WiGig and IEEE 802.

11ad For Multi-Gigabyte-Per-Second WPAN and WLAN

SaiShankar N, Debashis Dash, Hassan El Madi, and Guru Gopalakrishnan
Tensorcom Inc., 5900 Pasteur Court, Carlsbad, CA 92008
Abstract:
The Wireless Gigabit Alliance (WiGig) and IEEE 802.11ad are developing a multigigabit wireless
personal and local area network (WPAN/WLAN) specication in the 60 GHz millimeter wave
band. Chipset manufacturers, original equipment manufacturers (OEMs), and telecom
companies are also assisting in this development. 60 GHz millimeter wave transmission will
scale the speed of WLANs and WPANs to 6.75 Gbit/s over distances less than 10 meters. This
technology is the first of its kind and will eliminate the need for cable around personal
computers, docking stations, and other consumer electronic devices. High-definition
multimedia interface (HDMI), display port, USB 3.0, and peripheral component interconnect
express (PCIe) 3.0 cables will all be eliminated. Fast downloads and uploads, wireless sync, and
multi-gigabit-per-second WLANs will be possible over shorter distances. 60 GHz millimeter
wave supports fast session transfer (FST) protocol, which makes it backward compatible with 5
GHz or 2.4 GHz WLAN so that end users experience the same range as in todays WLANs. IEEE
802.11ad species the physical (PHY) sublayer and medium access control (MAC) sublayer of
the protocol stack. The MAC protocol is based on time-division multiple access (TDMA), and the
PHY layer uses single carrier (SC) and orthogonal frequency division multiplexing (OFDM) to
simultaneously enable low-power, high-performance applications.

Keywords: 60 GHz communications; IEEE standards; WiGig; 802.11ad; contention based access
protocol; scheduled protocol; Beamforming; power save

1 Introduction
There is a huge amount of unlicensed spectrum available worldwide in the 60 GHz band.
Academia and industry have turned to the 60 GHz spectrum because of the universal
availability of unlicensed spectrum, the ever-growing number of user applications creating
heavy data traffic, and the need to reduce data transfer times. Considerable efforts have been
made to use this spectrum and spur the development of silicon, similar to what happened with
the 2.4 GHz ISM band 15 years ago. 60 GHz millimeter wave technologies offers a way to
provide end users with guaranteed quality of service (QoS) for different applications. Fig. 1
shows the allocation for 60 GHz in different countries [1].
Figure 1. Spectrum allocation for WiGig.

60 GHz millimeter wave technologies create significant problems in designing the radio
frequency (RF) front-end, processing gigabit-per-second data, and migrating to 40 nm and 28
nm low-power technologies in designing the silicon, considerable progress have been made in
making it practical and feasible [4]-[6]. 60 GHz millimeter wave systems are needed to cater for
newer applications, such as streaming video in the home or office, that have flourished as a
result of last-mile access provided by internet service providers (ISPs). Such systems will also
eliminate the need for cables around docking stations, and this will reduce clutter and allow
easier connection between devices. There are multiple industry organizations involved in 60
GHz standardization, the notable ones being Wireless HD [7], IEEE 802.15.3c [8], WiGig [9], and
IEEE 802.11ad [10]. The last two of these organizations involve a large number of silicon, OEM,
and telecom companies that are motivated to have a single worldwide 60 GHz standard.
WiGig began standardization in 2008 and has recently released the WiGig 1.0 standard. IEEE
802.11ad also began standardization in 2008 and has recently released IEEE 802.11ad Draft 9.0
standard. These standards are similar, and in this paper, we will refer to 802.11ad as the
representative of both standards, pointing out when there is a feature that is unique to the
WiGig standard. Similar standardization efforts have been made by ECMA-387 and CMMW
Study Group [2], [3]. 60 GHz millimeter wave is the next wireless networking technology and
will appear in the market around 2014 [11]. It is poised to repeat the successes of Bluetooth
and Wi-Fi [12]. This explosive growth of the wireless industry in such a short time can also be
attributed to the opening of unlicensed bands in 60 GHz by the Federal Communications
Commission (FCC).

802.11ad aims to develop the protocol adaptation layers (PALs) to support a plethora of
applications that will arise from the elimination of cables and from fast wireless sync and
transfer. The PALs being considered by WiGig include wireless serial extension (WSE), which
eliminates USB 3.0 cables; wireless bus extension (WBE), which eliminates PCIe 3.0 cables;
wireless display extension (WDE), which eliminates high-definition multimedia interface (HDMI)
and display port cables; and wireless secure digital (WSD), which makes secure digital
input/output card (SDIO) disks wireless. The first important 60 GHz millimeter wave application
to enter the market as wireless docking based on PCIe 3.0with one second-generation lane
(also called x2)or USB 3.0. All devices with 802.11ad MAC/PHY/Radio use the corresponding
PALs between the application and MAC layers to seamlessly transfer information between
devices as if the devices were connected by wires. Another 60 GHz application is wireless HDMI
based on WDE, which allows transfer of uncompressed bits from devices such as set top boxes
and blue ray disc players to television screens and from laptops, desktops, or ultrabooks to
monitors via a display port cable replacement. The WDE also supports H264 compressed rates
for handling variations in the wireless channel and to ensure seamless content delivery to the
end users. Performance of the PHY and MAC protocols is analyzed in [13] and [14].
In this paper, we describe the novel features of the MAC and PHY sublayers of the protocol
stack defined in 802.11ad. In section 2, we describe the TDMA protocol and the need for
directionality in 60 GHz. In section 3, we outline the 802.11ad PHY layer, and in section 4, we
outline the MAC layer. In section 5, we outline the beamforming protocol, and in section 6 we
outline the power-saving protocol. In section 7, we describe the fast session transfer, and in
section 8, we show achievable rates using different MAC- and PHY-layer packet transmission
options. Section 9 concludes the paper.

