This overview portrays the evolution of orthogonal frequency division multiplexing (OFDM) research. The amelioration of powerful multicarrier OFDM arrangements with multiple-input multiple-output (MIMO) systems has numerous benefits, which are detailed in this treatise. We continue by highlighting the limitations of conventional detection and channel estimation techniques designed for multiuser MIMO OFDM systems in the so-called rank-deficient scenarios, where the number of users supported or the number of transmit antennas employed exceeds the number of receiver antennas. This is often encountered in practice, unless we limit the number of users granted access in the base station's or radio port's coverage area. Following a historical perspective on the associated design problems and their state-of-the-art solutions, the second half of this treatise details a range of classic multiuser detectors (MUDs) designed for MIMO-OFDM systems and characterizes their achievable performance. A further section aims for identifying novel cutting-edge genetic algorithm (GA)-aided detector solutions, which have found numerous applications in wireless communications in recent years. In an effort to stimulate the cross pollination of ideas across the machine learning, optimization, signal processing, and wireless communications research communities, we will review the broadly applicable principles of various GA-assisted optimization techniques, which were recently proposed also for employment in multiuser MIMO OFDM. In order to stimulate new research, we demonstrate that the family of GA-aided MUDs is capable of achieving a near-optimum performance at the cost of a significantly lower computational complexity than that imposed by their optimum maximum-likelihood (ML) MUD aided counterparts. The paper is concluded by outlining a range of future research options that may find their way into next-generation wireless systems.

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Multiuser MIMO-OFDM for Next-Generation Wireless Systems

Owing to the excessive demand on signal processing and space constraint, a full digital implementation of a large adaptive antenna array at millimeter wave frequencies is very challenging. Targeted at long range high data rate point-to-point link in the 70/80 GHz bands, a novel hybrid adaptive antenna array which consists of analogue subarrays followed by a digital beamformer is presented in this paper to overcome the digital implementation difficulty. Two subarray configurations, the interleaved subarray and the side-by-side subarray, are proposed, and two Doppler resilient adaptive angle-of-arrival estimation and beamforming algorithms, the differential beam tracking (DBT) and the differential beam search (DBS), are developed. Simulation results on the DBT and DBS performance are provided using a 64 element hybrid planar array of four 4 by 4 element subarrays with the two subarray configurations, respectively. Recursive mean square error (MSE) bounds of the developed algorithms are also analyzed and compared with simulated MSEs.

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A hybrid adaptive antenna array

A multi-antenna transmitting entity transmits data to a single- or multi-antenna receiving entity using (1) a steered mode to direct the data transmission toward the receiving entity or (2) a pseudo-random transmit steering (PRTS) mode to randomize the effective channels observed by the data transmission across the subbands. For transmit diversity, the transmitting entity uses different pseudo-random steering vectors across the subbands but the same steering vector across a packet for each subband. The receiving entity does not need to have knowledge of the pseudo-random steering vectors or perform any special processing. For spatial spreading, the transmitting entity uses different pseudo-random steering vectors across the subbands and different steering vectors across the packet for each subband. Only the transmitting and receiving entities know the steering vectors used for data transmission. Other aspects, embodiments, and features are also claimed and disclosed.

Transmit diversity and spatial spreading for an ofdm-based multi-antenna communication system

A 24-beams slot array antenna, which achieves 360-degrees coverage over azimuth direction, based on substrate integrated waveguide (SIW) technology is designed, fabricated and measured for supporting the concept of triangle cell, beam diversity and multiple input multiple output (MIMO) wireless communications. Constructed by six 4-beams single layer SIW antennas which integrate the 4times8 modified Butler matrix with a low side lobe level (SLL) amplitude distribution, measured SLLs are reduced as low as -14.5 dB for all beams in E-plane. The size of the SIW single layer multibeam antenna is 385 mm times 110 mm including feeding network. The usable bandwidth of the proposed antenna is 3.75% at 16 GHz.

A Multibeam Antenna Based on Substrate Integrated Waveguide Technology for MIMO Wireless Communications

For data transmission with spatial spreading, a transmitting entity (1) encodes and modulates each data packet to obtain a corresponding data symbol block, (2) multiplexes data symbol blocks onto NS data symbol streams for transmission on NS transmission channels of a MIMO channel, (3) spatially spreads the NS data symbol streams with steering matrices, and (4) spatially processes NS spread symbol streams for full-CSI transmission on NS eigenmodes or partial-CSI transmission on NS spatial channels of the MIMO channel. A receiving entity (1) obtains NR received symbol streams via NR receive antennas, (2) performs receiver spatial processing for full-CSI or partial-CSI transmission to obtain NS detected symbol streams, (3) spatially despreads the NS detected symbol streams with the same steering matrices used by the transmitting entity to obtain NS recovered symbol streams, and (4) demodulates and decodes each recovered symbol block to obtain a corresponding decoded data packet.

