Radio-Interface Physical Layer

Chapter 3 Radio-Interface Physical Layer Gerardo Gómez, David Morales-Jiménez, F. Javier López-Martínez, Juan J. Sánchez, and José Tomás Entrambasagu...
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Chapter 3

Radio-Interface Physical Layer Gerardo Gómez, David Morales-Jiménez, F. Javier López-Martínez, Juan J. Sánchez, and José Tomás Entrambasaguas Contents 3.1 Physical Layer Overview.............................................................................50 3.1.1 Physical Resource Structure.............................................................51 3.1.2 Reference Signals.............................................................................52 3.1.3 Synchronization Signals...................................................................53 3.1.4 Physical Channels............................................................................54 3.1.5 OFDM/SC-FDMA Signal Generation............................................58 3.2 Link Adaptation Techniques.......................................................................61 3.2.1 Adaptive Modulation and Coding...................................................62 3.2.2 Channel Coding..............................................................................63 3.2.3 CRC and Segmentation.................................................................. 64 3.2.4 Coding Techniques in LTE............................................................ 64 3.2.4.1 Tail Biting Convolutional Coding.................................... 64 3.2.4.2 Turbo Coding....................................................................65 3.2.4.3 Rate Matching.................................................................. 66 3.2.4.4 Coded Block Concatenation............................................. 66 3.2.5 Hybrid Automatic Repeat Request (HARQ)...................................67 3.2.6 Channel-Aware Scheduling..............................................................69 49

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50  ◾  Long Term Evolution: 3GPP LTE Radio and Cellular Technology

3.3 Multiple Antennas.......................................................................................70 3.3.1 Introduction....................................................................................70 3.3.2 Transmit Diversity...........................................................................73 3.3.2.1 Cyclic Delay Diversity (CDD)...........................................74 3.3.2.2 Space Frequency Block Coding (SFBC).............................74 3.3.3 Spatial Multiplexing........................................................................75 3.3.3.1 Space Division Multiplexing (SU-MIMO)........................78 3.3.3.2 Multiple Access..................................................................78 3.3.3.3 Precoding-Based Spatial Multiplexing...............................79 3.3.4 LTE Multiantenna Procedures.........................................................81 3.3.4.1 Antenna Mapping..............................................................81 3.3.4.2 LTE Multiantenna Configurations....................................82 3.3.4.3 Multiantenna Adaptation Procedures............................... 84 3.4 Physical Layer Procedures...........................................................................85 3.4.1 Synchronization...............................................................................86 3.4.1.1 Synchronization in OFDMA DL.......................................88 3.4.1.2 Synchronization in SC-FDMA UL....................................89 3.4.1.3 Cell Search Procedure....................................................... 90 3.4.1.4 Other Synchronization Procedures....................................91 3.4.2 Channel Estimation.........................................................................91 3.4.2.1 Channel Estimation in OFDMA DL................................93 3.4.2.2 Channel Estimation in SC-FDMA UL.............................94 3.4.3 Random Access................................................................................94 3.4.3.1 Random Access Preamble..................................................95 3.4.3.2 Random Access Procedure.................................................96 References............................................................................................................96

3.1 Physical Layer Overview The long term evolution (LTE) physical layer is targeted to provide improved radio interface capabilities between the base station* and user equipment (UE) compared to previous cellular technologies like the universal mobile telecommunications system (UMTS) or high-speed downlink packet access (HSDPA). According to the initial requirements defined by the 3rd Generation Partnership Project (3GPP; 3GPP 25.913), the LTE physical layer should support peak data rates of more than 100 Mb/s over the downlink and 50 Mb/s over the uplink. A flexible transmission bandwidth ranging from 1.25 to 20 MHz will provide support for users with different capabilities. These requirements will be fulfilled by employing *

In LTE, the base station is called enhanced NodeB (eNodeB).

