Orthogonal Variable Spreading Factor Research Articles

An Efficient Multicode Design for Real Time QoS Support in OVSF Based CDMA Networks

Orthogonal variable spreading factor (OVSF) codes are used for CDMA based networks to handle multimedia rates. While waiting time (or call establishment delay) is a significant QoS parameter for real time calls, its impact on non-real time calls is negligible. Traditionally, the call handling in OVSF based CDMA system has been accomplished with multi code approach, where the arriving call is divided into many quantized fractions and each fraction is handled by different code. These designs are very efficient for non-real time calls, but the code search corresponding to each fraction needs significant time that makes existing multi code approaches unsuitable for real time calls. This paper describes a novel multi code design, in which the waiting time for real time call is zero. The design initially assigns the incoming call to the vacant code with the capacity equal to call capacity, and subsequently looks for the vacant codes for each quantized fraction after code assignment. Once the vacant codes with capacities equal to the capacity of all rate fractions are available, the call is shifted to these codes. The information of this code assignment for every fraction needs to be sent to the receiver like existing schemes. Therefore, there is no additional hardware requirement for the proposed design. Simulation results show significant dominance of our scheme as compared to the existing multi code schemes.

Text Hiding in Images Based on QIM and OVSF

This paper presents an algorithm of digital image watermarking that uses a text as mark. Our proposal generates an optimal level of robustness according to the parameters considered for watermarking. The proposed algorithm consists in the insertion over an image of a text string, previously modified by permutation using a random key and an OVSF (Orthogonal Variable Spreading Factor) generator. The insertion is made in the wavelet domain and uses Quantization Index Modulation (QIM). Also, the conditions to reverse the process and restore the original information are set. Finally, the robustness of the proposed algorithm is evaluated implementing several attacks on the marked image.

OVSF Dynamic Code Allocation Quality based Glowworm Swarm Optimization Approach in WCDM

OVSF codes are used as channelization codes to support applications with different bandwidth requirements for the downlink of 3G mobile communication systems. The code assignment and reassignment problem focuses on the subject of minimizing the number of code relocations. OVSF (Orthogonal Variable Spreading Factor) code assignment method considering traffic characteristics in the WCDMA systems. The previously proposed Dynamic Code Assignment (DCA) scheme allows code reassignments to improve code utilization, but induces some service delay time to on-going calls, unlikely assumptions, including fixed service data rates and code-limited system capacities. Thus, OVSF code tree has insufficient number of available codes. Method/Statistical Analysis: In order to solve these problems and efficiently utilized OVSF codes, in this paper presents novel Glow warm Swarm Intelligence (GSO) based approach for active OVSF code assignment in WCDMA based networks. GSO algorithm is completely dependent on the behavioral pattern of glowworm (fireflies). The optimization algorithm requires an understanding of this behavioral pattern. The intensity of the emission of Lucifer in can be changed by glow worms which glow at different intensities. In order to improve the ability of the GSO it employs various phases until code allocation is completed. Results: By permitting only codes in the data branch to be reassigned the quality of service guarantee for real time traffic is possible. The simulation results shows that proposed GSO schemes not only guarantees the QoS but also gives high code utilization which is comparable with other existing techniques. Conclusion/ Application: Performances of these methods are evaluated in terms of blocking probability, spectral efficiency, delay and throughput. In addition to this different GSO operator are tested under varying traffic loads to increase the overall system performance.

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.

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.

Fixed capacity and variable member grouping assignment of orthogonal variable spreading factor code tree for code division multiple access networks

Orthogonal variable spreading factor codes are used in the downlink to maintain the orthogonality between different channels and are used to handle new calls arriving in the system. A period of operation leads to fragmentation of vacant codes. This leads to code blocking problem. The assignment scheme proposed in this paper is not affected by fragmentation, as the fragmentation is generated by the scheme itself. In this scheme, the code tree is divided into groups whose capacity is fixed and numbers of members (codes) are variable. A group with maximum number of busy members is used for assignment, this leads to fragmentation of busy groups around code tree and compactness within group. The proposed scheme is well evaluated and compared with other schemes using parameters like code blocking probability and call establishment delay. Through simulations it has been demonstrated that the proposed scheme not only adequately reduces code blocking probability, but also requires significantly less time before assignment to locate a vacant code for assignment, which makes it suitable for the real-time calls.

