Hybrid TDMA-FDMA Based Inter-Relay Communication in Cooperative Networks over Nakagami-m Fading Channel Umer Riaz Tanoli, Raheel Abbasi, Qasim Jan Utmani, Muhammad Usman, Imran Khan, Sadaqat Jan Department of Telecommunication Engineering University of Engineering and Technology Peshawar, Mardan Campus Khyber Pukhtoonkhwa, Pakistan
[email protected] [email protected]
Abstract² Hybrid Time division multiple access-Frequency division multiple access (TDMA-FDMA) based three time slots transmission protocol is proposed with inter-relay communication. In this paper an in-depth analysis of two protocols has been assessed for Nakagami-m fading channel. The system is also simulated at various relay locations to estimate the performance for optimization. Performance is analyzed in terms of Bit Error Rate (BER), Outage probability and Gain. Index Terms² Cooperative Networks, Hybrid FDMA-TDMA, Nakagami Fading, Network protocols.
I.
INTRODUCTION
The idea of Collaborative diversity has been presented in [13]. Cooperative or relaying networks feature an emerging technology which gained special attention in efficiently solving the three challenges. The concept of Cooperative networks is to establish virtual multiple antenna arrays by employing multiple relays. Hence diversity is achieved without using multiple antennas [4]. Cooperative or relaying networks have the manifestation of obtaining same capacity and diversity as in multiple antenna diversity techniques hence solving the size, cost and hardware limitations of multiple antennas. Additionally, Cooperative communication helps in reducing the effect of fading in a wireless channel thereby achieving high data rates because multipath fading and shadowing adversely affects the performance of wireless communication networks [5]. The signal received at relay from the source is amplified or decoded before further transmission. Relaying schemes can be categorized as Amplify and Forward (AF) and Decode and Forward (DF). AF simply amplifies the signal at the relay and then transmits to the destination while DF first decodes and demodulates the signal before retransmission. The extended work for AF mode is presented in [6] implying that it experiences less processing speed but outperforms the latter. We have considered DF mode in this paper because it is more sophisticated as compared to AF mode.
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Various access techniques have been proposed in the research community namely Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA) and Code Division Multiple Access (CDMA) [7-9]. Our work is concerned with the comparison of two protocols presented in [10]. In [11] comparative analysis of two TDMA based protocols has been investigated. In [12] a three time slot TDMA based protocol with a third time slot for inter-relay communication is analyzed. Outage analysis of space time block coding for MIMO cooperative networks for amplify and forward scheme is done in [13]. SER Analysis for hybrid FDMA-TDMA based user Cooperation diversity over Nakagami-m fading is presented in [14]. To the best of our knowledge Bit Error Rate (BER) and outage analysis for inter-relay communication has not been investigated for Decode and Forward mode. In our work, a comparative analysis of two Hybrid TDMA-FDMA based protocols over Nakagami-m fading channel is presented. The performance parameters used are BER, Outage probability and Gain. Decode and forward relaying scheme is considered in our analysis. The employment of inter-relay based communication protocol is able to demonstrate better performance of network than two time slot based communication protocol. Rest of the paper is organized as follows: Section II illustrates our concerned system model for hybrid TDMAFDMA based protocols for comparison with Nakagami-m fading channel. Relay optimization for different locations of both the relays is also established in section II. In section III, results of our intended system model are presented and discussed in detail. Synopsis of the paper is presented in section IV as Conclusion.
II.
SYSTEM MODEL
For our analysis we have considered two relays in our system model, a source and destination as demonstrated in Fig.1. Relays are assumed to be full duplex. Each terminal is equipped with single antenna while Decode and Forward is used at relays. The modulation scheme used is Binary Phase
Shift Keying (BPSK). MRC is used at destination to extract the signal.