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new technologies for cellular environments, such as orthogonal frequency division multiplexing (OFDM) or multiantenna schemes (3GPP 36.201). Additionally, channel variations in the time/frequency domain are exploited through link adaptation and frequency-domain scheduling, giving a substantial increase in spectral efficiency. In order to support transmission in paired and unpaired spectra, the LTE air interface supports both frequency division duplex (FDD) and time division duplex (TDD) modes. This chapter presents a detailed description of the LTE radio-interface physical layer. For that purpose, this section provides an introduction to the physical layer, focusing on the physical resource structure and the set of procedures defined within this layer. Link adaptation techniques, including adaptive modulation, channel coding, and channel aware scheduling, are described in Section 3.2. The topic of multiple antenna schemes for LTE is tackled in Section 3.3. Finally, Section 3.4 addresses other physical layer procedures like channel estimation, synchronization, and random access.

3.1.1 Physical Resource Structure Physical resources in the radio interface are organized into radio frames. Two radio frame structures are supported: type 1, applicable to FDD, and type 2, applicable to TDD. Radio frame structure type 1 is 10 ms long and consists of 10 subframes of length Tsubframe = 1 ms. A subframe consists of two consecutive time slots, each of 0.5 ms duration, as depicted in Figure 3.1. One slot can be seen as a time-frequency resource grid, composed of a set of OFDM subcarriers along several OFDM symbol intervals. The number of OFDM subcarriers (ranging from 128 to 2,048) is determined by the transmission bandwidth, whereas the number of OFDM symbols per slot (seven or six) depends on the cyclic prefix length (normal or extended). Hereinafter, we will assume a radio frame type 1 with normal cyclic prefix (i.e., seven OFDM symbols per slot). In cases of multiple antenna schemes, a different resource grid is used for each transmit antenna. The minimum resource unit allocable by the scheduler to a user is delimited by a physical resource block (PRB). Each PRB corresponds to 12 Frame (10 ms) Subframe #0 (1 ms)

Subframe #1

Subframe #9

Slot (0.5 ms) #0

#1

#2

#3

#18

#19

Figure 3.1  Radio frame structure type 1.

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52  ◾  Long Term Evolution: 3GPP LTE Radio and Cellular Technology

Slot #2 (0.5 ms)

Slot #1 (0.5 ms)

Time

Resource element (RE)

Subframe (1 ms)

Frequency

Physical resource block (PRB)

Figure 3.2  Subframe structure.

consecutive subcarriers for one slot (i.e., a PRB contains a total of 7 × 12 resource elements (REs)* in the time-frequency domain, as shown in Figure 3.2).

3.1.2 Reference Signals In LTE downlink, special reference signals are used to facilitate downlink channel estimation procedures. In the time domain, reference signals are transmitted during the first and third last OFDM symbols of each slot. In the frequency domain, reference signals are spread over every six subcarriers. Therefore, an efficient channel estimation procedure may apply a two-dimensional time-frequency interpolation to provide an accurate estimation of the channel frequency response within the slot time interval. When a downlink multiantenna scheme is applied, one reference signal is transmitted from each antenna in such a way that the mobile terminal is able to estimate the channel quality corresponding to each path. In this case, reference signals corresponding to each antenna are transmitted on different subcarriers so that they do not interfere with each other. In addition, resource elements used for transmitting reference signals on a specific antenna are not reused on other antennas for data transmission. An example of reference signal allocation for two-antenna downlink transmission is illustrated in Figure 3.3. The complex values of the reference signals in the time-frequency resource grid are generated as the symbol-by-symbol product of a two-dimensional orthogonal *

A resource element corresponds to an OFDM subcarrier in the time-frequency grid.