An Efficient Dynamic Orthogonal Variable Spreading Factor Code Allocation Approach in WCDMA through Modified Adaptive Genetic Algorithm

Orthogonal Variable Spreading Factor (OVSF) codes would give variable data rate transmissions for different bandwidth supplies in Wideband CDMA (WCDMA) networks. These OVSF codes are used for the channelization of codes in WCDMA. In WCDMA, effective utilization of OVSF codes has become an active area of research as the number of codes is very limited. It is a fact that the successor and predecessor codes of OVSF cannot be used simultaneously when a specific code is used in OVSF as their encoded sequences become indistinguishable. Consequently, OVSF code tree has inadequate number of available codes. Thus, this research study uses Adaptive Genetic Algorithm (AGA) based approach for dynamic OVSF code assignment in WCDMA networks. Different from existing Conventional Code Assignment (CCA) and dynamic code assignment schemes, population is adaptively constructed according to existing traffic density in the OVSF code-tree. In existing technique in order to improve the ability of the GA, ＂dominance and diploidy＂ structure is employed to adapt to changing traffic conditions. Because in SGA algorithm cannot convergence if the new user is included into the existing OVSF code tree while SGA is running to find optimum OVSF code tree, SGA cannot adapt its structure to this unexpected variation. This problem can be overcome by the Modified Adaptive Genetic Algorithm (MAGA). Performance of the proposed MAGA approach is evaluated in terms of blocking probability and spectral efficiency and is compared with SGA, D&D GA.

Modelling and Simulation of Interference Cancellation Receiver for MIMO Multicarrier CDMA based Cognitive Radio

In this paper, modeling and simulation of a receiver with interference cancellation technique for the downlink transmission of multiple input multiple output (MIMO) multicarrier code-division multiple access (MC-CDMA) based cognitive radio is investigated. Synchronous transmission of multiple users’ signals over frequency selective Rayleigh channel is considered. Linear MMSE frequency domain equalization is utilized to overcome channel effect. Orthogonal Variable Spreading Factor (OVSF) codes are used to support users of different data bit rates. An adaptive subcarrier deactivation method is proposed for deactivating a set of subcarriers used for downlink transmission by the cognitive radio base station that overlap with primary user’s transmission in order to avoid causing interference to the primary user. Orthogonality between the spreading codes is lost due to subcarrier deactivation which causes interference among secondary cognitive radio users. In this paper a modified Group-wise Successive Interference Cancellation (m-GSIC) technique is proposed to suppress the interference caused due to subcarrier deactivation. The simulation results show that the proposed interference cancellation technique results in better BER performance over conventional interference cancellation techniques

Two‐dimensional spreading scheme employing 2D orthogonal variable spreading factor codes for orthogonal frequency and code division multiplexing systems

Future 4G systems require transmission of richer multimedia services which inevitably implies an increase in data rate. Orthogonal frequency and code division multiplexing (OFCDM) technique has shown promising results in achieving a high data rate while simultaneously combating multipath fading. OFCDM is an amalgamation of orthogonal frequency division multiplexing and two-dimensional (2D) spreading. 2D spreading helps to achieve diversity gains in both time and frequency domains. The present OFCDM systems employ 1D orthogonal variable spreading factor (OVSF) codes to achieve the required 2D spreading in code multiplexed channels. However, 2D OVSF codes have better correlation properties in comparison to 1D OVSF codes. Motivated by this principle, the authors propose a spreading scheme for OFCDM systems using 2D OVSF codes. The spreading scheme is designed to increase the system throughput and reduce multi-code interference. Here, the authors study the OFCDM system performance using the proposed spreading scheme in a multipath fast fading channel with varying spreading factors in both time and frequency domains. The results are compared with the existing OFCDM systems using 1D OVSF codes.

