Enis Akay

A MIMO System With Multifunctional Reconfigurable Antennas

A multiple-input-multiple-output (MIMO) system equipped with a new class of antenna arrays, henceforth referred to as multifunction reconfigurable antenna arrays (MRAAs), is investigated. The elements of MRAA, i.e., multifunction reconfigurable antennas (MRAs) presented in this work are capable of dynamically changing the sense of polarization of the radiated field thereby providing two reconfigurable modes of operation, i.e., polarization diversity and space diversity. The transmission signaling scheme can also be switched between transmit diversity (TD) and spatial multiplexing (SM). The results show that the reconfigurable modes of operation of an MRAA used in conjunction with adaptive space-time modulation techniques provide additional degrees of freedom to the current adaptive MIMO systems, resulting in more robust system in terms of quality, capacity and reliability. A performance gain up to 30 dB is possible with the proposed system over conventional fixed antenna MIMO systems depending on the channel conditions

Achieving Full Frequency and Space Diversity in Wireless Systems via BICM, OFDM, STBC, and Viterbi Decoding

Orthogonal frequency-division multiplexing (OFDM) is known as an efficient technique to combat frequency-selective channels. In this paper, we show that the combination of bit-interleaved coded modulation (BICM) and OFDM achieves the full frequency diversity offered by a frequency-selective channel with any kind of power delay profile (PDP), conditioned on the minimum Hamming distance dfree of the convolutional code. This system has a simple Viterbi decoder with a modified metric. We then show that by combining such a system with space-time block coding (STBC), one can achieve the full space and frequency diversity of a frequency-selective channel with N transmit and M receive antennas. BICM-STBC-OFDM achieves the maximum diversity order of NML over L-tap frequency-selective channels regardless of the PDP of the channel. This latter system also has a simple Viterbi decoder with a properly modified metric. We verify our analytical results via simulations, including channels employed in the IEEE 802.11 standards

Bit Interleaved Coded Multiple Beamforming

In this paper, we investigate the performance of bit-interleaved coded multiple beamforming (BICMB). We provide interleaver design criteria such that BICMB achieves full spatial multiplexing of min( N, M) and full spatial diversity of NM with N transmit and M receive antennas over quasi-static Rayleigh flat fading channels. If the channel is frequency selective, then BICMB is combined with orthogonal frequency division multiplexing (OFDM) (BICMB-OFDM) in order to combat ISI caused by the frequency-selective channels. BICMB-OFDM achieves full spatial multiplexing of min(N, M), while maintaining full spatial and frequency diversity of NML for an NtimesM system over L-tap frequency-selective channels when an appropriate convolutional code is used. Both systems analyzed in this paper assume perfect channel state information both at the transmitter and the receiver. Simulation results show that, when the perfect channel state information assumption is satisfied, BICMB and BICMB-OFDM provide substantial performance or complexity gains when compared to other spatial multiplexing and diversity systems.

Diversity analysis of single and multiple beamforming

Multi-antenna communication systems have the potential to play an important role in the design of the next generation broadband wireless communication systems. In this paper, we study a single-user multi-antenna system with perfect channel state information (CSI) both at the transmitter and the receiver. Beamforming is used to exploit the perfect channel knowledge at both ends. We show that beamforming achieves the maximum diversity in space when only the best eigenmode is used (i.e., single beamforming). We extend our analytical results to multiple beamforming (i.e., sending more than one symbol simultaneously). Our main contribution is the analysis of the maximum achievable diversity order of beamforming systems.

Low complexity decoding of bit-interleaved coded modulation for M-ary QAM

It has been shown by Zehavi that the performance of coded modulation can be improved over a Rayleigh fading channel by bit-wise interleaving at the encoder output, and by using an appropriate soft-decision metric for a Viterbi decoder at the receiver. Caire et al presented the details of the theory behind bit-interleaved coded modulation (BICM). In this paper we show that for Gray encoded M-ary quadrature amplitude modulation (QAM) systems, the bit metrics of BICM can be further simplified. In QAM systems, the maximum likelihood (ML) detector for BICM uses the minimum distance between the received symbol and M/2 constellation points on the complex plane as soft-decision metrics. We show that soft-decision bit metrics for the ML decoder can be further simplified to the minimum distance between the received symbol and /spl radic/M/2 constellation points on the real line /spl Ropf/1. This reduces the number of calculations needed for each bit metric substantially, and therefore reduces the complexity of the decoder without compromising the performance. Simulation results for single carrier modulation (SCM), and multi-carrier modulation (MCM) systems over additive white Gaussian noise (AWGN) and Rayleigh fading channels agree with our findings. In addition, we tie this result to the decoding methods for bit interleaved convolutional code standards used in industry.

