Bit Error Rate Performance of TFT-OFDM Over Different Channels

IJSRD - International Journal for Scientific Research & Development| Vol. 4, Issue 05, 2016 | ISSN (online): 2321-0613

Bit Error Rate Performance of TFT-OFDM over Different Channels Swati J. Doifode1 Prof. S. A. Shirsat2 1 P.G. Student 2Assistant Professor 1,2 Department of Electronics & Telecommunication Engineering 1,2 Sinhgad College of Engineering, Vadgaon (Bk), Pune, Maharashtra, India Abstract— Various wireless communication applications use Orthogonal Frequency Division Multiplexing (OFDM) as modulation and multiplexing technique due to its high Spectral Efficiency (SE). Due to the growing need for wireless capacity such as more users and applications, spectral efficiency should be improved. The signal which is transmitted over wireless channel subject to various loss such as interference, and fading path loss. The demand for high capacity with higher quality of service is increased. In this case, OFDM is a better option to overcome the problem of capacity. The OFDM is having high bandwidth transmission of data. TFT-OFDM has training information both in time and frequency domain. TFT-OFDM utilizes time-domain training sequence without interference cancellation to obtain path delay information of the channel, whereas the path coefficients are obtained by using frequency-domain grouped pilots. The simulation is carried out in terms of BER performance of TFT-OFDM over different channels such as AWGN, Brazil D and Vehicular B channel. Result shows that as the signal to noise ratio increases the bit error rate performance of TFT-OFDM system improves.

Key words: Bit Error Rate (BER), Fast Fading Channel, Inter Block Interference (IBI) Cancellation, Inter Carrier Interference (ICI) Cancellation, Orthogonal Frequency Division Multiplexing (OFDM), Signal to Noise Ratio (SNR), Time-Frequency Training (TFT) I. INTRODUCTION OFDM is widely used in many applications such as digital audio broadcast TV (DAB-TV), digital video broadcast TV (DVB-TV), wireless local area networks (WLAN). It is quite difficult to handle frequency selective fading channels in the receiver. Orthogonal frequency division multiplexing (OFDM) is an effective technique used for communication over the channels with frequency selective fading. Also, OFDM technique is robust against the narrow band intercarrier interference and inter-block interference. The problem of frequency selective fading can be reduced by converting the whole frequency selective fading channel into small flat fading channels. These small channels are orthogonal to each other. These small flat fading channels are multiplexed with each other in frequency domain.

OFDM based block transmission scheme can be divided into four different types. These are Cyclic prefix (CP), Zero padding (ZP), Time-Domain Synchronous (TDS) and Time Frequency Training (TFT) OFDM scheme. TFT-OFDM makes use of both time and frequency domain for transmission of training sequence (TS) and pilots. Thus TFT-OFDM system is widely accepted for wireless communication.

II. RELATED WORK This section consists of the work related to different OFDM systems. The cyclic prefix is used by CP-OFDM to remove

the inter-block interference (IBI) and inter-carrier interference (ICI) [1]. In case of ZP-OFDM, the cyclic prefix is replaced by zero samples. This helps to improve the equalization performance of the system [2]. In the case of both CP-OFDM and ZP-OFDM, the frequency-domain pilots are required for the purpose of channel estimation and equalization. This causes the reduction in the spectral efficiency. This problem can be reduced, if the known training sequence (TS) such as pseudorandom noise (PN) sequence is used instead of CP [3]. This sequence is known by receiver. Thus it can be also used for channel estimation and synchronization. TDS-OFDM saves the large amount of frequency domain pilots used in ZP-OFDM and CP-OFDM. The spectral efficiency of TDS-OFDM is about 10% more than that of the CP-OFDM and ZP-OFDM [4]. Inter-block interference (IBI) in the time-domain TS and OFDM data block is the main disadvantage of TDS-OFDM. This interference can be removed by using iterative interference cancellation algorithm. This algorithm is used for channel estimation and equalization [5]. Problem of interference in TDS-OFDM can also be solved by duplicating the TS twice.

This results in the dual-PN OFDM (DPN-OFDM) scheme.

