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Review of IEEE-802.11n

IEEE-802.11n was designed to allow transmissions of up to 100Mbps, thanks to this work devices are theoretically capable of transmitting at rates of up to 600Mbps (PHY data rate). This section tries to summarize what 802.11n for developers.

Contents

Review of IEEE-802.11n
Spatial streams
MCS Rates
1. HT20 rates
2. HT40 rates
Block Acks
Aggregation
1. A-MPDU
2. A-MSDU

Spatial streams

802.11n capable hardware can have more than one spatial stream. You can think of each spatial stream hardware designated for generation of traffic for TX or RX. The idea is you can have multiple spatial streams all capable of transmitting and receiving concurrently. Each stream can use multiple RX or TX chains. Streams are different than chains, each TX or RX chain consists of a dedicated radio and at least one antenna (a chain can also use antenna diversity). One stream can use one chain or move between them intelligently (this is called selection diversity). Theoretically you can use however many chains for one stream. This differs from the notion of antenna diversity on 802.11abg hardware, for example, in that with antenna diversity you are switching which antenna you use to TX or RX dynamically.

The NxM nomenclature tells us about the number of TX/RX chains, and has nothing to do with streams. It should be noted that each stream increase the overall throughput. See below for nSS on the table.

MCS Rates

With 802.11n you can use a new set of rates designed specifically for high throughput (HT). Below are the HT Modulation and Coding Schemes (MCS) rates and its dependent symbols. These come from section 20.6 form 802.11n D5.

Legend

nSS	number of spacial streams
R	coding rate
nBPCSS(iSS)	Number of coded bits per signal carrier for each spatial stream, iSS = 1..., nSS
nSD	Number of complex data numbers per spatial stream per ODFM symbol
nSP	Number of pilot values per OFDM symbol
nCBPS	Number of coded bits per OFDM symbol
nDBPS	Number of data bits per OFDM symbol

HT20 rates

HT20 mode uses 20 MHz width channels.

MCS Index	nSS	Modulation	R	nBPCSS(iSS)	nSD	nSP	nCBPS	nDBPS	Mbps (800 ns GI)	Mbps (400 ns GI)
0	1	BPSK	1/2	1	52	4	52	26	6.5	7.2
1	1	QPSK	1/2	2	52	4	104	52	13.0	14.4
2	1	QPSK	3/4	2	52	4	104	78	19.5	21.7
3	1	16-QAM	1/2	4	52	4	208	104	26.0	28.9
4	1	16-QAM	3/4	4	52	4	208	156	39.0	43.3
5	1	64-QAM	2/3	6	52	4	312	208	52.0	57.8
6	1	64-QAM	3/4	6	52	4	312	234	58.5	65.0
7	1	64-QAM	5/6	6	52	4	312	260	65.0	72.2
8	2	BPSK	1/2	1	52	4	104	52	13.0	14.4
9	2	QPSK	1/2	2	52	4	208	104	26.0	28.9
10	2	QPSK	3/4	2	52	4	208	156	39.0	43.3
11	2	16-QAM	1/2	4	52	4	416	208	52.0	57.8
12	2	16-QAM	3/4	4	52	4	416	312	78.0	86.7
13	2	64-QAM	2/3	6	52	4	624	416	104.0	115.6
14	2	64-QAM	3/4	6	52	4	624	468	117.0	130.0
15	2	64-QAM	5/6	6	52	4	624	520	130.0	144.0
16	3	BSSK	1/2	1	52	4	156	78	19.5	21.7
17	3	QPSK	1/2	2	52	4	312	156	39.0	43.3
18	3	QPSK	3/4	2	52	4	312	234	58.5	65.0
19	3	16-QAM	1/2	4	52	4	624	312	78.0	86.7
20	3	16-QAM	3/4	4	52	4	624	468	117.0	130.0
21	3	64-QAM	2/3	6	52	4	936	624	156.0	173.3
22	3	64-QAM	3/4	6	52	4	936	702	175.5	195.0
23	3	64-QAM	5/6	6	52	4	936	780	195.0	216.7
24	4	BPSK	1/2	1	52	4	208	104	26.0	28.9
25	4	QPSK	1/2	2	52	4	416	208	52.0	57.8
26	4	QPSK	3/4	2	52	4	416	312	78.0	86.7
27	4	16-QAM	1/2	4	52	4	832	624	156.0	173.3
28	4	16-QAM	3/4	4	52	4	832	624	156.0	173.3
29	4	64-QAM	2/3	6	52	4	1248	832	208.0	231.1
30	4	64-QAM	3/4	6	52	4	1248	936	234.0	260.0
31	4	64-QAM	5/6	6	52	4	1248	1040	260.0	288.9

HT40 rates

MCS Index	nSS	Modulation	R	nBPCSS(iSS)	nSD	nSP	nCBPS	nDBPS	Mbps (800 ns GI)	Mbps (400 ns GI)
0	1	BPSK	1/2	1	108	6	108	54	13.5	15.0
1	1	QPSK	1/2	2	108	6	216	108	27.0	30.0
2	1	QPSK	3/4	2	108	6	216	162	40.5	45.0
3	1	16-QAM	1/2	4	108	6	432	216	54.0	60.0
4	1	16-QAM	3/4	4	108	6	432	324	81.0	90.0
5	1	64-QAM	2/3	6	108	6	648	432	108.0	120.0
6	1	64-QAM	3/4	6	108	6	648	486	121.5	135.0
7	1	64-QAM	5/6	6	108	6	648	540	135.0	150.0
8	2	BPSK	1/2	1	108	6	216	108	27.0	30.0
9	2	QPSK	1/2	2	108	6	432	216	54.0	60.0
10	2	QPSK	3/4	2	108	6	432	324	81.0	90.0
11	2	16-QAM	1/2	4	108	6	864	432	108.0	120.0
12	2	16-QAM	3/4	4	108	6	864	648	162.0	180.0
13	2	64-QAM	2/3	6	108	6	1296	864	216.0	240.0
14	2	64-QAM	3/4	6	108	6	1296	972	243.0	270.0
15	2	64-QAM	5/6	6	108	6	1296	1080	270.0	300.0

Note that MCS 16-76 exist going up to 600 Mbps but too lazy to write that from the docs, feel free to expand. These rates also require the addition of another entry in the table, the nES (number of BCC encoders for the DATA field).

Block Acks

The idea with Block Acks is to let you acknowledge a set of frames with a single new modified type of ACK. This is extremely useful when you use aggregation. If a transmitter did not get a block ack back it will send a block ack request to the receiver after a period of time.

Aggregation

802.11n introduces two types of aggregation to increase throughput.

A-MPDU

This is the most common form of aggregation supported and used by drivers. mac80211 has full support for this type of aggregation. With it you pack together frames and each frame has its own dedicated CRC. The receiver can send a BlockAck with a bitmap of each successful frame. In the bitmap it can also specify which frame failed to force the transmitter to retransmit the single unsuccessful frame.

A-MSDU

This is not commonly used, but the standard and WiFi certification requires support for receiving A-MSDUs. mac80211 has support for this, so drivers don't need to do anything. With A-MSDUs you cannot check each individual frame as each of them does not have a dedicated CRC. If a frame is corrupt you will have to retransmit the entire aggregate.