MIMO
Multiple-input multiple-output (MIMO) is the heart of 802.11n. It takes advantage of other techniques to improve the SNR at the receiver. One technique is transmit beamforming. When there is more than one transmit antenna, it is possible to coordinate the signal sent from each antenna so that the signal at the receiver is dramatically improved. This technique is useful only when transmitting to a single receiver.
Transmit beamforming is not something that can easily be done at the transmitter without information from the receiver about the received signal. This feedback is available only from 802.11n devices, not from 802.11a, b, or g devices. To maximize the signal at the receiver, feedback from the receiver must be sent to the transmitter so that the transmitter can tune each signal it sends. This feedback is not immediate and is only valid for a short time. Any physical movement by the transmitter, receiver, or elements in the environment will quickly invalidate the parameters used for beamforming. The wavelength for a 2.4-GHz radio is only 120mm, and only 55mm for 5-GHz radio. So, a normal walking pace of 1 meter per second will rapidly move the receiver out of the spot where the transmitter’s beamforming efforts are most effective.
Transmit beamforming is useful only when transmitting to a single receiver. It is not possible to optimize the phase of the transmitted signals when sending broadcast or multicast transmissions. For this reason, in general networking applications, the utility of transmit beamforming is somewhat limited, providing improved SNR at the receiver for only those transmissions that are sent to that receiver alone. Transmit beamforming can increase the data rate available at greater distances from the AP. But, it does not increase the coverage area of an access point, since that is determined, in large part, by the ability to receive the beacons from the AP. Beacons are a broadcast transmission that does not benefit from transmit beamforming.
Another technique adopted by MIMO system is spatial diversity, which dramatically improves the SNR, providing more flexibility for the WLAN system designer. A MIMO radio sends multiple radio signals at the same time and takes advantage of multipath. Each of these signals is called a spatial stream. Each spatial stream is sent from its own antenna, using its own transmitter. Because there is some space between each of these antennae, each signal follows a slightly different path to the receiver. Each radio can also send a different data stream from the other radios. The receiver has multiple antennas as well, each with its own radio. Each of the receive radios independently decode the arriving signals. Then, each radio’s received signal is combined with the signals from the other receive radios. With a lot of complex math, the result is a much better receive signal than can be achieved with either a single antenna or even with transmit beamforming.
Radio Enhancements
802.11n uses both 20-MHz and 40-MHz channels. Like the proprietary products, the 40-MHz channels in 802.11n are two adjacent 20-MHz channels, bonded together. When using the 40-MHz bonded channel,802.11n takes advantage of the fact that each 20-MHz channel has a small amount of the channel that is reserved at the top and bottom, to reduce interference in those adjacent channels. When using 40-MHz channels, the top of the lower channel and the bottom of the upper channel don’t have to be reserved to avoid interference. These small parts of the channel can now be used to carry information. By using the two 20-MHz channels more efficiently in this way, 802.11n achieves slightly more than doubling the data rate when moving from 20-MHz to 40-MHz channels.
802.11n continues to use orthogonal frequency division multiplex OFDM and a 4-microsecond symbol, similar to 802.11a and 802.11g. However, 802.11n increases the number of subcarriers in each 20-MHz channel from 48 to 52. This marginally increases the data rate to a maximum of 65 Mbps, for a single-transmit radio. 802.11n provides a selection of eight data rates for a transmitter to use and also increases the number of transmitters allowable to four. For two transmitters, the maximum data rate is 130 Mbps. Three transmitters provide a maximum data rate of 195 Mbps. The maximum four transmitters can deliver 260 Mbps. In total, 802.11n provides up to 32 data rates for use in a
20-MHz channel.
MAC Enhancements
There is a significant amount of fixed overhead in the MAC layer protocol, and in the interframe spaces and acknowledgements of each frame transmitted, in particular. At the highest of data rates, this overhead alone can be longer than the entire data frame. In addition, contention for the air and collisions also reduce the maximum effective throughput of 802.11. 802.11n addresses these issues by making changes in the MAC layer to improve on the inefficiencies imposed by this fixed overhead and by contention losses.
Every frame transmitted by an 802.11 device has fixed overhead associated with the radio preamble and MAC frame fields that limit the effective throughput, even if the actual data rate was infinite (see Fig. 1). To reduce this overhead, 802.11n introduces frame aggregation. Frame aggregation is essentially putting two or more frames together into a single transmission. 802.11n introduces two methods for frame aggregation: Mac Service Data Units (MSDU)
aggregation and Message Protocol Data Unit (MPDU) aggregation. Both aggregation methods reduce the overhead to only a single radio preamble for each frame transmission.
another 802.11 frame; since the collected frames already begin with an 802.11 MAC header (see Fig. 3).
Power Saving
Radios are power hungry. Operating several radios requires even more power. To address this situation, 802.11n has extended the power management capability of the 802.11 MAC. There are two extensions beyond the existing mechanisms established in the original standard and the automatic power save delivery added in 802.11e. The two new mechanisms provided by 802.11n are Spatial Multiplexing (SM) Power Save and Power Save Multi-Poll.
The SM power save mode allows an 802.11n client to power down all but one of its radios. This power save mode has two submodes of operation:
static operation and dynamic operation. The static SM power save mode has the client turn off all but a single radio, becoming essentially equivalent to an 802.11a or 802.11g clients. The client’s access point is notified that the client is now operating in the static single-radio mode, requiring the access point to send only a single
spatial stream to this client until the client notifies the access point that its additional radios are again enabled and operating. This notification of the access point is done using a new management frame, defined by 802.11n, telling the access point that the client is in static SM power save mode. The dynamic SM power save mode also turns off all but one of the client’s radios. But in this mode of operation, the client can rapidly enable its additional radios when it receives a frame that is addressed to it. The client can immediately return to the low power state by disabling its additional radios immediately after its frame reception is complete. In this mode of operation, the access point typically sends a request-to-send (RTS) frame to the client, to wake its radios, prior to sending the client a data or management frame. On receiving the RTS frame, the client enables its radios and responds with a clear-to-send (CTS) frame. All of its radios are now ready to receive the multiple spatial streams sent by all the radios in the access point. To use this power save mode, the 802.11n client sends a new management frame to its access point, informing the access point that it is in dynamic SM power save mode.
The Power Save Multi-Poll (PSMP) mode extends the Automatic Power Save Delivery (APSD) mechanism defined in 802.11e. Using APSD, a client informs an access point that frames of some specified QoS levels should be buffered (delivery enabled) until the client requests them, while frames sent by the client at another set of specified QoS levels are to be considered triggers that will cause the delivery of buffered frames. In 802.11e, this is typically used by WLAN handsets to reduce power expenditures during active calls, by specifying that voice packets be buffered at the access point until a voice packet is sent by the client. Once the access point receives the client’s voice packet, it delivers the voice packet that is buffered and waiting to be sent.