We consider the problem of relay selection in mmWave networks with wireless energy transfer, assuming a two-hop decode-and-forward relaying. We incorporate the energy stored at each node in relay selection decision. We propose a centralized algorithm that maximizes an objective function with linear complexity. The results show that incorporating the energy at nodes in relay selection decision increases the number of packets successfully received over network lifetime.

Wireless sensor networks demand the implementation of energy-aware transmission protocols. Recently, cooperative protocols have been suggested for use in sensor networks since they typically require less transmit power than the direct transmission for the same error probability. For sensor networks with short range transmissions, however, the energy consumed in receiving transmissions from cooperative partners may exceed that of transmission energy when using cooperation. Hence it is not clear if such implementation is energy efficient. The objective of this work is to study energy efficiency of multistage cooperative networks that accounts for both the transmission reliability in terms of outage probability and the total network energy consumed (including the transmit as well as receive energy) to achieve this performance. We analyze the effect of node density, network topology and the number of cooperation stages on the energy efficiency of multistage relay networks using numerical results. With respect to serial equidistant network topology, we observe that the largest energy efficiency is obtained with three cooperation stages and just few relays in the medium SNR range while it is best to allocate five cooperation stages and large number of relays when transmit SNR is low. It is also observed that uniform relay distribution, a more practical approach in many scenarios, can improve energy efficiency of sensor network with five cooperation stages and large number of relays.

User cooperation, whereby multiple users share their antennas and transmit to a common destination in a collaborative manner, has been shown to be an effective way to achieve spatial diversity. We propose in this paper, a strategy to minimize the total transmit power in a decode-and-forward (DF) multi-user, multi-relay cooperative uplink, such that each user satisfies its quality-of-service (QoS) data rate. Each user in the proposed system transmits its own data towards the base station and also serves as a relay for other users. The base station assigns one or more relays to each user in order to minimize total power in the uplink. The relay selection is based upon the instantaneous user to base station channels, inter-user channels and also the target rates of the users. The simulation results indicate significant power savings over a non-cooperative uplink, under proposed joint relay selection and power minimization algorithm in a DF cooperative uplink when using a space-time coded cooperative diversity.

This paper investigates the impact of inter-user non-orthogonality and asynchronous communication on the information-outage probability performance of multi-user decode-and-forward (DF) cooperative diversity in a code-division multiple-access (CDMA) uplink. Each user in the proposed system transmits its own data towards the base station and also serves as a relay for other users. We assume full-duplex communication so that each user can transmit and receive simultaneously at the same frequency. Each user attempts to decode the messages of a plurality of other users and forwards the superposition of multiple re-encoded and re-spread messages. Our cooperative scheme employs a sub-optimum decorrelating receiver to suppress the multi-user interference at both the base station and the relay-side. We evaluate the information-outage probability performance of the proposed scheme in an underloaded, fully-loaded and overloaded CDMA uplink. We consider combining schemes at the base station where the source information is code combined with the relayed information, while the information from multiple relays is either code combined or diversity combined. Under the system parameters contemplated in this paper, diversity combining of the relayed information is nearly as good as code combining because of the associated probabilities of decoding at the relays. We then examine the effect of using practical modulation formats on the information-outage probability performance of the proposed DF multi-user sharing scheme under diversity combining. We see that the performance loss due to modulation constraints and the use of diversity combining instead of code combining is relatively small.

Cooperative diversity is a form of distributed space- time macrodiversity capable of mitigating the detrimental effects of multipath fading in a wireless network. Prior work on cooperative diversity has focused on the case that all terminals have a single antenna. However, in many practical situations, it is feasible for one or more terminals to be equipped with an array of multiple antennas. This paper investigates the impact of the presence of a single antenna array in a three terminal orthogonal relaying network. The array may be at the source, relay, or destination. The information outage probability under diversity combining is derived, and closed form expressions given wherever possible. The numerical results suggest that it is best to place the array at the destination, and that it is better to place the array at the relay than at the source.

Abstract — In this paper, we compare two classes of linear interference suppression techniques for downlink TDD-CDMA systems, namely, linear multiuser detection methods (receiver processing) and linear precoding methods (transmitter processing). For the linear precoding schemes, we assume that the channel state information (CSI) is available only at the transmitter but not at the receiver (i.e., ultra simple receivers). We propose several precoding techniques and the corresponding power control algorithms. The performance metric used in the comparisons is the total power required at the transmitter to achieve a target SINR at the receiver. Our results reveal that in general multiuser detection and precoding offer similar performance; but in certain scenarios (e.g, low BER requirements or use of random spreading sequences), precoding can bring a substantial performance improvement. These results motivate the use of precoding techniques to reduce the complexity of the system and the mobile terminals (only a matched-filter to the own spreading sequence is required without CSI). Moreover, it is shown that the proposed chip-wise linear MMSE precoding method is optimal in the sense that it requires the minimum total transmitted power to meet a certain receiver SINR performance. Index Terms — Downlink CDMA, linear multiuser detection, linear precoding, power control. I.

