OMAP Wireless Connectivity WLAN rates

WiFi Data Rates versus Environment

The practical data rates or throughput in the WLAN differ depends on the specific standard (802.11a/b/g/n), the number of users in the network and the the distance from the access point. For example, in an 802.11g networks the data rate drops to as low as 1 or 2 Mbps at 50 meters, although the maximum rate is 54 Mbps. Physical barriers such as walls will further reduce the maximum rate possible at a given distance from the AP.

802.11a/b/g Data Rates

Protocol	Data rate (Mbit/s)	Modulation Method
802.11b 802.11g	1	DSSS/DBPSK
802.11b 802.11g	2	DSSS/DQPSK
802.11b 802.11g	5.5, 11	CCK/DQPSK
802.11g 802.11a	6, 9	OFDM/BPSK
802.11g 802.11a	12, 18	OFDM/QPSK
802.11g 802.11a	24, 36	OFDM/16-QAM
802.11g 802.11a	48, 54	OFDM/64-QAM

802.11n Data Rates - MCS (Modulation and Coding Scheme)

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

MCS index	Spatial streams	Modulation type	Code rate	Data rate (Mbit/s)
				20 MHz channel		40 MHz channel
				800 ns Guard interval	400 ns Guard interval	800 ns Guard interval	400 ns Guard interval
0	1	BPSK	1/2	6.50	7.20	13.50	15.00
1	1	QPSK	1/2	13.00	14.40	27.00	30.00
2	1	QPSK	3/4	19.50	21.70	40.50	45.00
3	1	16-QAM	1/2	26.00	28.90	54.00	60.00
4	1	16-QAM	3/4	39.00	43.30	81.00	90.00
5	1	64-QAM	2/3	52.00	57.80	108.00	120.00
6	1	64-QAM	3/4	58.50	65.00	121.50	135.00
7	1	64-QAM	5/6	65.00	72.20	135.00	150.00
8	2	BPSK	1/2	13.00	14.40	27.00	30.00
9	2	QPSK	1/2	26.00	28.90	54.00	60.00
10	2	QPSK	3/4	39.00	43.30	81.00	90.00
11	2	16-QAM	1/2	52.00	57.80	108.00	120.00
12	2	16-QAM	3/4	78.00	86.70	162.00	180.00
13	2	64-QAM	2/3	104.00	115.60	216.00	240.00
14	2	64-QAM	3/4	117.00	130.00	243.00	270.00
15	2	64-QAM	5/6	130.00	144.40	270.00	300.00
16	3	BPSK	1/2	19.50	21.70	40.50	45.00
17	3	QPSK	1/2	39.00	43.30	81.00	90.00
18	3	QPSK	3/4	58.50	65.00	121.50	135.00
19	3	16-QAM	1/2	78.00	86.70	162.00	180.00
20	3	16-QAM	3/4	117.00	130.70	243.00	270.00
21	3	64-QAM	2/3	156.00	173.30	324.00	360.00
22	3	64-QAM	3/4	175.50	195.00	364.50	405.00
23	3	64-QAM	5/6	195.00	216.70	405.00	450.00
24	4	BPSK	1/2	26.00	28.80	54.00	60.00
25	4	QPSK	1/2	52.00	57.60	108.00	120.00
26	4	QPSK	3/4	78.00	86.80	162.00	180.00
27	4	16-QAM	1/2	104.00	115.60	216.00	240.00
28	4	16-QAM	3/4	156.00	173.20	324.00	360.00
29	4	64-QAM	2/3	208.00	231.20	432.00	480.00
30	4	64-QAM	3/4	234.00	260.00	486.00	540.00
31	4	64-QAM	5/6	260.00	288.80	540.00	600.00

Folowing is the formula to calculate the 802.11n data rate:

802.11n data rate = Base 802.11n data rate * number of Spatial streams * 2.077 (if channel bonding) * 1.11 (if shorten Gurd Interval)

The Data rate elements is present in:

• Association requests (from Station)
• Beacons (from AP)
• Association responses (from AP)

Following is the Rates fields in Association Response frame:

BG band rates:



N Band rates:




Guard Time support is defined in <HT Capability info>-><HT Capability info> field


Number of Spatial Streams is defined in <Basic MCS Set>-><Tx Max Num. Spatial Streams Supported> field


Protection mode is defined in <HT Information>-><HT Info Element 2> field

Data rate example

Following is an example of data packets rate, such as data packet (ping), WLAN Acknowledgement, beacon and so on.



In addition per packet, the rate can be seen in the <Packet Info>->< Data Rate> field

Glossary

• Channel Type

There are two channel types - 20MHz and 40 MHz. Our chip currently supports 20 MHz channel.

• Guard Interval

Guard intervals are used to ensure that distinct transmissions do not interfere with one another.

• Spatial Streams

In the MCS table it indicates the number of antenas used to transmit and received.

In general, Spatial Multiplexing (seen abbreviated SM or SMX) is a transmission technique in MIMO wireless communication to transmit independent and separately encoded data signals, so-called streams, from each of the multiple transmit antennas. Therefore, the space dimension is reused, or multiplexed, more than one time.

• Code Rate

In telecommunication and information theory, the code rate (or information rate) of a forward error correction code is the proportion of the data-stream that is useful (non-redundant). That is, if the code rate is k/n, for every k bits of useful information, the coder generates totally n bits of data, of which n-k are redundant.

• MCS Index

Each MCS index value defines a set of Spatial Streams, Modulation Type and Code Rate.

Our chip supports MCS indexes 0 to 7 only.