Siavash M. Alamouti

OFDMA: A Broadband Wireless Access Technology

In this note, we review the design philosophies for uplink and downlink of OFDMA systems and demonstrate that OFDMA is a superior access technology for broadband wireless data network compared with traditional access technologies such as TDMA and CDMA. The main advantages of OFDMA over TDMA/CDMA stem from the scalability of OFDMA, the uplink orthogonality of OFDMA and the ability of OFDMA to take advantage of the frequency selectivity of the channel. Other advantages of OFDMA include its MIMO-friendliness and ability to provide superior quality of service (QoS).

New detection schemes for transmit diversity with no channel estimation

We present two new detection methods for the simple transmit diversity scheme proposed by Alamouti (see IEEE Journal on Selected Areas of Communications, Special Issue on Signal Processing for Wireless Communications, 1998). The new detection schemes require no channel state information at either the transmitter or the receiver. Using 2 transmit antennas and 1 receive antenna, the scheme provides a diversity order of two. Simulations have been included to show that the scheme has almost a 3 dB penalty compared to coherent detection.

A Pragmatic PHY Abstraction Technique for Link Adaptation and MIMO Switching

MIMO (multiple input multiple output) techniques are widely employed to improve the performance of wireless systems. These techniques are used to overcome multipath fading and/or improve the peak throughput of wireless systems. It is well known that there is a fundamental tradeoff between diversity gain and multiplexing gain [1]. Orthogonal space time codes such as the Alamouti code (also known as space time block codes (STBC)) exploit multiple antennas as a diversity source, and thus improve packet error rate (PER) and the average throughput. However, space time block codes are not designed to increase the peak data rate of the system. On the other hand, spatial multiplexing (SM) techniques offer higher peak throughput by transmitting parallel streams of data from different antennas. In order to successfully decode the parallel streams, the channel must exhibit a small eigenvalue spread. Otherwise, the streams will interfere with one another and it is difficult to decode the information data. The performance improvement from SM is therefore highly dependent on the channel characteristics. It is possible to use multiple encoders and rate control per layer to improve the SM performance. However, there are instances of channel where STBC is more beneficial than SM with a single encoder. In order to resolve the shortcomings of STBC and SM, a hybrid technique can be applied where depending on the instantaneous channel conditions either STBC or SM is selected. This technique is commonly referred to as adaptive MIMO switching (AMS) [5]-[11]. One important aspect of the technique is the switching criteria, namely the PHY abstraction method which estimates a packet error event as a function of the instantaneous channel condition, transmission profiles, and receiver characteristics. We show that the proposed algorithm outperforms other techniques such as determinant and Demmel condition number based techniques in flat fading channel. In selective fading channels where a codeword sees a finite number of multiple channel qualities, estimating the resulting PER is very challenging. Existing techniques rely on an estimate of the average of the channel qualities seen by each codeword. In this paper, we propose a PHY abstraction and switching algorithm hereby referred to by weighted sum of instantaneous qualities (WSIQ) whereby the channel qualities are ordered in order to reduce the variance of channel qualities and then a weighted sum of the qualities is applied. The weighting vector is chosen to minimize the variance of the linear sum. We have provided simulations to show the superiority of WSIQ even when channel statistics are unknown. In addition, computationally efficient techniques suited for practical implementation are proposed.

A Maximum Likelihood Doppler Frequency Estimator for OFDM Systems

This paper derives a maximum likelihood Doppler frequency estimator for orthogonal frequency division multiplexing (OFDM) systems in time-varying multipath channels. The proposed scheme is a frequency-domain approach that utilizes pilot subcarriers, which are commonly implemented in most practical systems. Time-varying fading causes intercarrier interference (ICI) in OFDM systems. Thus, in the proposed estimator, the effect of ICI is taken into consideration with a proper model for accurate results. The estimator can be implemented using a finite impulse response (FIR) filter bank whose coefficients can be pre-calculated and stored in order to lower the computational complexity. We evaluate various methods to improve the estimation accuracy and analyze their complexity-performance tradeoffs. We also derive the Cramér-Rao bound and provide simulation results to quantify the performance of the proposed algorithm.

Performance analysis and comparisons of antenna and beam selection diversity

Smart antennas can greatly improve the performance of wireless communication systems by providing better link quality and immunity to interference. A simple smart antenna structure is fixed beamforming and beam selection. More complex structures require combining of the signals from multiple receivers. In this paper we compare the performance of beam and antenna selection and combining techniques. Our results are based on recently reported statistical and measurement based spatial channel models and in most cases include impact of co-channel interference. We derive the probability density function (pdf) of SINR in space- and frequency-selective multiple clustered Rayleigh fading channel with finite angle spread (AS). We also analyze the outage capacity and probability of beam selection in flat fading assuming both correlated and independent fading between the antennas. Our analysis also includes the impact of erroneous beam or antenna selection. Furthermore, theoretical bounds for a hybrid of beam/antenna selection and maximal ratio combining (MRC) are presented. We show that beam selection techniques can be very effective against both multipath fading and co-channel interference. We also show that simple fixed beamforming networks such as Butler matrices are nearly as effective as more complex fixed beamforming networks.

Approximate comparative analysis of interference suppression performance between antenna and beam selection techniques

Multiple antenna systems can greatly improve the performance of wireless systems by providing better link quality, capacity and immunity to interference. A very simple structure for smart antennas is a fixed beamforming network (FBN) with beam selection. In Y.S. Choi and S.M. Alamouti (2004) the performance of antenna and beam selection/combining techniques are compared. The channel model in that paper is a statistical, clustered propagation model in a correlated Rayleigh fading channel. In this paper we derive the approximate density function of the signal-to-interference-and-noise power-ratio (SINR) of beam selection with eigen-beamforming. We also analyze the outage probability and capacity of beam selection with a fixed beamforming network (FBN) with correlated fading between the antennas. We compare the performance of this system with antenna selection avid independent fading between the antennas. We also investigate the impact of erroneous beam or antenna selection. We show that beam selection outperforms antenna selection in the presence of interference

MIMO-OFDM and its application

In the last decade, we have witnessed revitalization of multi-carrier techniques, and in particular orthogonal frequency division multiplexing (OFDM) for wireless applications. For wireless applications, an OFDM air-interface is of interest, because it can turn a wideband frequency selective channel into narrowband frequency flat channels. This makes the receiver design particularly easy since it eliminates the need for equalizers and produces robustness to fading and other channel artifacts. Additionally, because OFDM produces time-frequency blocks in a natural manner, it leads to multiple-access techniques such as orthogonal frequency division multiple access (OFDMA).