IEEE 802.11 Evolution
1. Chronology of IEEE 802.11 Amendments
2. Maths of WiFi Data Rates | Modulation and Coding Scheme
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3. 802.11a/b/g
4. 802.11n: Channel Bonding, Aggregation
5. 802.11ac: Multi-User MIMO
6. 802.11ax: OFDMA
q 802.11-1997: 2 Mbps Legacy WiFi in 2.4GHz (now extinct!) q 802.11b-1999: Higher speed modulation in 2.4GHz (11Mbps)
q 802.11a-1999: Higher speed PHY (OFDM) in 5GHz (54Mbps)
q 802.11g-2003: Higher speed PHY (OFDM) in 2.4GHz (54Mbps)
q 802.11n-2009: Enhancements for higher throughput: 2.4/5GHz (600 Mbps)
q 802.11ac-2013: Very high throughput: 5GHz (~7 Gbps)
q 802.11ax-2020 (expected): High efficiency: 2.4/5GHz (~9.6Gbps, but particularly efficient for short packets and dense deployments)
Evolution:
Key 802.11 Amendments
q Each WiFi version supports a range of specific data rates Ø E.g., 802.11bà11 Mbps,
Ø 802.11aà6,9,12,18,24,36,48,54 (Mbps)
q Data rate = symbol rate x data bits per symbol q Symbol rate = number of symbols per second
q Symbol rate calculation depends on PHY Ø Direct sequence spread spectrum (802.11b)
Ø OFDM (802.11a/g/n/ac/ax)
q Data bits per symbol depends on modulation and coding
q WiFi with MIMO can have more than one independent data stream => increases data rate linearly with number of such available streams
Data Rates
IEEE 802.11b
q Direct Sequence Spread Spectrum:
Data Bits Time
Signal 01001011011011010010 Time
Chips = Code bits Multi-bit symbols with appropriate code to minimize errors
q Complementary Code Keying (CCK):
q IEEE 802.11-1997: 1⁄2 rate binary convolution encoder, 2 bit/symbol, 11
chips/symbol, DQPSK = 1⁄2 ×22 × 1/11 × 2 = 2 Mb/s using 22 MHz q IEEE 802.11b-1999: 1⁄2 rate binary convolution encoder, 8 bit/symbol, 8
chips/symbol, CCK = 1⁄2 ×22 × 1/8 × 8 = 11 Mb/s using 22 MHz
Ref: P. Roshan and J. Leary, “802.11 Wireless LAN Fundamentals,” Cisco Press, 2003, ISBN:1587050773, Safari book
q A WLAN standard is employing a spread spectrum coding with only 1⁄2 rate, which produces chips at a rate of 1⁄2 chips per Hz. It uses 8 chips to code a symbol and 16 QAM modulation to modulate the symbol stream. What would be the data rate for 22 MHz channels?
Chip rate = 1⁄2 x 22 = 11 Mcps (cps = chips per second)
Symbol rate = 11/8 = 1.375 Msps (sps = symbols per second)
Bits per symbol = log2(16) = 4 [16 QAM produces 4 bits per symbol] Data rate = symbol rate x bits per symbol = 1.375 x 4 = 5.5 Mbps
Symbol Rate for OFDM
q OFDM adopted from 802.11a onwards
q OFDM symbol rate = 1/(symbol-interval)
q Symbol interval = data interval + guard interval
q Only the data interval contain the actual symbol
q Guard interval contains no data; it is used to protect against inter symbol interference
Ø More multipathàlonger delay spreadàlonger guard intervalà reduced symbol rate
Bits per symbol
q Bits per symbol in OFDM depends on modulation order and subcarrier structure
q Total # of subcarriers in a WiFi channel = channel-bandwidth/subcarrier-spacing Ø Subcarrierspacing312.5kHzin802.11a/g/n/ac
Ø Subcarrierspacing78.125kHzin802.11ax
q Total subcarriers are divided into three categories: data subcarriers + pilot subcarriers + guard subcarriers
Ø Onlydatasubcarrierscarrydata
Ø Pilotsestimatethewirelesschannel
Ø Guards protect against interference from adjacent channels
q Each OFDM symbol is carried over all data subcarriers in parallel q M-ary modulationàlog2M bits per data subcarrier for the symbol
Ø E.g., 16-QAM yields 4 bits per data subcarrier
q Number of raw (coded) bits per symbol = log2M ✘ #-of-data-subcarriers
OFDM subcarrier structure: Black: Data, Blue: Pilot, Yellow: Guard ©2020
Impact of error coding on
data bits per symbol
q Error coding is often applied to original data to detect/correct bit errors during transmission
q Coded bit stream = original data bits + code bits
Ø E.g., with 3⁄4 code, 4 bits are transmitted for 3 data bits
Ø With 2/3 code = 3 bits are transmitted for 2 data bits, and so on Ø 3⁄4, 2/3, etc. are referred to as coding rate
q Data bits per symbol = coding rate ✘ log2M ✘ #-of-data- subcarriers
Question: What is the data rate of an OFDM WiFi applying 64-QAM and a coding rate of 3⁄4 to its 48 data subcarriers? Assume a symbol interval of 4μs.
