TECHNICAL REPORT AND PAPER ABSTRACTS

Abstract-- Arithmetic units for Galois fields whose dimensions can be varied over a certain range have potential applications in data reliability and security of mobile communication and computing systems. The complexity of many reliability and security schemes, when implemented in hardware, depends on arithmetic operations in the fields over which the computations are performed. In this paper, a multiplier for fields GF(2^m), 1< m <= M is presented which allows us to vary the field dimension  without changing the hardware. The maximum dimension  that the multiplier can support is determined by the length of the registers and the width of the input arguments of the inner product module used in the multiplier. The proposed multiplier provides bit-serial and pipeline operations. It has a low circuit complexity and hence it is suitable for applications where area is of prime concern.

Using the multiplier, architectures for computing inverses of field elements and multiplication-and-division of polynomials of GF(2^m), 1< m <= M, are also presented. As an application, an encoder for Reed-Solomon codes is presented where both the code rate and field dimension can be varied. click here to download  

Abstract-- A piggy-backed profile format for a feedback-based handoff algorithm is proposed to reduce the impact of profile information on radio bandwidth. An efficient identification scheme for mobile switching centers and access points, which reduces the size of the identifiers required for the profile, is also proposed. The implementation of the proposed profile format is described in detail. Furthermore, the sojourn time and the feedback interval of the profile are analyzed extensively and evaluated by simulations using the OPNET Modeler/Radio package. Numerical results indicate that feedback-based handoff is a viable approach. click here to download  

Abstract-- A distributed polling and channel reservation scheduling algorithm and call admission control (CAC) algorithm, which make uses of the scheduling algorithm, for intra-cell resource allocation, and a novel Virtual Cluster Allocation with Scheduled Retry (VCA/SR) algorithm for inter-cell resource reservation are proposed. The interworking of the algorithms ensures optimal channel utilization and reduces forced connection dropping probability for multimedia applications in a wireless communication environment. The major strength of combining service scheduling and CAC to form a complete system is to guarantee a predefined quality of service to all classes of traffic without making conflicting assumptions about each other. click here to download  

Abstract-- The increasing reliance on information available on Internet, and the rapid growth of the wireless subscriber population suggest a need for Internet users to maintain communications as they move from place to place. However, a mobile user needs to have a stable IP address in order to be stably identifiable, and a stable IP address is counter to the concept of mobility. Mobile IP proposed by the Internet Engineering Task Force (IETF) is the emerging standard for mobile Internet applications. It allows a mobile user to change its location without restarting its applications and without disrupting any ongoing communications. Mobile IP is transparent to the physical medium over which a mobile user communicates. On the other hand, the wireless access to fixed wirelined IP networks gives rise to a number of issues. The nature of wireless data links and the user mobility have an evident impact on the performance and usage of IP. This report presents an exposition on wireless/IP interworking, a solution to the problem of transferring information through Internet to and from users anywhere and at anytime. click here to download  

Abstract-- The system capacity in an asynchronous direct-sequence code division multiple ac- cess (DS-CDMA) system is dominated by multiple access interference (MAI) which is proportional to the transmit power strength of other users. The transmit power there- fore becomes a major resource and requires careful planning and control if optimal system performance and maximal user capacity are to be achieved. This work investi- gates the power distribution law and studies its convergence condition. The problem is formulated in the context of third generation multimedia multirate wideband CDMA (WCDMA) mobile communications. It shows that the convergence condition for any power distribution or power control algorithm is a function of the spread bandwidth (resource), user data rates, and QoS requirements of connections (trafficc demands). The closer the demand is to the resource, the higher are the required transmit powers. If the demand exceeds the resource, no algorithm can converge, which means some transmitters may reach their saturation power level resulting in unsatisfactory quality of service (QoS). A new power control algorithm based on the convergence criterion is proposed. Numerical results are presented to validate the analytical results and the proposed power control algorithm. click here to download

Abstract-- A class of service scheduling schemes called the Reference Queue Tracking (RQT) schemes is identified, and their ability to provide delay bounds on a per-VC basis is studied. A statistical multiplexer appears to an incoming VC stream as a single server queue whose service rate (bandwidth) is controlled by the service scheduling scheme. We refer to this queue as the Virtual Connection Queue (VCQ). Corresponding to each VCQ, the RQT schemes use a fictitious single server Reference Queue (RQ) which is also fed with the VC stream but which is served at a constant rate equal to the allocated bandwidth of the VC. The RQT schemes serve VCs in such a way that some selected parameters of the VCQs track those of the corresponding RQs. An important property of a service scheduling scheme is the ability to guarantee that no cells depart from the VCQs later than from the corresponding RQs. In this paper, four schemes belonging to the RQT class are identified and their ability to satisfy the above property is investigated. click here to download

