LTE Physical Layer | ENodeB And UE Physical Layer In LTE

This LTE physical layer covers eNodeB and UE physical layer transmitter modules as per LTE standard.The processing of PDSCH channel through LTE physical layer modules have been described.

Introduction: The phy layer i.e. physical layer is used to carry upper layer information. It is interface between MAC layer i.e. layer-2 and RF (Radio Frequency) transceiver. It lies at layer-1 of OSI stack. It uses wired or wireless medium for transport of upper layer informations. The common functions of phy layer are as follows.
• It converts MAC layer format in the format suitable as per medium used.
• It adds FEC (forward error correction) functionality to enable error correction at the receiver.

 It performs modulation and demodulation functionalities at transmitter and receiver end respectively.
Refer PHY Vs MAC which mentions functional difference between phy layer and mac layers in general.

There are different MAC layer frames in LTE in the downlink (PDSCH, PDCCH, PBCH) and in the uplink (PRACH, PUSCH, PUCCH). Physical layer is used to process these channels.

LTE Physical Layer ENodeB Transmitter

Following figure depicts LTE physical layer transmitter part of eNodeB i.e. LTE base station. LTE eNodeB is similar to Base station of other technologies such as Wimax, GSM etc. eNodeB Physical layer consists of Channel coding,rate matching, scrambler, mapper, layer mapping, pre-coding, resource element mapper, ofdm module. CRC is appended to the data from MAC layer before being passed through the PHY layer.

Let us understand LTE physical layer with example of downlink shared channel(SCH). As shown in the figure for eNodeB Transmitter upper layer data in the form of transport block is the input to the physical layer.

At first the transport block is passed through a CRC encoder, we will use 24 bit CRC method. If the number of bits is more than 6144 bits then it is broken into smaller blocks. It is then turbo coded. Turbo coding is a form of concatenated coding, consisting of two convolutional encoders with certain interleaving between them. Rate matching acts as rate coordinator between preceding and succeeding blocks, it uses a buffer. Modulation used is QAM. It is then passed through a OFDM modulator. The same is shown below in the DL-SCH channel processing figure.

CRC

A cyclic redundancy check (CRC) is used for error detection in transport blocks. The entire transport block is used to calculate the CRC parity bits. The transport block is divided by a cyclic generator polynomial to generate 24 parity bits. These parity bits are then appended to the end of transport block. The polynomial is as follows:
G(x)= x24 + x23 + x18 + x17 + x14 + x11 + x10 + x7 + x6 + x5 + x4 + x3 + x + 1

Segmentation and 2nd CRC: If the input block size is greater than 6144 bits, it is split in to smaller blocks. Again CRC is performed and redundant parity bits are appended to each resulting smaller block. Also, filler bits are added so the code block sizes match a set of valid block sizes input to turbo code.

Turbo Coding

The constituent encoders used are convolutional encoders. The input to the first constituent encoder is the input bit stream to the turbo coding block. The input to the second constituent encoder is the output of the QPP interleaver, a permutated version of the input sequence.

Rate Matching And Modulation

The rate matching block creates an output bit stream with a desired code rate. The rate matching algorithm is capable of producing any arbitrary rate. The bit streams from the turbo encoder are interleaved followed by bit collection to create a circular buffer. Bits are selected and punctured from the buffer to create an output bit stream with the desired code rate.

Physical Channels: Actual Transmission

Each physical (phy) channel corresponds to a set of resource elements in the time-frequency grid that carry information from higher layers. The basic entities that make a physical channel are resource elements and resource blocks. A resource element is a single subcarrier over one OFDM symbol, and typically this could carry one (or two with spatial multiplexing) modulated symbol(s). A resource block is a collection of resource elements and in the frequency domain this represents the smallest quanta of resources that can be allocated. The transport channels need to be mapped in to actual physical channels.

PDSCH channel carries user data originating from the higher layer. It is associated to DL-SCH. It has various steps involved in it, such as scrambling, modulation mapper, layer mapper, precoding, resource mapping and OFDM modulation.

As shown in figure, Scrambling Produces a block of scrambled bits from the input bits according to the relation given by the equation.

b^=b+c mod 2

Where b^ denotes the scrambled bits, b denotes the input bits, c denotes the scrambling sequence.

Modulation Maps the bit values of input to complex modulation symbols with the modulation scheme specified. There are three modulation schemes for the PDSCH viz. QPSK (Quadrature phase shift keying), 16QAM (Quadrature Amplitude Modulation) and 64QAM (Quadrature Amplitude Modulation). Layer mapper splits the data sequence in to a number of layers. Precoding is used for transmission in multi-antenna wireless communications. In conventional single-stream beam forming, the same signal is emitted from each of the transmit antennas with appropriate weighting (phase and gain) such that the signal power is maximized at the receiver output. The resource-mapping block maps the actual data symbols, reference signal symbols and control information symbols into a certain resource element in the resource grid.

LTE Phy Layer UE Transmitter

Above figure depicts LTE physical layer modules in UE transmitter part. LTE UE is similar to subscriber station of other technologies such as Wimax,GSM etc. It consists of channel coding,rate matching,scrambler,mapper,transform precoder, resource element mapper and SC-FDMA. CRC is appended to the data before passed to the PHY.