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Radio-Interface Physical Layer  ◾  53 Frequency

e

m

Ti na

en

nt A #0 na

en

nt A #1

Reference signal Not used

Figure 3.3  Example of reference symbol allocation for two-antenna transmission in downlink.

sequence and a two-dimensional pseudorandom sequence. This two-dimensional reference signal sequence also determines the cell identity to which the terminal is connected. There are 504 reference signal sequences defined in the LTE specification, corresponding to 504 different cell identities. Additionally, frequency shifting is applied to the reference signals in order to provide frequency diversity. Reference signals are also used in the uplink to facilitate coherent demodulation (then called demodulation signals) as well as to provide channel quality information for frequency dependent scheduling (referred to as sounding signals). Demodulation signals are transmitted on the fourth OFDM symbol of each UL slot along the transmission bandwidth allocated to a particular user, whereas sounding signals utilize a larger bandwidth to provide channel quality information on other frequency subcarriers. The uplink reference signals are based on constant amplitude zero autocorrelation (CAZAC) sequences. Further details on CAZAC sequences can be found in Section 3.4.1.1.

3.1.3 Synchronization Signals The base station periodically sends synchronization signals in the downlink so that the mobile terminals may be always synchronized. These signals also help the terminal during the cell search and handover procedures. Synchronization signals consist of two portions: ◾◾ Primary synchronization signals are used for timing and frequency acquisition during cell search. The sequence used for the primary synchronization signal is generated from a frequency-domain Zadoff–Chu sequence (see

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54  ◾  Long Term Evolution: 3GPP LTE Radio and Cellular Technology

Time Frequency

Frame (10 ms)

Subframe #0

Subframe #5

Reference signals

PDCCH, PCFICH, PHICH

Primary synchronization signals (P-SCH)

PDSCH, PMCH

Secondary synchronization signals (S-SCH)

PBCH

Figure 3.4  Downlink radio frame structure.

Section 3.4 for further details) and transported over the primary synchronization channel (P-SCH). ◾◾ Secondary synchronization signals are used to acquire the full cell identity. They are generated from the concatenation of two 31-length binary sequences. Secondary synchronization signals are allocated on the secondary synchronization channel (S-SCH). Synchronization signals are transmitted on the 72 center subcarriers (around the DC subcarrier) within the same predefined slots, in the last two OFDM symbols in the first slot of subframes 0 and 5 (twice per 10 ms). Figure 3.4 shows the time allocation of both the synchronization and reference signals within the downlink radio frame.

3.1.4 Physical Channels LTE supports a wide set of physical channels that are responsible for carrying information from higher layers (both user data and control information). The complete set of downlink/uplink physical channels, together with a brief explanation of their purpose, is listed in Table 3.1. A simplified diagram showing the location of the physical channels and signals in the radio frame is provided in Figure 3.4 (downlink) and Figure 3.5 (uplink). For each physical channel, specific procedures are defined for channel coding, scrambling, modulation mapping, antenna mapping, and resource element mapping. A general structure of the whole downlink processing sequence is illustrated in Figure 3.6. This sequence is determined by the following processes:

*

1. Cyclic redundancy check (CRC). The first step in the processing sequence is the CRC attachment. A CRC code is calculated and appended to each transport block (TB),* thus allowing for receiver-side detection of residual A transport block is defined as the data accepted by the physical layer to be jointly encoded.

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Table 3.1  LTE Physical Channels Direction

Channel Type

Downlink PDSCH

PMCH

PBCH

PCFICH

PDCCH

PHICH

Uplink

Description Physical downlink shared channel

Carries downlink user data from upper layers as well as paging signaling Physical multicast Used to support point-tochannel multipoint multimedia broadcast multicast service (MBMS) traffic Used to broadcast a certain set of Physical broadcast channel cell or system-specific information Physical control Determines the number of OFDM format indicator symbols used for the allocation channel of control channels (PDCCH) in a subframe Physical downlink Carries scheduling assignments, control channel uplink grants, and other control information; the PDCCH is mapped onto resource elements in up to the first three OFDM symbols in the first slot of a subframe Physical HARQ Carries the hybrid automatic indicator channel repeat request (HARQ) ACK/NAK

PUSCH

Physical uplink shared channel

PUCCH

Physical uplink control channel

PRACH

Physical random access channel

Carries uplink user data from upper layers; resources for the PUSCH are allocated on a subframe basis by the scheduler PUCCH carries uplink control information, including channel quality indication (CQI), HARQ ACK/NACK, and uplink scheduling requests Used to request a connection setup in the uplink

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56  ◾  Long Term Evolution: 3GPP LTE Radio and Cellular Technology Time Frequency

Frame (10 ms)

Subframe #0

Demodulation reference signals PUSCH, PUCCH

Figure 3.5  Uplink radio frame structure.



