An overview of the LTE physical layer--Part III

Downlink Physical Channels

LTE defines a number of downlink physical channels to carry information blocks received from the MAC and higher layers. These channels are categorized as transport or control channels.

Transport Channels

1- Physical Broadcast Channel (PBCH): The PBCH broadcasts a limited number of parameters essential for initial access of the cell such as downlink system bandwidth, the Physical Hybrid ARQ Indicator Channel structure, and the most significant eight-bits of the System Frame Number. These parameters are carried in what's called a Master Information Block which is 14 bits long.

The PBCH is designed to be detectable without prior knowledge of system bandwidth and to be accessible at the cell edge. The MIB is coded at a very low coding rate and mapped to the 72 center sub-carriers (6 RBs) of the OFDM structure. PBCH transmission is spread over four 10 ms frames (over subframe #0) to span a 40 ms period as shown in Error! Reference source not found.. Each subframe is self decodable which reduces latency and UE battery drain in case of good signal quality, otherwise, the UE would 'soft-combine' multiple transmissions until the PBCH is decoded. The PBCH is transmitted using Space Frequency Block Code (SFBC), a form of transmit diversity, in case of multiple antennas thereby allowing for greater coverage.

2- Physical Downlink Shared Channel (PDSCH): The PDSCH is the main data bearing channel which is allocated to users on a dynamic and opportunistic basis. The     PDSCH carries data in what's known as Transport Blocks (TB) which correspond to a MAC PDU. They are passed from the MAC layer to the PHY layer once per Transmission Time Interval (TTI) which is 1 ms (i.e. 1 ms scheduling interval to meet low latency requirements).

To guard against propagation channel errors, convolutional turbo coder is used for forward error correction. The data is mapped to spatial layers according to the type of multi-antenna technique (e.g. closed loop spatial multiplexing, open-loop, spatial multiplexing, transmit diversity, etc.) and then mapped to a modulation symbol which includes QPSK, 16 QAM and 64 QAM.

Physical resources are assigned on a basis on two resource blocks for one TTI (1 ms). This is referred to by 'pair of resource blocks' which is the quantum of resources that can be allocated. It corresponds to 12 sub-carriers (180 kHz) for 14 OFDM symbols (normal CP mode).

The PDCH is also used to transmit broadcast information not transmitted on the PBCH which include System Information Blocks (SIB) and paging messages. 

3- Physical Multicast Channel (PMCH): This channel defines the physical layer structure to carry Multimedia Broadcast and Multicast Services (MBMS). However, MBMS are not included in the first release of LTE. The PMCH is designed for a single-frequency network and it requires that the base stations transmit with tight time synchronization the same modulated symbols. The PMCH is transmitted in specific dedicated subframes where the PDSCH is not transmitted.

Control Channels

Control occupy the first 1, 2, or 3 OFDM symbols in a subframe extending over the entire system bandwidth as shown in Error! Reference source not found.. In narrow band systems (less than 10 RBs), the control symbols can be increased to include the fourth OFDM symbol.

1- Physical Downlink Control Channel (PDCCH): The PDCCH carries the resource assignment for UEs which are contained in a Downlink Control Information (DCI) message. Multiple PDCCHs can be transmitted in the same subframe using Control Channel Elements (CCE) each of which is a nine set of four resource elements known as Resource Element Groups (REG). QPSK modulation is used for the PDCCH. Four QPSK symbols are mapped to each REG. Furthermore, 1, 2, 4, or 8 CCEs can be used for a UE depending on channel conditions to ensure sufficient robustness.

2- Physical Control Format Indicator Channel (PCFICH): This channel carries the Control Frame Indicator (CFI) which includes the number of OFDM symbols used for control channel transmission in each subframe (typically 1, 2, or 3). The 32-bit long CFI is mapped to 16 Resource Elements in the first OFDM symbol of each downlink frame using QPSK modulation.

3- Physical Hybrid ARQ Indicator Channel (PHICH): The PHICH carries the HARQ ACK/NAK which indicates to the UE whether the eNodeB correctly received uplink user data carried on the PUSCH. BPSK modulation is used with repetition factor of 3 for robustness. 

Uplink Physical Channels

There are three physical layer channels defined for the uplink in LTE as described below.