Log2M = log264 = 6
Coded bits per symbol = log2M ✘ #-of-data-subcarriers = 6×48 = 288
Data bits per symbol = coding rate x 288 = 3⁄4 x 288 = 216
Symbol rate = 1/symbol-interval = 1⁄4 Msps (0.25 million symbols per sec.) Data rate = symbol rate x data bits per symbol = 216 x 1⁄4 Mbps = 54 Mbps
5 Key Parameters
1. Modulation (affects number of bits per symbol) Ø Usually multiple options available
2. Coding (error correction overhead)
Ø Usually multiple options available
Ø MCS (integer number) defines combination of modulation and coding
3. Guard interval (affects symbol rate)
4. Channel width à # of OFDM data subcarriers
Ø Channel width can be increased via bonding from 802.11n onwards
5. MIMO streams (number of independent data streams that can
be sent in parallel)
Ø MIMO available from 802.11n onwards
data rates
IEEE802.11a
q To increase the data rate, 802.11a uses OFDM.
q 20 MHz divided into 64 subcarriers (20000/312.5=64). 6 subcarriers at each
side are used as guards and 4 as pilot, which leaves 48 for data.
q Each OFDM symbol is carried over 48 subcarriers in parallel.
q 802.11a OFDM has a symbol interval of 4μsà0.25 M symbols/s
Ø 3200ns (data pulse) + 800ns (guard interval) = 4000ns = 4μs
q With a binary modulation (e.g., BPSK) , there will be 1 coded bit per subcarrier for each OFDM symbol, or 48 coded bits per OFDM symbol in total (over 48 subcarriers)
q Data rate depends on the combination of modulation and coding
q 802.11a supports 4 different modulations: BPSK, QPSK, 16QAM, 64QAM
q 802.11a supports three coding rates, 1⁄2, 2/3, and 3⁄4
q 802.11a supports 8 different data rates, 6 Mbps up to 54 Mbps, by selecting
a combination of modulation and coding scheme (MCS) ©2020
IEEE802.11a
1999 Data Rates
Modulation
Coding Rate
Coded bits per subcarrier
Coded bits per symbol
Data bits per symbol
Data Rate (Mbps)
IEEE 802.11g
q 802.11a was great at max. data rate of 54Mbps, but
Ø Only operates at 5GHz and not backward compatible with 11b
q 802.11g achieved 54Mbps at 2.4GHz using OFDM
Ø And could fall back to 11b rates with CCK modulation
Ø Was cheaper than 11a (2.4GHz radio had economy of scale)
q OFDM data rates are identical with 11a: Ø 6, 9, 12, 18, 24, 36, 48, 54 Mbps
q CCK data rates:
Ø 1, 2, 5.5, 11 Mbps
IEEE 802.11n
1. First WLAN to use MIMO (Multi-input Multi-Output)
2. MIMO (Multi-input Multi-Output) Multiplexing: n×m:k Þ n transmitters, m receivers, k streams k = degrees of freedom = min(n,m)
Þ k times more throughput
E.g., 2×2:2, 2×3:2, 3×2:2, 4×4:4, 8×4:4
3. The AP is expected to have more antennas than the mobile
Other New Features of IEEE 802.11n
1. Frame Aggregation: Pack multiple input frames inside a frame Þ Less overhead Þ More throughput
2. Reduced Inter-Frame Spacing (SIFS=2 μs, instead of 10 μs)
3. : Optionally eliminate support for a/b/g (shorter and
higher rate preamble)
4. Dual Band: 2.4 and 5 GHz
5. Lower FEC Overhead: 5/6 instead of 3⁄4
6. Channel Bonding: Combine two 20MHz channels to achieve 40MHz
7. Shorter Guard Interval: 400 ns instead of 800 ns
8. More OFDM subcarriers: Shorter GI in time domain enables less guard carriers in frequency domain
Ø 4 instead of 6 guard carriers on either side of data carriers
Ø 52 instead of 48 data carriers with 20 MHz legacy channels
Ø 108 data carriers with 40MHz (no guard between two legacy channels)
