SDMA for 60GHz Gigabit Wireless Networks Candy Yiu Department of Computer Science Portland State University Portland, OR 97207 Email:candy@cecs.pdx.edu Suresh Singh Department of Computer Science Portland State University Portland, OR 97207 Email:singh@cecs.pdx.edu Abstract—With the opening of the 60 GHz spectrum for WLANs, there has been a great deal of interest in academia and industry on how best to exploit the more than 5 GHz of available bandwidth. One goal in the wireless community has been delivering gigabit data rates to end users. For this, a variety of approaches have been studied in the past. In this paper, we present two SDMA (Spatial Division Multiple Access) algorithms that exploit the peculiar propagation properties of this part of the frequency. We show that in typical indoor environments, one access point can deliver over 8 gbps total throughput while using only 640 MHz of the bandwidth. We generalize the algorithms to the case when multiple channels are available and show that with seven channels, we get aggregate throughput of over 31 gbps. I. I NTRODUCTION Providing gbps/user wireless connectivity in indoor environ- ments is an exciting problem that has received much attention in the recent past. Techniques such as MIMO (Multiple Input Multiple Output) and MIMO/SDMA have been extensively researched and have been shown to provide very high data rates in the WLAN. In this paper, we consider the same problem of providing gbps data rates but at the 60 GHz ISM band. Unlike lower frequency bands (typically between 2 and 6 GHz), the 60 GHz band has unique propagation properties. This band is very well absorbed by oxygen, and reflections are severely attenuated. Thus, as noted in [8], [1], the propagation at this frequency can be considered ray-like with a measured path loss exponent of 2.1 [2]. Also, multipath signals are severely attenuated making MIMO a poor choice for 60 GHz [4]. Various authors [11], [4] have therefore suggested using SDMA with highly directional smart antennas as the best architecture of such WLANs. In this paper, we compare two SDMA algorithms that are designed for this particular frequency band and show that we can achieve multi- gbps data rates indoors. The remainder of the paper is organized as follows. The next section describes related work on 60 GHz. Section III presents the system model we use and section IV presents the two algorithms we study. Results are described in section V where we first focus on achievable throughput for one channel and then study how the use of multiple channels affects throughput, section V-A. Finally, conclusions and future work are described in section VI. II. RELATED WORK The propagation characteristics of 60 GHz has been studied by several authors [13], [2], [3], [7] who have noted its ray-like propagation properties and almost complete lack of significant multipath. Indeed, [13] compares the LoS (Line of Sight) path to first-order reflections in typical indoor environments and notes that the strongest reflections (from windows) were almost 10 dB below the LoS path. If the surface is not polished, then the reflected component is much weaker. The penetration losses are also severe with many building materials such as concrete causing a 35 dB reduction in signal strength. The conclusion of these studies is that using SDMA with highly directional antennas will yield the highest throughput. SDMA has been studied over the past decade and numerous algorithms have been developed. However, all previous works look at a much lower frequency (2-5GHz) where multipath components are significant. Thus, a great deal of previous work [9], [12], [6] develops MIMO/SDMA algorithms to identify the strongest multipath components in a room and combine them to achieve high data rates. In our system, since multipath is negligible, these previous algorithms are not applicable. III. SYSTEM MODEL Smart antenna systems typically consist of M antenna elements placed in some geometry such as linear or circular or rectangular. These antenna elements are connected to k ≥ 1 beamforming modules. A beamforming module is responsible for appropriately phase shifting and weighting the input/output of each antenna element in order to beamform in some given direction. Generally, the beamforming modules occur after the analog-to-digital converter on the receiver side and before the digital-to-analog converter on the transmitter side. We consider a room with the AP (Access Point) located in the center of the ceiling. The AP and users are all assumed to be equipped with a smart antenna system where the antenna elements are arranged in a linear geometry with d = λ/2 inter- element spacing. The AF (Array Factor) of such a system is given by [5], AF = M  i=1 e j(i-1)kd(sin θ-sin θ0) where k =2π/λ, d is the inter-element spacing, θ 0 is the direction we are beamforming in and θ is the direction in