We treat utility-based joint power control and rate allocation in a downlink code division multiple access system, where either the matched-filter or the blind multiuser detection technique is employed at the mobile receivers. Rate allocation is performed on a per-frame basis; whereas within the frame, power control is performed at the symbol basis. A hierarchical rate allocation scheme is proposed, which together with the utility-based power control and the opportunistic fair scheduling scheme, maximizes the instantaneous weighted system throughput and the number of feasible users, and at the same time, guarantees the fairness among all users. In order to apply such a resource allocation framework to systems employing blind multiuser detection, we make use of the recent analytical results on the signal-to-interference-plus-noise-ratio performance of the blind multiuser detectors, assuming multiple spreading codes are used to realize multiple data rates. Our results show that under the utility-based joint power and rate allocation framework, the system capacity is significantly enhanced by using blind multiuser detection and the fairness can be well guaranteed.

In this paper, we treat the rate control and fairness scheduling problem for downlink UBPC systems. An hierarchical rate allocation scheme is proposed, which is based on the analytical performance analysis for the physical-layer constraints in the UBPC system. Using such a rate control algorithm together with the opportunistic fairness scheduling scheme, the UBPC system achieves rate allocation more fairly in the sense that the instantaneous system throughput and the number of feasible users can be maximized, and meanwhile the long-term fairness can be well guaranteed. We present the simulation results to demonstrate the performance of the proposed algorithms in terms of the throughput and the fairness.

The desire to reduce complexity at the mobile unit in cellular systems has led to the investigation of signal processing techniques that move computational complexity from the mobile unit to the base station. In this paper we investigate transmitter precoding for downlink time diversion duplex (TDD) code-division multiple-access (CDMA) communications. In particular, we develop a linear MMSE-based precoding strategy using blind channel estimation for fading multipath channels that allows for simple matched filtering at the mobile unit and is easy to make adaptive. We also present a performance analysis using tools developed for the analysis of conventional (receiver-based) linear blind multiuser detection in unknown channels. We compare the analytical and simulation results to traditional receiver-based blind multiuser detection. It is seen that transmitter precoding offers a reasonable alternative for TDD-mode CDMA when minimizing computational complexity at the mobile unit is a priority.

We develop a distributed power control algorithm for CDMA systems employing blind or group-blind multiuser detectors in heterogeneous traffic environments. In order to serve traffic with varying performance requirements we use a utility-based approach that allows users to optimize performance for their particular traffic type. In this development we make use of recent results on the analytical performance of blind and group-blind detectors. We also design an adaptive group-blind receiver incorporating the new power control algorithm and provide performance results. We will see that the utility-based power control algorithm allows us to efficiently set performance goals for users in multi-traffic environments.

We consider the problem of turbo multiuser detection for synchronous and asynchronous code-division multiple-access (CDMA) in the presence of unknown users. Turbo multiuser detectors, as previously developed, typically require knowledge of the signature waveforms of all of the users in the system and ignore users whose signature sequences are unknown, e.g., users outside the cell. We develop turbo multiuser detection for CDMA uplink systems and other environments in which the receiver has knowledge of the signature waveforms of only K/spl breve//spl les/ K users. Subspace techniques are used to estimate the interference from the unknown-users and the interference estimate is subtracted from the received signal. We see that the new receiver significantly outperforms the conventional turbo multiuser receiver for moderate and high signal-to-noise ratios. It is also seen that the traditional turbo receiver provides little gain through iteration when unknown users are present.

The demand for performance and capacity in cellular systems has generated a great deal of interest in the development of advanced signal processing techniques to optimize the use of system resources. In particular, much recent work has been done on space-time processing in which multiple transmit/receive antennas are used in conjunction with coding to exploit spatial diversity. In this paper, we consider space-time multiuser detection using multiple transmit and receive antennas. Specifically, we develop batch and sequential blind adaptive algorithms for space-time multiuser detection for synchronous CDMA in flat-fading channels. We also investigate the performance of the blind adaptive algorithm via simulation.

Wang and Host-Madsen (see IEEE J. Select. Areas Commun., vol.17, p.1971-84, 1999) developed group-blind multiuser detectors for use in code-division multiple-access (CDMA) uplink environments in which the base station receiver has the knowledge of the spreading sequences of all the users within the cell, but not that of the users from other cells. Yu and Host-Madsen (see Proc. IEEE Vehicular Technology Conf. (VTC99), Houston, TX, p.1042-46, 1999) later developed an adaptive version of this detector for synchronous CDMA channels. We develop a new low-complexity, high-performance subspace tracking algorithm and apply it to adaptive group-blind multiuser detection in asynchronous multipath CDMA channels. The detector can track changes in the number of users and their composite signature waveforms. We present steady-state performance as well as the ability of the receiver to track changes in the signal subspace. We also address the performance gain of the group-blind detector over its blind counterpart for this application.