Hybrid Ovsf Code Assignment Scheme In Wcdma Vs Cdma

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

P. Kavipriya *


  • Electronics and Communication Engineering, Sathyabama University, Chennai - 600 119, India

DOI: 10.17485/ijst/2015/v8i12/58562


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.


ABC, Dynamic Code Assignment, GSO, MAGA Performance Parameters, PSO, Resource Allocation, WCDMA

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DOI: http://dx.doi.org/10.17485/ijst%2F2015%2Fv8i12%2F58562


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In next generation mobile communications systems, a flexible support of multi-rate/multi-user broadband services is demanded [1, 2]. This can be achieved by multi-code code division multiple access (multi-code CDMA). The well-known CDMA techniques include single-carrier CDMA (SC-CDMA) by time-domain spreading [3, 4] and multi-carrier CDMA (MC-CDMA) by frequency-domain spreading [2, 4]. In both kinds of CDMA, the usage of frequency-domain equalization (FDE) based on the minimum mean square error (MMSE) criterion provides good bit error rate (BER) performance in a severe frequency-selective fading channel [5].

For the uplink (mobile to base station (BS)) transmission, because different users' signals are asynchronously received via different fading channels, multiple-access interference (MAI) occurs, which limits the uplink capacity. The two-dimensional (2D) block spread CDMA can be applied to solve the MAI and achieve frequency diversity gain in a frequency-selective fading channel [6]. In the 2D spreading, chip-level spreading and block-level spreading are implemented as different roles. The chip-level spreading plays the same part as traditional SC-(or MC-) CDMA, which can achieve the frequency diversity gain by using MMSE-FDE in the receiver. In 2D block spread CDMA, block-level spreading is performed to each block after chip-level spreading. Before the transmission, the guard interval (GI), which is larger than or equals to the maximum delay among different users, is inserted. At the receiver, after removing the GI, block-level de-spreading is performed in order to remove the MAI. If the maximum timing offset among uses is within the GI length, perfect removal of MAI is possible in the case of block fading (i.e., the channel stays constant during at least one block). Both chip-level spreading codes and block-level spreading codes can be constructed using the orthogonal variable spreading factor (OVSF) code tree [7].

The OVSF code tree has a unique property. The descendant and ancestor codes of the same root code cannot be used simultaneously because any two codes from the same mother code are not orthogonal to each other. Therefore, the OVSF code tree has a limited number of available codes (limited code capacity). Many code assignment algorithms were proposed for CDMA [8-19]. Recently, we proposed a new adaptive code assignment algorithm, which could minimize the code blocking probability due to the code capacity limitation [20]. However, even if the whole spreading codes constructed by OVSF code tree are successfully assigned to users, the received signal-to-interference plus noise power ratio may sometimes drop as a result of fading.

Therefore, the blocking probability is not only caused by the code capacity limitation but also due to the poor quality (i.e., the BER becomes higher than the required BER). Although the code assignment algorithms for a traditional CDMA system have been well studied, few papers investigated the achievable BER performance when code assignment algorithm is used. The code blocking probability is affected by BER performance [14]. However, because the system level evaluation is very complex, most of the papers only considered the code blocking probability caused by code capacity limitation assuming error free transmission. In Ref. [21], the BER performance is taken into account when evaluating the proposed code assignment algorithm, in 2D block spread CDMA system. However, in Ref.  [21], only a single cell was considered, and furthermore, a fairly large code space was assumed (i.e., no code limitation exists). To overcome the code limitation problem in a cellular system (i.e., multi-cell environment), a code reuse algorithm was first proposed in paper [22] for 2D block spread cellular CDMA uplink, where the same block-level spreading code was able to be reused in different cells. Reference  [22] does not provide theoretical analysis of the code reuse scheme in 2D block spread CDMA because of page limitation, and only discusses the block-level spreading factor reuse algorithm with the assumption that the same data rate for all users. This paper provides a theoretical analysis of the code assignment problems for 2D block spread CDMA in a multi-cell environment and considers different data rates. The main contributions in this paper are as follows:

  1. The code reuse scheme is proposed for 2D block spread CDMA in multi-cell environment. The code reuse region is theoretically discussed.
  2. The code assignment algorithm is presented taking into account both the chip-level spreading and block-level spreading. Impact of chip-level and block-level spreading factor on the BER performance is discussed.

The remainder of the paper is organized as follows. Section 2 briefly discusses the 2D code assignment and blocking in 2D block spread CDMA. The uplink transmission model is presented in Section 3. Then, the code reuse algorithm is proposed in Section 4. In Section 5, simulation results on code reuse efficiency and blocking probability are discussed. Section 6 offers some concluding remarks.


2.1 Two-dimensional code assignment

Figure 1 illustrates the OVSF code tree [7]; Cp,k denotes an OVSF code of the kth ( k = 0,1, … ,2p − 1 − 1) code in the pth layer. The root code is denoted as C1,0 = (1) and the second layer has two codes, C2,0 = (1,1) and C2,1 = (1, − 1). The codes in the pth layer are generated as from each code C of the ( p − 1)th layer; here, is the bit-wise complement of C. The number of available codes in the pth layer is 2p − 1, which is the same as the spreading factor of the layer; thus, the number of orthogonal codes increases with the increasing layer. All codes in the same layer are orthogonal to each other, whereas codes in different layers are orthogonal only if they do not have the same mother code. Thereby, to avoid MAI in uplink 2D block spread CDMA, orthogonal codes, that is, the codes in the same layer, are necessary for block-level spreading.

