Wireless LAN II
Mainstream IEEE 802.11a/b/g/n/ac/ax/be
1. IEEE 802.11 Amendments
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2. 802.11a/b/g
3. 802.11e: Enhanced DCF, Multiple Queues, Frame Bursting
4. 802.11n [WiFi 4]: Bonding, Aggregation
5. 802.11ac [WiFi 5]: Beamforming, Multi-User MIMO
6. 802.11ax [WiFi 6]: High efficiency
7. 802.11be [WiFi 7]: Extremely high throughput
Mainstream 802.11 Amendments
802.11 Amendment
Key Enhancements
Max. Data Rate
802.11-1997
Legacy WiFi in 2.4GHz (now extinct!)
802.11b-1999
Higher speed modulation in 2.4GHz
802.11a-1999
Higher speed PHY (OFDM) in 5GHz
802.11g-2003
Higher speed PHY (OFDM) in 2.4GHz
802.11n-2009
Higher throughput in 2.4/5GHz
802.11ac-2013
Very high throughput in 5GHz
802.11ax-2020
High efficiency in 2.4/5GHz
802.11be-2024 (expected)
Extremely high throughput in 2.4/5/6GHz
IEEE 802.11b
q Direct Sequence Spread Spectrum:
Signal 01001011011011010010
q Complementary Code Keying (CCK):
Multi-bit symbols with appropriate code to minimize errors
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
Data Bits Time
Chips = Code bits Time
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
Symbol rate = 11/8 = 1.375 Msps
Bits per symbol = log2(16) = 4 [16 QAM produces 4 bits per symbol] Data rate = 1.375 x 4 = 5.5 Mbps
IEEE802.11a
q To increase the data rate, 802.11a uses OFDM.
q 20 MHz divided into 64 subcarriers. 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 OFDM has a symbol length of 4 μsà0.25 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 8 different data rates, 6 Mbps up to 54 Mbps, by selecting
a combination of modulation and coding
q 802.11a supports three coding rates, 1⁄2, 2/3, and 3⁄4 (the ratio indicates the ratio
of data bits over all coded bits transmitted).
Ø E.g., 1⁄2 indicates that only half of the total transmitted bits contain data.
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 OFDM – Same as 802.11a Þ 54 Mbps
q 2.4 GHz band Þ Cheaper than 5 GHz 802.11a q Fall back to 802.11b CCK
IEEE 802.11e
1. Hybrid Coordination Function (HCF) w two components
a. Contention Free Access: Polling
b. Contention-based Access: Enhanced DCF (EDCF)
1. Multiple Priority levels with multiple FIFO queues
2. Frame bursting and Group Acknowledge
3. Direct link
2005 (Enhanced QoS)
Enhanced DCF
q Up to 4 queues. Each Q gets a different set of four Parameters: Ø CWmin/CWmax
Ø Arbitrated Inter-Frame Spacing (AIFS) = DIFS
Ø Transmit Opportunity (TXOP) duration
q DIFS replaced by Arbitrated Inter-frame Spacing (AIFS)
Multiple Queues
Cwmin[2] Cwmax[2] AIFS[2] TXOP[2] BO[2]
Cwmin[3] Cwmax[3] AIFS[3] TXOP[3] BO[3]
Cwmin[4] Cwmax[4] AIFS[4] TXOP[4] BO[4]
Cwmin[1] Cwmax[1] AIFS[1] TXOP[1] BO[1]
q EDCF allows multiple frame transmission Ø Both individual and group ACK allowed
q Max time = Transmission Opportunity (TXOP) Ø Covers total time including multiple data frames
q Voice/gaming has high priority but small burst size q Video/audio has lower priority but large burst size
Frame Bursting
Direct Link in 802.11e
q Direct Link is another feature introduced by 802.11e
q Direct link allows two WiFi devices to communicate directly without going through
the AP, which reduces latency (great for delay-sensitive applications)
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. MIMO Beamforming: Focus the beam directly on the target
antenna for increased coverage and signal strength
4. MIMO Power Save: Use multiple antennas only when needed
IEEE 802.11n
5. Frame Aggregation: Pack multiple input frames inside a frame Þ Less overhead Þ More throughput
6. Lower FEC Overhead: 5/6 instead of 3⁄4
7. Reduced Guard Interval: 400 ns instead of 800 ns
8. Reduced Inter-Frame Spacing (SIFS=2 μs, instead of 10 μs)
9. : Optionally eliminate support for a/b/g
(shorter and higher rate preamble)
10. Dual Band: 2.4 and 5 GHz
11. Space-Time Block Code
12. Channel Bonding: Use two adjacent 20 MHz channels
13. More subcarriers: 52+4 instead of 48+4 with 20 MHz, 108+6 with 40MHz
2009 (Cont)
Guard Interval
GI GI GI GI GI GI GI GI GI GI GI
q Rule of Thumb: Guard Interval = 4 × Multi-path delay spread q Initial 802.11a design assumed 200ns delay spread
Þ800 ns GI + 3200 ns data Þ20% overhead
q Most indoor environment have smaller 50-75 ns
q So if both sides agree, 400 ns can be used in 802.11n Þ400 ns GI + 3200 ns data Þ11% overhead
Ref: M. Gast, “802.11n: A Survival Guide,” O’Reilly, 2012, ISBN:978-1449312046, Safari Book
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 ©2021
802.11n Channel Bonding
q Two adjacent 20 MHz channels used
q OFDM: 52+4 instead of 48+4 with 20 MHz,
108+6 with 40MHz (No guard subcarriers between two bands)
q Primary 20 MHz channel: Used with stations not capable of channel bonding
q Channel bonding is achieved by combining a secondary 20 MHz channel. q Secondary 20 MHz channel: Just below or just above the primary channel
(indicated by the primary channel number and up/down indicator)
Ø E.g., in 5 GHz band, 36+ would indicate that channel bonding is achieved by combining 36 and 40 (both 36 and 40 are 20 MHz channels)
Modulation, Coding, Data Rates of 802.11n: Single Stream
Example: 802.11n
Question: 802.11n can use either 20 MHz channels or 40 MHz channels with channel bonding. For 40 MHz bandwidth, data rate can be improved by what factor if channel bonding is used?