B. Transmission Protocol B Consider the system model demonstrated in Fig 1. As it is clear from the figure we have only considered two full duplexed relays, a source and destination. Unlike Protocol A, discussed in the previous section, transmission is made in three time slots instead of two time slots. In the third extra time slot, both the relays exchange their information with each other. In the first time slot source broadcasts the signals to both the relays and to the destination through direct path. The received signal after first time slot at both the relays are ݕௌோଵ ൌ ݄ௌோଵ ݔ ݊ௌோଵ And ݕௌோଶ ൌ ݄ௌோଶ ݔ ݊௦ோଶ
Fig.1 (Three time slot inter-relay communication based protocol)
A. Transmission Protocol A Consider a wireless communication network where numbers of relays are used for relaying. For simplicity we have considered only two relays. The two time slot hybrid TDMAFDMA based protocol is investigated for two relays over Nakagami-m fading channel. In this protocol, source broadcasts a signal x to the relays R1 and R2 respectively in the first time slot. The signal is then decoded at the relays and retransmitted to the destination in the second time slot. The signals received at the destination are given by: ݕԢோଵ ൌ ݄Ԣோଵ ݕԢௌோଵ ݊Ԣோଵ And ݕԢோଶ ൌ ݄Ԣோଶ ݕԢௌோଶ ݊Ԣோଶ WhereݕԢௌோଵ and ݕԢௌோଶ are the signals received at R1 and R2 in 1st time slot, respectively. Similarly ݄Ԣோଵௗ and ݄Ԣோଶௗ are the fading channels from R1 to destination and R2 to destination, respectively. ݊Ԣோଵௗ and ݊Ԣோଶௗ are the Additive White Gaussian Noise (AWGN) added to the signals received at the receivers from R1 and R2, respectively and Maximal Ratio Combining (MRC) is used at the destination to extract the signal. Summarized form of two times slot TDMA based protocol [11] is illustrated in Table 1. TABLE 1 TWO TIME SLOT TDMA BASED TRANSMISSION PROTOCOL
Time Slot 1
Time Slot 2
SÆR1 SÆR2
R1ÆD R2ÆD
݄ௌோଵ and ݄ௌோଶ are the fading channels between the source and R1, R2 respectively. ݊ௌோଵ and ݊௦ோଶ are the Additive White Gaussian Noise (AWGN) added at the receivers. The received signal at the destination in first time slot will be ݕௌ ൌ ݄ௌ ݔ ݊ௌ ݄ௌ represents fading between source and destination and ݊ௌ is the AWGN added at receiver. TABLE 2 THREE TIMES SLOT TDMA BASED PROTOCOL
Time Slot 1 SÆR1,SÆR2 SÆD
Time Slot 2 R1ÆD, R2ÆD R1ÆR2, R2ÆR1
Time Slot 3 R1ÆD R2ÆD
In the next time slot R1 and R2 interchange the received signals with each other, meanwhile R1 and R2 transmits the received signals from previous time slot to the destination. The signal interchanged by R1 and R2 in this time slot will be of different frequencies and hence a hybrid TDMA-FDMA protocol will be fashioned. The interchange of signals between the two relays is called inter-relay communication. The received signals from R1 and R2 at destination in the second time slot will be ݕோଵ ൌ ݄ோଵ ݕௌோଵ ݊ோଵ And ݕோଶ ൌ ݄ோଶ ݕௌோଶ ݊ோଶ ݄ோଵ ݄ோଶ represent fading between the destination and R1, R2 respectively. While ݊ோଵ and ݊ோଶ represent the AWGN added on the receive antennas. The signal received from R2 at R1 in the second time slot will be ݕோଶோଵ ൌ ݄ோଶோଵ ݕௌோଶ ݊ோଶோଵ
Where ݄ோଶோଵ is the fading experienced by the signal from R2 to R1 and ݊ோଶோଵ is the AWGN. Similarly the received signal from R1 at R2 will be ݕோଵோଶ ൌ ݄ோଵோଶ ݕௌோଵ ݊ோଵோଶ ݄ோଵோଶ is the fading and ݊ோଵோଶ is the AWGN that signal experienced from R1 to R2. In the third and final time slot both the relays transmit the signals received in the previous time slot from each other. The signals received at destination from R1 will be ಅ ಅ ൌ ݄ோଵ ݕோଶோଵ ݊ோଵ ݕோଵ ಅ ݄ோଵ and ݊ோଵ represent fading and noise added at the receiver. Similarly the received signal at destination from R2 will be ಅ ಅ ݕோଶ ൌ ݄ோଶ ݕோଵோଶ ݊ோଶ ƍ ݄ோଶ and ݊ோଶ represent fading and AWGN added during transmission and at reception respectively. Summarized form of three time slot inter-relay communication based protocol is shown in Table 2. MRC is used to combine all the signals received at destination.
C. Relay Optimization Relay optimization enhances system performance; this section summarizes the description of two dimensional system model shown in Fig.2. The distance between source and destination is assumed as݀௧ . We have considered protocol B for this system model. R1 is set at fixed position 0.1݀௧ and R2 is varied from 0.1݀௧ to 0.9݀௧ . Then R2 is placed at 0.1݀௧ and R2 is moved from 0.1݀௧ to 0.9݀௧ for investigating the performance of Outage probability and Gain. The input/output equations for optimal relay locations will be as follows The received signal in the first time slot from source at R1 and R2 will be: ݕௌோଵ ൌ ሺ݀ௌோଵ ሻିఈ ݄ௌோଵ ݔ ݊ௌோଵ
Fig.2 System Model for Optimal Relay Location
Where ݀ௌோଵ and ݀ௌோଶ are the distances taken between source to R1 and source to R2, respectively. ןis the path loss exponent. In the first time slot the source also transmits the signal to the destination. The received signal at destination will be ݕௌ ൌ ሺ݀ௌ ሻିఈ ݄ௌ ݔ ݊ௌ Where ݀ௌ is the distance from source to destination. The received signal at destination from R1and R2 can be expressed as: ݕோଵ ൌ ሺ݀ோଵ ሻିఈ ݄ோଵ ݕௌோଵ ݊ோଵ And ݕோଶ ൌ ሺ݀ோଶ ሻିఈ ݄ோଶ ݕௌோଶ ݊ோଶ Where ݀ோଵ and ݀ோଶ are the distances from R1 to destination and R2 to destination. The received signals at R1 from R2 and at R2 from R1 can be expressed as: ݕோଶோଵ ൌ ሺ݀ோଶோଵ ሻିఈ ݄ோଶோଵ ݕௌோଶ ݊ோଶோଵ And
ݕௌோଶ ൌ ሺ݀ௌோଶ ሻିఈ ݄ௌோଶ ݔ ݊ௌோଶ
ݕோଵோଶ ൌ ሺ݀ோଶோଵ ሻିఈ ݄ோଵோଶ ݕௌோଵ ݊ோଵோଶ Here ݀ோଶோଵ is the distance from R2 to R1. The received signals in the third time slot at destination from R1 and R2 will be ಅ ಅ ൌ ሺ݀ோଵௗ ሻିఈ ݄ோଵௗ ݕோଶோଵ ݊ோଵௗ ݕோଵௗ
And
ಅ ಅ ݕோଶௗ ൌ ሺ݀ோଶௗ ሻିఈ ݄ோଶௗ ݕோଵோଶ ݊ோଶௗ
MRC is used to extract the required signal at the destination. III.
