3rd National Conference on Signal Processing, Communications and VLSI Design(NCSCV’11) 6th & 7 th May,2011
Simulation and analysis of low power low cost MIMO-MC CDMA transreceiver system and its implementation using FPGA kit Sayan Dey* Dept. of Electronics & Communication Engineering Techno India College of Technology West Bengal University of Technology Kolkata-700156 West Bengal, India E-mail:
[email protected]
Ms. Smita Datta**
Ms. Ritika Ray
Dept. of Electronics & Instrumentation Engineering
Dept. of Electronics & Communication Engineering
Techno India College of Technology West Bengal University of Technology Kolkata-700156 West Bengal, India E-mail:
[email protected]
Techno India College of Technology West Bengal University of Technology Kolkata-700156 West Bengal, India E-mail:
[email protected]
Abstract- With the growing need of communication between the people of the world, new techniques have been put forward for successful data transfer. The main problems which were faced in the earlier days included data loss, data insecurity, connection unavailability and most importantly, huge, high cost hardwares. The primitive technical solutions could not possibly handle the pressure of the growing number of users. There aroused a problem of lack of security, delay in sending the data, loss of data and interference of the signals having similar parameters. To eradicate these problems, the developers came up with new concepts like FDM (Frequency Division Multiplexing) and TDM (Time Division Multiplexing) which, to some extent, reduced the user allocation and accommodation problems but were not enough to solve all the problems faced by the communication professionals. They could not provide the efficiency required. Thus, newer concepts like OFDM (Orthogonal Frequency Division Multiplexing) and MC CDMA (Multi Carrier Code Division Multiple Access) were introduced. Code Division Multiple Access was introduced for the first time during the Second World War where the data was sent using certain codes via noise (as carrier). This used the concepts of Spread Spectrum where the spectrum of the baseband signal is deliberately spread to increase the frequency range (more effectively the bandwidth) to a very high value. This makes the signal behave
“close to” noise (as theoretically, noise signal has a bandwidth from -∞ to + ∞). According to literature, several attempts have been made in the past to design CDMA based communication systems using FPGA[1]. The main constraints of such designs have been the overall power consumption of the chip, the time complexity which are very important design issues when it comes to Very Large Scale Integrated Circuit designs. Moreover, the signal recovery at the receiver is also a major issue from the communication point of view. In this paper, a typical low power low cost MIMO-MC CDMA system has been designed and the performance of the chip was analyzed. The design was downloaded in the FPGA ASIC design kit (Spartan 3 series) and was tested under different circumstances and the effective variation of Bit Error Rate. Here, a comparatively advanced and less complex algorithm is used for the signal recovery which reduces the computational complexity during signal reception and signal recovery. Moreover, the critical chip design issues like power consumption, junction temperature, operating frequencies were also measured and compared statistically. KeywordsAlgorithm, Spectrum I.