Solution: Data rate is proportional to the number of OFDM subcarriers used for data.
# of subcarriers for 20 MHz channels (no channel bonding) = 52 # of subcarriers for 40 MHz channels with channel bonding = 108
Channel bonding improvement over 40 MHz bandwidth = 108/(52+52) = 1.04 or 4%
Frame Aggregation
q Frame Bursting: Transmit multiple PDUs together q Frame Aggregation: Multiple SDUs in one PDU
All SDUs must have the same transmitter and receiver address
802.11n Frame Aggregation
IP Datagram 1
IP Datagram 2
IP Datagram 3
MAC Header
MSDU Subframe 1
MSDU Subframe 2
MSDU Subframe n
MPDU Delimiter
PHY Header
MPDU Subframe 1
MPDU Subframe 2
MPDU Subframe m
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 ©2021
802.11n MAC Frame
CSI Feedback Opportunity
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
q Supports 80 MHz and 80+80 (channel bonding) MHz channels q 5 GHz only. No 2.4 GHz.
q 256-QAM 3/4 and 5/6: 8/6 times 64-QAM Þ 1.33X (of 11n) q 8 Spatial streams: 2X (of 11n)
q Multi-User MIMO
q Less pilots/more data subcarriers: 52+4 (20 MHz), 108+6 (40
MHz), 234+8 (80 MHz), 468+16 (160 MHz)
Ref: M. Gast, “802.11ac: A Survival Guide,” O’Reilly, July 2013, ISBN:978-1449343149, Safari Book
Bandwidth and Subcarriers of 802.11ac
# of Data Subcarriers
# of Pilot Subcarriers
Modulation, Coding, Data Rates of 802.11ac: Single Stream
Beamforming
q Direct energy towards the receiver
q Requires an antenna array to alter direction per frame
Þ A.k.a. Smart Antenna
q Implicit: Channel estimation using packet loss
q Explicit: Transmitter and receiver collaborate for channel estimation
q 802.11ac supports a more “standard” beamforming so multi-vendor products can cooperate easily
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
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!
Beamforming with Multi
q Single User MIMO: Colors represent transmission signals not frequency.
has 4 antennas has 1 antenna
has 1 antenna
q Multi User MIMO:
802.11n vs. 802.11ac
https://www.cisco.com/c/en/us/products/collateral/wireless/aironet-3600-series/white_paper_c11-713103.html
Goal of 802.11ax: Efficiency vs. Speed
q Up until 802.11ac, pushing the data rates had been the main goal Ø 3500X increase from 2Mbps in 1997 to 7Gbps in 2013
Ø 802.11ac increased data rates by ~11X compared to its immediate predecessor, 802.11n
q Instead of speed, 802.11ax seeks to solve two efficiency problems:
Ø Efficient WiFi in densely deployed scenarios (urban areas) Ø Efficient communications for machines (IoT)
q 802.11ax has a modest data rate increase of only 37% against its immediate predecessor, 802.11ac
Parameters of 802.11ax
q Band: 802.11ax supports both 2.4GHz and 5GHz bands.
q Coding rate: There is no change for the coding rate; 5/6 remains the maximum
allowed coding rate.
q Channel width: There is also no change for the allowed channel width, i.e. 40MHz and 160MHz remain the maximum for 2.4GHz and 5GHz bands, respectively.
q MIMO streams: Like its predecessor, 802.11ax maintains the maximum number of MIMO streams to 8 only.
q Modulation: 802.11ax supports an increased modulation rate of up to 1024 QAM.
q Symbol interval: 802.11ax uses increased symbol intervals to address longer delay spread in challenging outdoor environments. Symbol data interval is increased to 12.8μs (vs. 3.2μs in 11a/g/n/ac) while the guard interval is also increased to 0.8μs, 1.6μs, or 3.2μs (3 options).
q OFDM subcarrier: subcarrier spacing is reduced to 78.125 kHz (vs. 312.5kHz in 11a/g/n/ac), which yields a total number of subcarriers as follows: 256 for 20MHz, 512 for 40MHz, 1024 for 80MHz, and 2048 for 160MHz, which includes two new types of subcarriers, DC and null subcarriers, in addition to the conventional data, pilot, and guard subcarriers used in previous WiFi versions. The number of data carriers available are as follows: 234 for 20MHz, 468 for 40MHz, 980 for 80MHz, and
1960 for 160MHz.