Abstract-- In this paper, the capacity analysis for connection admission control is presented for the reverse-link transmission of a packetized indoor multimedia wireless communication system using direct sequence code division multiple access (DS/CDMA). Since CDMA is interference limited, the signal-to-interference-plus-noise ratio criterion is used to check if there is enough system resources (i.e., the CDMA channels and received signal power) for each new connection request. Taking into account the stochastical nature of multimedia traffic, the effective bit rate is used to characterize the resources required by each mobile user and a linear approximation is then used to find the total resources required by all the mobile users already admitted to the system and the new connection request. Transmission errors due to both base station buffer over ow and wireless channel impairments are considered. The capacity of multimedia traffic is determined in such a way that the utilization of the system resources is maximized and, at the same time, the required transmission bit error rate and transmission delay of all users admitted to the system are guaranteed. Computer simulation results are given to demonstrate the performance of the proposed method for capacity analysis. click here to download

Abstract-- In this paper, a new phase precoding technique is developed to combat the intersymbol interference (ISI) resulting from a frequency-selective slowly fading channel in a personal communication system using quadrature phase shift keying (QPSK). Based on a new dimension partitioning technique, the precoder predistorts only the phase of the transmitted signal to keep a constant transmitted signal amplitude and, therefore, to ensure the stability of the precoder even in equalizing a non-minimum-phase channel. Under the constraint of the constant amplitude, the dimension partitioning method is developed to guarantee the possibility of correct detection for all transmitted information symbols and to further improve the transmission accuracy by increasing the size of decision regions. Analytical and simulation results demonstrate that, over frequency-selective Rayleigh and Rician fading channels, the system using the proposed channel precoder can achieve a bit error rate comparable with that using a conventional decision-feedback equalizer (DFE). The precoder can outperform DFE in an indoor environment where there is a strong direct propagation path. The main advantage of using the precoder is that the impairment of ISI due to multipath propagation on the transmission performance can be mitigated without increasing the complexity of the portable unit receiver. The proposed technique is especially useful for personal communications, where ISI due to multipath fading channels can severely deteriorate the BER transmission performance and where the simplicity of portable units is a vital characteristic of the system. click here to download

Abstract-- Future wireless ATM (asynchronous transfer mode) networks present a number of mobility related challenges. Handoffs due to user mobility require network signaling to maintain the communication link and may result in packet loss due to packet misrouting and/or misordering. This paper proposes a handoff solution for an ATM- based code division multiple access (CDMA) packet switched wireless environment. The proposed solution combines soft handoff and the modied nearest common node rerouting (NCNR) to meet the challenges of wireless ATM. It is shown that, com- pared with the hard handoff approach, the proposed solution reduces the number of control signals required for handoff and the volume of buffered information packets during handoff mitigates the problem of packet misrouting and misordering, and signifficantly improves the radio link transmission accuracy. click here to download

Abstract-- This paper proposes a packetized indoor wireless system using direct sequence code division multiple access (DS-CDMA) protocol. The indoor radio environment is characterized by slow Rayleigh fading with or without lognormal shadowing. The system supports multimedia services with various transmission rates and quality of service (QoS) requirements, and allows for seamless interfacing to Asynchronous Transfer Mode (ATM) broadband networks. All packets are transmitted with forward error correction (FEC), using convolutional code for voice packets and BCH code for data packets with an automatic retransmission request (ARQ) protocol and for video packets without ARQ. A queueing model is used for servicing data transmission requests. A power control algorithm is proposed for the system, which combines closed loop power control with channel estimation to give the best performance. Cell capacity of each traffic type and various multimedia trac congurations in both single-cell and multiple-cell networks is evaluated theoretically under the assumption of perfect power control. The eect of power control imperfection on the capacity using the proposed power control algorithm is investigated by computer simulation. click here to download

Abstract-- This paper presents a channel sharing scheme, Neighbor Cell Channel Sharing (NCCS), based on region partitioning of cell coverage for wireless cellular networks. Each cell is divided into an inner-cell region and an outer-cell region. Cochannel interference is suppressed by limiting the usage of sharing channels in the inner-cell region. The channel sharing scheme achieves a traffic-adaptive channel assignment and does not require any channel locking. Performance analysis shows that using the NCCS scheme leads to a lower call blocking probability and a better channel utilization as compared with other previously proposed channel assignment schemes. click here to download

