Semih Serbetli

Transceiver optimization for multiuser MIMO systems

We consider the uplink of a multiuser system where the transmitters as well as the receiver are equipped with multiple antennas. Each user multiplexes its symbols by a linear precoder through its transmit antennas. We work with the system-wide mean squared error as the performance measure and propose algorithms to find the jointly optimum linear precoders at each transmitter and linear decoders at the receiver. We first work with the case where the number of symbols to be transmitted by each user is given. We then investigate how the symbol rate should be chosen for each user with optimum transmitters and receivers. The convergence analysis of the algorithms is given, and numerical evidence that supports the analysis is presented.

Distributed power allocation strategies for parallel relay networks

We consider a source-destination pair assisted by parallel regenerative decode-and-forward relays operating in orthogonal channels. We investigate distributed power allocation strategies for this system with limited channel state information at the source and the relay nodes. We first propose a distributed decision mechanism for each relay to individually make its decision on whether to forward the source data. The decision mechanism calls for each relay that is able to decode the information from the source to compare its relay-to-destination channel gain with a given threshold. We identify the optimum distributed power allocation strategy that minimizes the total transmit power while providing a target signal-to-noise ratio at the destination with a target outage probability. The strategy dictates the optimum choices for the source power as well as the threshold value at the relays. Next, we consider two simpler distributed power allocation strategies, namely the passive source model where the source power and the relay threshold are fixed, and the single relay model where only one relay is allowed to forward the source data. These models are motivated by limitations on the available channel state information as well as ease of implementation as compared to the optimum distributed strategy. Simulation results are presented to demonstrate the performance of the proposed distributed power allocation schemes. Specifically, we observe significant power savings with proposed methods as compared to random relay selection.

ASAP: a MAC protocol for dense and time-constrained RFID systems

We introduce a novel medium access control (MAC) protocol for radio frequency identification (RFID) systems which exploits the statistical information collected at the reader. The protocol, termed adaptive slotted ALOHA protocol (ASAP), is motivated by the need to significantly improve the total read time performance of the currently suggested MAC protocols for RFID systems. In order to accomplish this task, ASAP estimates the dynamic tag population and adapts the frame size in the subsequent round via a simple policy that maximizes an appropriately defined efficiency function. We demonstrate that ASAP provides significant improvement in total read time performance over the current RFID MAC protocols. We next extend the design to accomplish reliable performance of ASAP in realistic scenarios such as the existence of constraints on frame size, and mobile RFID systems where tags move at constant velocity in the readers field. We also consider the case where tags may fail to respond because of a physical breakdown or a temporary malfunction, and show the robustness in those scenarios as well.

Relay assisted F/TDMA ad hoc networks: node classification, power allocation and relaying strategies

This paper considers the design of relay assisted F/TDMA ad hoc networks with multiple relay nodes each of which assists the transmission of a predefined subset of source nodes to their respective destinations. Considering the sum capacity as the performance metric, we solve the problem of optimally allocating the total power of each relay node between the transmissions it is assisting. We consider four different relay transmission strategies, namely regenerative decode-and-forward (RDF), nonregenerative decode-and-forward (NDF), amplify- and-forward (AF) and compress-and-forward (CF). We first obtain the optimum power allocation policies for the relay nodes that employ a uniform relaying strategy for all nodes. We show that the optimum power allocation for the RDF and NDF cases are modified water-filling solutions. Weobserve that for a given relay transmit power, NDF always outperforms RDF whereas CF always provides higher sum capacity than AF. When CF and NDF are compared, it is observed that either of CF or NDF may outperform the other in different scenarios. This observation suggests that the sum capacity can be further improved by having each relay adopt its relaying strategy in helping different source nodes. We investigate this problem next and determine the optimum power allocation and relaying strategy for each source node that relay nodes assist. We observe that optimum power allocation for relay nodes with hybrid relaying strategies provides higher sum capacity than pure RDF, NDF, AF or CF relaying strategies.

ASAP : A MAC Protocol for Dense and Time Constrained RFID Systems

In this paper, we introduce a novel medium access control (MAC) protocol for Radio Frequency Identification (RFID) systems which exploits the statistical information collected at the reader. The protocol, termed Adaptive Slotted ALOHA Protocol (ASAP), is motivated by the need to significantly improve the total read time performance of the currently suggested MAC protocols for RFID systems. In order to accomplish this task, ASAP estimates the dynamic tag population and adapts the frame size in the subsequent round. We demonstrate that ASAP provides significant improvement in total read time performance over the current RFID MAC protocols. We extend the design to mobile RFID systems where tags move at constant velocity in the readers field, and show that ASAP performs well in mobile scenarios as well.

