CHAPTER 2 SUBCHANNEL POWER CONTROL THROUGH WEIGHTING COEFFICIENT METHOD

Transcription

1 CHAPTER 2 SUBCHANNEL POWER CONTROL THROUGH WEIGHTING COEFFICIENT METHOD 2.1 INTRODUCTION MC-CDMA systems transmit data over several orthogonal subcarriers. The capacity of MC-CDMA cellular system is mainly limited [I081 by the mutual interference generated by the mobile stations and the fading radio channel. A reduction in interference will obviously increase the system capacity. Interference is caused predominantly by the effects of near-far problem, co-channel interference and fast fading channel [l09]. These effects can be effectively compensated through perfect power control so that all the subcarrier signals from the mobile stations arrive at the same power level [I and in turn serve to maximize the system capacity. Though the same amount of power is transmitted through each subcarrier, the received power level at the receiver may have different values. This may lead to significant performance degradation, in terms of BER and reduct~on in system capacity [ and In order to alleviate the variations of received power level on different subcarriers [I 13, I141 subchannel (group of subcarriers lying within the coherence bandwidth) power control was introduced. The subchannel power controlled MC-CDMA systems can transmit data with variable power according to the received SINR for each subchannel at the receiver. It is used in the reverse link to balance the received SINR level and to maximize the reverse link capacity.

2 2.2 CLASSIFICATION OF POWER CONTROL FOR MC-CDMA SYSTEM Power control is an interference management technique to balance the received powers of the users so that no user creates excessive interference to others. It plays an important role in MC-CDMA systems. There are many classifications available for power control techniques. One of the major classifications is power control for reverse link (from mobile to base station) and power control for forward link (from base station to mobile). Power control for reverse link is essential [22] to combat the near far effect. For the fonvard link, no power control is required in a single cell system since all signals are transmitted together and hence vary together. It is, however, necessary to reduce the inter-cell interference in multicellular environment. Power control techniques can also be classified, based on the type of signal measured to determine the power control command, as signal strength based, SINR based and BER based. Power control techniques can be grouped into open-loop and closed-loop power control algorithms. The open-loop power control is designed to overcome the near far problem, while the closed-loop power control aims at reducing the effect of fading. The closed loop power control may be an inner loop power control or a combination inner and outer loop power control. The inner loop adjusts the transmission power according to the power control command. The outer-loop power control is used in a closed-loop power control to adjust the threshold SINR. Based on the power update strategies, power control algorithms can be classified as fixed step power control, multilevel power control, and adaptive step size power control. The above classifications, intended for CDMA are also applied to MC-CDMA systems. In addition. the power control schemes for multicarrier systems can be classified as (i) All carrier power control (ACPC) scheme (ii) Each carrier power control (ECPC) scheme (iii) Subchannel power control (SCPC) scheme

3 2.2.1 All Carrier Power Control Scheme The base station estimates the average received SINR of all subcarriers, compares it with the reference SINR, and determines the power control command in ACPC scheme. The reference SlNR should be less than the upper bound to avoid the potential of positive feedback in power control operation. The same power level is allocated to all subcarriers [42, 431 in a mobile station, through only one power control command. This scheme is not very effective, as the fading characteristics of the subcarriers differ drastically Each Carrier Power Control Scheme The base station estimates the received SINR values for each subcarrier separately, and compares it with the reference SINR, in ECPC technique. The power control commands are computed independently [43] for each subcarrier. As spreading is done in the frequency domain in MC-CDMA, the desired user data cannot be segregated prior to de-spreading operation. Also, it requires a higher feedback bandwidth on forward link. Hence ECPC is not a suitable transmission power control (TPC) mechanism for MC-CDMA systems Subchannel Power Control Scheme The band of subcarriers lying within the coherence bandwidth (B,) undergoing a highly correlated fading process are grouped together and is termed as a subchannel. The subchannels can be grouped according to the following condition. where KMC Af 5 Bc (2.1) I Af = - T, - T, Af is the intercarrier frequency spacing T, is the symbol period T, is the guard interval and KMC is the spreading factor

