Experimental Investigation of IEEE802.11n Reception with Fractional Sampling

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1 21st Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications Experimental Investigation of IEEE802.11n Reception with Fractional Sampling Ryosuke Nakamura, Yukitoshi Sanada Department of Electronics and Electrical Engineering, Keio University Yokohama, Japan Abstract In the IEEE802.11n WLAN standard, orthogonal frequency division multiplexing (OFDM) modulation is employed. Diversity techniques are implemented to overcome multipath fading in the OFDM systems. A fractional sampling (FS) scheme is one of the diversity techniques with a single antenna. This scheme also can be applied in a MIMO-OFDM system to increase its capacity. In this paper, the effect of FS in a WLAN system following the IEEE802.11n standard is investigated through experiments. Numerical results through the experiments indicate that diversity gain through FS can be obtained in the NLOS conditions. I. INTRODUCTION IEEE802.11n WLAN is one of broadband communication specifications ratified in In the IEEE802.11n WLAN standard, orthogonal frequency division multiplexing (OFDM) is adopted as a modulation scheme. OFDM has been a prevailing technology in broadband wireless communications such as terrestrial digital broadcasting, cellular systems, or wireless local area networks. This is because OFDM based systems can achieve high frequency utilization efficiency due to orthogonal subcarriers [1], [2]. A Multiple-Input Multiple-Output (MIMO) technique is also widely used in recent wireless communication standards such as IEEE802.11n to realize high-speed and reliable transmission [3]. The MIMO technique employs multiple antennas at both the transmitter and the receiver in order to increase the capacity of the MIMO system. Even though the capacity of the MIMO system can be increased with additional antennas, the form factor of the terminal may limit the number of antenna elements. On the other hand, for an OFDM receiver, a fractional sampling (FS) scheme is proposed to resolve this problem [4]. FS obtain diversity gain with a single antenna by sampling the received signal higher than the baud rate. Experimental investigation of FS in the IEEE802.11a/g WLAN standard has already been carried out [5]. However, its diversity effect has not been confirmed with the IEEE802.11n signal. Therefore, in this paper, the experimental investigation of FS with the IEEE802.11n signal is carried out. In NLOS conditions, diversity gain can be observed through FS in a receiver. This paper is organized as follows. Firstly, the FS scheme is described briefly in Section II. An experiment system is then explained in Section III. Numerical results are shown in Section IV. Finally, conclusions are presented in Section V. II. OFDM WITH FRACTIONAL SAMPLING Suppose there are M t transmit antenna elements and M r receive antenna elements in a MIMO-OFDM system. The following expression focuses on the m t th transmit and th receive antenna elements. The information symbol on the kth subcarrier from the m t th antenna is s mt [k] (k =0,..., N 1), and then the OFDM symbol is given as u mt [n] = 1 s mt [k]exp(j 2πnk N N ) (1) k=0 where n (n = 0, 1,..., N 1) is the time index, and N is the size of the inverse discrete Fourier transform (IDFT). After appending guard interval with the duration of N GI, the transmitted signal in a baseband form is given by x(t) = n= N GI u mt [n]p tx (t nt s ), where p tx (t) is the impulse response of the transmit filter and T s is the symbol duration. This signal is up-converted and transmitted through a multipath channel between the m t th transmit antenna and the th receive antenna with an impulse response, c mrm t (t). The received signal after the down-conversion is given as (t) = n= N GI u mt [n]h mrm t (t nt s )+v mr (t) (2) where h mrm t (t) is the impulse response of the composite channel including the baseband filters on the m t th transmit antenna and the th receive antenna and is given by h mrm t (t) := p tx (t) c mrm t (t) p rx (t), where denotes convolution, p rx (t) is the impulse response of the receive filter, and v mr (t) is the noise on the th antenna. If (t) is sampled at the rate of T s /G, where G is the oversampling rate, its polyphase components can be expressed as [n, g] = l= N GI (u mrm t [l]h mrm t [n l, g] +v mr [n, g]) (3) where g =0,..., G 1, and [n, g], h mrm t [n, g], and v mr [n, g] are the polynomials of sampled (t), h mrm t (t), /10/$ IEEE 1043