2 TDMA Protocol and the Need for Directionality

Interest in the 30300 MHz millimeter wave spectrum has increased significantly because of
low-cost, high-performance CMOS technology and because of low-loss, low-cost organic
packaging. A millimeter-wave radio can be empowered for the same cost as a radio operating in
the 5 GHz band or lower. This advantage, combined with wide available bandwidth, makes the
millimeter-wave spectrum more attractive than ever before for supporting new systems and
applications. A millimeter wave signal can propagate over a few kilometers at lower frequencies,
penetrating through different construction materials and deriving advantages from reflection
and refraction; however, they are highly directional and can be sustained only over short
distances. The reason for this directionality is explained by the Friis free space equation:
2 2
PR = PT GTGR /4R (1)
where PR, PT, GT, GR, and R is the receive power, transmit power, transmit antenna gain,
receive antenna gain, wavelength, and distance between the transmitter and receiver,
respectively. There is a 22 dB loss when we move from 5 GHz to 60 GHz. This loss is due to
lower wavelength and can be offset by using directional antennas with higher gains. If 2 GHz
bandwidth was used in 60 GHz and PT = 10 dBm, the noise figure Nf = 10 dB, and the shadow
fading margin = 6 dB, 1 Gbit/s throughput could not be achieved (Fig. 2). Therefore, the gains
of directional antennas must be exploited to achieve higher rates.
Figure 2. Shannon capacity versus transmit power at 60 Hz [1].

Directional communication requires complex discovery and beamforming protocols to establish

links between different devices. Scheduled protocols such as TDMA are needed at the MAC
layer to guarantee QoS at multi-gigabit-per-second rates. Randomized access protocols such as
CSMA come with a random overhead that can depend on the number of users contending for
the channel. Although CSMA/CA is still used to handle bursty traffic, allocation of
contention-based access periods (CBAPs) is based on TDMA.

3 Physical Layer
802.11ad defines four different PHY layers: Control PHY, SC PHY, OFDM PHY and low-power
SC PHY (LPSC PHY). Control PHY is MCS 0. SC starts at MCS 1 and ends at MCS 12; OFDM PHY
starts at MCS 13 and ends at MCS 24; and LPSC starts at MCS 25 and ends at MCS 31. MCS 0
to MCS 4 are mandatory PHY MCSs. Here, we briey describe the different PHYs and their
packet structures. The system clock rate is 2640 MHz, and this rate is used for OFDM also.
Control, SC and LPSC PHYs have a clock rate of 2/3 2640 = 1760 MHz.

3.1 General Packet Structure

As is common with all 802.11 packet formats, the packet consists of a short training sequence, a
channel estimation sequence, the physical layer convergence procedure (PLCP) header, MAC
packet, and cyclic redundancy check (CRC). Although there are different PHYs, they all have this
unique structure, which ensures implementers do not need to change packet formats when
using different PHYs. The only difference is that each PHY is a different size and uses a different
Golay code.
Figure 3. IEEE 802.11ad packet structure.

The short training field (STF) and channel estimation field (CEF) help signal acquisition,
automatic gain control training, predicting the characteristics of the channel for the decoder,
frequency offset estimation and synchronization. Both STF and CEF sequences use Golay codes.
The PLCP header indicates the size of the packet as well as the modulation structure (MCS) of
the packet. The MAC packet comprises MAC header and data and contains information about
the destination. CRC ensures that the packet is not corrupted while being transmitted through
the air. 802.11ad has a TRN field comprising Golay codes. This field is used in beam tracking and
refinement and is described in section 5. Fig. 3 shows a typical packet in 802.11ad.

3.2 Control PHY

The control PHY, MCS 0, is the minimum rate that all devices use to communicate with before
establishing a high-rate beamformed link. To help discovery and detection, the control PHY has
an STF comprising 50 Golay sequences, each of which is 128 samples long. The CEF that follows
the STF has nine Golay sequences. The STF comprises 48 Gb128(n) (each 128 samples long)
followed by a single repetition of -Gb128(n). The CEF comprises Gu512(n) and Gv512(n)
followed by Gv128(n). These sequences are represented as function of Ga128(n) and Gb128(n)
in section 21.11 of the IEEE 802.11ad draft specification [10]. Gu512(n) and Gv512(n) are given
by:

Gu512(n) = [-Gb128-Ga128Gb128Ga128] (2)

Gv512(n) = [-Gb128Ga128-Gb128-Ga128] (3)

The control PHY uses BPSK with a code rate of 1/2 and is spread using a 32 code to create a PHY
rate of 27.5 Mbit/s. The Control PHY is used for transmitting and receiving frames such as
beacons, information request and response, probe request and response, sector sweep, sector
sweep feedback, and other management and control frames. It provides reliability and exploits
gain of the transmit antenna. Additionally, the first frame transmitted during the beam
refinement protocol (BRP) phase is also a control PHY frame.

