Friday, 30 October 2015

LTE physical layer (FDD)

The 3GPP Long Term Evolution (LTE) represents a major advance in cellular technology. LTE is designed to meet carrier needs for high-speed data and media transport as well as high-capacity voice support well into the next decade. It encompasses high-speed data, multimedia unicast and multimedia broadcast services. Although technical specifications are not yet finalized, significant details are emerging. This paper focuses on the LTE physical layer(PHY).
The LTE PHY is a highly efficient means of conveying both data and control information between an enhanced base station (eNodeB) and mobile user equipment (UE). The LTE PHY employs some advanced technologies that are new to cellular applications. These include Orthogonal Frequency Division Multiplexing (OFDM) and Multiple Input Multiple Output (MIMO) data transmission. In addition, the LTE PHY uses Orthogonal Frequency Division Multiple Access (OFDMA) on the downlink (DL) and Single Carrier – Frequency Division Multiple Access (SC-FDMA) on the uplink (UL). OFDMA allows data to be directed to or from multiple users on a subcarrier-by-subcarrier basis for a specified number of symbol periods. Due to the novelty of these technologies in cellular applications, they are described separately before delving into a description of the LTE PHY.
Although the LTE specs describe both Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD) to separate UL and DL traffic, market preferences dictate that the majority of deployed systems will be FDD. This paper therefore describes LTE FDD systems only.

2LTE Physical Layer
The capabilities of the eNodeB and UE are obviously quite different. Not surprisingly, the LTE PHY DL and UL are quite different. The DL and UL are treated separately within the specification documents. Therefore, the DL and UL are described separately in the following sections.
2.0 Generic Frame Structure
One element shared by the LTE DL and UL is the generic frame structure. As mentioned previously, the LTE specifications define both FDD and TDD modes of operation. This paper deals exclusively with describing FDD specifications. The generic frame structure applies to both the DL and UL for FDD operation.
Figure 2.0-1 LTE Generic Frame Structure
LTE transmissions are segmented into frames, which are 10 msec in duration. Frames consist of 20 slot periods of 0.5 msec. Sub-frames contain two slot periods and are 1.0 msec in duration.
2.1 Downlink
The LTE PHY specification is designed to accommodate bandwidths from 1.25 MHz to 20 MHz. OFDM was selected as the basic modulation scheme because of its robustness in the presence of severe multipath fading. Downlink multiplexing is accomplished via OFDMA.
The DL supports physical channels, which convey information from higher layers in the LTE stack, and physical signals which are for the exclusive use of the PHY layer. Physical channels map to transport channels, which are service access points (SAPs) for the L2/L3 layers. Depending on the assigned task, physical channels and signals use different modulation and coding parameters.
2.1.1 Modulation Parameters
OFDM is the modulation scheme for the DL. The basic subcarrier spacing is 15 kHz, with a reduced subcarrier spacing of 7.5 kHz available for some MB-SFN scenarios. Table 2.1.1-1 summarizes OFDM modulation parameters.
Table 2.1.1-1 Downlink OFDM Modulation Parameters
Depending on the channel delay spread, either short or long CP is used. When short CP is used, the first OFDM symbol in a slot has slightly longer CP than the remaining six symbols, as shown in Table 2.1.1-2. This is done to preserve slot timing (0.5 msec).
Table 2.1.1-2 Cyclic Prefix Duration
Note that the CP duration is described in absolute terms (e.g. 16.67 μsec for long CP) and in terms of standard time units, Ts. Ts is used throughout the LTE specification documents. It is defined as Ts = 1 / (15000 x 2048) seconds, which corresponds to the 30.72 MHz sample clock for the 2048 point FFT used with the 20 MHz system bandwidth.
2.1.2 Downlink Multiplexing
OFDMA is the basic multiplexing scheme employed in the LTE downlink. OFDMA is a new-to-cellular technology. The groups of 12 adjacent subcarriers are grouped together on a slot-by-slot basis to form physical resource blocks (PRBs). A PRB is the smallest unit of bandwidth assigned by the base station scheduler.