2 and v mr (t), respectively, and are expressed as [n, g] := (nt s + gt s /G), h mrm t [n, g] := h mrm t (nt s + gt s /G), v mr [n, g] := v mr (nt s + gt s /G). After removing the guard interval (GI) and taking discrete Fourier transform (DFT), the received symbol on the kth subcarrier is given by z mrm t [k] =H mrm t [k]s[k]+w mr [k] (4) where z mrm t [k] =[z mrm t [k, 0]...z mrm t [k, G 1]] T, w[k] = [w mr [k, 0]...w mr [k, G 1]] T, and H mrm t [k] =[H mrm t [k, 0]...H mrm t [k, G 1]] T are G 1 column vectors. The gth component of the received symbol is given as [z mrm t [k]] g := z mrm t [k, g] = n=0 y 2πkn j m t [n, g]e N and similarly for w mrm t [k, g] and v mr [k, g]. The channel response of the gth sample on the kth subcarrier is given as H mrm t [k, g] = n=0 h m t [n, g]exp( j 2πkn N ). A. Measurement Setup III. EXPERIMENT SYSTEM AD boards Signatec PDA1000 Down converter Koden Electronics Co. A ESG vector signal generator Agilent E4438C TABLE I MEASUREMENT EQUIPMENTS Sampling frequency: 250 MHz Resolution: 8 bits Bandwidth of the baseband filter: 40 MHz Gain: +20 db (in LOS) / +40 db (in NLOS) Frequency: from 250kHz to 6 GHz Maximum output power: +17 dbm Figure 2 illustrates the experiment system. Table 2 shows the measurement equipments. The OFDM signals of the IEEE802.11n system are generated by the signal generators. The receive signals are down-converted to the baseband with the down-converter. In the down-converter, the bandwidth of the baseband filter is set to 40 MHz. The gain of the LNA is set to +20 db in the LOS condition and +40 db in the NLOS condition. The outputs of the down-converter are digitized and saved in the A/D boards with the sampling rate of 250 MHz and the resolution of 8 bits. The sampling period of the received signal is 0.5 sec/measurement. 2) Transmit Signal: In this experiment, the OFDM signal of the IEEE802.11n system is generated by the vector signal generator. Specifications of the OFDM signal used for the experiment are shown in Table II [6]. TABLE II SPECIFICATION OF SIGNAL Bandwidth of one channel 20 MHz Center frequency of a channel GHz Modulation scheme OFDM Number of subcarriers 64 Number of data subcarriers 56 Symbol duration 3.2 μs Guard interval 0.8 μs Number of transmission antennas 2 Number of receiving antennas 2 Fig. 1. Measurement room. 1) Measurement Environment: Figure 1 shows the measurement environment. The room is shielded from an electrical wave. The boxes and the shelves are made of steel. Through the measurement, line-of-sight (LOS) and non-los (NLOS) conditions are evaluated. The receiver is placed on 9 different positions with 6.14 cm (=wavelength) separation for each measurement. Fig. 2. Experiment system. The frame format on the high-throughput (HT)-mixed mode of the IEEE802.11n standard is illustrated in Fig. 3. In this experiment, HT-LTF is only used as a transmit signal. Fig. 3. Frame format on HT-mixed mode of IEEE802.11n. The OFDM signal is generated with the clock of 20 MHz and is upsampled to 100 MHz. After upsampling, the signal goes through a transmit filter. Two types of the transmit filter are used. Those filters have the frequency responses that satisfy the IEEE802.11n spectrum mask. The frequency responses of the filters are shown in Fig. 4(a) and in Fig. 4(b). 1044

3 (a) Filter A. N av is about DC offset cancellation can be achieved by subtracting ȳ mr from the received data samples as follows. y [n, g] =y mr [n, g] ȳ mr, (6) where y mr [n, g] is the nth sample for the gth branch at the th antenna of the receiver. 2) Sampling Rate Conversion: Sampling rate conversion (SRC) is carried out by the M sets direct insertion/cancellation scheme in [7]. The sample after Msets (Q γ)/q SRC is given as y [n, g] = M c=1 [( ) ] y nq γrc +1,g, Q γ where indicates the maximum integer number which do not exceeds the value and R c is a parameter which specifies the location of the inserted or cancelled samples, which is expressed as c 1 R c = (Q 1) + φ,φ< Q 1 M M. (8) In this experiment, 25 set 24/25 SRC (M =25, Q =24and γ =1) is carried out. (7) (b) Filter B. Fig. 4. Frequency response of transmit filter. Fig. 5. Delay correlator. Finally, the filtered signal is transmitted from the signal generator. B. Signal Processing in Receiver 1) DC Offset Cancellation: The DC offset of the received signal is estimated with the following equation. ȳ mr = Nav n=1 G 1 g=0 y [n], (5) N where y mr [n] is the nth sample of the received signal and N av is the number of the samples for averaging. Averaging is carried out over each measurement periods and the number of 3) Symbol Synchronization: Figure 5 shows the block diagram of a symbol synchronization scheme. N is the OFDM symbol duration and N GI is the length of the GI. In the OFDM symbol, the GI is appended at the front of the symbol. In addition, the GI has the same waveform as the last part of the symbol. Therefore, the received signal is put into the delay correlator and the peak of the output of the delay correlator indicates the synchronization timing. The output of the delay correlator is expressed with the following equation O[n] = N GI G 1 m=1 g=0 y [n + m 1,g] y [n + m 1+N,g]. (9) 1045

