Andrej Stefanov

Cooperative coding for wireless networks

User cooperation represents an effective way of introducing diversity in wireless networks. Spatial diversity gains are obtained through the cooperative use of antennas belonging to several nodes. We design and analyze the performance of channel codes that are capable of achieving the full diversity provided by user cooperation, with the constraint that they also provide the best possible performance in the interuser link. We show that even though the interuser channel is noisy, the codes provide substantial diversity and coding gains over the noncooperative case.

Turbo-coded modulation for systems with transmit and receive antenna diversity over block fading channels: system model, decoding approaches, and practical considerations

We study the use of turbo-coded modulation for wireless communication systems with multiple transmit and receive antennas over block Rayleigh fading channels. We describe an effective way of applying turbo-coded modulation as an alternative to the current space-time codes with appropriate interleaving. We study the performance with the standard iterative turbo decoding algorithm, as well as the iterative demodulation-decoding algorithm. In addition to the introduction of the turbo-coded modulation scheme, we consider a variety of practical issues including the case of large number of antennas, the effects of estimated channel state information, and correlation among subchannels between different transmit-receive antenna pairs. We present examples to illustrate the performance of the turbo-coded modulation scheme and observe significant performance gains over the appropriately interleaved space-time trellis codes.

Cooperative Regions and Partner Choice in Coded Cooperative Systems

User cooperation is an efficient approach to obtain diversity in both centralized and distributed wireless networks. In this paper, we consider a coded cooperative system under quasi-static Rayleigh fading and investigate the partner-choice problem. We find conditions on the interuser and user-to-destination channel qualities for cooperation to be beneficial. Using frame-error rate (FER) as a metric, we define the user cooperation gain (G) for evaluating the relative performance improvement of cooperative over direct transmissions when a particular channel code is used. We introduce the cooperation decision parameter (CDP), which is a function of user-to-destination average received signal-to-noise ratios (SNRs), and demonstrate that whether cooperation is useful or not (

Turbo coded modulation for wireless communications with antenna diversity

G≫1

Cooperative space-time coding for wireless networks

or

Turbo coded modulation for systems with transmit and receive antenna diversity

G≪1

Performance bounds for space-time trellis codes

) depends only on the CDP, not the interuser link quality. We use an analytical formulation of the CDP to investigate user cooperation gain and provide insights on how a user can choose among possible partners to maximize cooperation gain. We first consider the asymptotic performance when one or both partners have high average received SNR at the destination. We then provide conditions on user and destination locations for cooperation to be beneficial for arbitrary SNRs. We illustrate these cooperative regions and study geometric conditions for the best partner choice. We also define the system cooperation gain and illustrate cooperation benefit for both users. All of our theoretical results are verified through numerical examples.

Design and Performance Analysis of Underwater Acoustic Networks

We propose to use turbo codes for wireless communication systems with multiple transmit and receive antennas over Rayleigh fading channels. We show that a simple, arbitrarily picked, turbo coded modulation scheme with a sub-optimal decoding algorithm outperforms the space-time codes significantly. We present examples for both block and fast fading channels, and observe gains as high as 8 dB at a bit error rate of 10/sup -5/ for large interleaver lengths, suitable for data communications. Furthermore, we show that, depending on the channel model, the turbo code block size can be chosen small enough to be suitable for speech applications, and still offer a significant performance improvement in terms of bit and frame error rates.

On the performance analysis of cooperative space-time coded systems

We consider a cooperative system in which the partnering mobiles are equipped with multiple antennas. We present space-time codes that have the capability of providing full cooperation diversity, while achieving maximum possible diversity and best performance in the inter-user channel. Our codes also perform well when cooperation does not take place. We illustrate that cooperative space-time coding offers significant performance improvement over direct transmission even when the inter-user channel is noisy. We also consider code design for users with different numbers of antennas.

Performance bounds for turbo-coded multiple antenna systems

In a previous work Stefanov and Duman (see Proceedings of VTG-Fall, 1999) introduced turbo coded modulation for wireless communication systems with transmit and receive antenna diversity over block Rayleigh fading channels. We showed that a simple, arbitrarily picked, turbo coded modulation scheme using the 4-PSK constellation outperforms the space-time block and trellis codes significantly. In this paper we present further results on the use of turbo coded modulation for antenna diversity systems. In particular, we consider different constellation sizes and various spectral efficiencies. We observe very high gains over the corresponding space-time codes for large interleaver lengths, which is suitable for data communications. We also observe that, depending on the channel model, the turbo code block size can be chosen small enough to be suitable for speech applications, and still offer significant performance improvement in terms of frame error rates.