Zhu, X., Doufexi, A., & Koçak, T. (2011). A performance evaluation of 60GHz MIMO systems for IEEE 802.11ad WPANs. In IEEE 22ndInternational Symposium on Personal Indoor and Mobile RadioCommunications (PIMRC), 2011 (pp. 950 - 954). Institute of Electrical andElectronics Engineers (IEEE). https://doi.org/10.1109/PIMRC.2011.6140109

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A Performance Evaluation of 60 GHz MIMO Systems for IEEE 802.11ad WPANs

Xiaoyi Zhu*, Angela Doufexi*, and Taskin Kocak†

*Department of Electrical and Electronic Engineering

University of Bristol, Bristol, United Kingdom {X.Zhu, A.Doufexi}@bristol.ac.uk

†Department of Computer Engineering

Bahcesehir University, Istanbul, Turkey Taskin.Kocak@bahcesehir.edu.tr

Abstract— The IEEE 802.11ad task group has published its first draft to cope with the characteristics in 60 GHz millimeter-wave (mmWave) wireless communications. In this paper, three different 2×2 multiple-input multiple-output (MIMO) techniques are considered to enhance the performance of 60 GHz wireless personal networks (WPANs). Packet Error Rate (PER) and link throughput performance are simulated under different channel conditions. In addition, the system throughput over operation range is presented in the paper. Results show that significant enhancements in both coverage and capacity can be achieved by employing space-time block codes (STBC), spatial multiplexing (SM) and three different configurations of beamforming.

Keywords- WPAN; 60 GHz; IEEE 802.11ad; OFDM; MIMO

I. INTRODUCTION

The wireless local and personal communications have been experienced increasing interest with the tremendous growth in multimedia applications in recent years. The successful 2.4/5 GHz wireless systems have already increased the data rate to 600 Mbps with multiple-input multiple-output (MIMO) techniques. However, with the development of CMOS design and the availability of the 60 GHz unlicensed millimeter-wave (mmWave) band, it is possible to deliver even higher quality multimedia and data services. To accommodate the characteristics of the mmWave, the IEEE 802.11ad task group was formed to amend the existing standard on both physical (PHY) and medium access control (MAC) layers within the 57-66 GHz band [1]. The orthogonal frequency division multiplexing (OFDM) and single carrier (SC) transmission are specified in the standard. The SC mode is used for control information and low complexity transceivers, while the OFDM mode is designed for delivering high performance applications.

MIMO techniques have been studied extensively in the last decade, and in general, they can be classified into three types. One of the techniques is space-time block coding (STBC), which aims to improve the power efficiency by maximizing spatial diversity. The advantage of STBC is it enables the use of linear decoding at the receiver, and with OFDM, it can be easily implemented. Another technique is spatial multiplexing (SM), which increases the spectral efficiency by transmitting parallel data streams without the need of extra bandwidth. A higher throughput can be achieved by SM and the process does not introduce any coding redundancy. Beamforming is the third type of multiple antenna technique, and the objective is to utilize the directivity of the signal transmission and reception.

Compared to STBC and SM, beamforming only needs to utilize one spatial channel among a multiple of parallel channels. In OFDM system, beamforming can be carried out by three generic types, namely, subcarrier-wise beamforming, symbol-wise beamforming, and hybrid beamforming [2]. The first type performs beamforming in the frequency domain and the second type carries out in the time domain. The hybrid beamforming, which employs symbol-wise beamforming at the transmitter and subcarrier-wise beamforming at the receiver, compromises the complexity and performance.

The rest of this paper is organized as follows. The PHY and channel models of IEEE 802.11ad WPANs are presented in Section II. The OFDM based MIMO models are described in Section III. In Section IV, the packet error rate (PER) performance of STBC, SM and three different beamforming schemes are simulated using our IEEE 802.11ad PHY simulator. The link throughput and operation range results are also investigated. Section V concludes the paper.

II. WPANS PHY AND CHANNEL MODELS

In IEEE 802.11ad, the large spectrum around 60 GHz is equally divided into four channels. In this study, an OFDM system with 2.16 GHz bandwidth is considered to combat frequency selective fading. The OFDM is implemented by means of an inverse FFT, and a total number of 512 subcarriers are transmitted in parallel in the form of one OFDM symbol. In order to eliminate inter symbol interference (ISI), a length of 128 cyclic prefix is added to each symbol. The key parameters used for the simulation of the MIMO-OFDM PHY in this paper are shown in Table I.

TABLE I. PARAMETERS FOR OFDM SYSTEMS IN IEEE 802.11AD

Parameter Value

Sampling frequency (MHz) 2640

Number of subcarriers 512

Number of data subcarriers 336

Number of pilot subcarriers 16

Subcarrier frequency spacing (MHz) 5.156

Sample duration (ns) 0.38

IFFT and FFT period (ns) 194

OFDM symbol duration (ns) 242

Let be the received decision baseband signal for the mth subcarrier, which can be expressed as

, 1, … (1)

where is the transmitted data symbol, is the Gaussian noise vector with zero mean and variance σ2, N is number of subcarriers, and represents the frequency response of the equivalent channel matrix for the mth subcarrier.