Guard Interval
GI GI GI GI GI GI GI GI GI GI GI
Longer GI: 5 symbols Shorter GI: 6 symbols
q Rule of Thumb: Guard Interval = 4 × Multi-path delay spread q Initial 802.11a design assumed 200ns delay spread
Þ 3200ns data + 800ns GI Þ 20% overhead (800/4000=0.2)
q Most indoor environment have smaller delay spread ~50-75ns
q So if both Tx-Rx agree, 400ns GI can be used in 802.11n Þ 3200ns data + 400ns GI Þ 11.11% overhead (400/3600=11.11)
Ref: M. Gast, “802.11n: A Survival Guide,” O’Reilly, 2012, ISBN:978-1449312046, Safari Book
Example: 11n data rate improvement
q Compared to 802.11a/g, 802.11n has higher coding rate, wider channel bandwidth, lower coding overhead, and reduced guard interval. On top of this, 802.11n uses MIMO multiplexing to further boost the data rate. Given that 802.11a/g has a data rate of 54 Mbps, can you estimate the data rate for 802.11n that uses 4 MIMO streams (assume 64 QAM for both of them, i.e., there is no improvement in modulation)?
54 Mbps is achieved with 3⁄4 coding for 3200 Data+800 GI for a/g, which basically uses a single stream (no MIMO).
802.11n has the following improvement factors:
Ø Streaming factor = 4
Ø Coding factor = (5/6)/(3/4) = ~1.11
Ø OFDM subcarrier (plus wider bandwidth) factor = (108/48) = 2.25 Ø Guard interval factor = (3200+800)/(3200+400) = ~1.11
Ø Total improvement factor = 4×1.11×2.25×1.11 = ~11.1 Improved data rate for 802.11n =
4×[(5/6)/(3/4)]×(108/48)×[(3200+800)/(3200+400)]×54 Þ 600 Mbps ©2020
Example: 802.11n maximum data rate
Question: Calculate the maximum achievable data rate for 802.11n
Minimum guard interval: 400ns (data interval=3200ns)à3.6μs symbol interval Maximum modulation: 64 QAM
Maximum coding: 5/6
Maximum # of MIMO streams: 4 (4×4 MIMO)
Maximum # of data carriers: 108 (for 40MHz bonded channels)
Coded bits per symbol = log264 ✘ #-of-data-subcarriers = 6×108 = 648
Data bits per symbol = coding rate x 648 = 5/6 x 648 = 540
Symbol rate = 1/symbol-interval = 1/3.6Msps
Data rate (single MIMO stream) = symbol rate x data bits per symbol = 1/3.6 x 540 Mbps = 150 Mbps
Data rate with 4 streams = 4 x 150 = 600 Mbps
802.11n Data Rates
Single Stream)
Source: https://www.cablefree.net/wirelesstechnology/wireless-lan/data-rates-in-802-11n/
Data rates for (Multiple Streams)
Source: https://www.cablefree.net/wirelesstechnology/wireless-lan/data-rates-in-802-11n/
Frame Aggregation
q Each layer has Service Data Units (SDUs) as input
q Each layer makes Protocol Data Units (PDUs) as output to communicate
with the corresponding layer at the other end
Ø PDUs have a header specific to the layer (header means overhead)
q Frame Aggregation: Multiple SDUs in one PDU
Ø All SDUs must have the same transmitter and receiver address
Frame Aggregation
MAC Header
MSDU Subframe 1
MSDU Subframe 2
MSDU Subframe n
MPDU Delimiter
PHY Header
MPDU Subframe 1
MPDU Subframe 2
PSDU = A-MPDU PPDU
Ref: D. Skordoulis, et al., “IEEE 802.11n MAC Frame Aggregation Mechanisms for Next-Generation High-Throughput WLANs,” IEEE Wireless Magazine, February 2008, http://tinyurl.com/k2gvl2g
IP Datagram 1
IP Datagram 2
IP Datagram 3
MPDU Subframe m
CSI Feedback
Frame Control
Duration/ ID
Seq Control
High Thr CTL
Info <7955B
16b 16b 48b 48b
48b 16b 48b 16b 32b 32b
Link Adaptation Control
Calibration Pos | Seq