3.3 Single-Carrier PHY, OFDM PHY and LPSC PHY

MCSs 1 to 4 are mandatory and ensure that all devices, irrespective of their PHY, are
interoperable. All the MCSs, with the exception of LPSC PHY, use LDPC code, and the LPSC uses
Reed Solomon (RS) codes. The following two subsections describe the MAC header and data
packet encoding process for SC PHY. Other PHYs use a similar encoding process. All packets are
modulated using BPSK, QPSK, 16-QAM and 64-QAM (Table 1).
 Table 1. PHY modulation and coding scheme table

MCS Code NDBPS/ DataRate

Modulation NCBPS Repetitions NBPCS
Index Rate Coding (Mbit/s)
0 /2 BPSK 1 32 1/2 1 168 27.5
1 /2 BPSK 1 2 1/2 1 168 385.0
2 /2 BPSK 1 1 1/2 1 168 770.0
3 /2 BPSK 1 1 5/8 1 168 962.5
4 /2 BPSK 1 1 3/4 1 168 1155.0
5 /2 BPSK 1 1 13/16 1 168 1251.25
6 /2 QPSK 2 1 1/2 1 168 1540.0
7 /2 QPSK 2 1 5/8 1 168 1925.0
8 /2 QPSK 2 1 3/4 1 168 2310.0
9 /2 QPSK 2 1 13/16 1 168 2502.5
10 /2 16 QAM 4 1 1/2 1 168 3080.0
11 /2 16 QAM 4 1 5/8 1 168 3850.0
12 /2 16 QAM 4 1 3/4 1 168 4620.0
13 SQPSK 336 1 1/2 1 168 693.0
14 SQPSK 336 1 5/8 1 210 866.25
15 QPSK 672 1 1/2 2 336 1386.0
16 QPSK 672 1 5/8 2 420 1732.5
17 QPSK 672 1 3/4 2 504 2079.0
18 16-QAM 1344 1 1/2 4 672 2772.0
19 16-QAM 1344 1 5/8 4 840 3465.0
20 16-QAM 1344 1 3/4 4 1008 4158.0
21 16-QAM 1344 1 13/16 4 1092 4504.0
22 64-QAM 2016 1 5/8 6 1260 5179.0
23 64-QAM 2016 1 3/4 6 1512 6237.0
24 64-QAM 2016 1 13/16 6 1638 6756.75
RS(224,208)+ 626.0
25 /2 BPSK 392 1 13/16 6
BS(16,8)
RS(224,208)+ 834.0
26 /2 BPSK 392 1 13/16 6
BS(12,8)
RS(224,208)+ 1112.0
27 /2 BPSK 392 1 13/16 6
SPC(9,8)
RS(224,208)+ 1251.0
28 /2 QPSK 392 1 13/16 6
BS(16,8)
RS(224,208)+ 1668.0
29 /2 QPSK 392 1 13/16 6
BS(12,8)
RS(224,208)+ 2224.0
30 /2 QPSK 392 1 13/16 6
SPC(9,8)
 RS(224,208)+ 2503.0
31 /2 QPSK 392 1 13/16 6
BC(8,8)

3.4 Header Encoding

The header is encoded using a single SC block of NCBPB symbols with NGI guard symbols. The bits
are scrambled and encoded in the following steps:

1) The input header bits (b1, b2,...,bLH) LH = 64 are scrambled, starting from the eighth bit, in
order to create d1s = (q1, q2,...,qLH).
2) The LDPC code word c = (q1, q2,...,qLH, 01, 02,...,0504-LH, p1, p2,...,p168)is created by
concatenating 504-LH zeros to the LH bits of d1s and then generating the parity bits p1,
p2,...,p168 so that HcT = 0, where H is the parity-check matrix for the 3/4 LDPC code
specification in IEEE 802.11ad.
3) Bits LH + 1 through 504 and bits 665 through 672 of the code word c are removed to create
the sequence cs1 = (q1, q2,...,qLH, p1, p2,...,p160).
4) Bits LH +1 through 504 and bits 657 through 664 of the code word c are removed to create
the sequence cs2 = (q1, q2,...,qLH, p1, p2,...,p152, p161, p162,...,p168) and then to create XOR with a
PN sequence. The PN sequence is generated from the LFSR used for data scrambling, and the
LFSR is initialized to the all-ones vector.
5) cs1 and cs2 are concatenated to form the sequence (cs1, cs2). The resulting 448 bits are then
mapped as /2-BPSK, and the NGI guard symbols are prepended to the resulting NCBPB
symbols.

