International Journal of Engineering Technology and Computer Research (IJETCR) Available Online at www.ijetcr.org
Volume 5; Issue 5; September-October: 2017; Page No. 161-169
Adaptive Modulation of Multi Carrier Code Division Multiple Access (MC-CDMA) Technique in WIMAX system
T.Jaya1, V. Rajendran 2
1Assistant Professor, Department of Electronics and Communication Engineering, Vels University, Chennai.India
2Professor and Head, Department of Electronics and Communication Engineering, Vels University, Chennai.India
Abstract
Communication has become one of the important aspects of life in present day technology. With the rapid growth in technology, the need for larger, faster and error free data has become the order of the day. In this regard the communication system faces some problems that are time varying channel characteristics and spectrum bandwidth is a mainly expensive thing in that service. However the channel state information changes always which results in reduced stability of link throughout the transmission. The occurrence of fast multipath fading and intrusion from other customers, the Signal to Noise ratio is keep on varying in time on wireless transmission medium. The system performance is degraded due to above metrics. The proposed method is dynamically changing the modulation method that is Adaptive Modulation for Multi Carrier Code Division Multiple Access (AM based MC-CDMA) in WIMAX (Wide Band Interoperability for Micro wave Access) systems which is a hopeful method to enhance throughput of the system with stable transmission technique on time varying transmission mediums. Here the different modulation techniques are introduced that are BPSK, QPSK and M-ary PSK mapping schemes useful to a proposed MC-CDMA scheme in Rayleigh medium or channel surroundings. It measure up with the Bit Error Rate metric of different code rates in the MC-CDMA method and further select the most excellent modulation technique and code rate to get improved Bit Error Rate performance metric. Also it improves the spectral efficiency up to 6 bps and throughput is 120 bps in the system.
Index terms: Multiple Carrier Code division multiple access (MC-CDMA), Adaptive Modulation, Multipath fading, Wide Band Interoperability for Micro wave Access (WIMAX), Spectral efficiency, Bit Error Rate(BER)
I. INTRODUCTION
In cellular communication, the communication assets demote to the time and bandwidth. It is presented in a specified available system. With many number of users, the possessions required to share by the users available in order to set up communication channels between the mobile station (MS) and Base Station.
But, the accessible resources are frequently restricted in support of known users, so the total band of frequency is limited. To enhance the effectiveness of allocation of resources and multi access methods are developed. Adaptation and Diversity techniques are important techniques used to utilize full channel capacity. In adaptive modulation we adjust the modulation technique and coding rates dynamically to maximize the throughput and reduce the Bit Error Rate depending on the channel conditions. In The mobile communication the diversity technique offer a method for getting better the consistency of
information signal by two or more channels with dissimilar characteristics. Diversity is maximum exist in radio communication and it is a usual technique for compensating the fading and intrusion and avoiding error during burst conditions. The signal is broadcasted using a number of frequencies and channels on a wide bandwidth that is mostly affected by frequency selective type of fading [1].
The concept of OFDM comes up from the multiple numbers of Carriers in Modulation (MCM) transmission technology. It is a new form of bandwidth proficient MCM method, this occupy closely separated orthogonal carriers and overlapped bandwidth. The filtering is required in the OFDM because the orthogonality property of the carriers. So the limited spectrum availed in a efficient manner without any intrusion from others. Multi Carrier Code Division Multiple Access is a comparatively advanced idea and its growth expected at getting better performance on multiple links [2].
The features these techniques are given below;
• It allows multiple accessing methods.
• It is immune to intrusion and distortion in signals.
• Synchronization is easy in this technique because the symbol duration is higher than chip period.
• Frequency diversity is also very good in this method however a single symbol is transmitted across multiple numbers of high frequency carrier signals.
• It is robust against frequency selective fading channel. Because the coherent bandwidth is greater than the delay spread, this results in reducing Inter Symbol Interference (ISI).
Multiple-Carriers Code Division Multiple Access is a multi access method applied in Orthogonal FDM based mobile communication systems; permit the structure to support many users simultaneously. It is a combination of OFDM and CDMA. Multiple carriers CDMA technique spread the every user symbol in the frequency domain [3]. So as to every user symbol is passed through several parallel subcarriers, however it is phase shifted depending on a code value. The values of code vary per carrier and per user. The receiver section mixes all the carrier symbols, by considering these to recompense the varying signal strengths and runback the code shift. The receiver is able to separate symbols of different users, for the reason that these have orthogonal code values. Since each data symbol reside in a much broad bandwidth in Hz than the information rate (in bit/s), a signal to noise interference ratio
A. WiMAX
WiMAX functioning on the similar common values as WIFI it transmits the information from one point to another point through two-way radio signals. A computer capable of receiving data from the WiMAX transmission terminal, mostly using secret number keys to stop illegal users from theft access.