Data transmission with spatial spreading in a MIMO communication system

Space-time coded orthogonal frequency division multiplexing (OFDM) is a promising scheme for future wideband multimedia wireless communication systems. The combination of space-time coding (STC) and OFDM modulation promises an enhanced performance in terms of power and spectral efficiency. Such combination benefits from the diversity gain within the multiple-input-multiple-output ST coded system and the matured OFDM modulation for wideband wireless transmission. However, STC transmit diversity impairs the system's interference suppression ability due to the use of multiple transmitters at each mobile. We propose an effective co-channel interference (CCI) cancellation method that employs angle diversity based on null-steering beamforming or minimum variance distortion response beamforming. It is shown that the proposed method can effectively mitigate CCI while preserving the space-time structure, thereby, significantly improving the system's interference suppression ability without significant bit-error rate performance degradation. Furthermore, it is demonstrated that the proposed method can significantly combat the delay spread detrimental effects over multipath fading channels without the use of interleaving.

Co-channel interference cancellation for space-time coded OFDM systems

Space-time coded OFDM is a promising scheme for future wideband multimedia wireless communication systems. The combination of space-time coding (STC) and OFDM modulation promises an enhanced performance in terms of power and spectral efficiency. However, STC transmit diversity impairs the system's interference suppression ability because each transmitter generates multiple signals that appear independent to the receiver antenna arrays. In this paper, we propose a novel CCI cancellation method employing angle diversity based on beamforming. It is shown that the proposed method can effectively mitigate CCI while preserving the space-time structure, thereby, significantly improving the system's interference suppression ability without significant performance degradation.

Orthogonal frequency division multiplexing (OFDM) has been widely regarded as an effective modulation technique for mitigating the effects of intersymbol interference in a frequency selective fading channel and for providing reliable high-data-rate transmission over wireless links. Adaptive antenna arrays at the base and mobile stations can achieve further increases in system's capacity and bandwidth efficiency, as well as in quality-of-service improvement in conventional OFDM systems. The conventional adaptive antenna-arrays-based OFDM systems always use the subcarriers characterized by the largest eigenvalues to transmit the OFDM block symbols. And in contrast to previous work, we propose dynamic spatial subchannel allocation with adaptive beamforming for broadband OFDM wireless transmission systems. The proposed system adaptively selects the eigenvectors associated with the relatively large spatial subchannel eigenvalues to generate the beamforming weights at the mobile and base stations and then dynamically assigns the corresponding best spatial subchannels to transmit the OFDM block symbols. It is shown that the proposed system can achieve better performance than an adaptive antenna-arrays-based OFDM system without dynamic spatial subchannel allocation over multipath fading channels. Simulation results also reveal that the proposed system is far less susceptible to feedback delay in rapid time-varying channels and a little more sensitive to channel estimation errors than conventional adaptive antenna-arrays-based OFDM systems. The performance of the proposed system combined with adaptive modulation is also considered.

Dynamic spatial subchannel allocation with adaptive beamforming for MIMO/OFDM systems

Dynamic resource allocation combined with adaptive antenna arrays at both the transmitter and receiver can achieve further QoS improvement in conventional OFDM systems. In this paper, we propose two different dynamic resource allocation algorithms with adaptive beamforming for MIMO/OFDM systems and investigate their performance over multipath fading channels under perfect and imperfect CSI. These proposed approaches involve the use of adaptive modulation and adaptive frequency-domain power allocation. By employing this proposed methodology, significant performance improvement can be achieved as compared to the conventional adaptive antenna arrays based on OFDM systems. It is shown, however, that the adaptive modulation based approach is too sensitive to channel estimation errors and that its performance is worse than the adaptive frequency-domain power allocation approach under imperfect CSI.

Dynamic resource allocation with adaptive beamforming for MIMO/OFDM systems under perfect and imperfect CSI

Orthogonal frequency division multiplexing (OFDM) has been widely regarded as an effective modulation technique for mitigating the effects of ISI in a frequency selective fading channel and for providing reliable high data rate transmission over wireless links. Adaptive antenna arrays at the base and mobile stations can achieve further increases in system's capacity and bandwidth efficiency, as well as in QoS improvement in conventional OFDM systems. The conventional adaptive antenna arrays based OFDM systems always use the sub-carriers characterized by the first largest eigenvalues to transmit the OFDM block symbols. In this paper and in contrast to previous work, we propose dynamic sub-channel allocation with adaptive beamforming for broadband OFDM wireless transmission systems. The proposed system adaptively selects the eigenvectors associated with the relatively large sub-channel eigenvalues to generate the beamforming weights at the mobile and base stations and then dynamically assigns the corresponding best sub-channels to transmit the OFDM block symbols. It is shown by simulation that, without adding much complexity, the proposed system can achieve a better performance than an adaptive antenna arrays based OFDM system without dynamic sub-channel allocation over multipath fading channels. Simulation results also reveal that the proposed system is not too sensitive to channel estimation errors.

Zhigang Cao

Papers

Co-channel interference cancellation for space-time coded OFDM systems

Co-channel interference cancellation for space-time coded OFDM systems

Dynamic spatial subchannel allocation with adaptive beamforming for MIMO/OFDM systems

Dynamic resource allocation with adaptive beamforming for MIMO/OFDM systems under perfect and imperfect CSI

Dynamic sub-channel allocation with adaptive beamforming for broadband OFDM wireless systems