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

3G and beyond wideband code division multiple access networks use orthogonal variable spreading factor (OVSF) codes to handle multimedia traffic. OVSF codes suffer from the limitation of code blocking, which leads to new call blocking. Scattered vacant codes in the OVSF code tree are the main cause of code blocking. This study proposes compact single code and multicode assignment schemes to reduce code blocking. The vacant codes used for incoming calls are the ones surrounded by a minimum number of consecutive vacant codes. Furthermore, finding consecutive vacant codes at the leaves of the tree is sufficient to find the consecutive vacant codes for all other layers. Handling non-quantised rates with a single code assignment produces wastage of code capacity, which is avoided with the use of a multicode assignment. Multicode usage facility along with the use of vacant codes from the minimum consecutive vacant code groups results in minimum code blocking. Two categories of the multicode assignment schemes are considered: the first one uses the least number of codes and is suitable for rake limited OVSF system, and the second scheme uses maximum number of codes to reduce code blocking significantly.

Combined code reuse scheme with two‐dimensional OVSF codes assignment algorithm for uplink multi‐user/multi‐rate block spread multi‐cellular CDMA

ABSTRACTThe two‐dimensional (2D) block spread code division multiple access (CDMA) can avoid the uplink multiple‐access interference with low‐complexity single‐user detection in a slow fading channel and, therefore, is very attractive. In the 2D spreading, orthogonal variable spreading factor (OVSF) is used for spreading; an important problem is how to efficiently assign the limited resource of OVSF codes to users with different data rates, while meeting the requirement of quality of service in a multi‐cell environment. In this paper, it is shown that the code reuse can improve the code reuse efficiency and the proposed code reuse scheme combined with code assignment algorithm can allow flexible multi‐rate uplink transmission. The computer simulation confirms that the proposed code assignment algorithm improves the code reuse efficiency while achieving lower blocking probability than traditional CDMA. Copyright © 2012 John Wiley & Sons, Ltd.

Fair Single Code and Multi Code Designs for 3G and Beyond CDMA Systems

Multimedia rates are handled in 3G and beyond CDMA networks using orthogonal variable spreading factor (OVSF) codes. Multimedia rates can have non uniform distribution of real time and non real time users. The paper describes fair single code and multi code designs to handle these rates. The single code design is for quantized rates and divides the OVSF code tree capacity according to the arrival distribution. The change in the distribution is dynamically reflected in the division of the code tree capacity for different rates. The multi code design is preferred for the system dominated by non quantized rates. Simulation results are presented to show the superiority of the proposed designs with its existing counterparts.

(1+ε)-competitive algorithm for online OVSF code assignment with resource augmentation

This paper studies the online Orthogonal Variable Spreading Factor (OVSF) code assignment problem with resource augmentation introduced by Erlebach et al. (in STACS 2004. LNCS, vol. 2996, pp. 270---281, 2004). We propose a (1+1/?)-competitive algorithm with help of (1+???)lg? h trees for the height h of the OVSF code tree and any ??1. In other words, it is a (1+?)-competitive algorithm with help of (1+?1/??)lg? h trees for any constant 0<?≤1. In the case of ?=1 (or ?=1), we obtain a 2-competitive algorithm with 2lg? h trees, which substantially improves the previous resource of 3h/8+2 trees shown by Chan et al. (COCOON 2009. LNCS, vol. 5609, pp. 358---367, 2009). In another aspect, if it is not necessary to bound the incurred cost for individual requests to a constant, an amortized (4/3+?)-competitive algorithm with (11/4+4/(3?)) trees for any 0<?≤4/3 is also designed in Chan et al. (COCOON 2009. LNCS, vol. 5609, pp. 358---367, 2009). The algorithm in this paper gives us a new trade-off between the competitive ratio and the resource augmentation when ??3 (or ?≤1/3), although the incurred cost for individual requests is bounded to a constant.