A two dimensional (time and frequency) resource grid can be constructed to represent the transmitted downlink signal. Each block in the grid represents one OFDM symbol on a given subcarrier and is referred to as a resource element. Note that in MIMO applications, there is one resource grid for each transmitting antenna.
2.1.3 Physical Channels
Three different types of physical channels are defined for the LTE downlink. One common characteristic of physical channels is that they all convey information from higher layers in the LTE stack. This is in contrast to physical signals, which convey information that is used exclusively within the PHY layer.
LTE DL physical channels are:
• Physical Downlink Shared Channel (PDSCH)
• Physical Downlink Control Channel (PDCCH)
• Common Control Physical Channel (CCPCH)
Physical channels are mapped to specific transport channels as described in Section 2.1.5 below. Transport channels are SAPs for higher layers. Each physical channel has defined algorithms for:
• Bit scrambling
• Modulation
• Layer mapping
• CDD precoding
• Resource element assignment
Layer mapping and pre-coding are related to MIMO applications. Basically, a layer corresponds to a spatial multiplexing channel. MIMO systems are defined in terms of Ntransmitters x Nreceivers. For LTE, defined configurations are 1x 1, 2 x 2, 3 x 2 and 4 x 2. Note that while there are as many as four transmitting antennas, there are only a maximum of two receivers and thus a maximum of only two spatial multiplexing data streams. For a 1 x 1 or a 2 x 2 system, there is a simple 1:1 relationship between layers and transmitting antenna ports. However, for a 3 x 2 and 4 x 2 system, there are still only two spatial multiplexing channels. Therefore, there is redundancy on one or both data streams. Layer mapping specifies exactly how the extra transmitter antennas are employed.
Precoding is also used in conjunction with spatial multiplexing. Recall that MIMO exploits multipath to resolve independent spatial data streams. In other words, MIMO systems require a certain degree of multipath for reliable operation. In a noise-limited environment with low multipath distortion, MIMO systems can actually become impaired.
Physical Downlink Shared Channel
The PDSCH is utilized basically for data and multimedia transport. It therefore is designed for very high data rates. Modulation options therefore include QPSK, 16QAM and 64QAM. Spatial multiplexing is also used in the PDSCH. In fact, spatial multiplexing is exclusive to the PDSCH. It is not used on either the PDCCH or the CCPCH. Layer mapping for the PDSCH is described in Table 2.1.3-1.
Table 2.1.3-1 PDSCH Layer Mapping
Physical Downlink Control Channel
The PDCCH conveys UE-specific control information. Robustness rather than maximum data rate is therefore the chief consideration. QPSK is the only available modulation format. The PDCCH is mapped onto resource elements in up to the first three OFDM symbols in the first slot of a subframe.
Common Control Physical Channel
The CCPCH carries cell-wide control information. Like the PDCCH, robustness rather than maximum data rate is the chief consideration. QPSK is therefore the only available modulation format. In addition, the CCPCH is transmitted as close to the center frequency as possible. CCPCH is transmitted exclusively on the 72 active subcarriers centered on the DC subcarrier. Control information is mapped to resource elements (k, l) where k refers to the OFDM symbol within the slot and l refers to the subcarrier. CCPCH symbols are mapped to resource elements in increasing order of index k first, then l.
2.1.4 Physical Signals
Physical signals use assigned resource elements. However, unlike physical channels, physical signals do not convey information to/from higher layers. There are two types of physical signals:
• Reference signals used to determine the channel impulse response (CIR)
• Synchronization signals which convey network timing information
Reference Signals
Reference signals are generated as the product of an orthogonal sequence and a pseudo-random numerical (PRN) sequence. Overall, there are 510 unique reference signals possible. A specified reference signal is assigned to each cell within a network and acts as a cell-specific identifier.
Figure 2.1.4-1 Resource Element Mapping of Reference Signals
As shown in Figure 2.1.4-1, reference signals are transmitted on equally spaced subcarriers within the first and thirdfrom-last OFDM symbol of each slot. UE must get an accurate CIR from each transmitting antenna. Therefore, when a reference signal is transmitted from one antenna port, the other antenna ports in the cell are idle.
