M.O. Sunay

Calculating error probabilities for DS-CDMA systems: when not to use the Gaussian approximation

Error probabilities for various unbalanced DS-CDMA systems are calculated using the standard Gaussian approximation, improved Gaussian approximation and the Fourier series based schemes. A scarcely populated system, a system with a dominant interferer, a system in a fading channel and a multimedia system with a single high processing gain user are considered. It is seen that the standard Gaussian approximation in all of these cases gives inaccurate results, especially when the number of users active in the system is low. The improved Gaussian approximation on the other hand, gives more accurate results for a scarcely populated system and a system with a dominant interferer. For systems in fading channels and multimedia systems, neither of the Gaussian approximations are accurate. For all of these cases, the Fourier series based scheme gives very accurate results without computational complexity.

Video streaming over wireless DCCP

It is envisioned that access networks will be mostly wireless in the future. Hence, it is of interest to consider extensions of the datagram congestion control protocol (DCCP) for wireless networks. This paper focuses on the problems of video streaming over DCCP in the wireless domain and proposes a cross-layer solution in which the wireless packet loss information available in the medium access (MAC) layer is utilized by DCCP to distinguish congestion losses from wireless losses and behave accordingly. Tests performed with our modified DCCP confirm that using cross-layer loss information prevents unnecessary rate decreases and results in better video streaming experiences.

Secure User-Plane Location (SUPL) Architecture For Assisted GPS (A-GPS)

The aim of this paper is to analysis the new assisted GPS (A-GPS) architecture based on SUPL (secure user plane location). This architecture improves performance of A-GPS based location estimation approaches. We provide comparative field test results of the developed system with the conventional GPS in terms of both the position accuracy and the TTFF (time to first fix) delay times. We have also examined the effectiveness of initial position estimation on the TTFF and the accuracy of calculated position. Field tests are conducted on a real GSM network in urban areas and indoor environments. Executed analysis has provided that the developed SUPL based A-GPS system achieves high-performance on estimation accuracy and faster TTFF.

Sensitivity of a DS CDMA system with long PN sequences to synchronization errors

Use of direct sequence code division multiple access (DS CDMA) has been the object of much attention in the research and development of a multi-service personal communications network. The performance of such a system is usually given in terms of the bit error rate as a function of the number of active users for a given signal to noise ratio. We have investigated the degradation in the performance of a BPSK modulated, bi-phase spread DS CDMA system due to synchronization errors. A Fourier series approach has been used to find an infinite series representation for the probability of error when both chip timing and carrier phase errors are present. The analysis requires the characteristic functions of both the multiple access interference and self interference and these functions have been expressed in terms of the special functions of mathematical physics. Computation of the infinite series is very fast as a whole bit error curve can be generated in minutes on a Sparc IPX workstation. The infinite series is also compared with the standard Gaussian approximation and an alternative Gaussian approximation found in the literature. The series is then used to assess the reduction in system capacity when synchronization errors are present in the system. We have expressed this degradation in the system performance as an effective processing gain reduction.

Effects of Transmit Beamforming on the Capacity of Multi-Hop MIMO Relay Channels

In this paper we investigate multi-hop fading relay channels where the source, the destination and the multiple relay nodes are all equipped with multiple antennas. We study the ergodic capacities of multiple relay networks based on Amplify-and- Forward (AF) and Decode-and-Forward (DF) relaying modes. We examine multi-user beamforming (MU-BF), where each data stream is assumed to be matched to a specified relay node, based on the conventional eigen-mode transmission for both modes, and derive ergodic capacity expressions. We also examine the impact of the number of selected relay nodes on the network capacity both for modes of relaying. We show that by using MU-BF at the source node and a maximum number of relay nodes selected for cooperation, the network gains from multiplexing, while beyond that number of cooperating relay nodes, it only gains from relay selection.

Effects of carrier phase and chip timing errors on the capacity of a quadriphase spread BPSK modulated DS-CDMA system

Efficient series that are used to calculate the probability of error for quadriphase spread, BPSK modulated, synchronous, quasi-synchronous and asynchronous DS-CDMA systems in the presence of chip timing and carrier phase errors are derived. The derivations require the characteristic functions of the multiple access interference and of the self interference. These series are then used to assess the capacity of the systems at different levels of synchronization errors. The derivations are performed for both constant and random synchronization errors. Using the series, the asynchronous quadriphase spread system capacity is compared to that of an asynchronous binary spread system.

An Analysis of Constant Bitrate and Constant PSNR Video Encoding for Wireless Networks

In wireless networks, transmission of constant-quality, high bitrate video is a challenging task due to channel capacity and buffer limitations. Content adaptive rate control, is used as a solution to this problem. Instead of transmitting all of the video content at low quality, the most important content can be transmitted at high quality while still preserving an acceptable quality for the remaining segments. Furthermore, the rate control strategy inside the individual temporal segments plays a key role for the network performance and viewing quality. Although constant quality video encoding inside the temporal segments is preferable for the best viewing experience, it causes more network packet losses due to adverse bitrate fluctuations in the video stream. In cases when the network is too much loaded, it may be better to employ constant bitrate encoding for network friendliness. In this paper, a performance analysis of constant bitrate and constant peak signal-to-noise ratio encoding for content adaptive rate controlled video streaming over wireless networks is presented. Experimental results obtained using AVC/H.264 encoding in a CDMA/HDR multi-user environment with cross-layer optimized scheduling show performance comparisons of CBR and CPSNR encoding.

Wireless multicast with multi-user diversity

Efficient wireless packet data access is possible by exploiting multi-user diversity. This is done by using an opportunistic multiple access system that allocates system resources to one user at a time while using adaptive modulation and coding. A scheduling algorithm provides resource allocation in such a system, and its proper design is crucial in ensuring satisfactory system performance. Multicasting is a spectrally efficient method of supporting group communication by allowing transmission of packets to multiple destinations using fewer resources. When the wireless opportunistic multiple access system provides a multitude of services, at least one of which is a multicast service, the scheduler needs to incorporate the service type as well as the time-varying wireless channel states and the individual observed user latencies for proper resource allocation. The paper presents a number of efficient scheduling algorithms that enable such an operation. Extensive, detailed simulations are performed to show the feasibility of the multicast service provisioning in the opportunistic multiple access system.

Diversity combining for DS CDMA systems with synchronization errors

An efficient series that is used to calculate the probability of error for a BPSK modulated DS CDMA system with chip timing and carrier phase errors in a slowly fading multipath channel is derived. The receiver is assumed to be a coherent RAKE receiver. Three types of diversity schemes are considered: selection diversity, equal gain diversity combining and maximal ratio diversity combining. The error probability derivation does not resort to the widely used Gaussian approximation for the intersymbol interference and multiple access interference and is very accurate. Systems of 1.25 MHz and 5 MHz are considered for different numbers of diversity branches.

Performance of selection diversity for a DS CDMA system with synchronization errors

Direct-sequence code division multiple access (DS CDMA), which was primarily used in military communications until the late 80s, has been the center of attention in cellular radio communications. An efficient series that is used to calculate the probability of error for a BPSK modulated DS CDMA system with chip timing and carrier phase errors in a slowly fading, multipath channel is derived. The receiver is assumed to be a coherent RAKE receiver that employs selection diversity. The error probability derivations require the characteristic functions of the intersymbol interference and the multiple access interference and these functions have been expressed in terms of the confluent hypergeometric function. The series is then used to assess the reduction in the system capacity due to different levels of synchronization errors. Systems of 1.25 MHz, 5 MHz and 10 MHz bandwidths are considered for different number of diversity branches.