For broadband wireless networks based on OFDM (Orthogonal Frequency Division Multiplexing), an FRF (Frequency Reuse Factor) of 1 has been highly desirable for more improved channel throughput. However, due to the limited power budget of MSs (Mobile Stations) or the increase in ICI (Inter-Cell Interference), a required QoS (Quality of Service) may not be maintained. This paper addresses an optimal LCRTD (Loading Control based on Region-Time Division) over multi-cell environments for an efficient uplink QoS control. In the LCRTD scheme, a cell is divided into several regions by utilizing an optimization approach under QoS constraints, and users in each region are allowed to send their data at the allocated time slots. In the simulation, it is demonstrated that a decrease of 26% in the transmit power can be obtained.

This letter discusses how to enhance the capacity of uplink DS-CDMA networks that support multihop transmission. For this purpose, we derived simple theoretic conditions by which one can choose between singlehop- and multihop transmission. Numerical results show that we can significantly increase the radio network capacity by adopting only a few number of multihop transmissions.

An LDPC-coded FH-OFDMA system is proposed for the uplink of a packet-based cellular system, where the frequency hopping (FH) is based on a resource block (RB) for coherent demodulation. For the system, different RB types are employed either for better intercell interference (ICI) averaging capability or for better channel estimation performance. For the receiver, practical iterative channel estimation and decoding methods are proposed to improve the channel estimation performance without boosting the pilot power and to mitigate the adverse effects of the ICI. Extensive simulation results are provided to show the effect of the RB size on the channel estimation and ICI averaging performance as well as possible application of the proposed receiver in harsh mobile environments with dynamic packet allocation.

This letter deals with our investigations into improving the performance of a wireless uplink system when an orthogonal frequency-division multiple access (OFDMA) is used as an access scheme. To do this, the OFDMA-based uplink system adopts a frequency diversity coupled with a cyclic time shift (CTS) at the transmitter, which is named as the FD-OFDMA system with CTS. It is found that the multi-user FD-OFDMA system equipping with CTS can decrease the probability of destroying the orthogonality among the users and provide the MAI-robust reception without decreasing the bandwidth efficiency of the system.

The uplink synchronous transmission scheme was proposed to improve the uplink performance of DS-CDMA systems. However, previous performance analyses have assumed perfect uplink synchronization among main paths, which is impractical due to timing misalignment. Accordingly, this Letter evaluates the impact of imperfect synchronization on the performance of an uplink synchronized DS-CDMA system by deriving an orthogonality factor as a measure of the imperfection in synchronization.

In W-CDMA systems, distributions of the interference power and the total transmit power both measured at base stations are respectively used for capacity analysis in the uplink and downlink. For accurate capacity analysis, these quantities must be in proportion to the traffic amount. However, these quantities are no longer in proportion to the traffic amount since the transmit power control maintains the signal to interference power ratio at a constant level. Although the relationship between these measurements and the traffic amount has been investigated, there are still challenges to calculate the statistics such as the blocking probability or the outage probability accurately. This paper proposes a method to calculate the blocking probability by transforming the distributions of these measurements into distributions that are referred to as "traffic equivalent distributions," where the distributions are automatically adjusted according to the traffic amount. The calculated results show good agreement with the results obtained by dynamic computer simulations in the uplink, and show good agreement in the downlink as well when the traffic load is light. Accurate calculation of the blocking probability using a feedback loop and the observation of the traffic equivalents is also reported.

Uplink synchronous transmission has been proposed to improve the uplink capacity of DS-CDMA systems by means of canceling interference from the main paths of other intra-cell users. A significant capacity gain has been reported in a single cell environment. This Letter further investigates the uplink capacity in a multiple cell environment, where two crucial factors are taken into account, namely code shortage problem and soft handover. The impacts of the target Eb/Io and the other-to-own cell interference ratio, together with the number of channelisation codes, are discussed mathematically and then, confirmed through system level simulations with more realistic parameters.

Multi-Carrier Code Division Multiple Access (MC-CDMA) is one of candidates for the next generation wireless communication systems. In an uplink, the MC-CDMA system suffers from the different access timing (asynchronous transmission), the different fading, and the different frequency offsets of each active user. In this paper we analyze the effects of the frequency offset compensation with MMSE-MUD (minimum mean square error based multi-user detection) for MC-CDMA in a quasi-synchronous uplink. We consider the MC-CDMA system with two subcarrier mapping schemes, the continuous mapping scheme and the discrete mapping scheme. From our theoretical analysis and computer simulation, we show that the MMSE-MUD can compensate the different frequency offsets among users. We also show that the MMSE-MUD significantly improves the bit error rate (BER) for the MC-CDMA system with the continuous mapping scheme.