4 A highly correlated fading process can be ensured on all KMC number of subcaniers. Therefore, a single power control command (PCC) is theoretically justified [I IS] for all these K M number ~ of adjacent sub-carriers in the subchannel. Hence feedback bandwidth requirement and signalling overhead are reduced considerably. In the present work the received SINR is assumed to be available through the reverse link in the subchannel power control scheme catering sufficiently to slowly varying fading effects. 2.3 SYSTEM MODEL A subchannel power control scheme for MC-CDMA systems is depicted in Figure 2.1. The MC-CDMA transmitter spreads [116, 1171 the original data stream over different subcarriers using Walsh Hadamard spreading. The number of subcarriers in each subchannel for each user is fixed by the processing gain. It is assumed that for a single cell MC-CDMA system, the chip rate and bit rate of the message signals are set, so that the processing gain is computed by the ratio of the chip rate and the bit rate. The subchannel power control weighting coefficient bank is added before IFFT block at the transmitter, in order to control the transmitted power of the subchannel. The weighting coefficients based on the received SINR are computed for each subchannel group and the transmission power is adjusted accordingly. It is assumed that the subcaniers have a narrow band and each subchannel is modelled as a frequency flat fading channel. The residual IS1 resulting from this approximation can be eliminated using a guard interval (GI) to transmit a symbol prefix. The transmitted signal corresponding to the irh user for A: number of subchannels [12] is given by

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6 The transmitted signal for subchannel power control can be modified [46] by incorporating the transmitted power coefficient and the subchannel weighting control coefficients as where P W1 is the transmitted power is the subchannel power control weighting coefficient of the fh subchannel. is transmitted data symbol ofthe fhsuhchannel ofthe at time instant n is the pulse shape function with a rectangular shape and is the time index is the index of the subchannel is the spreading code of the P user is the index of the spreading factor is the symbol period This signal traverses through the time invariant multipath channel having three paths [I 181 consisting of a faded line of sight (LoS) path and two Rayleigh fading components The impulse response of the channel where RI and R2 are two independent Rayleigh random variables representing the attenuation of the two Rayleigh paths and r is the relative dela) between the two Rayleigh components. Q, a,, and alare the attenuation coefficients. Channel transfer function of the fh subchannel is given by,

7 The received signal, further corrupted by interference and noise No), is At the receiver the signal is down converted to the base band signal by multiplying it with the carrier frequency in the frequency domain. In this processing, it is assumed that the synchronization in frequency and timing are perfect and the delay spread is smaller than the GI. At the output, the received SlNR of the Kh subchannel in the subchannel power controlled MC-CDMA system can be expressed as where, 5 is the average energy per bit per noise power density No Sk is the received SlNR of the iih subchannel in the conventional system. The output is summed and the data is recovered by integrating over the symbol period using equal gain combining (EGC) technique. 2.4 COMPUTATION OF WEIGHTING COEFFICIENTS The two subchannels k, and k2, with the received SlNR value of S, lower than that of S2 are considered. The BER performance can be improved if the transmitted power of the k,'h subchannel is increased by (w,,and that of the kp subchannel is decreased by /w,,l which is set to be (2-lw,,) to make the transmitted power equal to that of the conventional OFDM [47]. When the received SlNR of both subchannels are same, the subchannel power control shows the same BER performance as conventional one, because the average BER performance is symmetric at

8 Iw,,l=lw,,l=l. It means that the subchannel power control gives a large performance gain, when the received SlNR differ from each other. A grouping coefficient (G) is defined as the number of subchannels in a group. This is used for calculating the subchannel power control weighting coefticients. Subchannels with the maximum distance equal to h'/g are grouped to provide more independence among grouped subchannels. This can be achieved by using the frequency spacing greater than channel coherence bandwidth. Figure 2.2 shows the method of grouping with G = 3, where the optimum weighting coefficient w, is multiplied to the Ph group. Figure 2.2 Computation of weighting coefficients for subchannel power control