3.3 Data Encoding

The data packet is encoded using LDPC, which includes deciding the number of
shortening/repetition bits in every code word, shortening, coding each word, and repetition of
bits. Data packet encoding occurs in the following steps:

1) The number of LDPC code words is given by NCW = (length 8 )/(LCW R). This is used to
calculate the number of datapad bits given by NDATA_pad = (NCW LCW R)/ (length 8),
where LCW = 672 is the LDPC code word length; length is the length of the PSDU defined in the
header field (in octets); is the repetition factor (1 or 2); and R is the code rate. The scrambled
PSDU is concatenated with NDATAP AD zeros, which are scrambled using the continuation of
the sequence that scrambled the PSDU input bits.
2) The output stream of the scrambler is broken into blocks of LCWD = LCW R bits so that the
mth data word is b1m ,b2m ,..., bLCWD
m
m < NCW
3) To each data word, n - k = LCW (R LCW) parity bits p1m , p2m ,..., pn-k
m
are added to create the
T
code word c m = b1m , b2m ,..., bLCWD
m
, p1m , p2m ,..., pn-k
m
so that Hc(m ) 0
4) The code words are concatenated one after the other to create the coded bitstream c1, c2,,
cLCWD NCW The number of symbol blocks is given by NBLK = (NCW LCW) / NCBPB , and the number
of symbol block padding bits is given by NBLKPAD = (NBLK NCBPB) (NCW LCW), where NCBPB is the
number of coded bits per symbol block.
5) The coded bitstream is concatenated with NBLKPAD zeros, which are scrambled using
the continuation of the scrambler sequence that scrambled the PSDU input bits. Table I
shows the MCS values allowed in the IEEE 802.11ad specification.

4 Overview of 802.11AD MAC Protocol

4.1 802.11ad Superframe
The 802.11ad superframe is called the beacon interval and comprises a beacon transmission
interval (BTI), a data transfer interval (DTI), and optional association beamforming training
(A-BFT) or announcement transmission intervals (ATI) (Figs. 4 and 10). The DTI can include one
or more service periods (SPs) and contention-based access periods (CBAPs).

4.2 Service Period Channel Access

An SP is a scheduled access period between two stations: a transmitter and receiver. SPs are
suitable for directional (high-gain) antenna use. New features introduced by 802.11ad include
spatial sharing, where SPs need not be out of synch, that is, an SP between stations A and B
may occur at the same time as another SP between stations C and D. IEEE 802.11ad also
introduces dynamic SP allocation, truncation, and extension. An SP allocation is dynamic if it is
not initially scheduled by the personal basic service set (PBSS) control point/access point
(PCP/AP) but is scheduled during an existing SP or CBAP. SP truncation occurs when the
transmitter station relinquishes the remaining time in its SP. SP extension occurs when the
transmitter station extends the SP duration it had been allocated.

4.3 CBAP Channel Access

During a CBAP, all stations contend for channel access using a hybrid TDMA-CSMA/CA scheme
based on 802.11 enhanced distributed channel access (EDCA). 802.11ad provides physical
carrier sensing mechanism, provided by the physical layer, and a virtual carrier-sensing
mechanism, provided by the MAC layer. Physical carrier sensing uses clear channel assessment
(CCA), and
BTI: beacon transmission interval
DTI: data transfer interval
CBAP: contention-based access periods
ATI: announcement transmission intervals
STA: peer station

Figure 4. Examples of 802.11ad beacon intervals.

Virtual carrier sensing uses a timer called network allocation vector (NAV). NAV indicates, in
microseconds, how long the channel is reserved by another station and counts down to 0. The
virtual carrier-sensing mechanism uses request-to-send/directional multigigabit clear-to-send
(RTS/DMG CTS) frames. When a station receives an RTS/DMG CTS frame, it sets its NAV to the
value in the Duration field in the MAC header of the RTS/DMG CTS. Stations also use the
Duration field of other frames to update their NAVs; however, the frames destination address
must be different from the receiving stations MAC address, and the value of the Duration field
must be greater than the current NAV value. Stations may have a unique NAV or may have one
NAV per sector. If a station has multiple NAVs and at least one has a non-zero value, the virtual
carrier-sensing mechanism considers the medium busy. The medium is considered busy if either
the physical or virtual carrier-sensing mechanism indicates it is busy; otherwise, it is considered
idle. 802.11ad defines four different access categories (ACs) that have different priorities based
on the user priority (UP) of the data being transferred. In order of increasing priority, these ACs
are background (BK), best effort (BE), video (VI), and voice (VO). Only BE is mandatory in the
standard, that is, only BE is implemented or all four are. When all four ACs are implemented, all
ACs within a given station have to contend with each other and with other stations for channel
access. Each AC contends for a channel in the following way: After the medium has been idle
for a period of time, called the arbitration interframe space (AIFS [AC]), the station contending
for access randomly sets its backoff timer to a between 0 and the contention window (CW [AC]).
CW [AC] is initialized to CWmin [AC] and is updated after every transmission. In case of
transmission failure, CW [AC] is updated using

CW [AC] = 2 CW [AC] + 1 (4)

When CW [AC] reaches CWmax [AC], it remains unchanged for any remaining retries. In the case
of transmission success, CW [AC] is reset to CWmin [AC]. At every slot time boundary, the
medium is sensed. If the medium is found to be idle, the backoff timer is decremented;
otherwise, it is suspended. When the backoff timer for a particular AC reaches 0, that AC
obtains exclusive channel access for a period of time called the transmit opportunity (TXOP
[AC]). During the TXOP [AC], only frames with UP mapping to that AC may be transmitted. If the
backoff timers of two or more ACs reach zero at the same time, channel access is granted to the
AC with the highest priority, and the other ACs treat this occurrence as if it were an external
collision that happened in the wireless medium. The other ACs then enter backoff phase. For
each AC, EDCA parameters such as CWmin [AC], CWmax [AC], AIFS [AC] and TXOP [AC] are
calculated by the PCP/AP and included in the DMG beacon, probe response, or (re)association
response frames transmitted by the PCP/AP. Higher-priority ACs are granted lower values for
CWmin [AC], CWmax [AC], AIFS [AC] so that they can gain channel access while lower-priority ACs
are still in backoff phase.