In this paper, two code trees are used for both chip-level spreading and blocking spreading; suppose that SFfmax and SFtmax are the maximum number of chip-level spreading factor and block-level spreading factor, respectively, and set SFmax = SFfmax × SFtmax as a constant determined by the system design, an example to show the relationship between SFmax, SFfmax, and SFtmax is shown in Figure 2. This paper presents the code assignment in multi-rate transmission.

One advantage of CDMA is to provide flexible multi-rates transmission by selecting different spreading factors. For example, assume that the lowest data rate is R and the chip time is Tc. So, if a code from the fourth layer (as shown in Figure 1) is chosen, it needs 8Tc to transmit one data; moreover, if a code from the third layer is selected, it takes 4Tc that can support the data rate 2R. It can be derived that the spreading factor is inverse of the data rate; hence, the highest layer supports the lowest data rate, and the lowest layer provides the highest data rate (as shown in Figure 1). In this paper, we assume that the lowest data rate R needs SFmax ⋅ Tc chip time, for mathematical convenience, taking SFmax instead of SFmax ⋅ Tc. Here, SFmax is the maximum number of spreading factor. If the data rate Ru of the uth user is Cu times the lowest rate, the spreading factor of the uth user is given by


To describe the presented algorithm clearly, Table 1 lists the variables used in this paper.

RLowest data rate
Ru(Ru = Cu ⋅ R)Data rate of uth user, which is Cu times the lowest data rate R.
SFmaxMaximum spreading factor
SFfmaxMaximum chip-level spreading factor
SFtmaxMaximum block-level spreading factor
SFu(SFu = SFu,f × SFu,t)Total spreading factor of the uth user
SFu,f(1 ≤ SFu,f ≤ SFfmax)Chip-level spreading of the uth user
SFu,t(1 ≤ SFu,t ≤ SFtmax)Block-level spreading of the uth user
Chip-level spreading code sequence of the uth user in the bth cell
Block-level spreading code sequence of the uth user in the bth cell
PblockBlocking probability
Ublock_by_CodeLimitedNo. of blocked users due to code capacity limitation
Ublock_by_FaildedInQoSNo. of blocked users due to poor quality of service
Utotal_comeNo. of arrival users in a measurement time interval

2.2 Blocking probability

Sometimes users cannot be served (or blocked) because of the capacity limitation of the OVSF code tree. Even if a user is successfully assigned a code, its transmission quality may drop below the required quality of service (QoS). In this paper, the transmission quality is represented by the BER. If the BER of user u becomes higher than the required BERreq, user u is declared to be blocked.

The blocking probability Pblock is defined as


where Ublock_by_CodeLimited is the total number of users who could not be served because of the code capacity limitation, Ublock_by_FaildedInQoS denotes as the total number of users whose link quality worse than the required QoS, and Utotal_come denotes the total number of arrival users over a measurement time interval.


The uplink transmitter/receiver structure of 2D block spread direct sequence CDMA is shown in Figure 3. In the receiver, the block-level de-spreading is first performed to remove the MAI, and the fast Fourier transform (FFT) algorithm transforms the received signal block into frequency-domain signal so that FDE is applied to achieve the frequency diversity. A detailed description of 2D block spread CDMA can be found in Ref. [6].

In the following discussion, the spreading factors of chip-level and block-level spreading for the uth user are denoted by SFu,f and SFu,t, respectively, whereas the total spreading factor of the uth user is SFu( = SFu,f × SFu,t). In this paper, ⌈a⌉ represents the largest integer smaller than or equal to a; ⌊a⌋is the smallest integer larger than a.

3.1 Transmission signal

The data symbol sequence to be transmitted from the uth user in the bth cell is denoted by , where Nc is the block size of FFT at a receiver. The data symbol sequence to be transmitted is spread by chip-level spreading code sequence with and is further multiplied by a binary scramble sequence to make the resultant signal white-noise like. The resultant SC-CDMA chip sequence is expressed as


In the block-level spreading, each Nc-chip block is repeated SFu,t times, and each block is multiplied by a chip taken from an orthogonal block-level spreading code sequence , which is used by the uth user in the bth cell. The equivalent lowpass representation can be expressed as


for t = 0 ∼ SFu,t × Nc − 1, where is the transmit power of the uth user belonging to the bth cell. After inserting an Ng-chip GI in every Nc-chip block, a sequence of Nc-chip blocks is transmitted over a frequency-selective fading channel.

3.2 Received signal

The GI inserted signal is transmitted over a frequency- and time-selective fading channel. Assuming a channel with chip-spaced L independent paths, its impulse response of the channel between the uth user in the bth cell, and its corresponding BS is expressed as


where hb_u,l and τb_u,l are respectively the complex-valued path gain and time delay of the lth path. In this paper, it is assumed that hb_u,l stays unchanged during block interval , but it changes block by block, τb_u,l is equal to with Tc being the chip length, and is the transmit timing offset. The maximum time delay is assumed to be shorter than the GI.

The sum of users' faded signals is received at the bth cell BS, and the GI-removed received signal can be expressed as


for t = 0 ∼ Nc ⋅ SFt,max − 1, where n(t) is the zero-mean complex-valued noise samples due to the additive white Gaussian noise (AWGN) with variance 2N0 ∕ Tc ( N0 is the AWGN one-sided power spectrum density), B is the number of cells, and Ub is the number of arriving users in a time interval of SFt,max consecutive blocks in the bth cell.

To recover the transmission SC-CDMA signal of the uth user, block-level de-spreading is carried out first as for t = 0 ∼ Nc − 1. Because a block fading (i.e., path gains stay almost constant over a time interval of SFt,max consecutive blocks) is assumed, it can be derived that


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