9 The subchannel power control weighting coefficients are calculated using the minimization of the average BER performance of the receiver. The probability of error [I 191 of the lib subchannel is expressed as given as The average probability of error performance of the subchannel group is The total transmitted power of the proposed system is made equal to that of the conventional one. The power ueighting coefficients in the rh group is normalized 1 *'+,/. 1 by G. The average BER of the ph subchannel group is minimized by partially differentiating the equation (2.9) uith respect to w Then, w of the fh subchannel group, after minimizing the average BER satisfies the following equation and it can be represented in a vector form as Wr =A;'BI where A,, B,, U', are given as

10 Because, S,1-, is a positive value, the determinant of A1 is never zero, which means 1, that the inverse matrix ofai always exists to provide a proper set of subchamel power control weighting coefficient. IV, The subchannel power control weighting coefficients must be positive and smaller than G. If they are negative or greater than G, an out-of-range may arise. In such cases equal power is assigned to each suhchannel of the group as in conventional transmission scheme. 2.5 SIMULATION RESULTS AND DISCUSSION The simulation is carried out using Matiuh 7 0 software package. It is assumed that channel characteristics do not change during the OFDM symbol period. The simulation parameters are chosen as given in Table 2.1. The simulation parameters are classified into system and variable parameters. S~stem parameters depend on bandwidth allocation and required data rate support. Variable parameters are chosen to suit the practical situation like vehicle speed, power control period, delay time etc.,

11 Table 2.1 Simulation parameters for subchannel power control tbrough weighting coefficient method PARAMETERS Data rate (Mbps) Required BER Carrier frequency (GHz) Number of subcarriers Number of users Modulation scheme Symbol duration ( its) Guard interval (ns) Chip rate (MHz) Bit rate (Hz) Processing gain Spreading code - OFDM Block length VALUES r to 100 BPSK Walsh Hadamard 100 symbols Comparison of the BER performance of the proposed subchannel power controlled MC-CDMA systems under the three-ray multipath channel with that of the conventional system, for different grouping coefficients (G) is shown in Figure 2.3. MC-CDMA systems without power control scheme achieves the BER of lr3, at SINR of 17dB. whereas the proposed subchannel power controlled scheme attains the same BER performance at a lower SINR of 14.5 db. for G of 4. The BER performance of the system improves with the increase of G. However, there is no significant variation in BER performance is observed when G exceeds 3. It is also evident from the Figure 2.3 that the BER curve corresponding to G of 4 dep~cts a deteriorating performance after 16 db. This poor performance at higher SINR region is due to the transmission using the conventional scheme without power control (out of range probability). Thus, the grouping coefficient of 3 is the optimum value for SCPC considering the trade off between the complexity and performance enhancement.

12 lo A v subchannel power control (G=4) lo', R. lo2 N. subchannel power control (G=3) ' subchannel power controi(g=2) wlthout power control - - Figure 2.3 BEK performance of suhchannel power control scheme w~th power control - w~thout Dower control l0;o Number of userr Figure 2.4 Capacity analysis with and without power control for SINK =14.5 db and G = 3

13 The capacity analysis of MC-CDMA systems with and without power control, with 128 subcarriers and GI of I% providing frequency non-selective fading over each subcarrier is given in the Figure 2.4. It is evident that the subchannel power control scheme with G=3 and an average SlNR of 14.5 db performs well and more number of users can be served by the network. 2.6 SUMMARY A subchannel transmission power control scheme for MC-CDMA systems has been proposed. The BER performance and the capacity have been evaluated under time invariant multipath channel. Significant reduction in BER is achieved through subchannel power control. This subchannel power control scheme has been found to offer an improved system capacity while transmining the same amount of data, power and bandwidth with guaranteed QoS compared to MC-CDMA systems without power control.