2.



3.



4.



5.

From MAC layer

errors in the decoded TB. The corresponding error indication, reported via uplink, can be used by the downlink hybrid automatic repeat request (HARQ) protocol to perform a retransmission. Channel coding. The goal of this process is to increase reliability in the transmission by adding redundancy to the information vector, resulting in a longer vector of coded symbols. This functionality includes code block segmentation, turbo or convolutional coding (depending on the channel type), rate matching, and code block concatenation (see Section 3.2 for further details). Scrambling. Scrambling of the coded data helps to ensure that the receiverside decoding can fully utilize the processing gain provided by the channel coding. Scrambling in LTE downlink consists of multiplying (exclusive or operation) the sequence of coded bits (taken as input) by a bit-level scrambling sequence. Modulation mapping. The block of scrambled bits is modulated into a block of complex-valued modulation symbols. Because LTE uses adaptive modulation and coding (AMC) to improve data throughput, the selected modulation scheme is based on the instantaneous channel conditions for each user. The allowed modulation schemes for downlink and uplink are shown in Table 3.2. Antenna mapping. Signal processing related to multiantenna transmission is performed at this stage. This procedure is responsible for mapping and

CRC

Channel Coding

CRC

Channel Coding

Scrambling

Modulation Mapping

Scrambling

Modulation Mapping

Resource Element Mapping

OFDM Signal Generation

Resource Element Mapping

OFDM Signal Generation

Antenna Mapping

Number of codewords

To radio interface

Number of antennas

Figure 3.6  Physical layer processing sequence in the base station (downlink).

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Table 3.2  Modulation Schemes



Direction

Channel

Modulation Schemes

Downlink

PDSCH PMCH PBCH PCFICH PDCCH PHICH

QPSK, 16 QAM, 64 QAM QPSK, 16 QAM, 64 QAM QPSK QPSK QPSK BPSK

Uplink

PUSCH PUCCH PRACH

QPSK, 16 QAM, 64 QAM BPSK, QPSK uth Root Zadoff–Chu

precoding the modulation symbols to be transmitted onto the different antennas.* The antenna mapping can be configured in different ways to provide different multiantenna schemes, including transmit diversity, beam forming, and spatial multiplexing. More details—regarding the antenna mapping procedure and, in general, related to LTE multiantenna schemes—are provided in Section 3.3. 6. Resource element mapping. Modulation symbols for each antenna will be mapped to specific resource elements in the time-frequency resource grid. 7. OFDM signal generation. The last step in the physical processing chain is the generation of time-domain signals for each antenna, which is addressed in detail in the next section.

In addition to the functionalities previously described, HARQ with soft combining is jointly used with CRC and channel coding to allow the terminal to request retransmissions of erroneously received transport blocks. When a (re)transmission fails, incremental redundancy is used to enable the combination of successively received radio blocks until the full block is correctly decoded. Previous functionalities may be dynamically configured, depending on the type of physical channel being processed. As an example, the downlink control channels (physical downlink control channel—PDCCH) are convolutionally encoded and use a transmit diversity configuration in the antenna mapping functionality. However, for downlink data channels (i.e., physical downlink shared channel— PDSCH—or physical multicast channel—PMCH), turbo coding is applied and other multiantenna schemes such as spatial multiplexing may be employed. *

LTE supports up to four transmit antennas.

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58  ◾  Long Term Evolution: 3GPP LTE Radio and Cellular Technology

From modulation mapping

N-Point DFT N

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