Performance Analysis of OFDM 60GHz System and SC-FDE 60GHz System

. In this paper, the performance of 60GHz wireless communication system with SC and OFDM is studied, the models of OFDM 60GHz system and SC 60GHz frequency domain equalization (SC-FDE) system are established, and the bit error rate (BER) performance of OFDM 60GHz system and SC-FDE 60GHz system in 802.15.3c channels is compared. The simulation results show that SC-FDE 60GHz system has a slight advantage over OFDM system in line-of-sight (LOS) channels, while OFDM 60GHz system has a slight advantage over SC-FDE system in non-line-of-sight (NLOS) channels. For 60GHz system, OFDM 60GHz system has a slight advantage over SC-FDE system in overcoming multipath fading, but the performance of both is close whether in the LOS or NLOS case.

computational complexity required for both the transmitter and receiver are different, in OFDM systems, both the transmitter and receiver should execute FFT or IFFT operations. In the SC-FDE system, the transmitter does not need to execute FFT / IFFT operations, while at the receiver, it need to perform two times of FFT / IFFT operations.
Because of OFDM system with high spectrum efficiency, which can help to achieve high-speed communications, OFDM and SC-FDE are two powerful candidate technologies that can realize gigabit transmission throughput. In 802.15.3c [7] and 802.11ad [8], both standards are chosen as the physical layer transmission modulation.802.15.3c standard divides 60GHz band into four channels band. The standard defines three physical layer technology, namely High Speed Interface (HSI) physical layer, Audio and Visual (AV) physical layer and SC physical layer, the first two kinds are based on OFDM modulation, SC physical layer uses SC-FDE modulation. The paper uses HSI and SC over 802.15.3c standards for an example, the performance of OFDM 60GHz and SC-FDE system are studied and simulated.
The remaining part of the paper is organized as follows: Section 2 analyze the 60GHz system with OFDM modulation, and conducts Monte Carlo simulations in LOS and NLOS scenes over 802.15.3c channel model; In Section 3, the 60GHz system with SC-FDE is analyzed, and compares bit error rate (BER) performance with 60GHz system with OFDM modulation; Concluding remarks are given in Section 4.

The 60ghz System With Ofdm Modulation
The 60GHz system model with OFDM modulation [9] is shown in Figure 1. The specific process is as follows: after LDPC channel coding, interleaving and constellation mapping, the frequency domain signal d(n) obtain the d ata symbols transmitted in N subcarriers, denoted as X (k) (k = 0, 1, 2, ..., N-1). After IFFT, the time domain signal x (n), n = 0, 1, 2 ..., N-1 is obtained.
Then the Guard Interval (GI) is added to x(n) [10]. GI length, denoted by TGI, is greater than the maximum delay spread of the wireless channel. The original symbol is used to cycle and expand by GI generally as the Cyclic Prefix, and by this way Inter-Carrier Interference (ICI) is suppressed, Inter-Symbol Interference (ISI) is eliminated, while the linear convolution of the channel and signal is transferred to cyclic convolution. After adding CP, the signal sequence is denoted as ( ) where NGI is the length of CP.
The signal ( ) (3) where h(n) is the sampling sequence of actual channel impulse response, w (n) is the sample sequence of additiv e white Gaussian noise with zero mean, the sampling rate is same with that of input data X (k). By removing CP fro m the received signal ( ) After FFT transform, we can obtain  Figure 2 and 3, under different physical layers MCS schemes, i.e., transmission rates are 1540Mb/s, 2310Mb/s, 2695Mb/s, 3080Mb/s, 4620Mb/s, 5390Mb/s, 5775Mb/s, the SINR required for the reliable transmission with bit error rate of 10 -6 under LOS CM1.3 channel of order are 3.3dB, 5.4dB, 6.8dB, 9.0 dB, 11.1dB, 12.9dB and 14.4dB. The SINR required for the reliable transmission with bit error rate of 10 -6 under NLOS CM2.3 channel are 5.1dB, 7.2dB, 9.1dB, 12.6dB, 15.2dB, 17.1dB and 18.7dB. As can be seen from the simulation, in the same modulation scheme, as the coding rate increases, the bit error rate performance will deteriorate correspondingly. In order to obtain higher transmission rates, a higher SINR requirements. In whatever rate transmission, with the same SINR, the package rate and bit error rate performance of OFDM 60GHz system under LOS channel has better performance than that of OFDM 60GHz system under NLOS channel. Comparing Figure2 (a) and Figure 3 (a), we can find that, with the increasing of coding rate and modulation order, to obtain the same error rate, the SINR difference between NLOS channel and LOS channel is growing.
a) The BER performance simulation b) The PER performance simulation The 60GHz system model employing SC-FDE [11] is depicted in Figure 4 which moves the IFFT module, in transmitting end of OFDM system, to the receiving end.
The same with the OFDM 60GHz system is that its equilibrium is also happened in frequency domain, so it is similar to the structure of the OFDM 60GHz system model.  Table 2 shows the parameters selection using the LDPC code. From Table 1 and Table 2, we can obtain that, when the pilot length is 64, the data transmission rates of SC MCS7 8 9 13 are the same with that of HSI MCS1 2 3 5.Using the MCS in Table 1 and Table 2, under the same data transmission rates, we compare the BER performance of SC-FDE and OFDM over 802.15.3c channels.
From Figure 5(a), we can see that the performance of SC-FDE 60GHz system is close to that of OFDM 60GHz system in LOS channels. With he increasing of coding rate and modulation order, the performance of SC-FDE 60GHz system has advantage over that of OFDM 60GHz system. This can be observed when SNR is large, but the advantage is small. When BER is even with 16QAM high order modulation and high coding rate, the gap between them is less than 1dB. SC-FDE 60GHz system has a slight advantage in the high-speed transmission in LOS channels, but the performance of OFDM 60GHz system can be compared with that of SC-FDE system. Figure 5(b) shows that, the performance of OFDM 60GHz system is better than that of SC-FDE 60GHz system in NLOS channels when SNR is low and QPSK (1/2LDPC) is used, and with the increasement of SNR, the performance of SC-FDE 60GHz system is slightly better than that of OFDM 60GHz system. When QPSK (3/4 LDPC) and QPSK (7/8 LDPC) are used, the performance of SC-FDE system is close to that of OFDM system. When 16QAM (3/4 LDPC) is used, the performance of OFDM system has a slight advantage over that of SC-FDE system, and the gap between them is less than 0.5dB when BER is 10-6. By comparing Figure  5, we can obtain that, SC-FDE has a slight advantage over OFDM 60GHz system in LOS channels, OFDM 60GHz system has a slight advantage over SC-FDE 60GHz system in NLOS channels. It shows that OFDM 60GHz system has a slight advantage in overcoming multipath fading, but the performance of both is close whether in the LOS or NLOS case.

Conclusions
The system models of OFDM 60GHz and SC-FDE 60GHz are established respectively, and their principle are elaborated. Physical layer communication scheme under 802.15.3c standard is analyzed, and the BER performance of OFDM 60GHz system and SC-FDE system in 802.15.3c channels is compared and simulated respectively based on 802.15.3c standard and channel model. The simulation results show that, SC-FDE has a slight advantage over OFDM in LOS channels, while OFDM has a slight advantage over SC-FDE in NLOS channels. For 60GHz system, OFDM has a slight advantage over SC-FDE in overcoming multipath fading, but the performance of both is close whether in the LOS or NLOS case.