4.4 Packet Aggregation

802.11ad is capable of a multi-gigabyte-per-second data rate because of features such as
packet aggregation and block acknowledgement in addition to directional antennas. The basic
MAC data unit is called MAC protocol data unit (MPDU) and comprises a MAC header and a
MAC service data unit (MSDU) or MAC payload. A PHY header and an MPDU comprise a PHY
PDU (PPDU). 802.11ad uses the Galois/counter mode (GCM) protocol for data encryption.
This protocol was designed for encryption at multi-gigabytes-per-second data rates.

MPDU: MAC protocol data unit

MSDU: MAC service data unit

Figure 5. 802.11ad MPDU structure.

GCMP: GCM protocol

Figure 6. 802.11ad MPDU structure with encryption turned on.

An encrypted MPDU includes a GCM protocol header and a MIC field. Fig. 5 shows the MPDU
structure. Fig. 6 shows the MPDU structure with encryption turned on. Packet aggregation
involves combining several packets into a single packet. When several MSDUs or MPDUs are
combined, the resulting packet is called aggregated MSDU (A-MSDU) (Fig. 7) or aggregated
MPDU (A-MPDU) (Fig. 8). 802.11ad uses a new type of packet aggregation called aggregated
PPDU (A-PPDU). In an A-PPDU packet, several PPDUs are transmitted back-to-back without
interframe spacing (IFS) and preamble in between. A-PPDU reduces overhead associated with
IFS and MAC/PHY header processing.

AMSDU: aggregated MAC service data unit

FCS: frame check sequence

Figure 7. 802.11ad A-MSDU structure.

 AMPDU: aggregated MAC protocol data unit
Figure 8. 802.11ad A-MPDU structure.

4.5 Acknowledgement Policies

802.11ad defines a frame acknowledgement (ACK) policy called block acknowledgement.
When block ACK is enabled, the transmitting station transmits a block of frames one frame at a
time immediately after each other without waiting for the receiver to acknowledge the
previous frame. After the entire block of frames has been transmitted, the receiving station
sends a control frame called block ACK that includes a bitmap. The bitmap, in which each bit
corresponds to a frame, indicates which frames were received successfully and which ones
were not. The receiver knows to send a block ACK frame when it receives a block ACK request
frame from the transmitter. This ACK policy allows the transmitter to use shorter IFS between
frames, and it eliminates the IFS between each frame and its individual ACK frame, as in a
typical stop-and-wait protocol. Fig. 9 shows normal ACK and block ACK policies.

Figure 9. Normal ACK policy and block ACK policy.

5 Beamforming Protocol
Because of the highly directional nature of 60 GHz communications, the transmitter and
receiver antennas need to be aligned in the right direction to obtain maximum gain. 802.11ad
supports up to four transmitter antennas, four receiver antennas, and 128 sectors.
Beamforming is mandatory in 802.11ad, and both transmitter-side and receiver-side
beamforming are supported.

Beamforming can be done at the transmitter side, receiver side, or at both sides [15].
Transmitter-side beamforming usually requires feedback from the receiver, especially when the
transmitter-to-receiver and receiver-to-transmitter channels are not reciprocal. The need for
feedback can be reduced or eliminated by using space time codes; however, this can cause
considerable overhead in the setting-up beamforming [16]. 802.11ad uses a selection-based
protocol in which the transmitter sends training from certain sectors that are pre-defined
according to distinct antenna patterns created by changing the antenna weights [17]. The
receiver antenna maintains an omnidirectional pattern and measures the strength of the
received signal from the different sectors. It responds with information about the best sector
and measured quality. With this feedback, the transmitter chooses the best sector to use while
transmitting to the receiver. Similarly, in receiver-side training, the receiver repeats the training
from the transmitter, which is sent using an omnidirectional antenna pattern, and measuring
the strength of the received signal through pre-defined receive sectors.

The station that starts the beamforming training is called the initiator, and the recipient is
called the responder. Beamforming in 802.11ad involves sector-level sweep (SLS), beam
renement protocol (BRP), and beam tracking (BT). Fig. 10 shows the sequence of this
beamforming. Each of the steps in the sequence is allowed in a particular part of the beacon
interval (Fig. 11). SLS enables reliable communication at the lowest supported rate (called MCS0
in 802.11ad). Usually, transmitter-side training is done during the SLS. The BRP enables receiver
training and iteratively trains the transmitter and receiver sides to improve on the values found
during the SLS. Both SLS and BRP phases use their own special packets for beamforming
training. By contrast, BT can be done during data transmission. It is used to track the
beamforming state and improve it during data transmission. BT is implemented by adding
training (TRN) elds to the back of a data packet.

