IEEE 802.11ah

IEEE 802.11ah is a wireless networking protocol that is an amendment of the IEEE 802.11-2007 wireless networking standard. It uses sub-1 GHz license-exempt bands to provide extended range Wi-Fi networks, compared to conventional Wi-Fi networks operating in the 2.4 GHz and 5 GHz bands. It also benefits from lower energy consumption, allowing the creation of large groups of stations or sensors that cooperate to share the signal, supporting the concept of the Internet of Things (IoT).[1] The protocol is intended to be competitive with Bluetooth with its low power consumption, but with a wider coverage range.[2]

The standard is expected to be finalized and arrive in 2016, with chips and systems based on 802.11ah hitting the market as early as 2015.[3][4]

In January 2016, the Wi-Fi alliance announced an extension of 802.11ah to be called Wi-Fi HaLow, pronounced "HAY-Low".[2][5]

Description

A benefit of 802.11ah is extended range, making it useful for rural communications and offloading cell phone tower traffic.[3] The other purpose of the protocol is to allow low rate 802.11 wireless stations to be used in the sub-gigahertz spectrum.[1] The protocol is one of the IEEE 802.11 technologies which is the most different from the LAN model, especially concerning medium contention. A prominent aspect of 802.11ah is the behaviour of stations that are grouped to minimize contention on the air media, use relay to extend their reach, use little power thanks to predefined wake/doze periods, are still able to send data at high speed under some negotiated conditions and use sectored antennas. It uses the 802.11a/g specification that is down sampled to provide 26 channels, each of them able to provide 100 kbit/s throughput. It can cover a one-kilometer radius . It aims at providing connectivity to thousands of devices under an access point. The protocol supports machine to machine (M2M) markets, like smart metering. [6]

Data rates

Data rates up to 347 Mbit/s are achieved only with the maximum of four spatial streams using one 16 MHz-wide channel. Various modulation schemes and coding rates are defined by the standard and are represented by a Modulation and Coding Scheme (MCS) index value. The table below shows the relationships between the variables that allow for the maximum data rate. GI (Guard Interval) : Timing between symbols.

2 MHz channel uses an FFT of 64, of which: 56 OFDM subcarriers, 52 are for data and 4 are pilot tones with a carrier separation of 31.25 kHz (2 MHz/64) (32 µs). Each of these subcarriers can be a BPSK, QPSK, 16-QAM, 64-QAM or 256-QAM. The total bandwidth is 2 MHz with an occupied bandwidth of 1.78 MHz. Total symbol duration is 36 or 40 microseconds, which includes a guard interval of 4 or 8 microseconds.[7]

Modulation and coding schemes
MCS
index[lower-alpha 1] 		Spatial
Streams		 Modulation
type		 Coding
rate		 Data rate (in Mbit/s)[lower-alpha 2][7]
2 MHz channels 4 MHz channels 8 MHz channels 16 MHz channels
8 μs GI[lower-alpha 3] 4 μs GI 8 μs GI 4 μs GI 8 μs GI 4 μs GI 8 μs GI 4 μs GI
0 1 BPSK 1/2 0.65 0.72 1.35 1.5 2.93 3.25 5.85 6.5
1 1 QPSK 1/2 1.3 1.44 2.7 3.0 5.85 6.5 11.7 13.0
2 1 QPSK 3/4 1.95 2.17 4.05 4.5 8.78 9.75 17.6 19.5
3 1 16-QAM 1/2 2.6 2.89 5.4 6.0 11.7 13.0 23.4 26.0
4 1 16-QAM 3/4 3.9 4.33 8.1 9.0 17.6 19.5 35.1 39.0
5 1 64-QAM 2/3 5.2 5.78 10.8 12.0 23.4 26.0 46.8 52.0
6 1 64-QAM 3/4 5.85 6.5 12.2 13.5 26.3 29.3 52.7 58.5
7 1 64-QAM 5/6 6.5 7.22 13.5 15.0 29.3 32.5 58.5 65.0
8 1 256-QAM 3/4 7.8 8.67 16.2 18.0 35.1 39.0 70.2 78.0
9 1 256-QAM 5/6 N/A N/A 18.0 20.0 39.0 43.3 78.0 86.7
0 2 BPSK 1/2 1.3 1.44 2.7 3.0 5.85 6.5 11.7 13.0
1 2 QPSK 1/2 2.6 2.89 5.4 6.0 11.7 13.0 23.4 26.0
2 2 QPSK 3/4 3.9 4.33 8.1 9.0 17.6 19.5 35.1 39.0
3 2 16-QAM 1/2 5.2 5.78 10.8 12.0 23.4 26.0 46.8 52.0
4 2 16-QAM 3/4 7.8 8.67 16.2 18.0 35.1 39.0 70.2 78.0
5 2 64-QAM 2/3 10.4 11.6 21.6 24.0 46.8 52.0 93.6 104
6 2 64-QAM 3/4 11.7 13.0 24.3 27.0 52.7 58.5 105 117
7 2 64-QAM 5/6 13.0 14.4 27.0 30.0 58.5 65.0 117 130
8 2 256-QAM 3/4 15.6 17.3 32.4 36.0 70.2 78.0 140 156
9 2 256-QAM 5/6 N/A N/A 36.0 40.0 78.0 86.7 156 173
0 3 BPSK 1/2 1.95 2.17 4.05 4.5 8.78 9.75 17.6 19.5
1 3 QPSK 1/2 3.9 4.33 8.1 9.0 17.6 19.5 35.1 39.0
2 3 QPSK 3/4 5.85 6.5 12.2 13.5 26.3 29.3 52.7 58.5
3 3 16-QAM 1/2 7.8 8.67 16.2 18.0 35.1 39.0 70.2 78.0
4 3 16-QAM 3/4 11.7 13.0 24.3 27.0 52.7 58.5 105 117
5 3 64-QAM 2/3 15.6 17.3 32.4 36.0 70.2 78.0 140 156
6 3 64-QAM 3/4 17.6 19.5 36.5 40.5 N/A N/A 158 176
7 3 64-QAM 5/6 19.5 21.7 40.5 45.0 87.8 97.5 176 195
8 3 256-QAM 3/4 23.4 26.0 48.6 54.0 105 117 211 234
9 3 256-QAM 5/6 26.0 28.9 54.0 60.0 117 130 N/A N/A