Based on the clustering phenomenon in both the temporal and spatial domains, a statistic channel model from a combination of measurements and ray-tracing was proposed for the 60 GHz WPANs [3]. We consider a 1-D uniform linear array consisting of Mt and Mr antenna elements at the transmitter and the receiver respectively. The antenna element spacing is half wavelength λ. The channels are generated with isotropic radiators in both line-of-sight (LOS) and non-line-of sight (NLOS) cases. To evaluate the performance of STBC and SM in IEEE 802.11ad, different degrees of channel correlation are considered. We assume the average channel matrix value across all channel realizations are 0.1 (low), 0.5 (medium) and 0.9 (high) respectively. It is assumed that the communication channel remains constant during an OFDM data packet transmission. The 60 GHz average path loss (PL) model considering shadow fading can be modeled as [3]:

dB 20 log 10 log (2) where for LOS scenario A = 32.5 dB, n = 2.0, and for NLOS scenario A = 51.5 dB, n = 0.6, f is the carrier frequency in GHz, and D is the distance between the transceivers in meter.

III. OFDM BASED MIMO MODEL

Fig. 1 describes a generic MIMO-OFDM block diagram of the transmitter for IEEE 802.11ad WPANs. It consists of Mt IFFT blocks before which the incoming bits are scrambled, encoded with LDPC, interleaved, constellation mapped, and MIMO processed. In this paper, MIMO systems with two transmit and two receive antennas (Mt = Mr = 2) are considered as a means of enhancing the performance and throughput of WPANs. The modulation and coding schemes (MCSs) we considered in this paper are listed in Table II.

Figure 1: Block diagram of MIMO-OFDM transmitter

A. Space-Time Block Coding (STBC)

In [4] Alamouti proposed a simple transmit diversity scheme to form STBC. These codes achieve the same diversity advantage as maximal ratio receiver combining. The transmit diversity can be easily applied to OFDM in order to achieve a diversity gain over frequency selective fading channels. For a 2×2 STBC architecture, the scheme uses a transmission matrix

, ∗; , ∗ , where and are two consecutive OFDM symbols.

TABLE II. MODULATION AND CODING SCHEMES CONSIDERED

Modulation & Coding Rate

Coded Bits/Symbol

Data Bits/Symbol

Data Rate (Mbps)

SM Data Rate

(Mbps)

QPSK 1/2 672 336 1386.00 2772.00

QPSK 5/8 672 420 1732.50 3465.00

QPSK 3/4 672 504 2079.00 4158.00

16-QAM 1/2 1344 672 2772.00 5544.00

16-QAM 5/8 1344 840 3465.00 6930.00

16-QAM 3/4 1344 1008 4158.00 8316.00

16-QAM 13/16 1344 1092 4504.50 9009.00

64-QAM 5/8 2016 1260 5197.50 10395.00

64-QAM 3/4 2016 1512 6237.00 12474.00

64-QAM 13/16 2016 1638 6756.75 13513.50

B. Spatial Multiplexing (SM)

Typically, SM scheme is used for increasing the peak data rate by transmitting separate data streams from each antenna. A 2×2 SM system can double the peak data rate. In 60 GHz WPANs, since the highest MCS mode has already reached to 6.7 Gbps, the data rate is no longer the major concern compared to other wireless systems operating at lower frequencies. We employ this scheme in order to increase the reliability and throughput of lower MCS modes. This comes at the expense of sacrificing diversity gain, and hence a higher signal-to-noise-ratio (SNR) is required. In MIMO detection, in order to obtain a good performance with reasonable complexity, a linear Minimum Mean Squared Error (MMSE) receiver is adopted. For both STBC and SM, an FFT/IFFT processor is required for each antenna element.

C. Beamforming

Beamforming uses knowledge of the channel state information (CSI) to create a set of weight vectors. The vectors are used on the data before transmitting to the channel, so that they can be extracted without interference at the receiver. Recall equation (1), we have

, 1, … (3)

where represents the response of the MIMO channel for the mth subcarrier, and w and c are the transmitter and the receiver beam steering vector respectively. In this work, we use the effective SNR as the criterion to choose the optimal w and c. The effective SNR defined as the average SNR across all subcarriers can be computed as [5]

ln ∑ exp / (4)

where γm is the symbol SNR experienced on the mth subcarrier, β is a parameter dependent on the coding rate, the modulation and the information block size. The SNR of the mth subcarrier after normalization can be calculated as follows [2]

| |

| |

| | (5)

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The authors wBlu-Wireless Tacknowledge thnology Ltd a

IEEE 802.11 Specification”, I

S. Yoon, T. Jeonfor OFDM BaseSelected Areas October 2009.

A. Maltsev, V. WLAN Systems

M. Alamouti, “CommunicationVol. 16, Issue. 8

Y. BlankenshipPrediction MethTechnology Conf

A. Pollok, W. CMIMO-OFDM and spatial Communication

J. Wang, et. alEvaluation for Technology Conf

IEEE 802.15 W2008.

throughput apnce should behe operation r

e tolerant distads the effectivNLOS, the SIm, but the hying range to ah data rate abThe throughp

ver, STBC is

V. CO

s presented a MIMO techniwave WPANIMO were preed by IEEE resented basedoperation rangs model. The st performancns. The 2×2 Sedium correlaigher data ratel. All three beance significanmforming provg reasonable h

ACKNOWL

would like to eTechnology fohe financial s

and Great Wes

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