Reference signals are sent on every sixth subcarrier. CIR estimates for subcarriers that do not bear reference signals are computed via interpolation. Changing the subcarriers that bear reference signals by pseudo-random frequency hopping is also under consideration.
Synchronization Signals
Synchronization signals use the same type of pseudo-random orthogonal sequences as reference signals. These are classified as primary and secondary synchronization signals, depending how they are used by UE during the cell search procedure. Both primary and secondary synchronization signals are transmitted on the 72 subcarriers centered around the DC subcarrier during the 0th and 10th slots of a frame (recall there are 20 slots within each frame).
2.1.5 Transport Channels
Transport channels are included in the LTE PHY and act as service access points (SAPs) for higher layers. Downlink Transport channels are:
Broadcast Channel (BCH)
• Fixed format
• Must be broadcast over entire coverage area of cell
Downlink Shared Channel (DL-SCH)
• Supports Hybrid ARQ (HARQ)
• Supports dynamic link adaption by varying modulation, coding and transmit power
• Suitable for transmission over entire cell coverage area
• Suitable for use with beamforming
• Support for dynamic and semi-static resource allocation
• Support for discontinuous receive (DRX) for power save
Paging Channel (PCH)
• Support for UE DRX
• Requirement for broadcast over entire cell coverage area
• Mapped to dynamically allocated physical resources
Multicast Channel (MCH)
• Requirement for broadcast over entire cell coverage area
• Support for MB-SFN
• Support for semi-static resource allocation
2.1.6 Mapping Downlink Physical Channels to Transport Channels
Transport channels are mapped to physical channels as shown in Figure 2.1.6-1. Supported transport channels are:
1. Broadcast channel (BCH)
2. Paging channel (PCH)
3. Downlink shared channel (DL-SCH)
4. Multicast channel (MCH)
Transport channels provide the following functions:
• Structure for passing data to/from higher layers
• Mechanism by which higher layers can configure the PHY
• Status indicators (packet error, CQI etc.) to higher layers
• Support for higher-layer peer-to-peer signaling
Figure 2.1.6-1 Mapping DL Transport Channels to Physical Channels
2.1.7 Downlink Channel Coding
Different coding algorithms are employed for the DL physical channels. For the common control channel (CCPCH), modulation is restricted to QPSK. The PDSCH uses up to 64 QAM modulation. For control channels, coverage is the paramount requirement. Convolutional coding has been selected for use with the CCPCH, though a final determination regarding code rate has not yet been made.
On the PDSCH, higher-complexity modulation is employed to achieve the highest possible downlink data rates. The PDSCH uses QPSK, 16QAM, or 64QAM depending on channel conditions. As a result, coding gain is emphasized over latency. Rate 1/3 turbo coding has been selected for the PDSCH.

2.2 Uplink
The LTE PHY uses Single Carrier - Frequency Division Multiple Access (SC-FDMA) as the basic transmission scheme for the uplink. SC-FDMA is a misleading term, since SC-FDMA is essentially a multi-carrier scheme that re-uses many of the functional blocks included in the UE OFDM receiver signal chain. The principle advantage of SC-FDMA over conventional OFDM is a lower PAPR (by approximately 2 dB) than would otherwise be possible using OFDM.
2.2.1 Modulation Parameters
In FDD applications, the uplink uses the same generic frame structure (see Section 2.0) as the downlink. It also uses the same subcarrier spacing of 15 kHz and PRB width (12 subcarriers). Downlink modulation parameters (including normal and extended CP length) are identical to the uplink parameters shown in Tables 2.1.1-1 and 2.1.1-2. Subcarrier modulation is, however, much different.
In the uplink, data is mapped onto a signal constellation that can be QPSK, 16QAM, or 64QAM depending on channel quality. However, rather than using the QPSK/QAM symbols to directly modulate subcarriers (as is the case in OFDM), uplink symbols are sequentially fed into a serial/parallel converter and then into an FFT block. The result at the output of the FFT block is a discrete frequency domain representation of the QPSK/QAM symbol sequence.
