Since their commercial debut in 1999, wireless LANs based on the IEEE 802.11 or “Wi-Fi” standards have been one of the major success stories in wireless. Freeing users from their network tethers increased the productivity of office workers, and allowed mobile unified communications solutions to take root. Along the way it also revolutionized home networking and launched the market for commercial Hot Spots.
The most exciting development has been the introduction of the new high capacity 802.11n radio link. While the exiting 802.11a or g radios have a maximum capacity of 54 Mbps, 802.11n could boost that to 289 Mbps in the same size channel (i.e. 20 MHz) and 600 Mbps in a double channel (i.e. 40 MHz). Not only will 802.11n boost the transmission rate, it will also increase the effective transmission range and improve the overall reliability. In radio, an increase in transmission rate normally comes at the expense of range or reliability, but 802.11n has added an important new factor. Let’s take a closer look
Magic Words: OFDM and MIMO
The two elements that we are seeing in most new radio interfaces are Orthogonal Frequency Division Multiplexing (OFDM) and Multiple Input-Multiple Output (MIMO) antenna systems. These are the keys to 802.11n, but they are also incorporated in WiMAX and will probably be used in the 4G cellular standards as well.
OFDM is the fundamental technique used to encode the data onto the radio carrier; it’s used with 802.11a and g as well as with 802.11n. In OFDM transmission, the data to be transmitted is first divided into a number of sub-channels or streams. The 802.11a and g standards use 48 sub-channels while 802.11n uses 52 (in a 20 MHz channel). The bandwidth (i.e. frequency range) of the radio channel is divided into an equal number of sub-carriers or "tones".
Each of the data streams is then sent on one of the sub-carriers. In essence, rather than using one modem whose signal occupies the entire transmission band, a number of narrowband modems (i.e. 48 or 52 plus some additional pilot tones) are sent in parallel each using a slice of the available bandwidth. The sub-carriers actually overlap to a degree, but they are arranged in a way that minimizes interference among them; that's where the orthogonal part comes in.
The advantage of the OFDM approach is that it is more bandwidth efficient (i.e. carriers more bits per second in the available radio bandwidth) and is less sensitive to multipath or interference caused by echoes created when the radio signal bounces off hard, flat surfaces.
The 802.11a and g radio links also use OFDM transmission, but what gives 802.11n its major capacity boost is an optional MIMO antenna system. The basic idea of a MIMO transmitter is that it divides the bitstream into a number of transmit chains (up to 4 in 802.11n), andsendseach as a separate, simultaneous radio transmission. None of the existing chipsets can generate four chains, so it is important to determine the maximum rate your devices can actually achieve. The key is that all of the transmit chains are OFDM modulated and sent on the same frequency channel.
Normally if we have four transmitters all sending on the same channel, they will interfere with each other and nobody gets through. The trick in a MIMO system is that the transmitting antennas are placed some distance apart, a technique called spatial multiplexing. As the signals are originating from different points in space, the receiver is able to distinguish each of them by itsunique arrangement of multipath images (i.e. the original signal and the delayed echoes of that signal produced by its bouncing off obstacles in the environment). The amazing thing is that the transmitters do not have to be spaced that far apart to produce recognizably different images; the minimum spacing is one-half wavelength or about 3-inches for 2.4 GHz or 1.5-inches for 5 GHz transmitters.
As well as having multiple transmitters or outputs, a MIMO system can also have multiple receivers or inputs. Signals from some transmitters will likely be stronger at some receivers than others.However, the total energy of each transmit signal is combined to improve reception; this capability also improves the performance of legacy a/b/g transmissions.The result is that a MIMO system can increase the transmission capacity through spatial multiplexing andincrease the range and the reliability by combining the receive signals from each input antenna.
Besides the inherent advantage of MIMO transmission, 802.11n incorporates some other important features. First, 802.11n can operate in either the 2.4 GHz ISM or the 5 GHz U-NII bands. As we noted, it can use the standard 20 MHz channel employed in 802.11a or g, or a 40 MHz double channel. The maximum transmission rate is 289 Mbps in a 20 MHz channel, and 600 Mbps in a 40 MHz channel. The obvious downside is that if you use 40 MHz channels, you will have roughly half as many channels to work with in laying out your network. The standard also reduces some of the timer intervals and adds other efficiencies to the MAC protocol. Finally, the 802.11n devices can share channels with legacy a/b/g devices, though that will result in a significant reduction in throughput for the n-devices.
Conclusion
Wireless LANs have been one of the great technology successes in the past decade, and that success has been driven by progressive advances in inter-operable products. However, with the introduction of 802.11n we are starting to see the limitations created by the requirement to maintain backwards compatibility with transmission rates that can go as low as 1 Mbps.
While public Hot Spot operators must continue to support every Wi-Fi device ever made, enterprise customers have greater flexibility in defining company-wide standards. However there are a number of issues that must be addressed in planning for an 802.11n deployment, but that will be the subject of another article.