Relay Access Point

A Relay Access Point (AP) is an entity that logically consists of a Relay and a networking station (STA), or client. The relay function allows an AP and stations to exchange frames with one another by the way of a relay. The introduction of a relay allows stations to use higher MCSs (Modulation and Coding Schemes) and reduce the time stations will stay in Active mode. This improves battery life of stations. Relay stations may also provide connectivity for stations located outside the coverage of the AP. There is an overhead cost on overall network efficiency and increased complexity with the use of relay stations. To limit this overhead, the relaying function shall be bi-directional and limited to two hops only.

Power saving

Power saving stations are divided into two classes: TIM stations and non-TIM stations. TIM stations periodically receive information about buffered traffic for them from the access point in so-called TIM information element, hence the name. Non-TIM stations use the new Target Wake Time mechanism which allows to reduce signaling overhead. [8]

Target Wake Time

Target Wake Time (TWT) is a function that permits an AP to define a specific time or set of times for individual stations to access the medium. The STA (client) and the AP exchange information that includes an expected activity duration to allow the AP to control the amount of contention and overlap among competing STA. The AP can protect the expected duration of activity with various protection mechanisms. The use of TWT is negotiated between an AP and a STA. Target Wake Time may be used to reduce network energy consumption, as stations that use it can enter a doze state until their TWT arrives.

Restricted Access Window

Restricted Access Window allows partitioning of the stations within a Basic Service Set (BSS) into groups and restricting channel access only to stations belonging to a given group at any given time period. It helps to reduce contention and to avoid simultaneous transmissions from a large number of stations hidden from each other. [9]

Bi Directional TXOP

Bi Directional TXOP allows an AP and non-AP (STA or client) to exchange a sequence of uplink and downlink frames during a reserved time (transmit opportunity or TXOP). This operation mode is intended to reduce the number of contention-based channel accesses, improve channel efficiency by minimizing the number of frame exchanges required for uplink and downlink data frames, and enable stations to extend battery lifetime by keeping Awake times short. This continuous frame exchange is done both uplink and downlink between the pair of stations. In earlier versions of the standard Bi Directional TXOP was called Speed Frame Exchange. [10]

Sectorization

The partition of the coverage area of a Basic Service Set (BSS) into sectors, each containing a subset of stations, is called sectorization. This partitioning is achieved through a set of antennas or a set of synthesized antenna beams to cover different sectors of the BSS. The goal of the sectorization is to reduce medium contention or interference by the reduced number of stations within a sector and/or to allow spatial sharing among overlapping BSS (OBSS) APs or stations.

Comparison with 802.11af

Another WLAN standard for sub-1 GHz bands is IEEE 802.11af which, as opposed to 802.11ah which operates in license-exempt bands, operates in TV white space spectrum in the VHF and UHF bands between 54 and 790 MHz using cognitive radio technology.[11]

