Low complexity iterative receiver design for OFDM systems
Creators: name: Namboodiri, Vamadevan
Publisher: name: No Publisher Supplied
Year: 2012

description: Single Input Single Output (SISO) Orthogonal Frequency Division Multiplexing (OFDM) systems have been adopted in many of the recent wireless communication standards such as European terrestrial broadcast systems based on DVB-H, DVB-T and DVB-T2. For OFDM systems, cyclic prefix  of sufficient length makes the receiver design simple in frequency-selective multipath environments.  Wireless communication based on Multiple Input Multiple Output (MIMO) systems has  gained popularity due to the  potential capacity  increases it can provide.  MIMO-OFDM based transmission systems can thus provide very high data rates with a relatively simple receiver design and are now adopted widely in recent wireless communication standards such as Long Term Evolution (LTE), WiMAX  and WiFi. Modern wireless communication applications, both SISO and MIMO, require high data rates at high carrier frequencies and at high levels of mobility.  This results in less intercarrier spacing and  severe time-varying frequency-selective multipath fading, which breaks the orthogonality of subcarriers and causes intercarrier interference (ICI) in the received signal thus severely impacting the BER performance of the receiver.  Hence, efficient receiver design which is  fundamental to any communication system is ever more relevant.     Turbo iterative receivers (IR) are based on the observation that performance of the system can be significantly improved if detection and decoding are combined together.  They, in general, are found to have superior performance compared to other solutions, however turbo IRs usually suffer from high computational complexity which makes their implementation expensive.  Such practical application challenges motivate us to propose a new, low complexity, Turbo IR for SISO and MIMO OFDM systems under time varying frequency selective channel conditions.     Motivated by the classical TE,  we first propose a sub-optimal, successive interference cancellation and MAP decoding (SIC-MAP) algorithm for SISO systems.  In SIC-MAP, copies of the received signal on the same and adjacent subcarriers are carefully combined to take advantage of  the frequency diversity (on account of the time variations of the channel) while eliminating the interference from the other transmit symbols leveraging the feedback information from the decoder.  The resulting system matrix becomes a single column vector which allows an easy MAP decoding.  BER performance, computation complexity, and convergence behavior of the proposed scheme has been contrasted with two other similar schemes.  It has been found that SIC-MAP, while having near identical  performance to the competing schemes,  can be implemented  approximately with only a third of their computational complexity.  Subsequently, we extend the above detection idea, SIC-MAP, to MIMO systems (SIC-MAP-MIMO).  Unlike single antenna systems, even under static multipath channel conditions, the received signal in a MIMO receiver is corrupted by the co-antenna interference (CAI), thus making the detection task more challenging.  SIC-MAP-MIMO algorithm achieves comparable BER performance to the competing equalization schemes but with even more  computational savings than SISO.  A low complexity Least Squares (LS) based iterative channel estimation scheme using soft feedback information has also been proposed.  This scheme is especially suitable when the number of significant channel taps is higher than the number of pilots, a phenomenon that is often encountered in practical  systems.