL: Synchronous H‐ARQ– Retransmissions are scheduled on a periodic interval– Requires no explicit signaling for H‐ARQ process number– Depends on the duplexing mode and H‐ARQ operation types
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Adaptive Coding and Modulation (AMC)
• AMC is to take advantage of fluctuations in the channel over time and frequency, i.e., exploiting fading– Transmit as high a data rate as possible when and where the channel
is good– Transmit at a lower rate when and where the channel is poor– Requires knowledge of instantaneous channel information
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Adaptive Coding and Modulation (AMC)
• A possible realization of AMC
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• With AMC, we approach the Shannon capacity within 2‐4 dB
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Scheduling and Resource Allocation
• Objective: to efficiently allocate the available radio resource to UEs
• Channel‐independent scheduling– The allocation is random and not based on channel condition, e.g.,
round‐robin scheduling
• Channel‐dependent scheduling– The allocation is based on channel condition, e.g., proportional
fairness scheduler, max CI (carrier‐to‐interference) scheduler– Frequency diverse scheduling: random in frequency domain, exploit
time selectivity and frequency diversity– Frequency selective scheduling: exploit both time and frequency
selectivity of the channel
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Scheduling and Resource Allocation
• Resource assignment consists of resource blocks (RB) and modulation and coding schemes (MCS)– Depends heavily on channel information– For DL: provided by uplink CQI feedback– For UL: by channel sounding at the eNode‐B
• DL supports both localized and distributed resource allocations– Distributed allocations provide better performance with high overhead– Localized allocation provides low overhead alternative
• UL only supports localized allocation– Preserve single‐carrier property– Exploit multiuser diversity in the frequency domain
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Scheduling and Resource Allocation‐‐Multiuser Diversity Gain
• For iid Rayleigh fading channels, with opportunistic scheduling– Sum capacity ~ loglog(K)
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CQI Feedback
• CQI (Channel Quality Indicator) from UE– Indicates a suitable downlink transmission data rate, i.e., a Modulation
and Coding Scheme (MCS)– Based on the observed SINR at the UE
• Wideband vs. Subband CQI– Wideband CQI: over the entire channel bandwidth, do not support
frequency‐selective scheduling– Subband CQI: consists of a vector of CQI values each representing
SINR over a subband, enables frequency‐selective scheduling
• Tradeoff between DL performance and UL bandwidth consumption
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CQI Estimation
• The UE computes SINR samples over multiple OFDM symbols and subcarriers to calculate an effective SINR
– I(.) maps the SINR to a performance metric
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CQI Index
• Based on effective SINR, the UE picks the CQI index that indicates the highest MCS level that can be supported with a 10% block error rate
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CQI Feedback Modes
• Two reporting modes in the time domain– Periodic reporting: the UE reports CQI, PMI, and RI with reporting
periods configured by the higher layer– Aperiodic reporting: can be used to provide large and more detail
reporting in a single reporting instance via PUSCH
• For subband CQI feedback– In periodic reporting
The UE cycles through different subbands from one reporting instance to the next, to reduce overhead
– In aperiodic reportingHigher layer configured subband reporting: UE reports the subband CQI for each band in a single feedback reportUE selected subband report: UE reports the subband CQI for the ‘M’ bands with the highest CQI values
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CQI Feedback Modes
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10 MHz FDD
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Cell Search or Synchronization
• When a UE powers on– It needs to acquire time and frequency synchronization with a cell and
detect the physical‐layer cell ID of that cell through the cell search procedure
• LTE uses a hierarchical cell search scheme
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Random Access Procedure
• Cell search – provides downlink synchronization• Random access – enables uplink synchronization, notifies the
eNode‐B that the UE has data to transmit, or transmit a small amount of control information and data packets
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Power Control in Uplink
• In LTE, orthogonal transmission in UL removes intra‐cell interference– No near‐far problem– Inter‐cell interference is the major issue Power control in UL
• Objective of Power Control– To control interference caused to neighboring cells, while maintaining
the required SINR at the serving cell
• Fractional Power Control (FPC)– Allows full or partial compensation of path loss and shadowing– Allows edge UEs to operate at lower SINR, with less interference to
other cells
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Power Control in Uplink
• Power control for PUSCH– Open‐loop component + closed‐loop component
• Open‐loop component: Fractional Power Control (FPC)
– M: # resource blocks, P0: mean received SINRα: path loss compensation factor (α=1, channel inversion, a=0, no PC)
• Open‐loop + closed loop
– f(.) performs closed loop power controlAccumulated:Absolute:
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Inter‐cell Interference (ICI) Coordination
• ICI randomization– Scramble the codeword after channel encoding with a pseudo‐random
sequence– Interference suppression is achieved through channel coding gain
• ICI cancellation– Decode the interfering signals first, and then subtract them– Achieved with multiuser detection, or multiple antennas
• ICI coordination/avoidance– Apply restrictions to the resource management in a coordinated way
between neighboring cells
• Coordinated Multi‐Point (CoMP) Transmission/Reception– Also called base station coordination, multicell processing– Have great potential, but not supported in LTE
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Inter‐cell Interference (ICI) Coordination• Example:
Possible downlink power levels of three neighboring cells. Edge users in each cell would be allocated to the higher power levels.
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Related 3GPP Specifications
• 3GPP TS 36.201, “Evolved Universal Terrestrial Radio Access (E‐UTRA); LTE Physical Layer ‐ General Description”.
• 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E‐UTRA); Physical channels and modulation”.
• 3GPP TS 36.212, “Evolved Universal Terrestrial Radio Access (E‐UTRA); Multiplexing and channel coding”.
• 3GPP TS 36.213, “Evolved Universal Terrestrial Radio Access (E‐UTRA); Physical Layer Procedures”.
• 3GPP TS 36.321, “Evolved Universal Terrestrial Radio Access (E‐UTRA); Medium Access Control (MAC) protocol specification”.
• Available on http://www.3gpp.org/specification‐numbering
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