MIMO, MC CDMA, LMS DSSS, PN Sequence, Spread
INTRODUCTION
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3rd National Conference on Signal Processing, Communications and VLSI Design(NCSCV’11) 6 th & 7 th May,2011 The advent of improved technology has made it possible that we can actually connect two or more users who are miles apart with the help of wireless communication systems. By wireless communication system we refer to the communication between various users located over a wide range without any physical connection between the two parties. Some frequently used wireless communication systems are Radio Communication, Microwave Communication and Satellite Communication. The primitive protocols suggested by IEEE for wireless communication is 802.11. The rate of data transfer is 1 to 2 Mbps and the radio frequency is 2.4GHz [2]. Low data transfer rate, insufficient security, and the signal being prone to radio wave, microwave of same frequency led to the advent of higher technological solutions like W CDMA, DS CDMA, MC CDMA, OFDM etc. As per literature, several attempts have been made to design different forms of modified CDMA designs like DS CDMA using Field Programmable Gate Array (FPGA) kit [3,4]. Most MC CDMA transmitter-receivers are governed by IEEE suggested protocols such as 802.11g and 802.11n. The data transfer rate of 802.11g is 54 Mbps with frequency 2.4GHz and that of 802.11n is 72.2 Mbps for 20 MHz bandwidth whereas for 40 MHz it is 150 Mbps with frequency 2.4 to 5 GHz. This ensures faster data transfer with enhanced data rate.MC CDMA or Multi Carrier Code Division Multiple Access is implemented by assigning unique codes to each part of the given signal. Among the transmitting signals, only those signals are mapped where the code matches. Hence, the communication takes place only among specific users. MC CDMA may be defined as a combination of Orthogonal Frequency Division Multiplexing (OFDM) and Code Division Multiple Access (CDMA) [1]. With an aim to transmit the modulating signal efficiently we modulate it with Pseudo Noise (PN) Sequence which is not exactly noise but tends to act as noise[5]. This is done to immune the signal from unwanted frequencies. The Spread Spectrum technology is employed to carry out the entire process where the signal is deliberately spread in its frequency domain to obtain maximum bandwidth[6]. The encoded signal from the transmitter end is decoded at the receiver end. Now with the growing demand of improved technological solutions like MC CDMA it is neither sufficient nor cost effective to design a Single Input Single Output (SISO) system. A
Single Input Single Output (SISO) system has only one transmitter and one receiver for effective communication to take place. In this case, the maximum data rate is around 256 Mbps as obtained in the readings in this work. However, it was observed that there is a considerable increase in the data rate if Multiple Input Multiple Output systems (MIMO) are used in place of SISO Systems. So, here, different order Multiple Input Multiple Output (MIMO) systems were designed and their data rates were compared. It was found that the data rate was increasing considerably and the maximum possible achievable data rate was found to be 1.026 Gbps at standstill. This is one of the very important issue in designing communication systems. Another constraint is the degree of accuracy of the data obtained at the receiver popularly known as the Bit Error Rate (BER). Again, SISO and different order MIMO systems were analyzed. It was found that as the order of the system (MIMO) increased, the error rate decreased to a considerable amount. Thus, this system provides very effective and non erroneous data transmission which is an absolute necessity in modern communication. Another important factor is the problems of bandwidth allocation or rather channel allocation. In a single carrier modulation system (pass band modulation), one user can be accommodated at a particular time and can be assigned a single carrier. The concept of Multi Carrier removes this difficulty. It is a very well known fact that in CDMA, each data stream is assigned with a specific code and is noise modulated my multiplying it with PN Sequence. Now, if this PN sequence code is vectorized to obtain several codes which are nothing but pieces of the same PN Sequence train, each code can then be used as an individual PN Sequence carrier to modulate a specific data. Thus, one PN stream can now be used to transmit data from multiple users at the same time. These are a few major difficulties in the Modern day communication systems which are removed to a considerable extent in this design. II. METHOD AND MATERIALS The design of low power, low cost MIMO MC CDMA is carried out in two stages. Firstly, a simulated model is made using Matrix Laboratory Simulink (MATLABT M version 2009b) and then the same is downloaded in a FPGA ASIC design kit (Spartan 3 series) using
603 Organized by Department of ECE, Anna University of Technology, Coimbatore
3rd National Conference on Signal Processing, Communications and VLSI Design(NCSCV’11) 6 th & 7 th May,2011 Xilinx System Generator to obtain the corresponding hardware System on Chip (SOC) design. The detailed block diagram of the transmitter is provided in the figure 1. It shows the whole transmitter and receiver communication systems. Each block is individually using MATLABT M Simulink library block sets.
worst case (most distorted) signals as output from the channels. Channel parameters like Doppler shift, Eb/N0 were varied and the distortions were observed. C. Receiver Now, the distorted output from the channel is sent to the receiver end for the purpose of decoding. At the receiver the received data i.e. the output is multiplied with PN Sequence to nullify the former effect. Then the Least Mean Square algorithm is applied to obtain the best fit value and is refined from the cyclic prefix [7] added before. Hence, the transmitted data is regenerated accurately at the receiver terminal. The receiver is shown in figure 2.