The PN sequence received second time is protected from interference. This OFDM data block can be directly used for channel estimation. In DPN-OFDM, the interference cancellation is replaced by reconstruction of cyclic prefix, which is accomplished by simple addition and subtraction operation [6]. This avoids the iterative interference cancellation algorithm, leading to the improved performance over fast fading channel. Since the DPN-OFDM consists of doubled length of TS, its spectral efficiency is reduced. The main purpose of this paper is to challenge the problems discussed above. A distinct OFDM scheme is mentioned in this paper to avoid the problems which are mentioned above. Such scheme is Time-frequency training OFDM (TFT-OFDM) scheme. TFT-OFDM achieves high spectral efficiency and also the system performance is reliable in high speed mobile environments. TFT-OFDM contains the training information in both time and frequency domain. It consists of training sequence in time domain and small amount of grouped pilots are present in frequency domain.

In TFT-OFDM, the frequency domain grouped pilots occupy only 3% of the total subcarriers, which helps to achieve higher spectral efficiency. The remaining paper is organized as given. The system model for TFT-OFDM is explained in section III. The simulation results are discussed in section IV. The section V consists of conclusion of work along with references.

III. SYSTEM MODEL The basic concept of TFT-OFDM is explained in this section. TDS-OFDM scheme contains inter-block interference (IBI) in OFDM block and in TS. The features of IBI from TS to OFDM data block and from OFDM block to TS are different. These features are shown in figure 1.

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Bit Error Rate Performance of TFT-OFDM over Different Channels (IJSRD/Vol. 4/Issue 05/2016/064)

Fig. 1: Different features of IBI’s in TDS-OFDM [1] If the channel estimation is perfect, then the interference can be removed completely. The training sequence is well known by transmitter. The length of TS is small, thus IBI can be calculated with low complexity. The length of OFDM data block is large thus its computational complexity is high. This causes the OFDM data block length as random and unknown. Thus even if the accurate channel estimation is obtained, the interference cannot be completely removed. Figure 2 and 3 shows the block diagram for the transmitter and receiver of TFT-OFDM system.

Fig. 4: Signal Structure of TFT-OFDM [1] As shown in figure 4, the training sequence is present in both time and frequency domain. TFT-OFDM consists of frequency domain grouped pilots and time domain TS. The grouped pilots and TS are scattered over the whole bandwidth of the signal. The ith TFT-OFDM symbol is represented by si. This symbol is given by �

đ?‘ đ?‘– = [đ?‘ đ?‘–,−đ?‘€ ‌ đ?‘ đ?‘–,−1 đ?‘ đ?‘–,0 đ?‘ đ?‘–,1 ‌ đ?‘ đ?‘–,đ?‘ −1 ] (1) This symbol is composed of the known timeđ?‘‡

domain TS đ?‘?đ?‘– = [đ?‘?đ?‘–,0 đ?‘?đ?‘–,1 ‌ đ?‘?đ?‘–,đ?‘€âˆ’1 ] . The OFDM data block given by đ?‘‡

Fig. 2: Transmitter of TFT-OFDM system [2] At transmitter end, the information signal is encoded by (n, k) low density parity check (LDPC) encoder.

Here, k is the number of information bits and n is the length of output codeword. The LDPC encoded codeword is fed to block interleaver. Then serial to parallel conversion of interleaved data is performed. BPSK modulation technique is used for constellation mapping of this parallelized data.

Then by using inverse Fourier transformation (IFFT), the frequency domain signal is converted into time domain.

Then this signal is again reshaped and training sequence is added to the signal. This TS added signal is an OFDM symbol which is then transmitted over the channel. The received signal is down converted at the receiver side. The receiver of the TFT-OFDM system is as shown in figure 3.

đ?‘Ľđ?‘– = [đ?‘Ľđ?‘–,0 đ?‘Ľđ?‘–,1 ‌ đ?‘Ľđ?‘–,đ?‘ −1 ] is given as below đ??ś đ??ź 0 đ?‘†đ?‘– = [ đ?‘– ] =[ đ?‘€ ] đ??ś + [ đ?‘€Ă—đ?‘ ] đ??š đ??ť đ?‘‹ (2) đ?‘‹đ?‘– đ?‘ƒĂ—1 0đ?‘ Ă—đ?‘€ đ?‘ƒĂ—đ?‘€ đ?‘– đ??źđ?‘ đ?‘ƒĂ—đ?‘ đ?‘ đ?‘– Where M is length of TS, N is length of OFDM data block. The total length of TFT-OFDM symbol is given

by P = M + N. The frequency domain OFDM symbol is �

denoted by đ?‘‹đ?‘– = [đ?‘‹đ?‘–,0 đ?‘‹đ?‘–,1 ‌ đ?‘‹đ?‘–,đ?‘ −1 ] and đ?‘Ľđ?‘– = đ??šđ?‘ đ??ť đ?‘‹đ?‘– is provided. In TFT-OFDM, the data subcarriers are given by Nd and BPSK modulated pilots are given by Ngroup. Each pilot group consists of 2d+1 number of pilots.