The discrete Fourier terms at the output of the FFT block are then mapped to subcarriers before being converted back into the time domain (IFFT). The final step prior to transmission is appending a CP. It is interesting to note that while the SC-FDMA signal has a lower PAPR in the time domain, individual subcarrier amplitudes can actually vary more in the frequency domain than a comparable OFDM signal.
2.2.2 Multiplexing
Uplink PRBs are assigned to UE by the base station scheduler via the downlink CCPCH. Uplink PRBs consist of a group of 12 contiguous subcarriers for a duration of one slot time.
2.2.3 Uplink Physical Channels
Uplink physical channels are used to transmit information originating in layers above the PHY. Defined UL physical channels are:
Physical Uplink Shared Channel (PUSCH)
Resources for the PUSCH are allocated on a sub-frame basis by the UL scheduler. Subcarriers are allocated in multiples of 12 (PRBs) and may be hopped from sub-frame to sub-frame. The PUSCH may employ QPSK, 16QAM or 64QAM modulation.
Physical Uplink Control Channel (PUCCH)
As the name implies, the PUCCH carries uplink control information. It is never transmitted simultaneously with PUSCH data. PUCCH conveys control information including channel quality indication (CQI), ACK/NACK, HARQ and uplink scheduling requests. The PUCCH transmission is frequency hopped at the slot boundary as shown in Figure 2.2.3-1 for added reliability.
Figure 2.2.3-1 PUCCH is Hopped at Slot Boundary
2.2.4 Uplink Physical Signals
Uplink physical signals are used within the PHY and do not convey information from higher layers. Two types of UL physical signals are defined: the reference signal and the random access preamble.
Uplink Reference Signal
There are actually two variants of the UL reference signal. The demodulation signal facilitates coherent demodulation. It is transmitted in the fourth SC-FDMA symbol of the slot and is the same size as the assigned resource. There is also a sounding reference signal used to facilitate frequency dependent scheduling. Both variants of the UL reference signal are based on Zadhoff-Chu sequences.
Random Access Preamble
The random access procedure involves the PHY and higher layers. At the PHY layer, the cell search procedure is initiated by transmission of the random access preamble by the UE. If successful, a random access response is received from the base station. The random access preamble format is shown in Figure 2.2.4-1. It consists of a cyclic prefix, a preamble and a guard time during which there is no signal transmitted.
Figure 2.2.4-1 Random Access Preamble Format
For the generic frame structure, the timing parameters are:
TRA: 30720 TS
TGT: 3152 TS
TPRE: 24576 TS
where TS = period of a 30.72 MHz clock
Random access preambles are derived from Zadoff-Chu sequences. They are transmitted on blocks of 72 contiguous subcarriers allocated for random access by the base station. In FDD applications, there are 64 possible preamble sequences per cell.
The exact frequency used for transmission of the random access preamble is selected from available random access channels by higher layers in the UE. Other information provided to the PHY by higher layers includes:
• Available random access channels
• Preamble format (which preamble sequences)
• Initial transmission power
• Power ramp step size
• Maximum number of retries
2.2.5 Uplink Transport Channels
As in the DL, uplink transport channels act as service access points for higher layers. Characteristics of UL transport channels are described below.
Uplink – Shared Channel (UL-SCH)
• Support possible use of beam forming
• Support dynamic link adaption (varying modulation, coding and/or Tx power)
• Support for HARQ
• Support for dynamic and semi-static resource allocation
Random Access Channel (RACH)
• Supports transmission of limited control information
• Possible risk of collision
2.2.6 Mapping Uplink Physical Channels to Transport Channels
Transport channels are mapped to physical channels as shown in Figure 2.2.6-1.
Figure 2.2.6-1 Mapping of UL Transport Channels to UL Physical Channels
2.2.7 Coding
The UL-SCH uses the same rate 1/3 turbo encoding scheme (two 8-state constituent encoders and one internal interleaver) as the DL-SCH.

Budi Prasetyo

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