Sunday, October 24, 2010

LTE Layer 1 Services

Multiple Access
The multiple access scheme for the LTE physical layer is based on Orthogonal Frequency Division Multiplexing (OFDM) with a cyclic prefix (CP) in the downlink, and on Single-Carrier Frequency Division Multiple Access (SC-FDMA) with a cyclic prefix in the uplink. To support transmission in paired and unpaired spectrum, two duplex modes are supported: Frequency Division Duplex (FDD), supporting full duplex and half duplex operation, and Time Division Duplex (TDD).

The Layer 1 is defined in a bandwidth agnostic way based on resource blocks, allowing the LTE Layer 1 to adapt to various spectrum allocations. A resource block spans either 12 sub-carriers with a sub-carrier bandwidth of 15kHz or 24 sub-carriers with a sub-carrier bandwidth of 7.5kHz each over a slot duration of 0.5ms.

The radio frame structure type 1 is used for FDD (for both full duplex and half duplex operation) and has a duration of 10ms and consists of 20 slots with a slot duration of 0.5ms. Two adjacent slots form one sub-frame of length 1ms. The radio frame structure type 2 is used for TDD and consists of two half-frames with a duration of 5ms each and containing each 8 slots of length 0.5ms and three special fields (DwPTS, GP and UpPTS) which have configurable individual lengths and a total length of 1ms.

A sub-frame consists of two adjacent slots, except for sub-frames 1 and 6, which consist of DwPTS, GP and UpPTS. Both 5ms and 10ms switch-point periodicity are supported.

To support a Multimedia Broadcast and Multicast Service (MBMS), LTE offers the possibility to transmit Multicast/Broadcast over a Single Frequency Network (MBSFN), where a time-synchronized common waveform is transmitted from multiple cells for a given duration. MBSFN transmission enables highly efficient MBMS, allowing for over-the-air combining of multi-cell transmissions in the UE, where the cyclic prefix is utilized to cover the difference in the propagation delays, which makes the MBSFN transmission appear to the UE as a transmission from a single large cell. Transmission on a dedicated carrier for MBSFN with the possibility to use a longer CP with a sub-carrier bandwidth of 7.5kHz is supported as well as transmission of MBSFN on a carrier with both MBMS transmissions and point-to-point transmissions using time division multiplexing.

Transmission with multiple input and multiple output antennas (MIMO) are supported with configurations in the downlink with two or four transmit antennas and two or four receive antennas, which allow for multi-layer transmissions with up to four streams. Multi-user MIMO i.e. allocation of different streams to different users is supported in both UL and DL.

Physical channels and modulation

The physical channels defined in the downlink are:
• the Physical Downlink Shared Channel (PDSCH),
• the Physical Multicast Channel (PMCH),
• the Physical Downlink Control Channel (PDCCH),
• the Physical Broadcast Channel (PBCH),
• the Physical Control Format Indicator Channel (PCFICH)
• and the Physical Hybrid ARQ Indicator Channel (PHICH).

The physical channels defined in the uplink are:
• the Physical Random Access Channel (PRACH),
• the Physical Uplink Shared Channel (PUSCH),
• and the Physical Uplink Control Channel (PUCCH).

In addition, signals are defined as reference signals, primary and secondary synchronization signals. The modulation schemes supported in the downlink and uplink are QPSK, 16QAM and 64QAM.

Channel coding and interleaving
The channel coding scheme for transport blocks in LTE is Turbo Coding with a coding rate of R=1/3, two 8-state constituent encoders and a contention-free quadratic permutation polynomial (QPP) turbo code internal interleaver. Trellis termination is used for the turbo coding. Before the turbo coding, transport blocks are segmented into byte aligned segments with a maximum information block size of 6144 bits. Error detection is supported by the use of 24 bit CRC. 

Physical layer procedures
There are several Physical layer procedures involved with LTE operation. Such procedures covered by the physical layer are;
- Cell search
- Power control
- Uplink synchronisation and Uplink timing control
- Random access related procedures
- HARQ related procedures
Through the control of physical layer resources in the frequency domain as well as in the time and power domain, implicit support of interference coordination is provided in LTE.

Physical layer measurements
Radio characteristics are measured by the UE and the eNode-B and reported to higher layers in the network. These include, e.g. measurements for intra- and inter-frequency handover, inter RAT handover, timing measurements and measurements for RRM and in support for positioning. Measurements for inter-RAT handover are defined in support of handover to GSM, UTRA FDD, UTRA TDD, CDMA2000 1x RTT and CDMA2000 HRPD.

Friday, October 22, 2010

LTE Layer 1

The radio interface is composed of the Layer 1, 2 and 3. Below figure shows the E-UTRA radio interface protocol architecture around the physical layer (Layer 1). The physical layer interfaces the Medium Access Control (MAC) sub-layer of Layer 2 and the Radio Resource Control (RRC) Layer of Layer 3. The circles between different layer/sub-layers indicate Service Access Points (SAPs). The physical layer offers a transport channel to MAC. The transport channel is characterized by how the information is transferred over the radio interface. MAC offers different logical channels to the Radio Link Control (RLC) sub-layer of Layer 2. A logical channel is characterized by the type of information transferred.

Service provided to higher layers

The physical layer offers data transport services to higher layers. The access to these services is through the use of a transport channel via the MAC sub-layer. The physical layer is expected to perform the following functions in order to provide the data transport service:

- Error detection on the transport channel and indication to higher layers
- FEC encoding/decoding of the transport channel
- Hybrid ARQ soft-combining
- Rate matching of the coded transport channel to physical channels
- Mapping of the coded transport channel onto physical channels
- Power weighting of physical channels
- Modulation and demodulation of physical channels
- Frequency and time synchronisation
- Radio characteristics measurements and indication to higher layers
- Multiple Input Multiple Output (MIMO) antenna processing
- Transmit Diversity (TX diversity)
- Beamforming
- RF processing.