LTE OFDM PHYSICAL LAYER
BPSK QPSK
16QAM 64QAM
Signalling functions only
Optional on uplink
1/3 Turbo Coding Traffic and most control channels
1/3 CC BCH only
Transport Block 24 bit CRC
Modulation and Error Protection
The range of modulation schemes used in E-UTRA comprises BPSK, QPSK, 16QAM (16-state Quadrature Amplitude Modulation) and 64QAM (64-state Quadrature Amplitude Modulation). BPSK is only employed for a limited set of signalling and reference functions, while 64QAM is optional on the uplink.
The range of error coding options used in E-UTRA devices is far more limited than those available to, for example, a UMTS device. For most channels the only option is one-third rate turbo coding based on convolutional coding.
Broadcast traffic channels are only permitted to use 1/3 Tail Biting convolutional coding. Various control channels have been assigned either convolutional coding, block coding or simple repetition as their error coding options.
In addition to error coding, transport blocks containing user and control traffic may also optionally have a CRC (Cyclic Redundancy Check) block attached. Transport blocks on connections that have CRC selected have a 24-bit CRC block appended to the end of the data container.
The familiar UMTS error monitoring levels of Bit Error Rate (BER), derived from the error coding service, and BLER (Block Error Rate), derived from CRC, continue to be available in E-UTRA.
Further Reading: 3GPP TS 36.211, 36.212, 36.300
Physical signals PSS/SSS Reference signals Physical
layer
MAC BCCH PCCH CCCH DCCH DTCH
BCH PCH RACH DL-SCH UL-SCH
PBCH PDCCH PUCCH PHICH PCFICH PRACH PDSCH PUSCH
MAC Control
Physical Channels
The physical layer involves the transmission and reception of a series of physical channels and physical signals. The physical signals relate to the transmission of reference signals, the PSS (Primary Synchronization Signal) and the SSS (Secondary Synchronization Signal).
The PBCH (Physical Broadcast Channel) carries the periodic downlink broadcast of the RRC MasterInformationBlock message. Note that system information from BCCH (Broadcast Control Channel) is scheduled for transmission in the PDSCH (Physical Downlink Shared Channel).
The PDCCH (Physical Downlink Control Channel) carries no higher layer information and is used for scheduling uplink and downlink resources. Scheduling decisions, however, are the responsibility of the MAC layer, therefore the scheduling information carried in the PDCCH is provided by MAC. Similarly the PUCCH (Physical Uplink Control Channel) is used to carry resource requests from UEs that will need to be processed by MAC.
The PHICH (Physical Hybrid ARQ Indicator Channel) is used for downlink ACK/NACK of uplink transmissions from UEs in the PUSCH (Physical Uplink Shared Channel). It is a shared channel and uses a form of code multiplexing to provide multiple ACK/NACK responses.
The PCFICH (Physical Control Format Indicator Channel) is used to indicate how much resource in a subframe is reserved for the downlink control channels. It may be either one, two or three of the first symbols in the first slot in the subframe.
The PRACH (Physical Random Access Channel) is used for the uplink transmission of preambles as part of the random access procedure.
The PDSCH and the PUSCH are the main scheduled resource on the cell. They are used for the transport of all higher-layer information including RRC signalling, service-related signalling and user traffic. The only exception is the system information in PBCH.
Further Reading: 3GPP TS 36.213, 36.211, 36.300
c. 32.5 ns T
s=
1 30,720,000
1
15,000 x 2048
T
s= T
s(Time unit) =
The Physical Layer Timing Unit
Almost all numbers, durations and calculations related to E-UTRA are derived from a fundamental parameter known as Ts or the basic ‘time unit’. Ts represents the ‘sampling time’ of the overall channel and is itself derived from basic channel parameters. The definition of Ts is based on a 20 MHz channel bandwidth with 15 kHz subcarrier spacing and an FFT size of 2048.
Ts is calculated to be the reciprocal of the subcarrier spacing multiplied by the total number of subcarriers in the FFT, or:
Ts = 1/(15,000 x 2048) seconds = 32.5 nsec
Frame, subframe and slot lengths, cyclic prefix durations and many other key parameters are defined as multiples of Ts.
Crucially, the value of Ts does not vary between E-UTRA physical layer configurations. As Ts stays constant, all of the key parameters used to define the E-UTRA structure also stay constant. This consistency reduces the overall complexity of E-UTRA and enables system manufacturers to scale their devices more easily to a variety of channel bandwidths and frequency bands.
Further Reading: 3GPP TS 36.211:4
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 (total per subframe 2048Ts)
0 1 2 3 4 5 6 (total per subframe 6144Ts) OFDM
There are two basic frame types employed in E-UTRA, which are common to both uplink and downlink.
Type 1 frames are employed for FDD full- and half-duplex systems, while Type 2 frames are reserved for TDD operation only.
The Type 1 frame duration is 10 ms and it is divided into 20 slots, each of 0.5 ms duration. More significantly, however, for most information transmission, two slots are combined to form a subframe.
Thus subframe duration is 1 ms, which corresponds to the TTI (Transmission Time Interval) for E-UTRA.
Type 1 slots contain either 7 or 6 symbols, depending upon which CP (cyclic prefix) type is in use.
Additionally, the length of the CP prefixed applied in a particular symbol within a slot varies, also dependent on which CP length is in use. With the normal CP, symbol 0 in each slot has a CP equal to 160 x Ts or 5.2 µsec, while the remaining symbols in the slot have slightly shorter CPs of just 144 x Ts or 4.7 µsec. When using the extended CP, all symbols are prefixed with a CP of 512 x Ts or 16.7 µsec.
Scheduling occurs across a subframe period. Up to the first three symbols in the first slot of each subframe can be defined as a ‘control region’ carrying control and scheduling messages. The remaining symbols of the first and all symbols in the second slot within the subframe are then available for user traffic.
Further Reading: 3GPP TS 36.211