Figure 1: MC CDMA Transmitter block diagram
A. Transmitter Figure 2: MC CDMA Receiver Block diagram
For the basic design of MC CDMA by MATLAB (Matrix Laboratory) Simulink, the Bernoulli Generator (base band signal) or any audio signal from outside world is multiplied with PN Sequence (noise modulation) for achieving the spreading of the spectrum and to prevent intervention of signals of different frequencies. Then, it is vectorised using IFFT (Inverse Fast Fourier Transform). A cyclic prefix is added to ensure security of the data transmitted so that even if there is an occurrence of data loss during transmission, the original data can be regenerated from the generator polynomial of the code word by general rules of coding theory. This is an effective coding technique which is applied to any form of communication. This transmitter allows a high speed data transmission and supports a large number of users over a single channel.
D. Mathematics of the whole (Modulation & Demodulation)
process
MC CDMA is implemented using the Spread Spectrum technology. DSSS or Direct Sequence Spread Spectrum is applied to make the signal immune to interference. The base band signal i.e. the transmitting signal[s(t)] is multiplied with the PN Sequence[p(t)]. This is a randomly generated sequence having the range from -∞ to +∞. Thus the signal is spread in the frequency domain. Now the modulated signal [µ (t)] is transmitted where it is prone to interference [i(t)] of the medium. The receiver receives this signal [r(t)] along with the interference from the medium. s(t)*p(t)= µ (t) r(t)= µ(t)+i(t)
B. Channel
The signal µ (t) is vectorized to obtain the multiple carrier property of the design. This received signal is multiplied again with the locally generated PN Sequence[p(t)] for the purpose of demodulation. The obtained signal is given by:
The channel for this systems consisted of different fading effects. The comparison of the effects of fading channels on the transmitting signals was effectively performed. There were mainly two types of fading channel, namely Raleigh Multipath Fading channel and Rician Fading Channel. The channel characteristics were varied along with varying signal strength to obtain the best case (least distorted) and the
D(t)=r(t)*p(t)
604 Organized by Department of ECE, Anna University of Technology, Coimbatore
3rd National Conference on Signal Processing, Communications and VLSI Design(NCSCV’11) 6 th & 7 th May,2011 The base band signal[s(t)] and PN Sequence[p(t)] are expressed in terms of their polar non return-to-zero representations where the threshold is considered as 0, maximum and minimum being +1 and -1 respectively. So, the signal is expressed in its domain as anything less than 0 to be +1 and greater than 0 as -1. It is done to reduce the effective power consumption.
MC CDMA ensures effective data transmission. The bit error rate determines the effectiveness of data transmission. Bit error rate is the ratio of error bits in a given data stream that has been altered due to noise to the actual number of bits sent in a given interval of time. Bit Error Rate (BER) is often expressed in percentage. Lesser the bit error rate higher is the efficiency to transmit data[9]. For example to transmit a data stream of 100011110 if noise, distortion or interference alter the underlined bits 100111100, the bit error rate will be given by:
D(t)=r(t)*p(t) =[µ(t)+i(t)]*p(t) =[s(t)p(t)+i(t)]*p(t) =p2 (t)s(t)+p(t)i(t) For using the no return-to-zero representation p2(t)=1 for both the values of -1 and +1. Therefore, D(t)=s(t)+p(t)i(t)
BER=number of altered bits/number of bits transmitted
Hence, the actual base band signal is regenerated along with an additive interference multiplied with PN Sequence.[8] As, the additive interference is in the form of product with p(t) hence the signal is a wide band signal. Then, an appropriate filter is added to bypass the additive term and hence regenerates the original base band signal at the receiver end.