The SLS involves initiator sector sweep (ISS), responder sector sweep (RSS), sector sweep
feedback (SSW-FB), and sector sweep acknowledgement (SSW-ACK). The BRP comprises setup,
multiple sector ID detection (MID), beam combining (BC), and BRP transactions. Of these, MID
and BC are optional features for 802.11ad supporting stations. BT comprises BT request and BT
response. The parameters for exchanging beamforming packets are obtained using the
capability element in the beacon packets, probe request/response packets, or information
request/response packets.
Figure 10. The sequence in which the different phases of beamforming occur in 802.11ad/WiGig standard.

Figure 11. Different transmission periods and BF phases allowed in each part of the beacon interval.

5.1 Beaconing and Sector-Level Sweep

At the start of every BTI, the PCP/AP MAC schedules an initiator transmit sector sweep (TXSS) to
transmit beacons through all sectors. The PCP/AP can also fragment the TXSS across multiple
beacon intervals if the BTI is insufficient and cannot complete the TXSS. A station without a
PCP/AP uses an omnidirectional receiving antenna configuration to scan non-associated
beacons or receive associated beacons from the PCP/AP and determine the best
sector/antenna ID using the sector sweep field at the end of TXSS. In a beacon, CDOWN is the
number of pending beacon transmissions for completion of TXSS, with 0 being completion.
Figure 12. A sample DMG beacon transmission by a PCP having one transmit antenna with four sectors.

If multiple transmit antennas are supported, a PCP/AP station cannot switch its transmit
antennas for beacon transmission within a BTI nor can it transmit a beacon more than once
using the same antenna conguration. To minimize potential interference, the PCP/AP
changes the order of sectors across beacon intervals if multiple directional beacon
transmissions are required or waits for a random delay at the start of beacon interval if only a
single beacon is to be transmitted (Fig. 12).

5.2 A-BFT Protocol

The PCP/AP announces the existence of an A-BFT period in the beacons, and this information is
used for association and beamforming training for new stations, such as PBSS stations, that join
the network. A-BFT may not be present in each beacon interval and may be periodically
inserted by the AP/PCP. The A-BFT period allows stations to perform RSS and SSW-FB phases of
beamforming with the PCP/AP. It is assumed that the new station has already used the beacons
sent from all the transmit sectors of the PCP/AP to perform an ISS with the PCP/AP during the
BTI. The A-BFT period is a slotted phase where each slot is a multiple of the time required for
RSS and SSW-FB. The new stations use random backoff to select the A-BFT slot for an RSS.
When beamforming is done during A-BFT, the SSW-ACK phase is skipped, and BRP is done
during the data transmission interval if necessary. There may be no chance to do RSS during
A-BFT because stations use random access; therefore, the AP/PCP may schedule an SP to
continue beamforming with the particular station.

5.3 Sector-Level Sweep

The SLS is the basic type of beamforming supported by 802.11ad. It comprises ISS, RSS, SSW-FB
and SSW-ACK. The link from the initiator to the responder is called the initiator link, and the link
from the responder to the initiator is called the responder link. During SLS, the ISS phase is used
to train the initiator link, and the RSS is used to train the responder link. The RSS contains
feedback about the best sector found during ISS, and the SSW-FB contains the best sector
found in the RSS. In SLS is concluded with an SSW-ACK (Fig. 13). A station can have separate
transmitter and receiver chains with their own antenna configurations. Hence, for each of the
initiator and responder links, the transmitter and receiver can be trained independently to
perform beamforming. The protocol used to train the transmitter during SLS is called TXSS, and
the protocol used to train the receiver is called RXSS. This gives rise to four possibilities; if an ISS
is used to the train the transmitter side of the initiator link, the phase is called ISS TXSS.
Similarly, the other three possibilities are ISS RXSS, RSS TXSS, and RSS RXSS.
During TXSS, the transmitter sends a separate SSW frame from different available transmit
sectors, the number of which can be pre-negotiated between stations. The receiver maintains a
quasi-omni receive configuration. If the receiver has multiple antennas, the transmitter repeats
this process for each receive antenna. The receiver measures the quality of the packet received
from each of the transmit sectors by cycling through all of its receive antennas in quasi-omni
mode. In the end, the receiver replies with the best sector. The standard allows a
vendor-specific algorithm to decide the best sector. Similarly, during RXSS, the transmitter uses
an omniantenna configuration, and the receiver changes receive sectors to determine which
the best receive sector.

The SLS phase can be initiated by the PCP/AP during the BTI by performing an ISS. Then, the RSS
and FB phases are completed during the A-BFT announced by the PCP/AP. In this case, the BRP
phase completed in an ATI or DTI. Alternatively, a station can either use a CBAP period (also
announced in the beacon) or schedule an SP to perform beamforming with another station. The
DTI can be used for all the phases of beamforming (Fig. 11).

Figure 13. Parts of the sector level sweep and beam refinement phases of the beamforming in 802.11ad.
D Beam Refinement

The BRP phase comprises a setup phase followed by a beam-renement phase based on
request-response (Fig. 13). The request-response packets of the setup phase are exchanged
until the responder (receiver) sets the capability-request field in the BRP packet at 0. This is
followed by a response from the initiator (transmitter) with the capability-request field set at 0.

