Design Adaptive MIMO Space-Time-Frequency Conjugate Two-Path OFDM Systems

Abstract

Multiple input multiple output (MIMO) systems can be designed for two operational modes: to improve bit error rate (BER) or to increase capacity without consuming additional power or bandwidth. In this paper, MIMO space-time-frequency (STF) conjugate two-path transmission orthogonal frequency division multiplexing (OFDM) systems with precoders are presented to combat inter-carrier interference and to adaptively switch between these two modes in mobile fading channels. First, the STF architecture is developed to form a 2 × 1 STF-OFDM system. Second, a conjugate cancellation (CC) two-path transmission scheme is integrated with the STF-OFDM system to form a new 2 × 1 STFCC-OFDM system. This system improves BER without expanding power or bandwidth in comparing with the CC scheme. Third, Walsh–Hadamard (WH) transform is employed as the precoder to form the 2 × 1 WHSTFCC-OFDM system. Finally, a 2 × 2 WHSTFCC-OFDM architecture is designed. It can be used to improve BER or switch to two 2 × 1 WHSTFCC-OFDM systems to increase capacity as needed. Simulations indicate that the 2 × 1 WHSTFCC-OFDM system has the best BER, the STFCC-OFDM system, and the STF-OFDM system follow in order. Similarly, the BER of the 2 × 2 WHSTFCC-OFDM system outperforms the 2 × 1 WHSTFCC-OFDM, and the 2 × 2 STFCC-OFDM system follow in order. All these STFCC- and WHSTFCC-OFDM schemes are simple and backward compatible with the existing OFDM system. They may serve as baseband building blocks in OFDM transceivers and can be adaptively switched between 2 × 1 and 2 × 2 architectures as required for the fourth generation and the future planned fifth generation MIMO systems. This adaptive MIMO system can also be applied to the regular or bad weather environment in the disaster conditions for increasing capacity or enhancing the reliability of the communication system.