Bit interleaved coded modulation with space time block codes for OFDM systems

Wireless systems often implement one or more types of diversity in order to achieve reliable communication. Different types of diversity techniques such as temporal, frequency, code, and spatial have been developed. Besides the destructive multipath nature of wireless channels, frequency selective channels pose intersymbol interference (ISI) while offering frequency diversity for well designed systems. Orthogonal frequency division multiplexing (OFDM) combats ISI very well by converting the frequency selective channel into parallel flat fading channels. On the other hand, bit interleaved coded modulation (BICM) has a high performance for flat fading Rayleigh channels. The combination of BICM and OFDM exploits the diversity that is inherited within frequency selective fading channels. Hence, BICM-OFDM is a very effective technique to provide diversity gain., employing frequency diversity. Orthogonal space-time block codes (STBC) make use of spatial diversity by coding in space and time. Thus, diversity in frequency and space can be taken advantage of by combining BICM-OFDM and STBC. We show and quantify both analytically and via simulations that, for frequency selective fading channels, BICM-STBC-OFDM systems can fully and successfully exploit frequency and space diversity to the maximum available extent.

High performance Viterbi decoder for OFDM systems

For systems that deploy conventional convolutional codes, a Viterbi decoder is the best solution, in the maximum likelihood sense, to decode an information sequence. Typically, a Viterbi decoder uses the Euclidean or Hamming distance as a metric. The use of a conventional metric leads to a high performance for systems that are employed for frequency nonselective channels (e.g., additive white Gaussian noise, AWGN, or Rayleigh fading ). However, if the system is based on orthogonal frequency division multiplexing (OFDM) and the channel has frequency selective multipath fading, then the performance can be further improved. We propose a simple modification to the conventional Viterbi metric (Euclidean distance) that improves the performance substantially if the channel is frequency selective. Simulation results on the IEEE 802.11a wireless local area network (WLAN) standard show that the performance is improved about 10 dB when the proposed metric is used. Furthermore, the proposed metric gives the same high performance as the conventional Viterbi metric if the channel is AWGN or flat fading.

Performance analysis of beamforming for MIMO OFDM with BICM

In this paper we show and quantify both analytically and via simulations that the use of channel knowledge at the transmitter, the technique known as beamforming, achieves the maximum diversity in space when the best eigenmode is used (single beamforming). Furthermore, we investigate beamforming in conjunction with next generation wireless local area networks (WLAN). It is known that the widely used technique in WLAN, bit interleaved coded modulation (BICM) with orthogonal frequency division multiplexing (OFDM), can achieve the maximum frequency diversity order that is inherited in the channel. We show that the combination of BICM, single beamforming, and OFDM also leads to the maximum diversity order in space and frequency domains. In other words, for systems with N transmit and M receive antennas, BICM beamforming-OFDM (BBO) can achieve a diversity order of NML over L-tap frequency selective channels by using an appropriate convolutional code. In addition to having a substantial diversity order, simulation results show that beamforming and BBO introduce significant coding gains when compared to other systems based on space time block codes (STBC) with the same diversity order.

Full frequency diversity codes for single input single output systems

Due to the severe conditions of wireless channels, it is crucial for wireless systems to accommodate some sort of diversity to achieve reliable communication. Different types of diversity techniques such as temporal, frequency, code, and spatial have been developed in the literature. In addition to the destructive multipath nature of wireless channel frequency selective channels pose intersymbol interference (ISI) while offering frequency diversity for successfully designed systems. Orthogonal frequency division multiplexing (OFDM) has been shown to fight ISI very well by converting the frequency selective channel into parallel flat fading channels. On the other hand, bit interleaved coded modulation (BICM) was shown by Zehavi (IEEE Trans. Comm.. vol. 40, no. 5, pp. 873-884, 1992) and later by Caire et al (IEEE Trans. Inform. Theory vol. 44, no. 3, 1998) to have high performance for flat fading Rayleigh channels. It is natural to combine BICM and OFDM to exploit the common ground of both techniques to improve overall system performance. In this paper we show both analytically and via simulations that for L tap frequency selective fading channels, BICM-OFDM can achieve a diversity order of min(d/sub free./ L), where d/sub free/ is the minimum Hamming distance of the convolutional code used for BICM.

Achieving full spatial multiplexing and full diversity in wireless communications

It is well-known that using multiple antennas provides a substantial capacity and diversity increase for wireless communication systems. A multi-input multi-output (MIMO) technique that utilizes the channel knowledge both at the transmitter and the receiver is known as beamforming. Beamforming separates a MIMO channel into parallel subchannels. It was previously shown that uncoded beamforming achieves a diversity order of (N - S + 1)(M - S + 1) if S symbols are transmitted simultaneously for N transmit and M receive antennas. Hence, there is a significant drop in the diversity order (and performance) of the system with increased spatial multiplexing. In this paper, we introduce bit interleaved coded multiple beamforming and name the system BICMB. We provide interleaver design criteria such that the resulting system achieves full spatial multiplexing of min(N, M) and full spatial diversity of NM. Simulation results show that BICMB, due to its ability of maintaining the maximum diversity order even at full spatial multiplexing, provides substantial performance gain when compared to the best spatial multiplexing systems