The TFT-OFDM receiver can be designed with the help of following steps: 1) Training Sequence Based Estimation of Path Delay.

2) Reconstruction of Cyclic Prefix.

3) Joint Time Frequency Channel Estimation.

4) Equalization of channel.

IV. SIMULATION RESULTS

Fig. 3: Receiver of TFT-OFDM system [2] The transmitted information sequence is reconstructed at the receiver side. Once the signal is received, the training sequence is removed from the OFDM symbol and by using fast Fourier transformation (FFT), the time domain signal is again converted into frequency domain. Then by demodulating and decoding the signal, the information signal is obtained at the end of receiver. The signal structure of TFT-OFDM is as shown in figure 4.

The performance of TFT-OFDM system is evaluated with the help of Matlab R2013a. The bandwidth of signal is 7.56 MHz having central radio frequency of 770 MHz. The subcarrier spacing is 2 kHz. The size of message is 4096.

Other system parameters are N= 3780, M=320, Q=1, d=1, S=20, J0=3. The simulation parameters are listed in table 1.

Parameter Value IFFT size 4096 TS length 320 Signal Bandwidth 7.56 MHz Central Frequency 770 MHz Modulation Technique BPSK Table 1: Simulation Parameters This section consists of the BER performance of TFT OFDM over different channels. These channels are

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Bit Error Rate Performance of TFT-OFDM over Different Channels (IJSRD/Vol. 4/Issue 05/2016/064)

AWGN channel, Brazil D channel and Vehicular B channel.

The two different 6-tap multi-path channels used for evaluation are Brazil D channel and Vehicular B channel.

The velocity of Brazil D channel is 28 km/h. The velocity of Vehicular B channel is 140 km/h. Figure 5 shows the BER versus SNR performance of TFT-OFDM system over AWGN channel.

Figure 6 shows the BER versus SNR performance of TFT-OFDM system over Brazil D channel. The Brazil D channel is having Doppler spread of 20 Hz and velocity of 28 km/h. The performance of Brazil D channel is poor as compared to the AWGN channel due to velocity of receiver.

Figure 7 shows the BER versus SNR performance of TFTOFDM system over Vehicular B channel.

The Vehicular B channel is having Doppler spread of 100 Hz and velocity of 140 km/h. Since the velocity is more, which means that the channel is fast fading channel, the BER performance of Vehicular B channel is poor than that of the Brazil D channel. The BER value decreases as the SNR ratio increases. The BER performance of AWGN channel and Brazil D channel are nearly similar to each other. But in the case of Brazil D and Vehicular B channel, the BER performance of Brazil D channel is good as compared to that of Vehicular B channel due to less velocity of receiver.

V. CONCLUSION

Fig. 5: BER performance of TFT-OFDM system over AWGN channel Result shows that as the signal to noise ratio (SNR) increases, the bit error rate (BER) of the system decreases.

Figure 6 shows the BER versus SNR performance of TFT-OFDM system over Brazil D channel.

The performance of TFT-OFDM system in terms of BER is evaluated over AWGN, Brazil D and Vehicular B channel by using MATLAB R2013a. In the TFT-OFDM scheme, training sequence is present in time domain while the grouped pilots are present in frequency domain. The grouped pilots occupy 4% of total signal bandwidth. Thus better performance is obtained over the time varying channels. The BER performance of TFT-OFDM system degrades over the high speed mobile environments. The threshold value of SNR can be predicted from the analysis of BER. This helps to determine the transmission power of the system at the transmitter end.

ACKNOWLEDGMENT

Fig. 6: BER performance of TFT-OFDM system over Brazil D channel

Fig. 7: BER performance of TFT-OFDM system over Vehicular B channel

I would like to express respect and gratitude towards Prof.

S. A. Shirsat for her kind co-operation and encouragement which help me in the completion of this project. I would like to thank my friends who have always stood with me and helped me through my career. I consider it as my good fortune to have got an opportunity to work under guidance of such wonderful and great person. I would like to acknowledge my Institute, Sinhgad College of Engineering, Vadgaon (Bk), Pune, for providing good facilities to complete my work.

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