MMSE transmitter design for correlated MIMO systems with imperfect channel estimates: power allocation trade-offs

We investigate the transmit precoder design problem for a multiple input multiple output (MIMO) link with correlated receive antennas, considering the effect of channel estimation. We work with the total mean squared error (MSE) as the performance measure, and develop transceiver structures considering the effect of channel estimation and the correlation of the MIMO link. The proposed transceiver structures are optimum in the sense of minimizing the total MSE and distributing the total MSE equally among the parallel data streams. Motivated by the substantial effect the channel estimation process can have on the system performance, we next investigate the problem of how the correlated MIMO link should distribute its total available power between power expended for channel estimation versus data transmission. The optimum power allocation problem between the training sequences for channel estimation and data transmission for the correlated MIMO link is shown to have a unique solution, that is different than the uncorrelated case. It is observed that the proposed transceiver achieves near-minimum MSE values via a relatively wide range of power allocation parameters. This is in contrast to the transceiver that is oblivious to the estimation errors when a more precise power allocation strategy is needed to achieve the best performance. Our results demonstrate that the correlation structure of the MEMO link has a profound effect on the performance, and that the transceiver optimization should be done by taking both the correlation and the channel estimation process into account

Distributed power allocation for parallel relay networks

We investigate power allocation strategies for distributed decode-and-forward (DF) parallel relay networks. We first propose a distributed decision mechanism for the relay nodes to make decisions on whether to forward the source data. Specifically, we identify the optimum distributed power allocation strategy that minimizes the total transmit power while providing a target signal-to-noise ratio (SNR) at the destination with a target outage probability. We also consider two simple distributed power allocation models, where the source does not contribute to the relay selection in the first model, and single relay is employed in the second model. Simulation results are presented to demonstrate the performance of the proposed distributed power allocation schemes, and the considerable power savings they provide with respect to random relay selection.

On Secure Signaling for the Gaussian Multiple Access Wire-tap Channel

We consider the Gaussian Multiple Access WireTap Channel (GMAC-WT) where multiple users communicate with the intended receiver in the presence of an intelligent and informed wire-tapper (eavesdropper). The wire-tapper receives a degraded version of the signal at the receiver. We assume that the wire-tapper is as capable as the intended receiver, and there is no other shared secret key. We consider two different secure communication scenarios: (i) keeping the wire-tapper totally ignorant of the message of any group of users even if the remaining users are compromised, (ii) using the secrecy of the other users to ensure secrecy for a group of users. We first derive the outer bounds for the secure rate region. Next, using Gaussian codebooks, we show the achievability of a secure rate region for each measure in which the wire-tapper is kept perfectly ignorant of the messages. We also find the power allocations that yield the maximum sum rate, and show that upper bound on the secure sum rate can be achieved by a TDMA scheme. We present numerical results showing the new rate region and compare it with that of the Gaussian Multiple-Access Channel (GMAC) with no secrecy constraints.

OFDM symbol synchronization using frequency domain pilots in time domain

In this paper, we describe and evaluate a novel time synchronization algorithm for OFDM systems robust to channels with long echoes. Most of the OFDM based standards provide known pilots in the frequency domain for channel estimation purposes. The basic idea of our contribution consists of interpreting the frequency domain pilots in the time domain and using them to obtain a rough channel estimation prior to the receiver discrete Fourier transform (DFT). The rough channel estimation is then used to perform the fine time synchronization.

Optimal power allocation for relay assisted F/TDMA ad hoc networks

In this paper, we study the power allocation problem at the relay nodes for two-hop F/TDMA networks with multiple sources and destinations. Considering the sum capacity as the performance metric, we solve the problem of optimally allocating the total power of each relay node between the transmissions it is assisting. We consider regenerative decode-and-forward (RDF), nonregenerative decode-and-forward (NDF), amplify-and-forward (AF) and compress-and-forward (CF) at the relay nodes. We observe that the optimum power allocation for the RDF and NDF cases are modified water-filling solutions. In RDF, the optimum power allocation considers both the direct links of the users and the relay to destination links, whereas the optimal power allocation for the NDF relaying considers only the relay to destination links. We also observe that relay nodes employing AF or CF may provide higher sum capacities than relay nodes employing DF techniques when sufficient power is available at the relay nodes. Motivated by the optimum power allocation identified for each case, we provide insights to relay selection strategies for relay assisted F/TDMA networks.