Joseph Binamira Soriaga

Multicell downlink capacity with coordinated processing

We study the potential benefits of base-station (BS) cooperation for downlink transmission in multicell networks. Based on a modified Wyner-type model with users clustered at the cell-edges, we analyze the dirty-paper-coding (DPC) precoder and several linear precoding schemes, including cophasing, zero-forcing (ZF), and MMSE precoders. For the nonfading scenario with random phases, we obtain analytical performance expressions for each scheme. In particular, we characterize the high signal-to-noise ratio (SNR) performance gap between the DPC and ZF precoders in large networks, which indicates a singularity problem in certain network settings. Moreover, we demonstrate that the MMSE precoder does not completely resolve the singularity problem. However, by incorporating path gain fading, we numerically show that the singularity problem can be eased by linear precoding techniques aided with multiuser selection. By extending our network model to include cell-interior users, we determine the capacity regions of the two classes of users for various cooperative strategies. In addition to an outer bound and a baseline scheme, we also consider several locally cooperative transmission approaches. The resulting capacity regions show the tradeoff between the performance improvement and the requirement for BS cooperation, signal processing complexity, and channel state information at the transmitter (CSIT).

On the achievable information rates of finite state ISI channels

In this paper, we present two simple Monte Carlo methods for estimating the achievable information rates of general finite state channels. Both methods require only the ability to simulate the channel with an a posteriori probability (APP) detector matched to the channel. The first method estimates the mutual information rate between the input random process and the output random process, provided that both processes are stationary and ergodic. When the inputs are iid equiprobable, this rate is known as the Symmetric Information Rate (SIR). The second method estimates the achievable information rate of an explicit coding system which interleaves m independent codes onto the channel and employs multistage decoding. For practical values of m, numerical results show that this system nearly achieves the SIR. Both methods are applied to the class of partial response channels commonly used in magnetic recording.

Downlink Macro-Diversity in Cellular Networks

In this paper, we study the potential benefit of base-station (BS) cooperation for downlink transmission in a modified Wyner-type multicell model. Besides the dirty-paper-coding (DPC) precoder, we also analyze several linear precoding schemes, including co-phasing, zero-forcing (ZF) and MMSE precoders. For the nonfading case, analytical sum rate expression is obtained for each scheme. In networks of a large number N of cells, a high signal-to-noise-ratio (SNR) asymptotic gap is shown between the sum rate performances of DPC and ZF precoders. Moreover, the MMSE precoder sum rate expression in large networks indicates different behaviors of MMSE precoder in different SNR regimes: in the SNR > N² regime, it coincides with the ZF precoder, while, in the SNR < N² regime, it coincides with the co-phasing precoder. For the Rayleigh fading case, Monte-Carlo simulations demonstrate the effectiveness of linear precoding schemes with the proposed user selection criterion.

Determining and Approaching Achievable Rates of Binary Intersymbol Interference Channels Using Multistage Decoding

By examining the achievable rates of a multistage decoding system on stationary ergodic channels, we derive lower bounds on the mutual information rate corresponding to independent and uniformly distributed (i.u.d.) inputs, also referred to as the i.u.d. information rate. For binary intersymbol interference (ISI) channels, we show that these bounds become tight as the number of decoding stages increases. Our analysis, which focuses on the marginal conditional output densities at each stage of decoding, provides an information rate corresponding to each stage. These rates underlie the design of multilevel coding schemes, based upon low-density parity-check (LDPC) codes and message passing, that in combination with multistage decoding approach the i.u.d. information rate for binary ISI channels. We give example constructions for channel models that have been commonly used in magnetic recording. These examples demonstrate that the technique is very effective even for a small number of decoding stages

Distributed Beamforming Based on Signal-to Caused-Interference Ratio

This paper presents a distributed beamforming technique that addresses the effect of inter-cell interference on the downlink of cellular communications systems. The beamforming weights are computed in a distributed manner at each transmit sector antenna array without the need for inter-sector communication. The beamforming weights are chosen to compromise between maximizing the power to the served user from each sector while minimizing the interference caused to users served in adjacent sectors. The extensions of this method for variable levels of channel state information feedback and multiple receiver antennas are introduced. Beamforming codebooks with power variations across antennas are presented. We show how users can additionally feed back the fraction of interference caused by each interfering sector to incorporate the urgency of interference avoidance into the transmitter optimization.

On achievable rates of multistage decoding on two-dimensional ISi channels

The achievable information rates for multilevel coding (MLC) systems with multistage decoding (MSD) are examined on two-dimensional binary-input intersymbol interference (ISI) channels. One MSD scheme employs trellis-based detection, while another involves zero-forcing equalization and linear noise prediction. Information rates are determined by examining the output statistics at each stage of MSD. The first scheme is shown to achieve rates very close to known information-theoretic limits. Systems with low-density parity-check codes are then optimized to approach these rates

WLC04-4: Network Performance of the EV-DO CDMA Reverse Link with Interference Cancellation

This paper addresses the network level aspects of incorporating interference cancellation into the CDMA2000 1xEV-DO Revision A reverse link. We illustrate how a physical layer analysis of interference cancellation can be applied to an extensive network simulation environment that models inter-cell interference, hybrid-ARQ, multipath fading channels and the MAC-layer dynamics of power control, rate allocation, and rise- over-thermal control. An investigation of reverse link pilot channel performance and rise-over-thermal distribution shows that interference cancellation can be added to the base station processing without modifying the overall network operation or system stability. Network simulation results demonstrate how interference cancellation increases the data rate of each user to significantly improve the throughput achieved with both 2 and 4 receiver antennas per sector.

Receiver Architectures and Design Tradeoffs for CDMA Interference Cancellation

This paper investigates the design of commercially viable CDMA basestation receivers that incorporate interference cancellation. Results are given for cdma2000 1xEV-DO but also apply to WCDMA HSUPA. The effect of receiver memory size is determined for canceling packets transmitted with hybrid-ARQ. Performance-complexity tradeoffs are presented for implementing iterative and successive interference cancellation of asynchronous user transmissions. Multipath channel estimation techniques of varying complexity are compared based on sector throughput. Practical interference cancellation designs with moderate complexity are shown to achieve a large fraction of the gains of more elaborate techniques.

On near-capacity coding systems for partial-response channels

We present a near-capacity coding system for higher-order partial-response channels, consisting of an outer set of interleaved low-density parity-check codes, an inner rate-1 shaping code, and a multistage decoder. The inner shaping code, which may be noninvertible, is designed to generate an output process similar to a binary Markov process that maximizes the mutual information for a given order. On the EPR4 channel, our system exhibits an iterative decoding threshold and a simulation BER of 10/sup -5/ within 0.19 and 0.33 dB, respectively, of the information-theoretic limit for a third-order input process.

On the low-rate Shannon limit for binary intersymbol interference channels

For a discrete-time, binary-input, Gaussian channel with finite intersymbol interference, we prove that reliable communication can be achieved if, and only if, E/sub b//N/sub 0/>log2/G/sub opt/, for some constant G/sub opt/ that depends on the channel. To determine this constant, we consider the finite-state machine which represents the output sequences of the channel filter when driven by binary inputs. We then define G/sub opt/ as the maximum output power achieved by a simple cycle in this graph, and show that no other cycle or asymptotically long sequence can achieve an output power greater than this. We provide examples where the binary input constraint leads to a suboptimality, and other cases where binary signaling is just as effective as real signaling at very low signal-to-noise ratios.