Physical Uplink Shared Channel (PUSCH): This channel carries user data. It supports QPSK and 16 QAM modulation with 64QAM being optional. Information bits are first channel-coded with a turbo code of mother rate of 1/3 before being adapted by a rate matching process for a final suitable code rate. Adjacent data symbols are mapped to adjacent SC-FDMA symbols in the time domain before being mapped across sub-carriers. After this interleaving process, bits are scrambled before modulation mapping, DFT-spreading, sub-carrier mapping and OFDM modulation. Channel coding is similar to that of the downlink.

The uplink scheduling interval is 1 ms, similar to the downlink. However, it is possible to 'bundle' a group of 4 TTIs to improve performance at cell edge and reduce higher layer protocol overhead. In this case, a MAC PDU is segmented for transmission over multiple TTIs. In the frequency domain, transmissions are allocated based on multiples of 180 kHz resource blocks. Uplink resources corresponding to the same set of sub-carriers are assigned for the two slots of a subframe. However, inter-slot frequency hopping is an option whereby different sub-carriers are used for the second slot resulting in a frequency diversity gain and averages interference provided different hopping sequences are used in neighboring cells.

The PUSCH carries in addition to user data any control information necessary to decode the information such as transport format indicators and MIMO parameters. Control data is multiplexed with information data prior to DFT spreading.

Physical Uplink Control Channel (PUCCH): Control signaling comprises uplink data transmitted independently of traffic data which include HARQ ACK/NACK, channel quality indicators (CQI), MIMO feedback (Rank Indicator, RI; Precoding Matrix Indicator, PMI) and scheduling requests for uplink transmission. This channel transmits in a frequency region at the edge of the system bandwidth as shown in Error! Reference source not found.. It consists of one RB per transmission at one end of the system bandwidth followed by a RB in the following slot at the opposite end of the channel spectrum thus making use of frequency diversity with an estimated gain of 2 dB. A PUCCH Control Region comprises every two such RBs.  Error! Reference source not found. shows the typical number of PUCCH regions for different system bandwidths. BPSK or QPSK are used for modulation of PUCCH information.

Physical Random Access Channel (PRACH): This channel carries the random access preamble a UE sends to access the network in non-synchronized mode and used to allow the UE to synchronize timing with the eNodeB. It consists of 72 sub-carriers in the frequency domain (six Resource Block, 1.08 MHz) as shown in Error! Reference source not found..  FDD LTE defines four different Random Access (RA) preamble formats with different preamble and cyclic prefix duration to accommodate different cell sizes. Preamble Format 0, which is well suited for small to medium size cells (up to approximately 14 km), uses a full 1 ms subframe and has a preamble duration of 800 ms with 103 ms cyclic prefix and 97 ms guard time, as shown in Error! Reference source not found..

CONCLUSIONS

The LTE physical layer implements a number of technologies to deliver on requirements for high data rates and spectral efficiency. OFDMA with MIMO allows the downlink to provide as high as 100 Mbps in link throughput while SC-FDMA on the uplink reduces design complexity for the user terminals by reducing PAPR. The design of the physical layer and system parameters are well matched with the characteristics of mobile propagation channel to allow optional downlink and uplink frequency selective scheduling thereby enhancing throughput performance. Adaptive modulation and coding maximizes throughput to individual subscribers and increases overall cell capacity.

Aside from capacity, the physical layer is structured to provide low latency. The 1 ms subframe duration provides low latency through small scheduling intervals while maintaining low overhead related to higher layer protocols. The PHY is also well designed to provide high cell-edge performance with specific features such as dynamic bandwidth allocation to users and the design of reference signals and control channels which take into account more challenging path loss and interference environment at the cell edge.

About the Author

Frank Rayal is the Chief Technology Officer and Co-Founder at Telesystem Innovations. Based in Canada, Mr. Rayal assists clients with technology and vendor evaluation, business plan and financial modeling, RFI/RFP process, RF network planning and dimensioning, and project management for field trials and network deployment, and product requirement development. Prior to founding TSI, he was Director of Product Management at Redline Communications, where he developed base stations targeted at different market segments and applications and launched end-to-end broadband wireless access networks in emerging markets. Mr. Rayal holds a BS in Electrical Engineering from Case Western Reserve University, Cleveland, OH, and a MASc and MBA from the University of Toronto.