Abstract-- This paper studies a packetized direct sequence code division multiple access (DS-CDMA) wireless network for providing multimedia services to mobile users. A strategy for integrating transmission error control with power control is presented and evaluated for the reverse link transmission where orthogonal signaling and noncoherent demodulation are necessary. For delay insensitive traffic requiring a very low bit error rate (BER), convolutional cod- ing and an automatic retransmission request (ARQ) protocol are used to guarantee the transmission accuracy. By using the modified Viterbi decoder for decoding error detection, power control based on the received Eb=I0 (the ratio of signal energy per bit to interference- and-noise density) is employed to minimize the total received power at the base station, resulting in a maximal frequency spectrum efficiency. Numerical results are presented to evaluate the optimal received Eb=I0 value, the system radio frequency spectrum efficiency, and the optimal packet length, given the required BER and the traffic type. It is shown that the spectrum eciency can be significantly increased by using an ARQ protocol with the integrated optimal power control and convolutional coding for delay insensitive traffic. click here to download

Abstract-- In this paper, a new adaptive H1 ltering algorithm is developed to recursively update the tap-coecient vector of a decision feedback equalizer (DFE) in order to adaptively equalize the time-variant dispersive fading channel of a high-rate indoor wireless personal communication system. Different from conventional L2 (such as the recursive least squares (RLS)) filtering algorithms which minimize the squared equalization error, the adaptive H filtering algorithm is a worst-case optimization. It minimizes the eect of the worst disturbances (including input noise and modeling error) on the equalization error. Hence, the DFE with the adaptive H ltering algorithm is more robust to the disturbances than that with the RLS algorithm. Computer simulation demonstrates that better transmission performance can be achieved using the adaptive H algorithm when the received signal-to-noise ratio (SNR) is larger than 20 dB. click here to download

Abstract-- In a high-rate indoor wireless personal communication system, the delay spread due to multipath propagation results in intersymbol interference (ISI) which can significantly increase the transmission bit error rate (BER). Decision feedback equalizer (DFE) is an ecient approach to combating the ISI. Recursive least squares (RLS) algorithm with a constant forgetting factor is often used to update the tap-coecient vector of the DFE for ISI-free transmission. However, using a constant forgetting factor may not yield the optimal performance in a nonstationary environment. In this paper, an adaptive algorithm is developed to obtain a time-varying forgetting factor. The forgetting factor is used with the RLS algorithm in a DFE for calculating the tap- coecient vector in order to minimize the squared equalization error due to input noise and due to channel dynamics. The algorithm is derived based on the argument that, for optimal ltering, the equalization errors should be uncorrelated. The adaptive forgetting factor can be obtained based on on-line equalization error measurements. Computer simulation results demonstrate that better transmission performance can be achieved by using the RLS algorithm with the adaptive forgetting factor than that with a constant forgetting factor previously proposed for optimal steady-state performance or a variable forgetting factor for a near deterministic system. click here to download

Abstract-- This paper presents a novel phase precoding (pre-equalization) technique to equalize frequency- selective Rayleigh and Rician slowly fading channels for personal communication systems using phase modulation. In order to achieve intersymbol interference (ISI)-free transmission, the precoding technique predistorts the signal transmitted from a base station to a portable unit. The novelty of the technique lies in using a spiral curve design i) to ensure the stability of the precoder even in equalizing a non minimum-phase channel, ii) to obtain an ISI-free received signal, and iii) to keep a constant transmitted signal amplitude. Using the precoder can improve the bit error rate (BER) transmission performance without increasing the complexity of the portable unit receiver. The BER performance of coherent quadrature phase-shift-keying (QPSK) with the channel pre-equalization is analyzed theoretically for both Rayleigh and Rician fading channels. Analytical and simulation results demonstrate that, coherent QPSK using the proposed channel precoder has a significantly lower BER than that using a conventional decision-feedback equalizer (DFE) because the precoder does not suer from error propagation. click here to download

Abstract-- This paper proposes a noncoherent hybrid parallel pseudonoise (PN) code acquisition scheme for code-division multiple access (CDMA) mobile communication systems and analyzes the effect of multiple access interference (MAI) on the code acquisition performance for Rayleigh and Rician fading channels. The hybrid acquisition scheme combines parallel search with serial search to cover the whole uncertainty region of the input code phase. It has a much simpler acquisition hardware structure than the total parallel acquisition, and can achieve the mean acquisition time slightly inferior to that of the total parallel acquisition in the case of severe MAI; on the other hand, it provides the exibility in the trade-off between the mean acquisition time and system complexity if no MAI is considered. The closed-form expressions of the detection and false-alarm probabilities and mean acquisition time are derived. Numerical analysis quantifies the severe performance degradation of code acquisition due to both MAI and channel fading, and demonstrates the dependence of the increase of mean acquisition time (due to MAI) on the number of users in the CDMA system, system design parameters, and channel fading statistics. click here to download