Ho et al ., MIMO beamforming in millimeter-wave directional Wi-Fi 1 MIMO Beamforming in Millimeter-Wave Directional Wi-Fi Keang-Po Ho, Senior Member, IEEE , Shi Cheng, Member, IEEE and Jianhan Liu, Member, IEEE arrays are used in both transmitter and receiver, as shown in Abstract —Beamforming is indispensable in the operation of Figure 1, the whole array with many antennas are used to 60-GHz millimeter-wave directional multi-gigabit Wi-Fi. Simple transmit one SISO data stream. Analog phase shifters are used power method and its extensions enable the transmitting and to form a single spatial beam, basically pointing the receiving antenna arrays to form a beam for single spatial transmitter and receiver toward each other, providing multi- stream. To further improve the spectral efficiency in future 60- GHz directional Wi-Fi, alternating least square (ALS) algorithm gigabit yet directional transmission. SISO beamforming based can form multiple beams between the transmitter and receiver on the simple power method is well-known and can be very for multi-input-multi-output (MIMO) operations. For both effective for frequency-flat channel. Beamforming in shared and split MIMO architecture, the ALS beamforming frequency-selective channel is less well-known but can be algorithm can be operated in both frequency-flat and frequency- performed using alternating least square (ALS) method that selective channels. In the split architecture, MIMO beamforming finds iteratively the transmitter beamforming vector by fixing approximately maximizes the capacity of the beam-formed MIMO channel. that of the receiver, and then the other way around. Index Terms —Millimeter wave, MIMO, beamforming, SISO operation for 60-GHz millimeter wave is a mature directional multi-gigabit Wi-Fi technology, defined and operated in all 60-GHz standards. The future of 60-GHz Wi-Fi requires MIMO operation to support multiple independent data streams and enhance the spectral I. INTRODUCTION efficiency. Ideally, the channel throughput can be increased nlicensed 60-GHz millimeter-wave band is very suitable proportional to the number of spatial streams. Existing SISO Ufor in-room wireless transmissions. With high wall loss, 60-GHz Wi-Fi can support up about 7 Gb/s, the future MIMO the 60-GHz signal is confined inside a room, reducing 60-GHz Wi-Fi should be able to support up to 28 Gb/s in a interference with neighboring systems using the same 4× 4 MIMO beamforming architecture [3]. With many frequency band [1][2]. With slight different regional challenging problems to solve, this paper explores the constraints, 60-GHz unlicensed band is almost available beamforming issues, focusing on the MIMO beamforming worldwide. The channel allocation is also the same in almost algorithm using antenna arrays. all standards, including WirelessHD for video area networking MIMO operation of antenna arrays tries to form multiple (VAN) [3], IEEE 802.15.3c for personal area network (PAN) beams to support multiple independent data streams using [4], and IEEE 802.11ad for wireless local area network only analog phase shifters. The phase shifters are operated (WLAN, or commonly referred to as Wi-Fi) [5]. Having a with each set of antennas to enhance the SNR for each spatial channel separation of 2160 MHz, the channel bandwidth is stream. In the shared MIMO beamforming architecture, each sufficient to provide multi-gigabit single-input-single-output antenna is driven by the signals from multiple data streams (SISO) wireless transmission. Multiple beams can also be and the beamforming algorithm is just a simple extension from formed to provide multi-input-multi-output (MIMO) the power method or the ALS method for SISO beamforming. operations. In the split MIMO architecture, each data stream drives a SISO operations for 60-GHz Wi-Fi have been defined in the different set of antennas and each antenna is driven only by IEEE 802.11ad, once called Wireless Gigabit Alliance one data stream. Also based on the ALS method, the MIMO (WiGig), as an extension of Wi-Fi from below 6-GHz to 60 beamforming algorithm maximizes the signal strength and GHz. In addition, SISO operations are defined in both minimizes interference at the same time. For simplicity, only WirelessHD [3] and IEEE 802.15.3c [4]. As explained later, the algorithm for 2× 2 MIMO beamforming is presented in beamforming is required in 60-GHz millimeter wave, mostly detail but it can be extended to any K × K split MIMO because of the free-space path loss due to the limited effective beamforming cases, where K is the number of spatial antenna aperture in millimeter wavelength. When antenna channels. The remaining parts of this paper are organized as Manuscript received April ??, 2014, revised ??, 2014. following: Sec. II briefly explains why beamforming is K.