Code scattering and reduction in OVSF code blocking for 3G and beyond mobile communication systems

Scattered vacant codes in Orthogonal Variable Spreading Factor (OVSF) code based 3G and beyond WCDMA wireless networks leads to code blocking which further gives call blocking. The paper proposes four single code assignment schemes to reduce code scattering. The use of assignment scheme depends upon the type of input calls. The code reservation assignment (CRA) is used to efficiently handle one or higher rate class calls. It assigns priority number to the children of priority class codes such that the future availability of vacant codes in the priority layer is the highest. The remaining three code assignment schemes favors low to medium calls. The code assignment using group leaders uses busy codes (capacity) under predefined leaders to handle future calls. The code blocking in group leader approach can be reduced further if the used capacity of all the parents of the eligible vacant codes is examined and the code whose parent has maximum used capacity is used for new call. In adjacent vacant codes grouping scheme, the eligible vacant codes are listed to find code with least adjacent vacant codes. If unique result does not exists, the code (among the codes producing same adjacent vacant codes) with the least elapsed time of the busy neighbors is used for incoming call.

The Construction of Orthogonal Variable Spreading Factor Codes from Semi-Bent Functions

In a recent paper spreading codes for CDMA systems were constructed based on Hadamard matrices and semi-bent functions. These codes were applicable to synchronous CDMA systems and to quasi-synchronous CDMA systems when a loosely-synchronized code is used. The codes consist of the union of rows of four Hadamard matrices. The codes can be assigned to a regular tessellation of hexagonal cells in such a way that the correlation between codewords assigned to adjacent cells is zero, while the correlation between codewords assigned to non-adjacent cells is small. The use of codewords from four Hadamard matrices allows greater codeword re-use distances than the use of codewords from a single Hadamard matrix. This has advantages for both satellite and terrestrial systems. These codes only support a single data rate. The current paper shows how orthogonal variable spreading factor codes can be created from them by a careful selection of the semi-bent functions used.

2차원 확산을 사용하는 다중전송률 MC-DS/CDMA 시스템

직교 주파수분할 다중(OFDM)시스템과 직접확산 코드분할다중접속(DS/CDMA)시스템이 결합된 다중반송파(MC) DS/CDMA 시스템은 대역폭 효율, 고 전송률과 페이딩에 강한 특성으로 많은 주목을 받고 있다. 본 논문에서는 시간영역과 주파수영역에서 확산을 수행하는 다중전송률 MC-DS/CDMA 시스템에서 복잡성이 감소된 다중사용자 검출기를 살펴본다. 여기서 시간영역과 주파수영역의 확산부호로는 2차원 직교 가변확산부호(OVSF)로부터 만들어진 1차원 OVSF 코드를 사용한다. 제안된 시스템에서는 다중사용자간섭(MUI)을 감소시키기 위해 코드그룹 간섭제거(CGIC) 수신기를 사용하는데, 이 CGIC 수신기는 1차원 OVSF 코드들의 상관특성을 이용하여 코드 그룹이라는 개념을 사용하여 다른 사용자들의 정보 없이도 사용자 선호를 검출할 수 있는 수신기이다. CGIC를 갖는 다중사용자 검출기가 시간 및 주파수영역에 따로 또는 같이 적용될 것이다. AWGN 채널 환경 하에서 시스템의 성능을 분석하며, 단일(다중) 전송률, 다중사용자를 갖는 시간/주파수 영역에서 확산된 MC-DS/CDMA과 비교할 것이다. 이러한 결과로서 기존의 정합검출기에 비해 제안된 시스템의 성능이 향상됨을 알 수 있다. Multicarrier-Direct Sequence/Code Division Multiple Access(MC-DS/ CDMA) which is a combination of Orthogonal Frequency Division Multiplexing(OFDM) and DS/CDMA has been of significant interest as a means to take such advantages as bandwidth efficiency, high bit rate and robustness against multipath fading. In this paper we study a reduced-complexity multiuser detection aided multirate MC-DS/CDMA with time(T)-domain and frequency(F)-domain spreading. The one- dimensional orthogonal variable spreading factor(1D OVSF) code extracted from 2D OVSF code are used as a spreading code in T/F-domain. The proposed system will use code grouping interference cancellation(CGIC) receiver to reduce Multiuser Interference(MUI). The CGIC receiver uses code grouping by the correlation properties of 1D OVSF code and dose not requires the code information and activity of other user. The multiuser detector with CGIC receiver will be analyzed in Time- and Frequency-domain separately(jointly). The system performance is analytically derived in Additive White Gaussian Noise(AWGN) channel and we also compare the system performance between proposed system and T/F spreaded single(multi) rate multiuser MC-DS/CDMA system. In the computer simulation results, the proposed receiver of demonstrated huge performance improvement over conventional matched filter receiver.

