Vipin Balyan

Top Down Code Search to Locate An Optimum Code and Reduction in Code Blocking for CDMA Networks

In this paper, a top down code search scheme is proposed that identify an optimum OVSF code for assignment at the base station of CDMA wireless networks. An optimum vacant code is the one whose usage produces least code blocking compared to other eligible codes. This scheme provides least code blocking compared to existing schemes without reassignments. In addition, the codes searched during locating the optimum code are significantly less than other existing schemes. The call establishment delay which is a significant factor for real time applications is directly proportional to the number of searches and should be low. The design is explained for single code, and extended to multi code assignment to improve code blocking. The multi code assignment is done using four ways. The first and second multi code schemes uses minimum and maximum rakes for a fixed rate system. The third scheme called scattered multi code scheme divide the incoming call into rate fractions equal to number of rakes available in the system, and each rate fraction is handled in a similar way in which the new call is handled in single code scheme. The rate fractions may be scattered or grouped in the code tree. The fourth multi code scheme, namely grouped multi code scheme allocates codes to all the fractions as close as possible. This maximizes future higher rate vacant codes availability by leaving a complete sub tree vacant when call using multi code ends.

Same rate and pattern recognition search of orthogonal variable spreading factor code tree for wideband code division multiple access networks

Orthogonal variable spreading factor-based code division multiple access systems allocate vacant codes when new call(s) arrive. This study proposes a same rate assignment scheme which allocates a new call to most favourable code in the region crowded by same rate calls. This does not add to blocked codes because of previous calls but utilise the unused scattered capacity created by previous calls. A vacant code whose parent code is handling maximum number of ongoing calls of same rate is the most favourable code for new call assignment under that sub tree. This reduces higher rate blocking because of scattering of lower rate calls due to the fact that same rate calls will be assigned in the same portion of code tree. If a tie occurs for two parent codes it is resolved using pattern recognition of codes. In the proposed scheme, the most favourable search starts from higher layers that reduce number of code searches before assignment significantly. The proposed single code and multi code schemes are compared with other schemes based on code blocking probability, throughput and number of code searches required before assignment. Simulation results indicate that the proposed scheme outperforms other schemes for various traffic distributions.

Neighbour code capacity and reduction in number of code searches

Orthogonal Variable Spreading Factor (OVSF) based networks require careful assignment of code to a new call. In this paper an assignment design is proposed which reduces number of code searches with less code blocking probability. The proposed design partitioned code tree into two parts and for a new call the algorithm needs to check one portion of the code tree for most of the calls. If no vacant code is available in that portion then second portion is checked. Also, it starts code search from bottom to top along the tree both these factors leads to less number of code searches before assignment. Further, code blocking is reduced as it jumps to that portion of code tree where a vacant code is available, blocking almost those codes blocked by previous calls. Simulation results shows improvement in terms of code blocking probability and number of code searches.

Quantized-Non Quantized Code Tree Partitioning and Reduction in Internal-External Fragmentation

The use of orthogonal variable spreading factor codes as channelization codes in downlink give rise to code blocking probability due to orthogonal nature of codes. This will further lead to increase in call establishment delay to locate a suitable code which can be assigned to new call request. Code blocking is mainly due to fragmentation of lower rate calls in code tree, which blocks capacity of higher rate codes and consequently blocking of higher rate call requests. In this paper, an assignment approach is proposed which addresses fragmentation due to scattered codes i.e. external fragmentation and internal fragmentation associated with non-quantized rates too. The code tree is virtually divided in two portions, one for quantized and another for non quantized rates. This will lead to reduction in number of code searches before assignment of call to a code. In addition, data calls are queued in buffer to handle voice calls. The data calls share the capacity of a assigned code, to reduce code blocking of real time calls. The data calls are handled using a code of higher capacity for most of the time which reduces their call duration. The proposed scheme is compared with existing novel schemes to show its ascendancy over them.

Fast OVSF Code Assignment Scheme for WCDMA Wireless Networks

In 3G and beyond wireless networks orthogonal variable spreading factor (OVSF) codes are used to handle multimedia rates. These codes suffer from code blocking limitation which reduces the throughput and spectral efficiency of the system. Also, real time calls suffer from call processing delay. We propose a single OVSF code assignment design which utilizes offline optimum code selection algorithm so that the optimum code is available at the arrival of new call. The design finds the optimum code using a variable code index for all the parents of the available vacant codes. If a tie occurs for two or more parents for code index, the parent with the higher code index is chosen. The procedure repeats till a unique solution is reached. If still tie is not resolved the average elapse time of busy children of these parents can be used to choose single optimum code option.