E. The chip Generator)
BER=2/9=0.222=22.2% approximately. design
(Xilinx
System
The simulated part is downloaded in Field Programmable Gate Array (FPGA) kit using the Xilinx System Generator. A FPGA kit consists of Programmable Logic Blocks (PLB) connected via buses. The PLBs in turn consists of logic gates both basic and universal logic gates. In accordance with the simulated circuit the corresponding hardware circuit is realized via FPGA ASIC design kit (Spartan 3 series) using Xilinx system generator. The relevant output is seen in a digital Cathode Ray oscilloscope (CRO). This output is a bit distorted than the output obtained after simulation. It happens because of the noise present in the communication channel. The system generator is used for analyzing different design issues required for the design of the chip.
Now, another important aspect of MC CDMA is secure data transfer. To ensure security frequency hopping spread spectrum is employed while transmitting the data through any medium. Since the range of the modulated signal is from -∞ to +∞, so the transmitting frequency can be randomly hopped within this infinite range of frequencies. This makes unauthorized data access practically impossible and secured data transmission is obtained. III. RESULTS
in table 1 that as the order of the system increases; the data rate also increases to a considerable amount (practically doubles for every system). Now, is should be noted that a maximum design of 12th Order system can be achieved. This is because of two factors:
Table 1: Data rates for different systems (SISO and MIMO) Order of the System Data rate SISO (1 X 1) 64.0035 Mbps MIMO (2 X 2) 128.007 Mbps MIMO (4 X 4) 256.014 Mbps MIMO (8 X 8) 512.028 Mbps MIMO (12 X 12) 1.025 Gbps
(i) There is no considerable change in the data rate of the system after 12th order. The data rate almost remains constant. (ii) As the order increases, the hardware becomes bulky and consumes more power which is not
The table 1 shows the effective change in the data rate with the increase in the order of the system. It can be noticed clearly from the results
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3rd National Conference on Signal Processing, Communications and VLSI Design(NCSCV’11) 6 th & 7 th May,2011 desired. Thus, an optimum order is chosen which happens to be 12th order.
accuracy is achieved in the 12 X 12 MIMO systems, where the BER is almost insignificant. Thus, this depicts the fact that as the order of the system increases, the bit errors are reduced and we obtain an almost perfect non erroneous communication system.
The table 2 represents the Bit Error Rate comparison between different ordered systems. It is observed that the BER decreases to a considerable extent in each system. The highest
Table 2: BER calculation for MIMO MC CDMA SNR (dB) 10 20 30 40 50 60 70 80
SISO 0.7439 0.7535 0.6823 0.6023 0.5564 0.4881 0.4324 0.3816
MIMO 2 0.0387 0.0247 0.0098 0.00589 0.001478 0.000687 0.000176 0.0000852
MIMO 4 0.00926 0.00549 0.00249 0.00081 0.000325 0.0001011 0.0000624 0.00002592
The graph 1 shows a comparison between the Bit Error Rates of different ordered systems when varied with the signal strength. Clearly from the graph, it is observed that any MIMO system is much better as compared to the conventional SISO systems in terms of BER. Now, as the order of the system increases, it is observed that the effective error decreases to a considerable amount. This proves the fact that the system is highly effective in high speed digital data communication where error is a major evil to be avoided by the system designers.
MIMO 8 0.000554 0.000432 0.0001035 0.00006 0.00000934 0.0000052 0.000000486 0.000000032
MIMO 12 0.00001152 0.0000094 0.00000725 0.00000315 0.000001016 0.00000062 0.000000085 0.00000001
1 0
20
40
60
80
100
0.1 0.01 S IS O
BE R
0.001
MIMO 2 0.0001
MIMO 4 MIMO 8
0.00001
MIMO 12
0.000001 0.0000001 0.00000001 S ig na l S tre ng th
Graph 1: Comparison of B ER with increasing Signal Strength
The VLSI chip design issue is another important factor which is studied. For this, the system generator is used to calculate the different chip level constraints. Tables 3 and 4 shows the different readings obtained regarding the chip design issues.
