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PHYSICAL CHANNELS

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.

Page 2: Uplink Physical Channels

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