4 After symbol synchronization, the received signal is downsampled with the ratio of 1/6 as [n, g] =y [6n, g]. (10) 4) Calculation of Correlation: The correlation between the fractional samples (g = 0 and 1) is calculated for each subcarrier. The received signals after DFT are given as Y mr [k, 0] = Y mr [k, 1] = n=0 n=0 [n, 0] exp( 2πj n 1 (k 1)), N k =1, 2,,N, [n, 1] exp( 2πj n 1 N (k 1)), k =1, 2,,N. (11) (12) The correlation coefficient on the kth subcarrier is calculated as E[Y mr [k, 0]Y mr [k, 1] ] ρ[k] = (E[Ymr [k, 0]Y mr [k, 0] ])(E[Y mr [k, 1]Y mr [k, 1] ]). (13) IV. NUMERICAL RESULTS A. Correlation in LOS and NLOS Conditions Fig. 6. Correlation in the LOS condition with Filter A. Figures 6 to 9 show the correlation between the demodulation branches on each subcarrier with different channel conditions and filters. The subcarrier indexes from 5 to 32 and from 34 to 61 are data subcarriers. Figures 6 and 7 show the correlation in the LOS and NLOS conditions with Filter A as the receive filter. Figures 8 and 9 show the correlation Fig. 7. Correlation in the NLOS condition with Filter A. Fig. 8. Correlation in the LOS condition with Filter B. coefficient in the LOS and NLOS conditions with Filter B as the receive filter. If the value of the correlation coefficient is low, diversity gain can be achieved [8]. In Figures 6 and 8, the correlation on the data subcarriers is close to 1. Therefore, diversity gain cannot be obtained through FS in the LOS condition. This is because the direct path is strong and no multipath component is available. In Figures 7 and 9, the correlation is a little smaller as compared to those in Figures 6 and 8. This is because the multipath component is available. Consequently, in the NLOS condition, path diversity can be achieved through FS. Since the difference of the correlation between the LOS and NLOS conditions is small, diversity gain is limited. Figure 10 shows a measurement result of a low correlation case in the NLOS condition with Filter A as the receive filter. In this case, the correlation coefficient of the signals which 1046

5 Fig. 9. Correlation in the NLOS condition with Filter B. ACKNOWLEDGMENTS The authors acknowledge and express appreciations to Dr.Takeda at Toshiba Corporate Research & Development Center for his support in the experiment. This work is supported in part by a Grant-in-Aid for the Global Center of Excellence for high-level Global Cooperation for Leading-Edge Platform on Access Spaces and Grant-in-Aid for Scientific Research (C) under Grant No from the Ministry of Education, Culture, Sport, Science, and Technology in Japan. REFERENCES [1] Biao Chen and Hao Wang, Blind Estimation of OFDM Carrier Frequency Offset via Oversampling, IEEE Trans. on Signal Processing, vol. 52, no. 7, pp , July [2] Jian Wang, Jian Song, Zhi-Xing Yang, Lin Yang, and Jun Wang, Frames Theoretic Analysis of Zero-Padding OFDM Over Deep Fading Wireless Channels, IEEE Trans. on Broadcasting, vol. 52, no. 2, pp , June [3] H. Bölckei, D. Gesbert, and A.J. Paulraj, On the Capacity of the OFDM- Based Spatial Multiplexing Systems, IEEE Trans. on Communications, vol. 50, no. 2, pp , February [4] C. Tepedelenlioĝlu and R. Challagulla, Low-Complexity Multipath Diversity Through Fractional Sampling in OFDM, IEEE Trans. on Signal Processing, Vol.52, No.11, pp , Nov [5] T. Shinkai, H. Nishimura, M. Inamori, and Y. Sanada, Experimental Investigation of Fractional Sampling in IEEE802.11a WLAN System, accepted to the Eleventh IEEE International Conference on Communications Systems, Guangzhou, China, Nov [6] IEEE Std P802.11n/D11.0 Jun [7] A. B. Bostaman, Y. Sanada, and H. Minami, Modified Direct Insertion/Cancellation Method Based Sample Rate Conversion for Software Defined Radio, IEICE Trans. Commun., Vol.E91-B, No.8, Aug [8] L. Fang, G. Bi, and A. C. Kot, New Method of Performance Analysis for Diversity Reception with Correlated Rayleigh-fading Signals, IEEE Tran. on Vehicular Technology, Vol.49, No.5, Sep Fig. 10. Measurement result of a low correlation case in the NLOS condition with Filter A. are measured at one measurement point is calculated. From the Figure 10, in most of the subcarriers, the correlation is less than 0.95 and then path diversity gain can be expected. V. CONCLUSIONS In this paper, the effect of the FS scheme in the IEEE802.11n WLAN system is investigated through the experiment. Diversity in the FS scheme is evaluated through the correlation between the fractional samples on each subcarrier in OFDM. In the experiment, it has been found that the correlation decreases in the NLOS condition. Consequently, it has been proven through the experiment that diversity gain in the FS scheme can be obtained in the IEEE802.11n WLAN system. 1047