See also

IEEE 802.11 network standards

802.11 network PHY standards
802.11
protocol		 Release
date[12]		 Fre-
quency		 Band-
width		 Stream data rate[13] Allowable
MIMO streams		 Modulation Approximate
range
Indoor Outdoor
(GHz) (MHz) (Mbit/s) (m) (ft) (m) (ft)
802.11-1997 Jun 1997 2.4 22 1, 2 N/A DSSS, FHSS 20 66 100 330
a Sep 1999 5 20 6, 9, 12, 18, 24, 36, 48, 54 N/A OFDM 35 115 120 390
3.7[A] — — 5,000 16,000[A]
b Sep 1999 2.4 22 1, 2, 5.5, 11 N/A DSSS 35 115 140 460
g Jun 2003 2.4 20 6, 9, 12, 18, 24, 36, 48, 54 N/A OFDM 38 125 140 460
n Oct 2009 2.4/5 20 400 ns GI : 7.2, 14.4, 21.7, 28.9, 43.3, 57.8, 65, 72.2 [B]
800 ns GI : 6.5, 13, 19.5, 26, 39, 52, 58.5, 65 [C]		 4 MIMO-OFDM 70 230 250 820[14]
40 400 ns GI : 15, 30, 45, 60, 90, 120, 135, 150 [B]
800 ns GI : 13.5, 27, 40.5, 54, 81, 108, 121.5, 135 [C]		 70 230 250 820[14]
ac Dec 2013 5 20 400 ns GI : 7.2, 14.4, 21.7, 28.9, 43.3, 57.8, 65, 72.2, 86.7, 96.3 [B]
800 ns GI : 6.5, 13, 19.5, 26, 39, 52, 58.5, 65, 78, 86.7 [C]		 8 35 115[15]
40 400 ns GI : 15, 30, 45, 60, 90, 120, 135, 150, 180, 200 [B]
800 ns GI : 13.5, 27, 40.5, 54, 81, 108, 121.5, 135, 162, 180 [C]		 35 115[15]
80 400 ns GI : 32.5, 65, 97.5, 130, 195, 260, 292.5, 325, 390, 433.3 [B]
800 ns GI : 29.2, 58.5, 87.8, 117, 175.5, 234, 263.2, 292.5, 351, 390 [C]		 35 115[15]
160 400 ns GI : 65, 130, 195, 260, 390, 520, 585, 650, 780, 866.7 [B]
800 ns GI : 58.5, 117, 175.5, 234, 351, 468, 702, 780 [C]		 35 115[15]
ad Dec 2012 60 2,160 Up to 6,912 (6.75 Gbit/s) [16] N/A OFDM, single carrier,
low-power single carrier		 60 200 100 300
ah Est. 2016[12] 0.9
aj Est. 2016[12] 45/60
ax Est. 2019[12] 2.4/5 MIMO-OFDM
ay 2017 60 8000 Up to 100,000 (100 Gbit/s) 4 OFDM, single carrier,
 60 200 1000 3000

Notes

  1. ↑ MCS 9 is not applicable to all channel width/spatial stream combinations.
  2. ↑ A second stream doubles the theoretical data rate, a third one triples it, etc.
  3. ↑ GI stands for the guard interval.

References

  1. 1 2 "Wi-Fi Advanced 802.11ah". Qualcomm.com. Retrieved 2014-06-25.
  2. 1 2 "There's a new type of Wi-Fi, and it's designed to connect your smart home". theverge.com. 2016-01-04. Retrieved 2015-01-04.
  3. 1 2 Tammy Parker (2013-09-02). "Wi-Fi preps for 900 MHz with 802.11ah". FierceWirelessTech.com. Retrieved 2014-06-25.
  4. ↑ "Wi-Fi for the Internet of things gets a name: Wi-Fi HaLow". Computech Technology Services Blog. Computechtechnologyservices.com. Retrieved 2016-01-07.
  5. ↑ Wi-Fi Alliance introduces low power, long range Wi-Fi HaLow; wi-fi.org; January 4, 2016.
  6. ↑ AustPrasadNiemegeers 2012.
  7. 1 2 Sun, Weiping; Choi, Munhwan; Choi, Sunghyun (July 2013). "IEEE 802.11ah: A Long Range 802.11 WLAN at Sub 1 GHz" (PDF). Journal of ICT Standardization. 1 (1): 83–108. doi:10.13052/jicts2245-800X.125.
  8. ↑ SunChoiChoi 2013, p. 43, 5.2 Power Saving.
  9. ↑ KhorovLyakhovKrotovGuschin 2014, 4.3.2. Restricted Access Window.
  10. ↑ KhorovLyakhovKrotovGuschin 2014, 4.3.1. Virtual carrier sense.
  11. ↑ Flores, Adriana B.; Guerra, Ryan E.; Knightly, Edward W.; Ecclesine, Peter; Pandey, Santosh (October 2013). "IEEE 802.11af: A Standard for TV White Space Spectrum Sharing" (PDF). IEEE. Retrieved 2013-12-29.
  12. 1 2 3 4 "Official IEEE 802.11 working group project timelines". June 2, 2014. Retrieved 2014-06-03.
  13. ↑ "Wi-Fi CERTIFIED n: Longer-Range, Faster-Throughput, Multimedia-Grade Wi-Fi® Networks" (registration required). Wi-Fi Alliance. September 2009.
  14. 1 2 "802.11n Delivers Better Range". Wi-Fi Planet. 2007-05-31.
  15. 1 2 3 4 "IEEE 802.11ac: What Does it Mean for Test?" (PDF). LitePoint. October 2013.
  16. ↑ "WiGig and IEEE 802.11ad For Multi-Gigabyte-Per-Second WPAN and WLAN" (PDF). Tensorcom Inc.

Bibliography

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