The beam-refinement request can be a transmitter- or receiver-refinement request. A

transmitter-refinement request indicates the need for transmitter antenna training by the
transmission station and vice versa. The transmitter station adds TRN-T subfields to the BRP
frame. The receiver station holds data that it obtained by measuring the TRN-T fields. The
receiver station responds to a receiver-refinement request (sent by the transmitter station) by
appending TRN-R subfields to its response frames, that is, an ACK or block ACK frame.

The SLS and BRP phases of beamforming usually precede data transmission. They are
completed right at the beginning of beamforming and are repeated periodically as needed.
Beamtracking is used for beamforming training during data transmission to accommodate
channel changes between two SLS/BRP beamforming training phases. In beamtracking, training
elds comprising CE and STF fields are attached to the back of data packets or, for example,
ACK/BA, to train the transmitter or receiver (Fig. 3). 802.11ad allows for three types of
beamtracking, and the type of beamtracking is signaled using three parameters in the PLCP
header. The three parameters of interest are packet type, training length, and beamtracking
request. Of these, the training length is always greater than zero. If the training length is zero,
the other two fields are reserved, and the packet does not contain any beamtracking training or
request. Table 2 shows the types of training indicated by the PLCP parameters.

Table 2. Types of beamtracking indicated by the PLCP parameters.

Parameters BT type Explanation

The transmitter attaches TRN-T

BT request = 1 elds to the current packet and the
Send TRN-T
Packet Type = 1 receiver sends back a BRP frame
with the feedback.

The transmitter attaches TRN-R

BT request = 0
Send TRN-R elds to the current packet and the
Packet Type = 0
receiver nds its own best sector.

The transmitter is requesting

BT request = 1 Request receiver training. In the next
Packet Type = 0 TRN-R packet, the receiver attaches TRN-R
elds.

6 Power-Save Protocol
Dedicated SPs in 802.11ad allow battery-powered stations to hibernate during data
transmission periods that are not assigned to them. An 802.11ad station can be in one of the
two power-save states: doze or awake. When awake, a station is fully powered; when in doze,
the station is powered off. A stations power-save state in various sections in a beacon interval
depends on whether the station is in active mode or power-save mode. In power-save mode,
stations with or without PCP/AP can doze for one or more consecutive BIs, or sections of a BI,
more if they were permanently in active mode. A station must check its peers wakeup
schedule before sending any individually addressed MPDUs to the peer station because it may
be in doze mode. A station without PCP/AP can always use information request/response
frames to request the wakeup schedule from any of its peers if required. Fig. 14 shows power
management modes and state transitions.
Figure 14. Power management-mode/state transitions.

a) b)
Figure 15. Fast session transfer done by peer stations (STAs) A and B in an a) 2.4 GHz channel and b) a 60 GHz
channel.

A station without PCP/AP that has not established a wakeup schedule with its peer is in active
mode. To switch from active to power-save mode, the station establishes a wakeup schedule
with PCP/AP. It does this by including a wakeup schedule (WS) element in its power-save
conguration request. To switch from power-save mode to active mode, the station without
PCP/AP sends a power-save configuration request in which the power management bit is set at
0. The station immediately switches to active mode upon receipt of the ACK frame from
PCP/AP.

A PCP/AP station includes its WS in its beacon or announcement frames before switching to
power-save mode. When switching back to active mode, it ceases including the WS in these
frames. The PCP/AP station keeps track of the wakeup schedules of all associated stations
without PCP/AP. In addition, APs also have to buffer MPDUs addressed to associated stations in
doze state and forward these MPDUs at designated times.

7 Fast Session Transfer Protocol

Fast session transfer (FST) protocol allows different streams or sessions to transfer smoothly
from one channel to another in the same band or different bands. This protocol makes
802.11ad compatible with the forthcoming 802.11ac standard and other existing standards,
such as 802.11a/b/g/n. The protocol allows different radios in the same device to operate
simultaneously or not simultaneously. Devices with 802.11ac and 802.11ad can have same MAC
address or different MAC addresses. If the same MAC address is used for all the radios in
different bands then FST is in transparent mode. If the MAC addresses differ according to
channel/band, then FST is not transparent.
A simple example of FST is a video stream to be established between STA A and STA B in the 2.4
GHz band using direct-link setup (DLS). The STAs are 40 m apart. The video uses 802.11n radio
at 144.4 Mbit/s and H.264 compression. After some time, the user of STA A moves very close to
STA B so that the separation is less than 3 m. Both STA A and STA B understand that they have
60 GHz radio, which was discovered during in 60 GHz discovery mode. They then transition to
60 GHz channel 2 and use an uncompressed stream by closing their link at MCS 12, which is
4.62 Gbit/s (Fig. 15).
This video stream established in 60 GHz channel 2 can be moved to 60 GHz channel 1 if there is
congestion in channel 2. It can also be moved to channel 4 in 5 GHz or channel 6 in 2.4 GHz
when STA A starts to move away from STA B. In this example, video compression, such as H.264
and that used in the WiGig WDE specication, ensures that the session does not drop because
of large range or insufficient bandwidth. The application and MAC layers interact with PHY to
optimize the smooth delivery of content to the end application. They compress the video
whenever the band is not 60 GHz by using IEEE 802.11ac or IEEE 802.11n and then transition to
uncompressed video using 60 GHz SC or OFDM modes when the range is less than three meters.
This ensures the highest QoS. The FST also ensures that a subset of streams can be transferred
from one channel/band to another while the remaining streams are in the original
channel/band.
Fig. 16 shows the basic FST protocol. The initiator and responder are assumed to have one
station management entity (SME) and two MAC layer management entities (MLMEs) that
correspond to two different radios. In device discovery, both the STA and PCP recognize that
they have multiple radios through beacon/information request and response frames. Then, the
STA decides to establish an FST session with the PCP/AP in the other band or new channel.
Any signal exchanges before the FST decision box is to establish the FST session with PCP/AP
and is done in the current channel. Once the decision to transfer the session or stream has
been decided at the predetermined times at both the STA and PCP/AP, the actual transition to
the new channel occurs. Then, the basic FST ACK information is sent and exchanged to signal
that both the STA and PCP/AP have transitioned to the new channel.
FST: fast session transfer
MLME: MAC layer management entities
PCP: PBSS control point
SME: Station management Entity
STA: peer station

