RESEARCH PROJECTS INCLUDE:
Optimum Resource Allocation for Access Networks
The appearance of increased-bandwidth applications with quality-of-service (QoS) requirements has brought about the emergence of multi-service access networks. In these networks service is provided to user applications according to their QoS requirements. A very important aspect is that of resource (bandwidth) management. A careful management of resources is needed in order to accommodate a large amount of users and still be able to comply with the demanded QoS. Our research proposes a centralized bandwidth allocation scheme that works with access networks regardless of the physical medium they use, and the multiple-access scheme employed. It takes advantage of the statistical characteristics of the traffic generated by the applications being serviced in order to minimize the bandwidth dedicated to control signals (such as bandwidth reservation). This is done with the purpose of achieving high bandwidth utilization. At the same time, the proposed approach complies with the QoS requirements of the serviced applications. As part of our work, we also delineate a procedure to obtain a statistical model of the traffic generated by an application. We use this procedure to obtain a statistical model for a very popular application: Internet video game. The traffic generated by several video game applications was studied and the resulting traffic model is presented. The performance of our bandwidth allocation approach was evaluated in a DOCSIS-based cable network environment using computer simulation. The metrics used to evaluate performance are the excess bandwidth, and the total delay experienced by the packets transmitted by the user stations. Results are obtained for the traffic models obtained for the video game applications, as well as for some traffic models commonly found in the literature. These results are shown for networks servicing a single application, as well as for the case when two applications with different delay requirements are serviced simultaneously. Results show that our approach achieves high bandwidth utilization, and still complies with the delay level required by the delay sensitive application. This compliance comes at the detriment of delay tolerant application.
Related Papers
Effect of Partial Coherent Carrier Recovery in Wireless Systems
In
mobile radio communications it is desirable to use
bandwidth and power resources most efficiently in the
sense that maximizing the bandwidth efficiency while
achieving a predetermined bit error probability with a
minimum power expense. This should be achieved in the
presence of a variety of channel impairments including
fading, intersymbol interference, adjacent and
co-channel interference, and thermal noise. The
modulation techniques to be used play a primary role in
this issue. Among those techniques coherent and
differentially coherent PSK (CPSK, DCPSK) systems are of
great interest and currently being used in wireless
communications. Our research is the investigation of
those modulation schemes performances under diversity
reception taking into account the above mentioned
channel impairments. For coherent systems the
carrier phase is often imperfectly recovered, and
results in degradation in system performance. To see how
much this degradation is, we evaluate the performance of
CPSK with imperfect phase and compare it to the
performance obtained with DCPSK. Generalized fading
channels including Rayleigh, Rice, and Nakagami-
are investigated. The analysis is
based on a convergent infinite series for the cumulative
distribution function of a sum of independent random
variables, and the metric used to evaluate performance
is the bit error probability. The analytical results are
validated by using simulation methods.
Equalization and Coding in Wireless Systems
Large demand for wireless services drives the current and future radio technologies to operate at or near optimum link efficiency possible. The current work is driven by the motivation to design high performance radio links at affordable complexity. In narrow band cellular systems, the receiver has to deal with inter-symbol interference (ISI), co-channel interference (CCI) and multi-path fading, which eventually limit the performance measured in terms of bit error rate, throughput etc. In this thesis, we investigate optimum and sub-optimum equalization methods for ISI and CCI suppression, turbo equalization concepts that exploit coding for ISI equalization, and their low complexity derivatives. In particular, we pay special attention to real modulation alphabets, such as BPSK, GMSK. New equalization and interference suppression techniques that exploit the special properties of real modulation alphabets will be investigated. The impact of these receiver link enhancements on overall system capacity, throughput will be assessed.
System Capacity with Soft Handoff
With the growing demand of 3G and 4G services, the forward link becomes the bottleneck of the CDMA systems. The high data rate in 3G and 4G CDMA systems, e.g. video service, requires careful and optimal utilization of the base station (BS) power, since the high data rate can easily exhaust the system resource in deep fading. To rise to this new challenge, a new analytical model to characterization the BS power under the power control and soft handoff , which are the key technologies used in CDMA systems, is proposed. Using this new model, we find out that the probability of the outage, which happens when the mobile station (MS) can not maintain reliable communications with the BS, will increase in order to achieve a higher system capacity in the forward link of CDMA systems. The most important result obtained by using our new model indicates that whether the soft handoff can increase system capacity or not depends on the properly design of the soft handoff threshold and the limitation of maximum fraction of power allocated by the BS to the MS, which reconciles the arguments in the literature. We further show that power control error can simply increase outage and decrease system capacity. Based on the new model and knowledge obtained there, the optimal power control algorithm is proposed for the multiple BSs in the soft handoff. Comparing to the commonly used power control model in the literature, we show that the proposed optimal power control algorithm can achieve less outage and more system capacity. Recently, a new adaptive video transmission scheme is under research. By jointly shaping the video data rate and the power transmitted from the BS to the MS, this new video transmission scheme can decrease the power requirement from the BS while maintain a certain video quality. To further improve the video quality and achieve a better video rate control, we are researching on the channel estimation and a better algorithm to estimate channel properties, such as the level crossing rate and average fading duration. Further research on how to improve the system capacity and video quality is being extended to CDMA systems including the transmitter diversity, multi-antenna, adaptive coding, and multi-user detection technologies.
Multiple Input Multiple Output Receivers in Wireless Systems
Mobile communication systems based on CDMA are inherently subject to multiple-access interference (MAI), since it is impossible to maintain orthogonal spreading codes in mobile environments. MAI limits the capacity of conventional detectors and brings on strict power control requirements to alleviate the near-far problem. Multi-user Detector (MUD) techniques exploit the character of the MAI by removal of the multi-user interference from each user’s received signal before making date decision, and thus offer significant gains in capacity and near-far resistance over the conventional receiver. However, complete interference cancellation may result in biased decision when the estimated interference is poor in the early stages. Since the decision bias has the strongest effect in the first stage and diminishes its influence in the subsequence stages, an effective scheme to improve the performance of receivers is to cancel only a fraction of MAI with the amount being canceled increasing as the iteration toward to the final decision. We proposed a practical real-time implementation of a coherent multiuser detector based on the improved partial parallel cancellation approach, along with the simulation results that confirm the significant performance improvements over the traditional receivers. Further research is going on the improvement of more efficient schemes and its application on MIMO systems.