Name of the Constraints Static power consumption Quiescent power consumption Time delay Operating temperature Operating frequency
Value obtained 0.035 W 0.0091W 0.35 µsec 27.1º C 325.26 KHz
IV. DISCUSSION The study conducted above deals with the simulation, analysis and implementation of the MIMO-MC CDMA System in an FPGA Kit. According to literature, several work has been performed to increase the performance of wireless systems [9, 10], but in this work, the
problem of data transfer speed is increased to a considerable extent. The data rate of around 1 Gbps at standstill is obtained here. Moreover, a secured data transmission is obtained as noise modulation is the major aspect of communication using CDMA technique.
606 Organized by Department of ECE, Anna University of Technology, Coimbatore
3rd National Conference on Signal Processing, Communications and VLSI Design(NCSCV’11) 6 th & 7 th May,2011 Effective usage of bandwidth by spreading technique is another advantage of this design. The VLSI chip design issues show effective usage of FPGA resources and satisfy the conditions for the design on a perfect
semiconductor chip. Thus, a low cost low power design can be obtained from the design proposed in the above study to enhance high speed wireless data communication.
Reference [1] LaRoche, I. Roy, S. Fortier, P. Beaumont “Base band MIMO receiver architecture for MC CDMA and its FPGA implementation”, IEEE 6 th Inter national Conference on Wireless and mobile Computing, Networks and Communications. 11-13th October ,2010, pp no. 749-752 [2] Jiancun Zhou Tao Wang Ming Wu Ke Wang, “Modeling of IEEE 802.11 Wireless communication protocol “, 2 nd Computer Engineering and Technology International Conference (ICCET), 16-18th April,2010, Volume 7. [3] D. Zhou-Zhi et al, “The FPGA implementation of high speed DS-CDMA receiver”, 2010 First international conference on pervasive computing, signal processing and applications, 2010, pp np. 751-754. [4] A. Dahmane, L. Mejri, R. Beguenane“FPGA Implementation of BP-DF-MPIC Detectors for DS-CDMA Systems in Frequency Selective Channels”, Circuits and Systems and TAISA Conference, 2009. NEWCAS-TAISA '09. Joint IEEE North-East Workshop on Digital Object Identifier, 2009, pp no. 1-4. [4] Rui Fa, Bayan S. Sharif and Charalampos C. Tsimenidis, “Performance Assessment of MC CDMA Systems in Implusove Noise”. [5] Po-Rong Chang, Chin-Feng Lin,” Design of Spread Spectrum Multicode CDMA Transport Architecture for Multimedia Services.”, pp no. 99-111, January 1, 2000 [6] Demosthenes Ikonomou, Luc Vandendorpe, “Performance Comparison of receivers for the Up-link of cyclic prefixed and non prefixed MC-CDMA systems”, pmrc 2002, pp no. 829 -833 [7] Communication Systems, Simon Haykin 4 th Edition, pg no.488 [8] Muhammad Ahsan Ullah, Kyesan Lee, “BER Performance Comparison between FHPB/MCCDMA and MC-CDMA under Multipath Rayleigh Fading Channels”, IEEE Xplore digital library 2006 [9] Y. Mehdaoui, M. Mrabti, “Improvement of MC CDMA receiver using a DSP implementation of the CORDIC on fixed point”, International journal of research and reviews in mechatronic design and simulation, Vol. 1 No. 1, March, 2011 [10] S Le Nours et al, “Design and implementation of MC CDMA systems for future wireless networks”, EURASIP journal on applied signal processing, 2004, pp no. 1604-1615
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3rd National Conference on Signal Processing, Communications and VLSI Design(NCSCV’11) 6 th & 7 th May,2011
608 Organized by Department of ECE, Anna University of Technology, Coimbatore
