Reducing code wastage in orthogonal variable spreading factor‐based wideband code division multiple access networks

Abstract

Most third-generation and beyond wideband code division multiple access networks use the orthogonal variable spreading factor code tree for channelization codes. The codes in this code tree are limited and the performance of a wireless network depends upon the code assignment for new calls. In this paper, we introduce a term called 'wastage capacity', which gives us the amount of wastage caused when a code (single or multiple) with a data rate higher than the rate of the incoming call is assigned to it. We suggest two methods to keep wastage capacity below an arbitrary threshold value or zero. In the first method, we devised an algorithm in which wastage up to a certain threshold would be tolerated and the minimum rakes to get this wastage capacity were identified. In the second approach, we reduced the wastage capacity to zero irrespective of the number of rakes at the expense of higher cost and complexity.

Noise‐based spreading in code division multiple access systems for secure communications

“…For γ g 1 ¼ þ1, the sampled value of random variable z g 1 can be easily found from (6). Having in mind relations (6) and (7), and statistical independence between the noise in the channel and generated Gaussian chips, the mean value of this random variable may be expressed as…”

“…Assuming that the information bit of the g th user is equal to 1, the variance of random variable z g 1 , which is expressed by (6), is…”

“…In contrast to ordinary CDMA systems based on application of binary sequences [1,[6][7][8][9][10], sequences with a zero-correlation zone [11], and the systems based on applications of chaotic and Gold sequences [12][13][14][15][16][17][18], this paper presents a theoretical model of a noise-based phase shift keying (NPSK) CDMA system that uses random sequences as the carriers of users' information, which allow a higher level of security in signal transmission. Related to this aspect, the system proposed is substantially different from the systems described in literature.…”

“…APPENDIX A Assuming that the information bit of the g th user is equal to 1, the variance of random variable z g 1 , which is expressed by (6), is The fourth, fifth, and the sixth terms in expression (A2) are equal to zero due to the statistical independence of the chips of users' sequences and the samples of the noise in the channel.…”

Reduction in code blocking using scattered vacant codes for orthogonal variable spreading factor‐based wideband code division multiple access networks

“…This almost guarantees that the calls will be terminated at almost the same time and the large capacity portion isvacated. A novel multicode design to reduce code wastage is proposed in [18]. Code wastage is reduced significantly by increasing the number of rakes.…”