Waveform diversity and design: Part I - issues for multi-antenna systems

Abstract

This paper, forming the first part of a two-part series, focuses on the design of error control codes (ECC) that are suitable for use in multiple-input multiple-output (MIMO) based systems. It has been shown in the past that the use of multiple antennas at both the transmitter and receiver can improve communication link performance. The use of higher spectral efficiency (increased bits per second per hertz of bandwidth) and link reliability via the implementation of diversity leads to significant increases in data throughput that can be sustained without additional bandwidth or transmit power. In order to further exploit the diversity introduced by MIMO, it is important to optimise use of the physical channel via the design of error control codes specific to the MIMO channel. Codes that have been optimised for use over such channels are termed space-time codes. An investigation of various space-time codes formed from a number of different underlying codes is presented herein. Space-time codes based on both turbo codes and low density parity check (LDPC) codes are investigated. Turbo codes and LDPC codes are discussed as these are two of the most powerful ECC derived for the single input single output (SISO) additive white Gaussian noise (AWGN) channel. The use of MIMO introduces diversity in the spatial domain. Fading across multiple antennas is optimally required to be independent in order to fully utilise the redundancy introduced via multiple antenna use. Diversity in the temporal domain is introduced via error control coding. Space-time codes operate across multiple antennas in order to maximise diversity in the spatial domain (diversity advantage) but also maximise diversity in the temporal domain (coding advantage). The paper explains design rules for maximising both diversity and coding advantage in order to design powerful space-time codes. Two specific examples are presented within the paper, one based on turbo codes and another based on LDPC codes. Turbo codes and LDPC codes provide maximum coding advantage for SISO based systems achieving performance extremely close to the AWGN channel capacity and therefore it makes sense to investigate the re-use of such coding schemes within the MIMO environment. There are many uses for MIMO including beamforming techniques typically used in sensor arrays for directional signal transmission or reception but the sole focus of this paper is on the code design for the space-time channel. Furthermore, MIMO and space-time codes have been developed principally for use in terrestrial applications where fading environments are exploited. This paper concludes with ideas for the application of MIMO to problems in satellite communications. (27 pages)