Kambiz C. Zangi

Reduced-complexity MAP equalizer for dispersive channels

We present a computationally efficient soft-output equalizer for communication over dispersive channels. The proposed equalizer has a forward and backward recursion structure similar to the classic BCJR MAP algorithm, but its complexity is reduced by constructing a reduced-state trellis, as in the decision feedback sequence estimator (DFSE). We are able to achieve consistent state definitions by running the backward pass on the same trellis as the forward pass. The performance of this reduced-complexity equalizer is evaluated within an iterative receiver in which the intersymbol interference (ISI) channel and channel code are treated as serially concatenated codes. The EDGE air interface is used as an illustrative example, for which we are able to reduce the number of states from 4096 to 8.

Physical-layer issues for deploying transmit diversity in GPRS/EGPRS networks

Lindskog and Paulraj (see ICC-2000, June, 2000) proposed a transmit diversity scheme for dispersive channels which by utilizing two transmit antennas and only one receive antenna achieves the same diversity benefit (except for the 3 dB array gain) as can be achieved by using one transmit antenna and two receive antennas. This scheme generalizes Alamoutis (see IEEE JSAC, vol.16, no.8, p.1451-58, 1998) transmit diversity scheme (which works best for flat channels) to dispersive channels. We show how the transmit diversity (TXD) scheme of Lindskog et al. can be deployed in GPRS/FGPRS networks. A low-complexity receiver structure for demodulating Lindskog and Paulraj (LP) encoded signals is presented based on a combination of a novel pre-filter and a delayed decision feedback sequence estimation (DFSE) equalizer. This pre-filter is designed to simultaneously achieve three desired effects: (1) the effective baseband channel seen by the equalizer has half the number of taps of the original baseband channel; (2) the effective response of the baseband channel becomes minimum-phase; and (3) the effective baseband noise is whitened. An information-theoretic analysis is also presented to demonstrate the advantage of the LP scheme over other TXD schemes (e.g. over delay diversity). A burst format for transmitting LP encoded signals which is compatible with the existing GPRS/EGPRS burst format is also presented. Finally, simulations results are presented to verify the benefit of using the LP scheme in the typical urban channel of GSM/EDGE.

Maximizing data rate over M-input/1-output channels

We focus on transmit diversity schemes for a communication system with M transmit antennas and one receive antenna (i.e. an M-input/single-output (MISO) system) where the transmitter knows all these M channels, and we derive a transmission scheme that maximizes the data rate, in an information-theoretic sense, that can be reliably transmitted over this MISO channel. An important example of the type of system we consider is a cellular wireless system on the downlink with M transmit antennas at the base station and one receive antenna at the mobile station. Our main result is the identification of the structure of the capacity-achieving transmitter. The capacity-achieving transmitter can be broken into two parts: (I) an optimal encoder for a single-input/single-output (SISO) channel, and (II) an M-dimensional matched filter. We show that the M-dimensional matched filter turns the original MISO channel into a SISO channel with exactly the same capacity as the capacity of the original MISO channel, and the 1-dimensional encoder is just the well known optimal encoder of Gallager (1966) for the SISO channel created by the M-dimensional matched filter. We also derive a closed-form expression for the channel capacity of MISO channels which is valid under a rather general set of assumptions: (1) each channel can be dispersive or flat, and (2) the additive noise at the receiver can be a Gaussian process with arbitrary power spectrum. In a typical urban environment, to achieve 1% block error rate at 4 Mbits/second, the MISO system with our transmission scheme requires 10 dB less SNR than todays SISO system.

Joint Optimization of the Transmit Antenna Array Geometry and Linear Precoding for MIMO Systems

For a cellular system with a fixed number of transmit antennas at each base station, we investigate how the data rate on the downlink varies as a joint function of the geometry of the transmit array and the linear precoding that precedes the transmit array. The maximum average data rate that can be reliably transmitted over the downlink channel (i.e. the ergodic capacity of the downlink channel) is used in this paper to compare various configurations of transmit antennas at the base stations. Using the 3GPP Spatial Channel Models [4] for a macro-cellular environment, we find the optimum configuration of transmit antennas with a total of 2 or a total of 4 transmit antennas at various SNRs. We offer guidelines for choosing the best transmit antenna geometry for different SNR regions.

Interference mitigation for MIMO systems employing user-specific, linear precoding

User-specific, linear precoding is used extensively by almost all existing and emerging wireless MIMO standards [1], [2], [3], [4]. With user-specific, linear precoding, the data symbols to be transmitted to each user are passed through a linear transformation before being sent to the transmit antennas, and a different precoder is used for each user depending on his/her channel. For example, with 4 transmit antennas at the BS, 2 receive antennas at the mobile, and instant channel quality indicator (CQI), the cell capacity of a 4times2 2-clustered system with user-specific, linear precoding is more than 80% higher than the cell capacity of a system with one transmit antenna [10]. But such schemes are vulnerable to delayed CQI due to fast variations of interference, leading in some cases to performance that is worse than a SISO system. In this paper, we will present a method that mitigates the degradation due to fast-varying interference in MIMO systems that use linear precoding. We will show that the temporal variation of other-cell interference is almost eliminated with this method. Our results indicate that for a 4times2 2-Clustered transmit array with the proposed method, the performance loss due to delayed CQI is reduced from 35% to 5%.

Impact of wideband ADCs on the performance of multi-carrier radio receivers

Multi-carrier wideband radio receivers offer many advantages over their narrowband counter parts by pushing the digital boundary closer to the antenna. In multi-carrier wideband receivers, channel selection and filtering are done in the digital domain after analog to digital conversion (ADC); hence, such a wideband receiver requires only a single analog RF chain and a single wideband ADC for reception of many channels. The impact of the quantization noise of the ADC on the performance of a multi-carrier wideband receiver is very different from the impact of the quantization noise of the ADC in a traditional single-carrier receiver. In this paper, we derive analytical expressions that quantify this impact in terms of bit error rate (BER) for a Rayleigh fading channel.