Abstract-Orthogonal variable spreading factor (OVSF) codes for wide-band CDMA (WCDMA) systems have been proposed in the third generation (3G) mobile communications' standard to support services of variable rates. To assign OVSF codes, there are single-code and multi-code approaches. In this paper, the multi-code placement and replacement issue is addressed. We propose a tree partition policy for managing the OVSF code tree to reduce code fragments and the number of code reassignments. Besides, we adjust the multi-code rate according to a unit-based method, which is capable of decreasing the number of code fragments because fewer low-rate codes are utilized. Through extensive simulations, it turns out that the tree partition policy and the unit-based multi-code method jointly perform better than the left-most and crowded-first schemes.
It is well known that a cognitive radio network (CRN) may further enhance the sum spectral efficiency using temporarily idle spectrum. However, we usually ignore the spatial domain behaviors to fully utilize the radio resource to approach available system capacity. When a signal is transmitted, we usually treat its occupancy of spectrum along all propagation directions. However, up to certain range (distance), the signal strength is weak and possibly allow another co-existing transmission happening with a satisfactory signal to interferenceand-noise ratio (SINR) at both transmit-receive (T-R) pairs. In this paper, we realize this concept for a CRN consisting of a primary (PR) T-R pair and a cognitive radio (CR) T-R pair; furthermore, we consider to what condition concurrent transmission can happen. We use the Lagrange multiplier method to determine the optimal power allocation scheme for such a CRN and then determine to what extent can the sum spectral efficiency be enhanced. It is found that the CR transmitter (CR-TX) may either transmit at its maximum allowable power or just turn off with no transmission power. Concurrent transmission of the PR T-R and CR T-R pairs with distance as a new dimension of multiplexing can be considered as a new version of CRN.
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