The 3G Long-Term Evolution RADIO INTERFACE CONCEPTS

The ability to provide high bit rates is a key measure for LTE. Multiple parallel data stream transmission to a single terminal, using multiple-input-multiple-output (MIMO) techniques, is one important component to reach this. Larger transmission bandwidth and at the same time flexible spectrum allocation are other pieces to consider when deciding what radioaccess technique to use. The choice of adaptive multi-layer OFDM, AML-OFDM, in downlink will not only facilitate to operate at different bandwidths in general but also large bandwidths for high data rates in particular. Varying spectrum allocations, ranging from 1.25 MHz to 20 MHz, are supported by allocating corresponding numbers of AML-OFDM subcarriers. Operation in both paired and unpaired spectrum is possible as both time-division and frequency-division duplex are supported by AML-OFDM.

A. Downlink – OFDM with Frequency-Domain Adaptation

The AML-OFDM-based downlink has a frequency structure based on a large number of individual sub-carriers with a spacing of 15 kHz. This frequency granularity facilitates to implement dual-mode UTRA/E-UTRA terminals. The ability to reach high bit rates is highly dependent on short delays in the system and a prerequisite for this is short sub-frame duration. Consequently, the LTE sub-frame duration is set as short as 0.5 ms in order to minimize the radio-interface latency. In order to handle different delay spreads and corresponding cell sizes with a modest overhead the OFDM cyclic prefix length can assume two different values. The shorter 4.7 ms cyclic prefix is enough to handle the delay spread for most unicast scenarios. With the longer cyclic prefix of 16.7 ms very large cells, up to and exceeding 120 km cell radius, with large amounts of time dispersion can be handled. In this case the length is extended by reducing the number of OFDM symbols in a sub-frame.

OFDM is suitable for broadcast services. To support such services the same information is transmitted from several (synchronized) base stations to the terminal. The total signal the terminal receives from the base stations will appear as multipath propagation and thus implicitly be exploited by the OFDM receiver. The longer cyclic prefix allows combining
broadcast signals from a large number of distant base stations. Techniques to exploit channel variations in the time domain have been successfully implemented for HSDPA. This has resulted in a substantial increase in spectral efficiency. For EUTRA, the channel-based adaptation can be extended to also include transmission adaptation in the frequency domain thanks to the use of OFDM. When the radio channel varies significantly over the system bandwidth, large performance gains can be achieved.

B. Uplink – Single-Carrier FDMA with Dynamic Bandwidth

A key requirement for uplink transmission is that the transmission should allow for power-efficient user-terminal transmission to maximize coverage. To reach this, single-carrier frequency-division multiple access (FDMA) with dynamic bandwidth is a good choice. In order to achieve intra-cell orthogonality, the base station assigns a unique time-frequency interval to the terminal for the transmission of user data. This is done for each time interval. The users are separated primarily by time-domain scheduling; however, if the terminal has a limited transmission power or not enough data to transmit, also frequency-domain scheduling is used.

C. Multi-Antenna Solutions

Advanced multi-antenna techniques will play an important role in fulfilling the 3G LTE requirements on increased data rates and improved coverage and capacity. This includes both beamforming and multi-layer transmission solutions to better exploit the potential of using the spatial domain. This potential is large and not always fully exploited in existing radio access technologies. Increasing data rates can be achieved by transmitting multiple parallel streams or layers to a single user. This Multi-layer transmission is often referred to as MIMO. The preferred use for MIMO is in conditions with favorable signal-to-noise ratio and rich scattering in the radio channel, e.g., small cells or indoor deployments. Multi-layer transmission may be applied for downlink as well as uplink transmission. The receiver has the possibility to separate the multiple data streams by using the channel properties and knowledge of the coding scheme. In order for the receivers to solve this task it is necessary to standardize the multi-layer transmission scheme selected for the long-term 3G. Selective per-antenna rate control (S-PARC) is an interesting technique where the number of layers and the data rate per layer, is adapted to the instantaneous channel conditions.

Beamforming implies that multiple antennas are used to form the transmission or reception beam and, in this way, increase the signal-to-noise ratio at the receiver. This technique can both be used to improve coverage of a particular data rate and to increase the system spectral efficiency. The increased signal-to-noise ratio is not only due to a larger gain in the direction of the desired user, but also due to a better control of the spatial interference distribution in the cell. Beamforming can be applied both to the downlink and the uplink. It is possible to make beamforming transparent to the terminal, which would eliminate the need to standardize a particular solution. Instead, the exact algorithms can evolve over time and be tailored to particular needs. Alternatively, one could include some explicit support for a specific beamforming solution, especially if that would increase the efficiency of the system and enable low complexity implementations.

It is also possible to combine multi-layer transmission and beamforming. An example of this would be to transmit two data streams with two groups of antennas, where beamforming is employed within each group. Beamforming is then used to increase the received signal-to-noise ratio and multi-layer transmission is applied to convert the increased signal-to-noise ratio into a higher data rate.