LTE Physical Layer

Step1:

LTE PHYSICAL LAYER

Intended audience:

• Students
• RF Engineer
• RF Optimization Engineer

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LTE physical layer is designed to support high data rate transmission(100Mbs DL/50Mbs UL). it is based on OFDM (orthogonal frequency division multiplex) and MIMO (multi input multi output).

OFDM:
               In OFDM a wide band  is  divided into multiple  narrow band Sub carrier. these sub carrier are orthogonal to each other so they eliminate inter symbol interference(ISI).

Cyclic Prefix:
                         suppose time duration of OFDM Symbol is Tu. as Radio Channel is time dispersive means  receiver  received multiple copies of transmitted signal with different path delays because reflections incurs along the path between transmitter and reciever. this leads to partial loss of orthogonality between sub carriers which creates ISI.
To prevent this a guard interval Tg is added at the beginning of  OFDM symbol. this guard interval is called cyclic prefix(CP). so total OFDM symbol length becomes Ts = Tu + Tg.


Physical  Layer Parameters:

 LTE radio frame hase a length of 10ms. each frame is divided in 10 equally sub frame. scheduling is done on per sub frame basis. each frame consists of 2 time slots. so 1 frame = 20 time sltos.
 each slot  consists of 7 OFDM symbol in case of normal CP and 6 OFDM symbol in case  of extend CP.The normal CP is used in urban cells and for high data rate while extended CP is used in  cases like multi-cell broadcast and in very large cells.

the range of sub carrier varies from 128 to 2048 depending on channel BW. commonly  sub carrier spacing is 15 khz.

Resource Block(RB):
                                a group of 12 consecutive sub carrier for the duration of one time slot is called a resource block.so


1 RB = 12x15=180 khz

Resource Element(RE):
                                   An OFDM symbol is a resource element.so


1 RB = 12x7= 84 RE



DL Reference Signals:
                                         Reference symbols are inserted in RB for coherent demodulation and channel estimation. there are for reference symbols per RB as shown in below figure.

SYNCHRONIZATION SEQUENCES:

3 synchronization requirements in LTE.

1. Symbol timing acquisition.

2. Carrier frequency synchronization.

3.Sampling Clock synchronization.

                                    two cell search procedures in LTE
1.initial synchronization.
2.detecting neighbor cells in preparation for handover.

Primary Synchronization Sequence (PSS) and Secondary Synchronization Sequence (SSS) are used for time and frequency synchronization.they are transmitted twice per frame.

PHYSICAL CHANNELS:

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

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

3- Physical Downlink Control Channel (PDCCH):
The PDCCH carries the resource assignment for UEs which are contained in a Downlink Control Information (DCI) message.

4- 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).

5- 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.


Uplink physical Channels:
                                                   Single Carrier FDMA (SC-FDMA) is used in LTE with cyclic prefix on the uplink.as name suggests there is a single carrier as opposed to N carrier as shown in figure.

why SC-FDMA ??
 well because OFDMA has some certain draw backs like
1. High sensitivity to frequency offset i.e doppler effect because of mobility.
2.High peak to average power ratio(PAPR) because of  constructive addition on sub carrier.
the method used to eliminate these problems are expensive on user equipment(UE). while
SC-FDMA is not sensitive to frequency instability and Doppler Effect as OFDM because of its single carrier .
RE: 

 Resource element is the smallest unit which consists of one SC-FDMA data block length on one sub-carrier.
RB:

       A resource block consists of 12 REs for the duration of a slot(0.5 ms). The minimum allocated bandwidth to a UE is, therefore, 180 kHz

1-Physical Uplink Shared Channel (PUSCH):

This channel carries user data. It supports QPSK and 16 QAM modulation.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.

2-Physical Uplink Control Channel (PUCCH):

It carries control signaling ,HARQ ACK/NACK, channel quality indicators (CQI), MIMO feedback (Rank Indicator, RI,Precoding Matrix Indicator, PMI) and scheduling requests.

3-Physical Random Access Channel (PRACH):

It 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 Figure