Figure 16. FST protocol exchange between STA and PCP when moving from one band/channel to another.

8 Packet Throughput

The modulation and coding schemes in section 3 along with the normal ACK, A-MSDU, and
A-MPDU packet structures in section 4 allow the 60 GHz radios to change between very
different achievable throughputs depending on the packet size. Fig. 17 shows the MAC layer
throughput versus packet size sent at different MCS values. Similarly, Fig. 18 shows the
throughput versus packet size when A-MPDU is used. In Figs. 17 and 18, BTI, ABFT and AT
overheads are not taken into account. The parameters described in section 3 can also be found
in section 21.3 of [9].
All the throughput curves saturate with respect to the packet size. In general, longer packets
can go through the channel without error if the channel remains constant for the entire
duration; that is, longer packets need longer channel coherence time. Throughput curves
saturate for larger packet sizes, and even though the channel might be good enough to support
longer packets, it is better to use a higher MCS than bigger packets (through aggregation), once
the length is close to the saturation point.
Figure 17. Single carrier throughput as a function of the packet size for different MCS values for non-aggregated
packets.
Figure 18. Single carrier throughput as a function of the packet size for different MCS values for A-MPDU packets.

9 Conclusion
802.11ad are standardizing 60 GHz technology to facilitate multi-gigabit-per-second
communications over shorter distances. This standard has many new features to improve and
sustain high-speed communications with TDMA single-carrier and OFDM schemes. They allow
for scheduled and contention-based access, beamforming, and power-save mechanisms that
decrease power consumption and increase throughput. Future evolution of 802.11ad towards
full MIMO support and channel bonding can further increase its data rate. With the advent of
new technologies to make these protocols practical, and with standardization by bodies such as
WiGig and IEEE, truly wireless broadband will be achieved with 60 GHz, and all wires in PANs
will be eliminated.

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Biographies

Sai Shankar N received his Ph.D from Indian Institute of Science, Bangalore in the year 1998 and
worked on DAAD fellowship on Queueing approaches in manufacturing at Dept. of Mathematics,
University of Kaiserslautern, Germany. In 1999, he joined Philips Research, Eindhoven, Netherlands to
work on IEEE 802.15 HFC networks and on Differentiated services. In 2001 he was transferred to Philips
Research, New York and worked on IEEE 802.11e, IEEE 802.11n, MBOA UWB and IEEE 802.22. To this
end he was nominated as one of the five finalists in EE Times for his contributions to UWB MAC. In 2005,
he worked at Qualcomm in San Diego on IEEE 802.11s and RLC and MAC-hs issues in HSPA. In 2007 he
worked at Broadcom Corporation working on 802.11 AMP in Bluetooth SIG and 60 GHz. Currently he is
with Tensorcom leading its 60 GHz FW and MAC HW solution. He can be reached at
nsai@tensorcom.com.

Debashis Dash received his B. Tech. degree from the Indian Institute of Technology, Kanpur, India, in
2004 and his M.S. degree from Rice University, Houston, in 2007. He is a Ph.D. candidate with the
Department of Electrical and Computer Engineering, Rice University, Houston, TX. He currently works at
Tensorcom, San Diego, CA. His research interests include information theory and graph theory and their
applications in wireless systems.

Hassan El Madi received his B.S. degree in computer engineering from the University of California, San
Diego in 2007 and his M.Eng. degree in electrical engineering with an emphasis in wireless
communications from Virginia Tech, Blacksburg, VA in 2010. He currently works as a software staff
engineer at Tensorcom, San Diego, CA. His interests include cognitive radios (spectrum sensing,
automatic modulation classification, geolocalization) as well as the design and implementation of WLAN
and WPAN MAC and PHY layers.

Guru Gopalakrishnan received his B.E. degree in Electronics and Communication from Anna University,
India in 2006 and his M.S. degree in Electrical Engineering (Computer Networks) from University of
Southern California, Los Angeles in 2009. He earlier worked at Broadcom Corporation, San Diego in
Bluetooth and Bluetooth low energy technologies and is currently at Adeptence, San Diego. His research
interests include throughput and power optimizations for wireless systems.