1b 1b 4b 3b
2b 2b 2b 2b 1b 5b 1b 1b
q 802.11n introduced a “High Throughput Control” field to exchange channel state information (CSI)
q Receivers can derive CSI from the pilots embedded in the transmissions (e.g., OFDM pilot subcarriers), but the transmitters cannot learn it unless receivers explicitly feedback this information. This new field in 802.11n provides this opportunity
IEEE 802.11ac
IEEE 802.11ac
q 5 GHz only (2.4 GHz not supported)
q Enhanced channel bonding: 20, 40, 80, 160 MHz channels
q More data subcarriers: 52+4 (20 MHz), 108+6 (40 MHz), 234+8 (80 MHz), 468+16 (160 MHz)
Ø More data subcarriers achieved due to wider channels
Ø Subcarrier spacing remains 312.5 kHz (same as 11a/g/n)
q Higher modulation: up to 256-QAM
q More MIMO streams: up to 8 streams allowed
Ref: M. Gast, “802.11ac: A Survival Guide,” O’Reilly, July 2013, ISBN:978-1449343149, Safari Book
Question: Calculate the maximum achievable data rate for an 802.11ac mobile client with a single antenna.
Single antennaàonly 1 stream possible (even if the AP has many antennas) Minimum guard interval: 400ns (data interval=3200ns)à3.6μs symbol interval Maximum modulation: 256 QAM
Maximum coding: 5/6
Maximum # of data carriers: 468 (for 160MHz bonded channels)
Coded bits per symbol = log2256 ✘ #-of-data-subcarriers = 8x468 = 3744
Data bits per symbol = coding rate x 3744 = 5/6 x 3744 = 3120
Symbol rate = 1/symbol-interval = 1/3.6Msps
Data rate (single MIMO stream) = symbol rate x data bits per symbol = 1/3.6 x 3120 Mbps = 866.67 Mbps
802.11ac data rate (1)
Question: An 802.11ac mobile client fitted with two antennas is connected to a wireless LAN via an 802.11ac access point equipped with four antennas. Calculate the maximum achievable data rate for the mobile client.
Max. # of streams = min(2,4) = 2
Max. data rate with single stream(from previous example) = 866.67 Mbps Therefore, max. data rate with 2 streams = 2x866.67 Mbps = 1.733 Gbps
802.11ac data rate (2)
802.11ac Data Rates in Mbps (Single Stream)
Source: https://www.cablefree.net/wirelesstechnology/wireless-lan/data-rates-802-11ac/
Question: What is the maximum achievable data rate in 802.11ac?
802.11ac allows a maximum of 8 MIMO streams
Maximum achievable with single stream = 866.67 Mbps Maximum achievable data rate of 802.11ac = 8x866.67 = 6.9 Gbps
maximum data rate
q MIMO: Multiple uncorrelated spatial beams Multiple antenna’s separated by l/4 or l/2 (absolute minimum)
Ø Cannot put too many antennas on a small device; also cost increases with number of antennas
802.11ac supports
q MU-MIMO: Two single-antenna users can act as one multi- antenna device. The users do not really need to know each other. They do not even know that their antennas are used in a MU-MIMO system!
Directing streams with Beamforming in
q Single User MIMO: Colors represent transmission signals (streams) not frequency.
has 4 antennas has 1 antenna
has 1 antenna
q Multi User MIMO:
802.11n vs.
Data Rate Enhancements in 3 Dimensions
https://www.cisco.com/c/en/us/products/collateral/wireless/aironet-3600-series/white_paper_c11-713103.html
[Affects # of data subcarriers]
IEEE 802.11ax
(expected)
High Efficiency (HE):
Motivation
q Up until now, 802.11 evolution was purely driven by pushing the data rates and throughput (we were crazy about speed!)
