We consider the problem of channel estimation and symbol and frequency synchronization for orthogonal frequency division multiplexing (OFDM) systems in multipath fading environment with pilot data. We developed an algorithm to estimate the channel, symbol timing and frequency offset simultaneously by using a technique based on maximum-likelihood theory. Unlike existing joint algorithms which operate on frequency domain (FFT demodulated data), our algorithm works on raw received data, avoiding the need of a coarse synchronization algorithm. Thus, it is much simpler yet robust. Simulation results also show that our algorithm is more accurate and reliable than existing algorithms.

An analytical formulation of the outage probability in terms of bit error rate specification for variable bit rate (VBR) multiclass services in the uplink of a wideband CDMA-based cellular system is presented. The analytical framework is formulated for the general case in which different traffic classes have different spreading gains. The analytical work leads to the determination of the capacity region of a wideband CDMA system with VBR traffic. Numerical results of the capacity region corresponding to typical parameter values are also presented.

The delay jitter performance of voice traffic, modeled using an exponential on/off source, in a cellular wireless ATM network is analyzed. The uplink MAC (medium access control) protocol is implemented by a P-TDMA/DR (prioritized time division multiple access with dynamic reservation) MAC protocol. Voice, compressed video and data are used as test traffic. The delay jitter (or cell delay variation) of an exponential on/off voice is evaluated using approximate analysis and computer simulation. Numerical results indicate that, with very high probability of 0.92 (simulation) or 0.96 (analysis), the voice delay jitter is practically zero and, with very low probability, it is at other values. 99.1% of the voice delay jitter are within /spl plusmn/10 TDMA frame intervals. This shows that the voice delay jitter is well-behaved due to the excellent design of the P-TDMA/DR MAC protocol for voice traffic.

Wormhole routing networks have become increasingly popular for low latency, high-speed interconnection of supercomputer and workstation clusters. An example is the Supercomputer SuperNet (SSN) at UCLA, which interconnects supercomputers across campus and metropolitan area distances. The SSN employs a two-level network architecture in which an optical backbone network interconnects several high-speed, wormhole-routing local area networks (Myrinets). The SSN applications such as scientific visualization and rendering require that the network support reliable delivery of traffic characterized by quality of service (QoS) parameters. Motivated by this requirement, we investigate QoS support in Myrinet-like high-speed, wormhole routing networks. Since native Myrinet protocols do not provide QoS support, we explore several novel strategies including (a) the use of a separate subnet for carrying such traffic (along with source pacing), (b) the overlay of a virtual synchronous system on the asynchronous network, and (c) the introduction of virtual channels. We discuss the tradeoffs among the different options and evaluate them via selected simulation experiments.

Optical networks can provide higher throughputs while high speed electronic networks possess the intelligence for network control and management. We present a two level high speed network architecture that combines the throughput advantage of optical networks and the intelligence of electronic processing. One level is a high speed mesh LAN which uses wormhole routing, source routing and hop-by-hop flow control mechanisms with mesh routers (asynchronous pipelined crossbar switches) to provide a high speed electronic network. The second level is an optical star network interconnecting high speed mesh networks distributed across metropolitan area distances. We obtain analytical expressions for the average message (worm) delays for the GTDM (group time division multiplexing) multi-access protocol (which includes as special cases TDM and DAS) for single-hop packet switching in the optical network of such an architecture. We use a two state discrete time Markov chain to model the arrival of messages to the optical network. Results for both uniform traffic and non-uniform traffic are presented. Finally, a modified dynamic allocation scheme is presented for single-hop packet switching which handles the message as a unit rather than sending a message as several fixed sized packets.

The authors propose a media access protocol for an optical star-coupler network in which the stations are equipped with fixed transmitters and tunable receivers and use wavelength-division multiplexing. The protocol, called the group TDM scheme (GTDM), is a reservation-based scheme in which the nodes are assembled into groups. The nodes within a group communicate with the nodes of other groups using the TDM scheme, but they make reservations using the dynamic allocation scheme (DAS), which has been proposed earlier. GTDM tries to overcome the drawbacks of TDM (unnecessary high delays under low load) and DAS (high signaling costs) by providing a trade-off between packet delay and signaling costs. Analytical and simulation results show that GTDM can achieve good performance with low signaling complexity. It is also shown to perform better than hybrid TDM (HTDM), a hybrid of TDM and DAS, for a given signaling complexity.>