Satya Prakash Ponnaluri

Signature sequence and training design for overloaded CDMA systems

The focus of this paper is on training and signature sequence design in an overloaded synchronous CDMA system. The channel fading gains are assumed to be unknown and are estimated using training sequences. We derive a maximum-likelihood (ML) estimator and design sequences to minimize the mean-squared error (MSE) of the estimate. Two design scenarios are considered. One case assumes that the spreading sequences are fixed due to existing system constraints and optimal training sequences are designed using an iterative algorithm in order to minimize the MSE of the estimate. Performance of the iterative algorithm is examined using Welch-bound equality (WBE) sequences as the pre-designed spreading sequences. In the other scenario, spreading sequences and training sequences are designed jointly to minimize the MSE of the estimate. The jointly designed training and spreading sequences achieve optimum performance in the sense of minimizing the MSE. It is observed that when WBE sequences are used as spreading sequences the performance of the iterative algorithm is close to optimum, or even optimum, in certain situations

Effects of Spreading and Training on Capacity in Overloaded CDMA

We address the issue of sum-capacity in an overloaded, uplink, synchronous CDMA system in a quasi-static fading environment. Part of each fading block is reserved for channel training and the remainder of the block is used for data transmission. We evaluate bounds on sum-capacity using the estimates from channel estimation, and consider the effects on the lower bound caused by the spreading-coding-training tradeoff.

Cooperative Relaying Using OFDM in the Presence of Frequency Offsets

We study the performance of cooperative relaying using orthogonal frequency division multiplexing (OFDM) in the presence of frequency offsets due to Doppler shifts and oscillator instabilities. Through this study, several aspects of transmitter and receiver design, including channel coding, subcarrier mapping and channel estimation are brought to light. We develop two linear front-end receiver architectures based on practical single-user OFDM receivers, and demonstrate the performance using simulations under quasi-static multipath fading channel conditions.

Effect of spreading and training on sum-capacity in overloaded synchronous CDMA

This work addresses the issue of sum-capacity in an overloaded, uplink, synchronous code-division multiple access (CDMA) system in a quasi-static Rayleigh-fading environment when the receiver does not have a priori knowledge of the fading gains. Part of each fading block is reserved for channel training and the remainder of the block is used for data transmission. Two modes of operation are considered: one in which the users are allowed to update their spreading sequences based on the channel estimates, and one in which their sequences cannot be modified. For both schemes we derive the optimal training scheme and then evaluate a lower bound on sum-capacity. When all the users have the same average received power, we consider the effects on this bound caused by the training-spreading-coding tradeoff.

Spread spectrum based cooperative communication transceiver on FPGA platform

In this paper we describe the implementation of a spread spectrum based cooperative relaying on an FPGA platform. We focus on a network comprising one source, one or more relays, and one destination. Once the sources message is received by the relays, the relays simultaneously transmit the information to the destination. In order to avoid interference between the various relay transmissions, each relay node spreads its information using a unique spreading code chosen from a set of sequences with good cross-correlation and autocorrelation properties. These properties allow separation of the signals in the presence of frequency and timing offsets between the various transmissions. This platform is tested through an audio application using a commercially available RF front end. The platform not only enables testing of cooperative communications, but also other related concepts relying on software-defined radio such as Cognitive Radio.

Cooperative Relaying with Carrier Frequency Uncertainty

We investigate the performance of several suboptimal detectors in a differential space-time coding based cooperative relaying scenario with two relays wherein the carrier frequencies of the relays are not identical due to native oscillator instability. The relays cooperate using a differential Alamouti space-time block code with QPSK symbols transmitted over an unknown quasi-static Rayleigh fading channel. In such a scenario we observe that an optimal detector is prohibitively complex to use and hence we describe various suboptimal detectors and observe that there is no loss in the diversity order which is 2 in the case of using two relays.

ALOHA for Wireless Sensor Networks with Random Frequency Offsets

The impact of random frequency offsets on low-end sensor networks is considered. Results indicate that opening up receiver bandwidth not only helps improve throughput as measured in bits per second but can also result in improved bandwidth efficiency as measured in bits/sec/Hz. Implementing such a system will not only drive down node costs but also increase their operational lifetime.

Network Control and Rate Optimization for Multiuser MIMO Communications

Abstract We present a network control and rate optimization solution for multiuser multiple input multiple output (MU-MIMO) communications in wireless networks. This solution is decentralized and includes scheduling and routing of the MU-MIMO communication links that adapt to dynamic channel, interference, and traffic conditions. We start with analyzing the ergodic sum rates of MIMO multiple access channel (MAC) and interference channel (IC) configurations by considering the error, and overhead effects due to channel estimation (training) and quantization (feedback). By taking the practical considerations such as channel estimation, quantization error and in-network interference into account, we show the rate gain with an increasing number of antennas compared with single-input single-output (SISO) systems. Next, we present a distributed channel access protocol to select and activate MU-MIMO configurations with the maximum achievable sum rates using local information on channel, interference, and traffic conditions. Then, we extend this scheduling algorithm to routing via a cross-layer solution based on a decentralized version of the backpressure algorithm. After accounting for the control message overhead, we show that the proposed MU-MIMO scheduling and routing solution improves the stable throughput over the minimum distance routing based on frequency of encounters and single user MIMO communications in a mobile ad hoc network (MANET) setting.

Optimal Network-Centric Planning for Airborne Relay Communications

This paper presents a network-centric mission planning system for airborne relay communications. Examples of airborne relays include aircrafts, such as the battlefield airborne communication node; unmanned aerial vehicles, such as Facebooks Aquila; balloons, such as Google Project Loon; and nano/microsatellites. There are two integrated components of mission planning: 1) the subscriber planner (run on each subscribers radio, phone, or attached computer) that finds the optimal transmission scheme for each subscriber communicating to/through an airborne relay by accounting for realistic channel, traffic, and energy effects and 2) the orbit (force-level) planner (run at a centralized controller prior to deployment of the airborne relay) that optimally designs the orbit of an airborne relay to maximize the overall network performance. For a given orbit of an airborne relay, the subscriber planner combines channel measurement and prediction (by accounting for terrain, aircraft, and antenna effects) to calculate signal-to-noise-ratio (SNR) and link rate. Then, the subscriber planner decides when and at what rate to transmit while optimizing the expected throughput for uplink/relay communications. The subscriber planner is shown to significantly outperform random and SNR threshold-based transmissions. On the other hand, the orbit planner first selects the optimal parameters of an elliptical orbit to maximize the network performance measured by the subscriber planner. This elliptical orbit is converted to an operational orbit by optimally selecting waypoints and locating smooth turning points while minimizing the airborne relays fuel consumption. The orbit planner software with an interactive graphical user interface is implemented to return the optimal orbit for the airborne relay that optimizes the network performance measured by the subscriber planner.

Distributed Interference Cancellation

Distributed interference cancellation is considered for a single source and multiple collaborative receivers in the presence of multiple unknown interferers. Receivers observe interference in a distributed setting and collaborate to mitigate the common interference from multiple unknown interferers. Under Rayleigh frequency-flat fading conditions, the ergodic capacity is shown to improve with collaboration surpassing the communication overhead due to collaboration. In particular at SINR of -15 dB, the performance improvement relative to the case without collaboration is a factor of 15. Numerical results are also provided for constrained alphabet signaling, along with simulation results based on DVB-S2#x002F;T2 standard. Finally, results from hardware-in-the-loop tests using USRPs are presented to show the feasibility of practical implementation. Both simulation and radio tests demonstrate the benefit of collaboration for distributed cancellation of unknown interference.