The WIFI transmission speed is up to 54 Mbps below most favorable situations. WIMAX transmission speed is up to 75Mbps. But the main difference for both is not speed it is different from coverage range.
The WIFI coverage area is maximum 100 feet (30 m).
The WIMAX coverage area is maximum of 30 miles or 50 km with without cable access. Because this is possible by increasing the frequencies and the transmitting power of the sender.
II. RELATED WORKS:
The author presented in paper [4], OFDM is combined with Code Division Multiplexing and have proposed a new modulation technique called MC-CDMA. This scheme effectively mitigates multipath interference
while providing multi access capability. The drawback of Multi Access Interference and inter carrier interference due to a carrier offset are main factors that go down the response of multicarrier code-division multiple access technique in communications. In [5], the authors present a novel statistical investigation of the bandwidth efficiency of asynchronous MC-CDMA transmission on a frequency selective fading channels. It makes use of precise arithmetical form for MAI that report for the similarity between neighbouring carriers to develop a easy expressions. The approximated values for the bandwidth efficiency in addition provided which decreases the difficulty. The large spreading codes and Carrier Frequency Offset on the response of asynchronous Multiple Carriers DS-CDMA scheme with related fading between channels were investigated in the paper [6].
The authors determined a closed solution of the best combining filter by taking the mean value of covariance of matrix for the distortion including unwanted noise vector on whole random factors. They can explain how the ICI eliminated by using most favourable combination and by allocating a general random distribution of series over whole high frequency carrier signals of a given user. But the response of the system based on amount of high frequency components. They are finalized that it is difficult to predict the consistent of carrier frequency offset. Recompense of this will prove the response of the system is highly increased. The need of data rate and bandwidth efficiency in current communication is compensated by an Multiple Carriers-CDMA system based dynamic modulation is used in [7] and to achieve regularity ,diversity and increase the efficiency. Also they presented a real dynamic modulation algorithm with less implementation difficulty and a simple delay, appropriate for MC- CDMA downlink communication. The output response signifies the dynamic MC-CDMA scheme do better than usual MC-CDMA scheme.
The coded MC-CDMA with dynamic modulation is analysis as good-looking method in support of fourth generation mobile phone systems, normally during reception the signal is affected by multiple path propagation. Steele and Webb [8] suggested burst to burst dynamic Quadrature AM (AQAM) for make use of the time deviation Shannon capacity of narrowband fading medium. This method also extended to adaptive orthogonal frequency division multiplexing (AOFDM) method. Keller and Hanzo are suggested a group of QPSK and QAM modulation schemes for OFDM in bidirectional [9].Channel Quality
Estimation In order to appropriately select the transmission parameters to be employed for the next transmission, a reliable prediction of the channel quality during the next active transmit timeslot is necessary. The mobile WiMAX is utilized in support of 802.16e standards and it will offer handy and full movable connectivity for customers[10]. The goal of this work is the assessment near the output response of Fixed and Mobile WiMAX standards. Fixed WiMAX is most strong against multiple path transmission since it is used an air medium support of orthogonal frequency division multiplexing (OFDM). The standard is IEEE 802.16 and this resolve originally function in frequency range of 2.3 GHz, 2.5 GHz, and 3.4 to 3.8 GHz band of frequencies[11].
III. PROPOSED WORK
The adaptive algorithm and modulation based multiple carriers CDMA system, the modulation scheme is changed dynamically according to the channel conditions provided by the channel estimator.
The channel estimator estimates the current channel related information from the channel. These information are fed to modulation selector. The modulation selector decides the suitable digital modulation scheme depending on the channel condition and provides this information to modulator and demodulator [12]. The Flow diagram of proposed Adaptive Algorithm in MC-CDMA system is shown in figure 1.
A. Steps in proposed algorithm
1. First the channel estimator predicts the channel conditions by calculating the signal to noise ratio. The NR value will be determined via average value and the value of variance of the signal. The signal level is estimated by measuring the square value of average for magnitude of signal from the receiver side. The noise level is estimated by taking variance of the signal in the receiver section. Finally find out the SNR value by using both the mean value and variance of presently received data. By using PSK modulation scheme, the mean of current data block is
𝑍𝑍 =𝑁𝑁1∑ |𝛾𝛾𝑁𝑁𝑖𝑖=1 𝑖𝑖| --- [1]
The average value of the signal is Z and N stands for the number of symbols.
