Relation between Channel bandwidth and transmission bandwidth configuration

Transmission scheme

The multiple access scheme for the LTE physical layer is based on Orthogonal Frequency Division Multiple Access (OFDM) with a Cyclic Prefix (CP) in the downlink and a Single Carrier Frequency Division Multiple Access (SC-FDMA) with CP in the uplink.

OFDMA technique is particularly suited for frequency selective channel and high data rate. It transforms a wideband frequency selective channel into a set of parallel flat fading narrowband channels, thanks to CP. This ideally, allows the receiver to perform a low complex equalization process in frequency domain, i.e. 1 tap scalar equalization.

The baseband signal representing a downlink physical channel is defined in terms of the following steps:

• scrambling of coded bits in each of the code words to be transmitted on a physical channel

• modulation of scrambled bits to generate complex-valued modulation symbols

• mapping of the complex-valued modulation symbols onto one or several transmission layers

• precoding of the complex-valued modulation symbols on each layer for transmission on the antenna ports

• mapping of complex-valued modulation symbols for each antenna port to resource elements

• generation of complex-valued time-domain OFDM signal for each antenna port

The baseband signal representing the physical uplink shared channel is defined in terms of the following steps, as shown in the below figure:

• scrambling

• modulation of scrambled bits to generate complex-valued symbols

• transform precoding to generate complex-valued symbols

• mapping of complex-valued symbols to resource elements

• generation of complex-valued time-domain SC-FDMA signal for each antenna port

Uplink physical channel processing

RF Related Requirements

In TS 36.101 and TS 36.104 different requirements can be found for the RF. In particular the following classification can be done.

For the UE transmitter, requirements are given for the following quantities: 

• Transmit signal quality (Error Vector Magnitude (EVM));

• Maximum Output Power (MOP), Maximum Power Reduction (MPR), Output power dynamics;

• Output RF spectrum emission: (Occupied bandwidth, spectrum emission mask. Out of band emission, Adjacent Carrier Leakage Ratio (ACLR), spurious emission);

• Transmit intermodulation.

For the UE receiver, requirements are given for the following quantities:

• Reference sensitivity power level

• Maximum Input level

• Adjacent Channel sensitivity

• Blocking characteristic

• Spurious response

• Intermodulation characteristics

• Spurious emissions.

For the BS transmitter, requirements are given for the following quantities:

• Base station output power and output power dynamics, Transmit ON/OFF power;

• Transmitted signal quality,  EVM and frequency error;

• Unwanted emissions (occupied bandwidth, ACLR, Operating band unwanted emissions, transmitter spurious emissions );

• Transmitter intermodulation.

For the BS receiver, requirements are given for the following quantities:

• Reference sensitivity level;

• Dynamic range;

• In-channel selectivity, Adjacent Channel Selectivity (ACS), blocking and narrow band blocking;

• Receiver spurious emissions;

• Receiver intermodulation.

In E-UTRA, the variable bandwidth of the system presents a special problem in the definition of the RF requirements; there may be the need to define some parameters as many times as there are bandwidth modes.  Special attention has been given also to the coexistence issue, since LTE has to coexist with all the other already existing systems.

Performance requirements can be found in TS 36.101 and TS 36.104.

TS 36.113 covers the assessment of E-UTRA base stations, repeaters and associated ancillary equipment in respect of Electromagnetic Compatibility.

TS 36.141 specifies the Radio Frequency (RF) test methods and conformance requirements for E-UTRA Base Stations (BS) operating either in the FDD mode (used in paired bands) or the TDD mode (used in unpaired bands). These have been derived from, and are consistent with the E-UTRA Base Station (BS) specifications defined in TS 36.104.

Layer 1

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 0.5 ms.

The radio frame structure type 1 is used for FDD (for both full duplex and half duplex operation) and has a duration of 10 ms and consists of 20 slots with a slot duration of 0.5 ms. Two adjacent slots form one sub-frame of length 1 ms. The radio frame structure type 2 is used for TDD and consists of two half-frames with a duration of 5 ms each and containing each 8 slots of length 0.5 ms and three special fields (DwPTS, GP and UpPTS) which have configurable individual lengths and a total length of 1 ms. A sub-frame consists of two adjacent slots, except for sub-frames 1 and 6, which consist of DwPTS, GP and UpPTS. Both 5 ms and 10 ms 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 7.5 kHz 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.

Under Single-User MIMO, the base station allocates 1 or 2 streams to the selected user, in the case of Multi-User MIMO the allocation of different (1 or 2) streams is done to different users. This is supported in both UL and DL.

The physical channels defined in the downlink are:

• Physical Downlink Shared Channel (PDSCH);

• Physical Multicast Channel (PMCH);

• Physical Downlink Control Channel (PDCCH);

• Physical Broadcast Channel (PBCH);

• Physical Control Format Indicator Channel (PCFICH);

• Physical Hybrid ARQ Indicator Channel (PHICH).

The physical channels defined in the uplink are:

• Physical Random Access Channel (PRACH),

• Physical Uplink Shared Channel (PUSCH),

• 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.

The channel coding scheme for transport blocks in LTE is Turbo Coding as for UTRA, 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.

There are several Physical layer procedures involved with LTE operation:

• Cell search;

• Power control;

• Uplink synchronization 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.

See TS 36.201 and TS 36.211 for more information.