-P. Ho and S. Cheng are with Silicon Image, 1040 E. Arques Ave., required for 60-GHz Wi-Fi, and defines the mathematical Sunnyvale, CA 94085, USA (e-mail: [email protected] , [email protected] ). J. Liu is with Mediatek USA Inc., 2860 Junction Ave., San Jose, 95134 models and notations for later sections. Sec. III describes the ([email protected] ). SISO beamforming algorithm, especially those for frequency- Ho et al ., MIMO beamforming in millimeter-wave directional Wi-Fi 2 selective channels. Sec. IV presents the MIMO beamforming transmitting power than 5-GHz Wi-Fi. Combining the free algorithm, first for the shared architecture and later for the space loss, signal characteristic, and circuitry limitation, 60- split architecture. Secs. V and VI are discussion and GHz Wi-Fi may require an addition gain of about 25 dB conclusion, respectively. compared with 5-GHz Wi-Fi. Beamforming using an antenna array is essential to bridge II. BEAMFORMING REQUIREMENT FOR 60-GH Z WI-FI the gap due to channel loss between 60-GHz and below 6- In this section, the requirement for beamforming in 60-GHz GHz Wi-Fi. Ideally, N transmitting antennas can provide a Wi-Fi is first explained. Afterward, the beamforming problem beamforming gain of N given the same total transmitting 2 is expressed by its mathematical models, both for frequency- power, and the total gain becomes N because N PAs emit N flat and frequency-selective channels, representing as channel times more power. At the same time, M receiving antennas matrix and tensor, respectively. This section also defines the can provide a SNR gain of M. The total ideal gain provided by notations that are used in later sections. two N × M antenna arrays is 20log 10 N + 10log 10 M in decibel unit. For example, for antenna arrays with N= M = 8 to 16 ( A. Antenna Array for Millimeter Wave 8×8 to 16 ×16 ) can provide an ideal beamforming gain of 27 According to the Friis’ formula for antenna transmission, to 36 dB. the received power is proportional to the square of the In 60-GHz SISO beamforming, multiple antennas use the wavelength. Physically, the received power is always same number of PAs as shown in Figure 1, increase the power proportional to the effective aperture of the receiving antenna consumption by a factor of N without corresponding increases that is proportional to the square of the wavelength. Because in data rate. The increase in data rate in 60-GHz Wi-Fi is of this physical limitation for free space path-loss, compared mostly due to the availability of bandwidth and beamforming with typical below 6-GHz Wi-Fi signals, 60-GHz millimeter provides SNR improvement. In 5-GHz MIMO Wi-Fi, the wave signal is about 28 or 20 dB weaker (calculated using number of independent data streams is ideally proportional to 2.45 and 5.8 GHz) for the same distance. However, due to the minimum of transmitting and receiving antennas, small wavelength of 60-GHz millimeter wave, many antennas increasing the data rate proportionally. Technically, multiple can be arranged as an array and packed into a small package to antennas in 60 GHz using SISO beamforming can only provide beamforming gain. Figure 1 shows the schematic provide diversity and SNR gain but multiple antennas in 5 diagram of a typical 60-GHz transmitter and receiver using GHz may ideally provide multiplexing gain (diversity gain as antenna arrays. The antenna arrays can be used to compensate well a choice [8]). for the transmission loss if proper beamforming is applied MIMO beamforming in 60-GHz Wi-Fi is essential to using the phase shifters in Figure 1. further increase the system throughput. Figure 2 shows the In Figure 1, the transmitting signal splits to many power measurement of the impulse response energy profile between amplifiers (PAs) and antennas. When a transmitting signal in two 36 × 36 antenna arrays. Each impulse corresponds to different antennas is phase shifted by phase shifters, the signal different reflectors that can support different beams. With up can be steered to different direction, constructively enhanced to 7 reflections within 2-3 dB, the antenna arrays definitely in certain direction, and destructively nulled in other directions can support MIMO operations and the remaining question is to [6][7]. Using phase shifters in different antennas, the receiving find a method to utilize them. antenna array can also enhance the signal in certain direction and null the signal in other directions. B. Beamforming Model and Notations Comparing the 802.11ad using 60 GHz with 802.11ac using The SISO beamforming model is the simplest for 5 GHz, in additional to free space loss, 802.11ad based 60- frequency-flat channel with a time (or frequency) independent GHz Wi-Fi also requires different power. Specially, a 4.6 Gb/s N × M channel matrix H that can be processed using methods single-carrier signal using 16-quadrature amplitude from matrix analysis [9].