This paper compares the forward link performance of a direct spread CDMA system with a spreading bandwidth of 3.6864 MHz and a multicarrier system consisting of 3 carriers each with spreading bandwidth of 1.2288 MHz. These systems are under consideration for the forward link of the next generation CDMA cellular/PCS systems. This paper summarizes performance comparison results of the two systems for a data rate of 9.6 kbps with the IMT-2000 vehicular and pedestrian channel models. Performance results are given for speeds of 3 and 30 km/h. The results indicate that the two systems have comparable performance for the above data rate and the IMT-2000 channel models.

The paper considers optimal forward link power allocation for data transmission in DS-CDMA systems. Acceptable quality voice communications in DS-CDMA can be achieved through the application of the forward error correction (FEC). Due to stringent requirements on signal delay, no retransmission is allowed in voice only systems. Highly reliable data transmission over wireless channels, however, can be achieved with hybrid automatic-repeat-request (ARQ) schemes. The average power allocated to data users then needs to be calculated as the actual link transmitted power times the average number of times that a data packet must be transmitted before it is accepted. Since a base station's total power available to mobiles is limited, there is an optimum power allocated to every data user in the cell, which maximizes the system capacity. The optimal power allocation can be described as the stochastic nonlinear programming optimization problem. Numerical results are presented to illustrate the problem. The powers allocated to data users are location dependent, but the numerical results demonstrate that the mean signal-to-interference ratio (SIR) is location independent when the optimal power allocation is achieved.

Two rapid synchronization acquisition algorithms applicable to spread spectrum links of code division multiple access (CDMA) personal communication systems are proposed and evaluated. The algorithms operate within a self-referencing matched filter synchronizer structure, and are particularly useful in reducing synchronization overhead on links designed to carry packet-type services. The main distinguishing characteristic between the two schemes is that one uses hard-decision while the other uses soft-decision detection. The proposed schemes are especially applicable to reverse link transmissions in quasisynchronous CDMA systems in which timing at portable terminals is established via pilot and synchronization signals received on respective code-division channels from the home base. If discontinuous (bursty) transmission is used on reverse links, the acquisition process is required for each transmission burst because of the propagation time uncertainty. Analysis of the algorithms on additive white Gaussian noise (AWGN) and Rayleigh fading channels reveals that their performance depends significantly on the choice of synchronizer parameters and the average despread signal-to-interference ratio (SIR). When this choice is proper, acquisition over a single preamble of relatively short duration can be achieved with high probability. The soft-decision scheme introduces a performance advantage of between 4-9 dB depending on the length of the synchronizing preamble.

We consider the transmission of QCIF resolution (176/spl times/144 pixels) video signals over wireless channels at transmission rates of 64 kb/s and below. The bursty nature of the errors on the wireless channel requires careful control of transmission performance without unduly increasing the overhead for error protection. A dual-rate source coder is presented that adaptively selects a coding rate according to the current channel conditions. An automatic repeat request (ARQ) error control technique is employed to retransmit erroneous data-frames. The source coding rate is selected based on the occupancy level of the ARQ transmission buffer. Error detection followed by retransmission results in less overhead than forward error correction for the same quality. Simulation results are provided for the statistics of the frame-error bursts of the proposed system over code division multiple access (CDMA) channels with average bit error rates of 10/sup -3/ to 10/sup -4/.

An improved synchronization acquisition algorithm applicable to spread-spectrum links of code division multiple access (CDMA) personal communication systems is proposed. The algorithm operates within a self-referencing matched filter synchronizer structure, and is particularly useful to reduce synchronization overhead on links designed to carry packet-type services. The proposed scheme involves a soft-decision detection algorithm which improves the acquisition performance by between 4 and 9 dB (depending on the choice of system parameters) in comparison to the one described previously by the authors (1994).

Measured channel impulse response estimates are used to predict the parameters of the channel model needed to evaluate the performance of Rake receivers in CDMA systems. A channel model consisting of a number of resolvable multipath components is assumed. The parameters of interest are the relative average powers of the multipath components, the probability distribution of multipath fading on each path, and the cross-correlation coefficient of multipath fading among the different multipath components. The paper presents statistics on these parameters for measurements made in outdoor urban and indoor microcellular environments at 910 MHz.>

We evaluate the capacity and bandwidth efficiency of microcellular CDMA systems. Power control, multipath diversity system bandwidth, and path loss exponent are seen to have a major impact on the capacity. The CDMA system considered uses convolutional codes, orthogonal signalling, multipath/antenna diversity with noncoherent combining, and fast closed-loop power control on the uplink (portable-to-base) direction. On the downlink (base-to-portable), convolutional codes, BPSK modulation with pilot-signal-assisted coherent reception, and multipath diversity are employed. Both fast and slow power control are considered for the downlink. The capacity of the CDMA system is evaluated in a multicell environment taking into account shadow fading, path loss, fast fading, and closed-loop power control. Fast power control on the downlink increases the capacity significantly. Capacity is also significantly impacted by the path loss exponent. Narrowband CDMA (system bandwidth of 1.25 MHz) requires artificial multipath generation on the downlink to achieve adequate capacity. For smaller path loss exponents, which are more likely in microcellular environments, artificial multipath diversity of an order of as high as 4 may be needed. Wideband CDMA systems (10 MHz bandwidth) achieve greater efficiencies in terms of capacity per MHz.< >