The variance is
𝑇𝑇2=𝑁𝑁−11 �∑ �|𝛾𝛾𝑁𝑁𝑖𝑖=1 𝑖𝑖| − �𝑐𝑐𝑖𝑖,��2� --- [2]
T2 signify the variance of signal.
Ci'
indicate the estimation of channel.
The Signal strength is estimated by following formula, Γ = �N−3N−1�|Z|T22−N1 [3]
2. After the prediction of channel conditions, the proposed algorithm checks the SNR value, If it is less than 9dB then the mode selector choose the BPSK modulation and at the same time determine the code rate for Walsh Hadamard (WH) Code by using below expression
Code rate = (k+1)/2k ...[4]
k is the number of bits
Then this suitable modulation technique and full code rate is applied back to the adaptive modulator and the spreader present in the transmitter side. Similarly the same information to be fed back to adaptive demodulator and despreader[13].
3. If the SNR value is not less than 9dB then again the system checks it is less than 14dB.If it is less than 14dB then the selector block choose the slightly high rate modulation scheme i.e QPSK and the code rate is 3/4. This information is fed back to adaptive modulator and spreader in transmitter and adaptive demodulator and despreader in receiver. Suppose the SNR is not less than 14dB further the system check the SNR is less than 21dB .
4. If the SNR is less than 21dB then the mode selector choose the high rate modulation technique is 8-PSK and the code rate is 1/2 and this value is fed back to transmitter and receiver. However the SNR is not less than 21dB then it will choose the best high rate 16- PSK scheme. If the SNR beyond 30dB then 64-PSK scheme will be selected by the mode selector block.
This process will be repeated for whole transmission and reception.
B. Block Diagram Of Proposed AM MC-CDMA Transmitter
The schematic illustration of adaptive algorithm and modulation based Multiple Carriers Code Division Multiple Access transmitter section is revealed in figure 2. The source data available in binary form is initially spreaded by Walsh coding, further follow the Serial-to-Parallel converter change the high rate bits into low rate bits. Each bit stream is modulated by the suitable modulation method. In spreading each information bit is multiplied with corresponding code available in the frequency domain sequence. Here we need signal processing processes at the output of the spreading section to bring out the chip associated process. Then these multiplied data are applied to the S/P conversion block and further it is applied to the
Inverse Fast Fourier Transform. After that it is applied to the Parallel to Serial conversion block and the frequency domain signals are encoded and broadcasted to the receiver side. The adaptive algorithm and modulation based Multiple Carriers- CDMA model include an adaptive on/off switch. The input data bits are is transformed in to word amount depends on the type of modulation need in support of transmission time. The Signal to Noise ratio of signal is obtained by using the standard formula then this information is straight away applied to the selector block through the channel estimator. From this information the selector block chooses the modulation type and it assure the switching level threshold value.
C. Block Diagram Of Proposed AM MC-CDMA Receiver
This diagram indicates the Adaptive algorithm and modulation for Multiple Carriers-CDMA reception is mentioned in the figure 3. The receiver section evaluate the information packets ie Signal to noise
ratio or bit error rate predict the channel oriented information, after that the feedback to transmitter side to recover itself for the subsequent data block to send.
Based on these target Signal power, selected, it has to feed back the selection mode to transmitter during the adaptation can be performed well. However, the challenge related with adaptive algorithm and modulation is that the radio channel is continuous time varying medium, so the feedback of the channel information becomes a preventive factor. So the assumption is slowly time varying channel conditions need for a reliable feedback channel to attain an precise output performance of the current Adaptive algorithm and modulation based schemes. We know that without channel prediction, the choice of the good modulation scheme is difficult based on channel conditions. AM- MC-CDMA is a mixture of adaptive algorithm and coding rate considerably enhanced the performance of the system superior than the enhancement that was get from fixed one [14].
Figure: 1 Adaptive algorithm for proposed work
Figure: 2 Block Diagram of Proposed AM MC-CDMA Transmitter
Figure: 3 Block Diagram of Proposed AM MC-CDMA Receiver
IV. RESULTS AND DISCUSSION
Figure: 4 shows BER Vs SNR performance of the adaptive modulation of OFDM system over AWGN channel, from the graph the estimated error rate is 10-
4.9 in support of 10dB Signal power and 10-5 error rate in support of 20dB power. This adaptive technique achieves more than the maximum error rate in support of 10dB signal power and improved the SNR to 21dB then the error rate is 10-5. So the BER is improved by using adaptive algorithm based Orthogonal FDM system.