It is well known that diversity performance of communication systems using signals with high dimensions in time, frequency and/or spatial domains depends on correlation of the channel characteristics along signal dimensions. On the other hand, it has not been payed due attention how the coherent receiver which combines the signals is greatly affected by the erroneous channel estimation which can undermine the diversity gain. In this paper, assuming that the estimator is given the a priori probability of the channel characteristics, we propose an optimum estimation scheme based on MAP criterion, in an uplink-MC/CDMA system on channels with frequency selective fading, with an array of antennas at the receiver. The MAP estimator effectively takes into account the correlation of the channel characteristics that the conventional estimator neglects. We also propose a signal design in pilot symbol periods that enables the receiver to separately obtain the sufficient statistic for estimating the channel characteristics without MAI. Using computer simulation, we obtained MSE error performances of the proposed estimator compared with the conventional estimator and their effect on BER performances of the diversity combining receiver. It was observed that using the conventional estimator for combining greater number of signals than the effective channel dimension deteriorated the BER performance while using the proposed estimator kept the optimum performance just as the error-free estimator did. Also obtained for MC/CDMA systems are BER performances of the single user matched filter and MMSE receivers using the proposed and the conventional estimators. A considerable improvement of the MMSE performance was achieved owing to the optimum estimator. It remains for the a priori probability of the channel characteristics to be properly assumed and dealt with in sequential estimation.

We have proposed an intracell uplink of a spread-spectrum code-division multiple-access (SS-CDMA) flexible wireless network based on approximately synchronized (AS) CDMA. Since the AS-CDMA has no co-channel interference, complicated transmission power control (TPC) is not required. A modem of the AS-CDMA has been designed and implemented for the Japanese 2.4 GHz industrial, scientific and medical (ISM) band. Using the implemented modem, the degradation of Eb/N0 from the theoretical limit is 1.0 dB at a bit error rate (BER) of 10-3. Under 2-user environment, the degradation of carrier-to-noise ratio (CNR) is 0.5 dB at a BER of 10-3 when the desired-to-undesired signal ratio (DUR) is -20.3 dB. We have evaluated BER performances in cases of varying carrier frequency offset and median DUR with computer simulation. Under 8-user environment, at the carrier frequency offset of 0.3 ppm, the BER with the DUR of -16 dB is found to be 10-3. Using the AS-CDMA with a 4-step open-loop TPC technique, the design of intracell uplink is available.

This paper proposes a new coherent CDMA channel structure for the uplink with staggered burst pilot and its detection algorithm. In the uplink, mobiles in a cell share a pilot channel by transmitting periodic bursts in nonoverlapping time slots, which enables coherent detection. We analyze the uplink capacity and derive the capacity formula. The dual mode channel estimator (DMCE) consists of pilot-based channel estimation (PBCE) and data-based channel estimation (DBCE). The proposed DMCE algorithm is very stable in the presence of Gaussian noise and Doppler shift because the pilot burst initiates the DMCE operation periodically. A negligible loss (0.068 dB) in Eb/N0 results from the introduction of the burst pilot. When compared with ideal (0 km/h) coherent detection, the required Eb/N0 in Doppler shift, corresponding to the speed of 160 km/h, is degraded less than 2.0 dB. The simulation result also shows increased channel capacity. The burst pilot can be implemented without added complexity even though some extra correlators are needed for the DMCE. This improvement is significant compared to previously published studies of coherent CDMA detectors with non-shared pilots.

An uplink synchronised CDMA system through transmission timing control at mobile users has been proposed to improve the uplink capacity. This Letter mathematically investigates its capacity, considering perfect fast TPC and two antenna diversity reception in a single cell environment and compares it with that of a conventional CDMA system.

This paper describes the effects of traffic distributions on uplink and downlink communications qualities in CDMA cellular systems. Many researches have been done from the viewpoint of the system capacity under ideal conditions in both uplink and downlink. However, there are few studies regarding traffic distributions that concurrently affect the uplink and downlink quality. The characteristics in both links are different even in a spatially uniform traffic distribution because the system structures are not symmetric between both links. When non-uniform radio environments are assumed, both link qualities become very different from each other. It is therefore important to design systems in consideration of link-specific characteristics in whole service area. This paper clarifies the difference in both link characteristics in CDMA systems regarding traffic distributions.