Performance Improvement in OVSF Based CDMA Networks Using Flexible Assignment of Data Calls

Orthogonal variable spreading factor (OVSF) codes are used to support multimedia calls in CDMA wireless networks. The inefficient use of OVSF code tree reduces the system throughput. The paper discusses a novel code assignment design to provide flexibility of rate variation in data calls maximizing channel utilization. The ongoing call uses different codes at different times depending on the code tree status and instantaneous traffic load. For low traffic loads, the high rate codes can be utilized. For medium to high instantaneous load conditions, the vacant codes with capacity less than the call rate can be utilized. For same call reassignments are done to assign different codes at different times. The utilization of the code tree can reach close to 100%. Simulation results are given to verify the superiority of the proposed design.

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

SUMMARY 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. Copyright © 2011 John Wiley & Sons, Ltd.

Vacant codes grouping and fast OVSF code assignment scheme for WCDMA networks

Channelization codes used in WCDMA are Orthogonal Variable Spreading Factor (OVSF) codes. These codes suffer from code blocking limitation. Many designs are proposed to avoid this limitation but most of them do not consider number of codes searched, which affects call establishment delay prior to handling a call. We propose a fast OVSF code assignment design which aims to reduce number of codes searched with optimal/suboptimal code blocking. The code assignment scheme aims to use those vacant codes whose parents are already blocked. This leads to occurrence of more vacant codes in groups, which ultimately leads to less code blocking for higher rate calls. The number of codes searched increases linearly in our design compare to most of other novel proposed single code methods like crowded first assignment, where it increases exponentially with increase in user rates. Also the calculation of vacant codes at one layer will be sufficient to identify the vacant code adjacency for all the layers which reduces complexity. Simulation results are presented to verify the superiority of the design.

Novel Scheme of Multi-Rate Quasi-synchronous Coherent Time-spreading OCDMA System

Abstract We propose a novel scheme of multi-rate quasi-synchronous coherent time-spreading OCDMA system, employing orthogonal variable spreading factor (OVSF) codes with a zero-correlation zone. Within permissible time delay, the aperiodic cross-correlation of OVSF with a zero-correlation zone equals zero. Therefore, multiple-access interference and beat noise can be removed in multirate quasi-synchronous coherent time-spreading OCDMA system, and the near-far problem can also be eliminated. Simulation of eight simultaneous users with 2.5 Gbit/s, 1.25 Gbit/s and 0.625 Gbit/s is demonstrated, where the fiber link consists of 20 km single-mode fiber, plus 2 km Dispersion Compensating Fiber. The results show that OVSF codes with a zero-correlation zone can be employed to multi-rate quasi-synchronous coherent time-spreading OCDMA system.