Modulation, Coding, Data Rates of 802.11ax: Single Stream
Multiple Access in 802.11ax: OFDMA
q Up until 802.11ac, CSMA had been used for channel access
q OFDMA had been used in cellular networks for many years, but for WiFi it
was introduced for the first time in 802.11ax Ø OFDMA is available as an option
q 802.11ax OFDMA
Ø Centrally allocate channel resources using fine-grained time-frequency resource
units (Rus)
Ø Subcarriers are called tones
Ø Each tone = single subcarrier of 78.125 kHz bandwidth
Ø The tones are then grouped into 6 different sizes of resource units (RUs): 26, 52, 106, 242, 484, or 996 tones
Ø 26 tones = ~2MHz (26x78.125kHz = 2031.25kHz)
Ø 996 tones = ~80MHz (996x78.125kHz = 77812.5kHz)
Ø A WiFi client can be allocated a maximum of 2 996 tones = ~160 MHz
802.11ax Resource Units
160(80+80)MHz
where +n means ‘plus n 26-tone RUs`. For example, to allocate ~20MHz, the access point would allocate 4 52-tone RUs plus 1 26-tone, which results in a total of 4x52+26=234 subcarriers allocated to the station.
Example 802.11ax
Question: A single antenna 802.11ax client receives a 26-tone RU allocation from the AP when trying to transmit a 147-byte data frame. What
could be the minimum possible time required to transmit the frame assuming at least 2 non-data subcarriers?
Single antenna means single stream
Number of data subcarriers = 26-2 = 24
Symbol length = data-interval+guard = 12.8+0.8 = 13.6μs
Maximum data rate for single-stream 26-tone 0.8μs GI ) = symbol-rate x (bits/symbol) x coding-rate
= (1/13.6) x (10x24) x (5/6)
= 14.7Mbps
Data frame length in bits: 147x8 bits
Minimum frame transmission time: (147x8)/14.7μs = 80μs
802.11be: Next Generation
q 802.11ax is perfectly capable of serving today’s needs, but
q Work on next generation WiFi must continue to keep WiFi future
q 802.11be is the next generation WiFi
Ø work has already begun; expected to release in 2024
q Data rates will be increased by enhancing several parameters Ø Increase bandwidth from 160MHz to 360MHz (6GHz band)
Ø Increase number of bits per symbol (4096 QAM)
Ø Increase MIMO streams to 16
Ø Multi-band communications (better throughput and reliability)
Ø Multi-AP coordination (better spectral efficiency and quality of experience)
802.11be vs. previous
generations
Table 5.10 Comparison of 802.11be with previous amendments
2.4/5/6GHz
Max. Channel Bandwidth
Max Modulation
16 streams
Max. Data Rate
Example 802.11be
Question: Calculate the maximum data rate of 802.11be. Solution:
Enhancements against 802.11ax:
Channel bandwidth factor: 320MHz/160MHz = 2
Modulation factor: 12 bits/symbol (log21024=10)/10 bits/symbol (log24096=12) = 1.2 MIMO factor = 16 streams/8 streams = 2
Therefore 802.11be is expected to achieve a 4.8X (2x1.2x2 = 4.8) improvement against 802.11ax.
Given that 802.11ax has a maximum data rate of 9.6Gbps, 802.11be is expected to achieve a maximum data rate of 4.8x9.6 = 46.08Gbps.
802.11be Multi
band Communications
2.4GHz 5GHz
2.4GHz 5GHz
Top: Improving throughput by allocating data from one traffic stream to multiple bands; Bottom: Improving reliability by sending duplicate data from one traffic stream over multiple bands;
802.11be Multi
AP Coordination Example
Downlink is handled by AP1, while AP2 handles the uplink.
1. 802.11a/g use OFDM with 64 subcarriers in 20 MHz, which includes 48 Data, 4 Pilot, 12 guard subcarriers.
2. 802.11e introduces 4 queues with different AIFS and TXOP durations and a QoS field in frames to provide enhanced support for QoS.
3. 802.11n adds MIMO, aggregation, dual band, and channel bonding.
4. IEEE 802.11ac supports multi-user MIMO with 80+80 MHz channels with
256-QAM and 8 streams to give 6.9 Gbps
5. IEEE 802.11ax supports 1024QAM, reduces OFDM carrier spacing to 78.125kHz and increases data symbol interval to 12.8μs. It introduced OFDMA.
6. 802.11be expects to increase data rates up to 46Gbps by using 4096 QAM, 320MHz channel bandwidth, and 16 MIMO streams. It uses 6GHz band along with 2.4GHz and 5GHz.
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