RESULTS AND DISCUSSION
This section provides the simulation results of the system models discussed in the previous section. The three performance metrics are BER, Outage probability and Gain. Binary Phase Shift Keying (BPSK) is used to modulate the signal; the AWGN samples are real Gaussian variables with zero mean and unit variance. A. BER Analysis Fig.3 shows BER curve in which Nakagami-m channel is considered for comparison of two protocols. The results clearly shows better performance for three time slots inter-relay based protocol as compared two time slot protocol. When m=1 the Nakagami channel behaves like Rayleigh fading channel; when m is increased the system shows better performance. System model in Fig.1 is simulated for BER calculation.
Fig.4 Comparison between Protocol A and B
Projection of relays means the relative distance of relays from the source. The comparison of Protocol A and B for outage probability shows better performance for protocol A. At 1 value both protocols have the same performance but as the value of SNR is increased, an increased performance for Protocol B is observed.
Fig.3 BER Comparison for Protocol A and B
The results are plotted in terms of BER vs SNR. ͳͲହ numbers of symbols have been simulated for plotting the BER curves. B. Outage Probability Outage probability is used for characterization of system performance. In outage probability the mutual information of both the source and destination does not reach a certain predefined threshold. Both the system models have been simulated for outage probability in our work. Fig.4 shows the comparison of both the TDMA and hybrid TDMA-FDMA based protocols while outage probability for system model with distance factor is calculated as shown in Fig.4.
Fig.5 Outage probability analysis for relay optimization
In Fig.5 3D graph of outage probability is shown considering Nakagami-m fading channel. C. Cooperative Gain Cooperation gain can be expressed as Gain= ܴܧܤ /ܴܧܤ Where ܴܧܤ is the Bit error rate with no cooperation
while ܴܧܤ is the Bit error rate using cooperative techniques. Cooperation gain has been simulated for different relay locations and 3D graph is originated for various values of relay location. Fig.6 shows the gain performance at different projections of R1 and R2.
Fig.6 Cooperation gain For Nakagami-m fading channel with relay optimization
This section can be synopsized by discussing the overall results achieved in our analysis and by emphasizing on the scope of our work. A profound analysis of two protocols has been done and results showed an increased BER and Outage analysis for the Protocol B. IV.
CONCLUSION
A hybrid TDMA-FDMA based protocol with inter-relay communication has been investigated in our work. It has been illustrated that inter-relay communication based protocol shows better BER and outage performance as compared to non-inter-relay based protocol for Nakagami-m fading channel. Our work also emphasizes on analysis of BER and Outage performance at various locations of relays. Mathematical modeling of this system is left for future work. The performance of the network can be improved by removing the assumptions made in our work and also by considering multiple relays to ensure the performance. REFERENCES >@-1/DQHPDQ'1& 7VHDQG* : :RUQHOO³&RRSHUDWLYH diversity in wireless networks efficient protocols and outage EHKDYLRU´,(((7UDQV. Inf. Theory, vol. 50, no. 12, pp. 3062±3080, 2004. >@-1/DQHPDQDQG*:RUQHOO³(QHUJ\-efficient antenna sharing DQG UHOD\LQJ IRU ZLUHOHVV QHWZRUNV´ LQ 3URF :LUHOHVV &RPPXQ Networking Conf. 2000, vol. 1, pp. 3062±3080. [3] J. N. Laneman and G. WRUQHOO ³'LVWULEXWHG VSDFH-time coded SURWRFROVIRUH[SORLWLQJFRRSHUDWLYHGLYHUVLW\LQ ZLUHOHVVQHWZRUNV´ in Proc. Glob. Telecomm. Conf. 2002, vol. 1, pp. 77±81.
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