This paper presents a new CLSP/CC (Channel Load Sensing Protocol/Channel Clearance) in order to obtain better performances in spread slotted ALOHA networks. This protocol can decrease a wasteful utilization of channel by prohibiting continuous transmission of the packets destroyed in previous slot, with little added network complexities. For channel clearance technique, hub station broadcasts NAK to all mobile stations when the number of packets transmitted in a slot exceeds the channel capacity, and mobile stations cease own packet transmissions at NAK broadcast. The performances of the proposed CLSP/CC are obtained and compared with those of conventional CLSP by simulations. The performances include network throughput, average processing time and the number of mobile stations in backlogged state. As simulation results, the proposed CLSP/CC outperforms conventional CLSP, especially in highly offered load environment. Also as the proposed CLSP/CC obtain maximum throughputs at higher offered loads and the larger number of mobile stations in backlogged state than the conventional CLSP, it is found that our protocol can support more mobile stations and higher occurrence probabilities.

We propose a method of enhancing the performance of a cross-talk canceller for a four-speaker system with respect to sweet spot size and ringing effect. For the large sweet spot of a cross-talk canceller, the speaker layout needs to be symmetrical to the listener's position. In addition, a ringing effect of the cross-talk canceller is reduced when many speakers are located close to each other. Based on these properties, the proposed method first selects the two speakers in a four-speaker system that are most symmetrical to the target listener's position and then adds the remaining speakers between these two to the final selection. By operating only these selected speakers, the proposed method enlarges the sweet spot size and reduces the ringing effect. We conducted objective and subjective evaluations and verified that the proposed method improves the performance of the cross-talk canceller compared to the conventional method.

In a multi-bandwidth CDMA system, the performance of a multiple order selection combining rake receiver is analyzed according to the spreading bandwidth of the system and the delay spread of a Rayleigh fading channel. The results for various channel environments indicate a tradeoff between total received signal energy and multipath fading immunity. Increasing the occupied bandwidth of the system (wide-bandwidth spreading) gives better performance for small delay spread environments, while gathering more energy (narrow-bandwidth spreading) gives better performance for large delay spread environments. It is also shown that the performance difference between low and high order selection combining grows larger as the spreading bandwidth is increased. It is noted that performance degrades by increasing the bandwidth above a certain point and the optimum spreading bandwidth for each channel environment decreases as the delay spread of the channel increases.

In this paper, we suggest the interference cancellation (IC) technique suitable for turbo coded code division multiple access (CDMA) systems, that merges IC processes into turbo decoding processes to improve system performance and reduce system complexity. To ensure the reliability of the temporary decision bits for cancellation, we use cyclic redundancy code (CRC) check as a measure. Prior to design turbo coded CDMA system, we first derive the optimized polynomials of low-rate turbo codes appropriate to CDMA systems. According to the simulation results with setting the processing gain (PG) to 120, the turbo coded CDMA system with the proposed IC technique can accommodate 60 users over additive white Gaussian noise (AWGN) channel when signal to noise ratio (SNR) is about 2. 5 dB and required frame error ratio (FER) is 10-2. Compared this result with the performance of single user's system, it requires only additional 1 dB SNR.