Figure 4: BER Vs SNR performance of adaptive modulation based OFDM
The proposed method was simulated by using MATLAB software to discover the output performance of the Multiple Carriers-CDMA system.
The result obtained from the simulation signify the output performance of Multiple Carriers-CDMA system designed for Rayleigh channel with Walsh spreading and adaptive algorithm, the amount of active users considered up to 32. The graph 4 signify the Bit error rate of the adaptive algorithm with Multiple Carriers-CDMA system and fixed modulation with Multiple Carriers-CDMA system using Walsh coded data in the fast fading channel.
The layer one modulation schemes are Binary PSK, Quadrature PSK, 8-PSK, 16- PSK and 64-PSK. The control for the modulation schemes are set to achieve better error rate. The system initially assign the highest order modulation set-up and as the users are increased, the system alter the modulation properly according to the predicted path variants. If the calculated carrier noise ratio is within the stored switching level used for the modulation scheme under process it maintains the similar modulation set-up.
Figure 5: Adaptive Modulation of MC-CDMA System
The simulation result shows the BER is 0.00000316 for proposed Multiple Carriers-CDMA and 0.000039 for fixed modulation based MC-CDMA. Thus the proposed adaptive modulation based Multiple
Carriers-CDMA system gives enhanced error rate than conventional one.
Figure 6: Percentage of BER Vs SNR in Adaptive Modulation of MC-CDMA System
Figure 6 represents percentage of BER Vs SNR in proposed Adaptive Modulation of Multiple Carriers - CDMA System. It indicates the percentage of Error Rate improvement while increasing SNR value compared with the conventional system.
Table 1 imply the Error Rate Vs Signal Power of proposed Adaptive algorithm and modulation used for Multiple Carriers -CDMA scheme and also it shows the percentage of Bit Error Rate improvement measure up to the conservative Multiple Carriers- CDMA system. The BER improvement is 98.8% for 18dB SNR in the proposed system. Thus we proved that the proposed Adaptive modulation for Multiple Carriers-CDMA system performance is greatly improved with adaptive algorithm.
Table 2 represents the allocation of bit rates in support of special modulation schemes applied in the MC- CDMA scheme. Here maximum bit rate is 120 Mbps achieved by using 64-PSK modulation.
Table 1: Error Rate Vs Signal Power of Adaptive algorithm Based Multiple Carriers-CDMA System
Table: 2 Bit Rates for different modulation schemes used in MC-CDMA system
Table: 3 Comparison of performance improvements in OFDM and MC-CDMA
Parameters
OFDM MC-CDMA
Theoretical value Simulated value Theoretical value Simulated value
Spectral efficiency 3.7 bps 3.3 bps 6.4 bps 6.0 bps
Throughput 74 Mbps 66 Mbps 124 Mbps 120 Mbps
Spectral improvement in MC-CDMA 82%
Throughput improvement in MC-CDMA 81.8%
The Table: 4 show that the SNR value is varied in time and the suitable mode of modulation is selected according to the received SNR value. At the same time the proposed adaptive algorithm choose the code rate depending on the modulation scheme.
Table 4: Simulated SNR values for time varying channel
SL NO Time Period in ms SNR obtained in time varying channel
Selection of Modulation Code rate
1 0-10 15.6 dB 8-PSK 1/2
2 10-20 34 dB 64-PSK 7/64
3 20-30 25 dB 16-PSK 5/16
4 30-40 7.8 dB BPSK 1
5 40-50 9.5 dB QPSK 3/4
6 50-60 12 dB QPSK 3/4
V. CONCLUSION
Thus we have proved that the proposed adaptive modulation based MC-CDMA system achieves better BER performance i.e 105.5 equal to 0.00000316 for 18 dB SNR value. This is very high (98.8%) compared to the conventional MC-CDMA system using Rayleigh channel. Also we proved that the spectral efficiency of MC-CDMA system is 82% superior improvement than the efficiency of OFDM system. Further the throughput performance also improved up to 81.9%
(maximum 120Mbps) compared to the OFDM system (maximum 66Mbps). It is emphasized that the accomplished proposed method relatively results in far better performance than to the current system used in fourth generation. Hence it justified that the proposed work got its own high metrics in application oriented performance qualitatively and quantitatively.
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