Ø From humble 2Mbps in 1997 (802.11 legacy) to ~7Gbps in 2013 (11ac) an increase of 3500x in just 16 years!
q WiFi has become so popular and dense that we cannot really use all that speed due to congestion, collisions, and interference
q Need a new WiFi that can work efficiently in dense deployments, in outdoors, for short message communications between IoT machines etc.
q 802.11ax is more about efficiency for such new environments than pushing the data rates
q 802.11ax has only a modest data rate increase of 37% against its predecessor 802.11ac; whereas 11ac increased data rate by 10x compared to 11n
Parameters of 802.11ax
q Supports both 2.4 and 5GHz bands
q No change for coding rate: 5/6 max.
q No change for channel width: 160 MHz max for 5GHz
Ø Upto 40MHz for 2.4GHz band
q No change with MIMO stream numbers: 8 streams max.
q Increased modulation rate: 1024 QAM max.
q Increased symbol interval to address longer delay spread in challenging outdoor environments
Ø Symbol data interval increased to 12.8μs (vs. 3.2μs in 11a/g/n/ac)
Ø Guard interval increased to 0.8μs, 1.6μs, or 3.2μs (3 options allowed)
q OFDM subcarrier spacing reduced to 78.125 kHz (vs. 312.5kHz in 11a/g/n/ac)
Ø Total subcarriers: 256 (20MHz), 512 (40MHz), 1024 (80MHz), 2048 (160MHz) Ø Total subcarriers = data+pilot+guard+DC+null (5 types of subcarriers)
Ø Data subcarriers = 234 (20MHz), 468 (40MHz), 980 (80MHz), 1960 (160MHz)
802.11ax OFDM Parameters
NBPSCS: number of coded bits per data subcarrier R: coding rate
Nss: number of spatial streams
Nsd: number of data subcarriers
TDFT: symbol data interval TGI: guard interval
Question: Calculate the maximum achievable data rate for 802.11ax OFDM
Minimum guard interval: 8μs (data interval=12.8μs)à13.6μs symbol interval Maximum modulation: 1024 QAM
Maximum coding: 5/6
Maximum # of MIMO streams: 8
Maximum # of OFDM data subcarriers: 1960 (for 160MHz channels)
Coded bits per symbol = log21024 ✘ #data-subcarriers = 10x1960 = 19600
Data bits per symbol = coding rate x 19600 = 5/6 x 19600 = 16333.33
Symbol rate = 1/symbol-interval = 1/13.6Msps
Data rate (single MIMO stream) = symbol rate x data bits per symbol = 1/13.6 x 5/6 x 19600 Mbps = 1.2 Gbps
Data rate with 8 streams = 8 x 1.2 = 9.6 Gbps
802.11ax OFDM maximum
Data Rates in Mbps (Single Stream)
Source: https://www.cablefree.net/wirelesstechnology/wireless-lan/802-11ax/
OFDMA: new access control for
q OFDMA: Orthogonal Frequency Division Multiple Access
q 802.11ax introduces OFDMA as an option to centrally allocate channel resources to each competing station using fine-grained time and frequency resource units (RUs) like cellular networks
q Channel bandwidth is first divided into many narrow subcarriers
Ø Subcarrier spacing = 78.125kHz (vs. 312.5kHz in previous WiFi)
q Subcarriers are grouped into RUs called tones
q 26, 52, 106, 242, 484, or 996 tones per station. Each tone consist of a single
subcarrier of 78.125 kHz bandwidth.
q Smallest resource allocated to an OFDMA communication: 26x78.125kHz = 2031.25kHz (~2MHz)
q Largest tone has: 996x78.125kHz = 77812.5kHz (~80MHz) q A station can have a maximum of TWO 996 tones allocated
OFDM Freq.
OFDMA Freq.
OFDMA Illustrated
802.11ax RUs
Source: A Tutorial on IEEE 802.11ax High Efficiency WLANs, Khorov et al., IEEE Communications Surveys and Tutorial, 2019.
802.11ax OFDMA Parameters
NBPSCS: number of coded bits per data subcarrier R: coding rate
Nss: numbe
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