The proliferation of wireless networking in the office, at home and in public spaces has generated considerable interest in the technology's potential for serving an ever-expanding list of applications. Many 802.11-based wireless-networking products have become available in different flavors, reaching standards-compliant data rates of up to 54 Mbits/second. But applications in need of higher data throughput-such as high-definition video streaming and home theater connectivity-as well as those for environments with high user density will require better performance than that provided by the 802.11a and .11g standards.
This has prompted the IEEE to begin work on yet another extension to the 802.11 standards list-802.11n-allowing for higher throughput and more efficient use of spectrum. The project authorization request for Task Group "n" was ratified within IEEE 802.11 last year.
Target applications for high-speed wireless networking fall into two basic groups: those requiring a fast data stream (greater than 100-Mbit/second throughput), along with a high quality of service, and those that require the performance equivalent of a wired network in a high-density environment such as a large enterprise or an apartment complex. The work being done now is focused on sorting through the technical issues that need to be addressed in developing this next-generation standard. The challenge for designers will be to develop a high-speed WLAN solution that will meet the feature and performance requirements for new consumer applications at absolute minimum cost, while maintaining backward compatibility to 802.11a/b/g systems.
Each cell of an 802.11b, a or g network typically includes an access point and several clients that use the same channel for communications. Increasing data throughput speeds can help avoid channel congestion by reducing the amount of airtime used for transmission.
In the 802.11a and g standards, the maximum data rate at the physical layer is 54 Mbits/s. The actual speed of data through the entire system (throughput), however, taking into account the overhead of the 802.11 protocols, is around 30 Mbits/s. The next-generation standard would seek to improve actual throughput. This can be done through a combination of increasing the physical-layer speed and improving the efficiency of the 802.11 protocols.
Techniques to increase the physical-layer data rate include:
- Creating more "air paths" for the data to be transmitted on the same channel. Each air path can carry a different set of data using coded RF signals and multiple antennas, a technique referred to as multiple input, multiple output. MIMO requires additional transmit and receive paths along with antennas, all of which have an impact on system cost.
- Widening the channel or bundling channels to something greater than 20 or 25 MHz, which are the channel widths currently set for 802.11a and b/g, respectively. Reducing the number of channels limits the overall system capacity. Defining flexibility in different modes can address specific application needs with respect to channel widths.
- Increasing the modulation level. Current 802.11a and g systems use a maximum modulation of 64 quadrature amplitude modulation (QAM), allowing 6 bits to be coded on each carrier. One idea is to increase this to 256 QAM, allowing 8 bits to be coded on each carrier, for a maximum data rate of 72 Mbits/s in one 20-MHz non-MIMO channel. The ability to reduce chip impairments to a level where the range in this mode is greater than about a meter is highly debated, however.
Techniques to improve the efficiency of 802.11 systems include:
- Improved coding. With today's cheaper processing power, some coding techniques that were considered too calculation-intensive can now be explored. Turbo codes, block codes and other iterative processing techniques can also be considered. The added processing power may increase chip set costs because of the additional gates required.
- Reducing the protocol overhead through larger packets and shorter time between transmissions. Currently the 54-Mbit/s rate has a 66 percent airtime usage for the actual user data. With the same protocol and overhead, this usage will diminish to only 35 to 50 percent airtime once raw data rates reach 162 to 216 Mbits/s. Protocol improvements are needed to make efficient use of airtime and meet the required throughput.
"Network capacity" deals with the total capacity of an installed base of access points, taking into consideration the number of channels available and the susceptibility of the system to signals from adjacent WLAN cells. Each cell can "hear" other co-channel cells and therefore will not transmit while another cell is transmitting. Sensitivity of a cell to the signal from a neighboring cell decreases the overall capacity of the network. In areas with co-channel interference, higher data rates (requiring more complex modulations) are often not reached.
Techniques to improve network capacity include:
- Increasing the number of channels available. As regulatory bodies recognize the market success of WLAN systems, they have been working to make more spectrum available to WLAN systems.
- Limiting channel bandwidth. Current channel widths are 25 MHz for 802.11b and g systems and 20 MHz for 802.11a systems. As mentioned, any increase in those bandwidths will reduce the number of channels available, affecting network capacity. This constraint on bandwidth works in direct contradiction to one of the easiest ways to increase the data rate, which is simply to take up more bandwidth. Careful consideration of these trade-offs must be made.
- Reducing the sensitivity to co-channel interference. This too contradicts increasing the modulation level to bump up the data rate. By increasing the modulation level, the system's sensitivity to co-channel interference worsens.
- Increasing the overall data rate of each transmission to limit the time each station is "on the air."
In the last few evolutions of the 802.11 standards, higher data rates were achieved with more complex modulation schemes, hurting range. The next-generation standard should attempt to minimize that effect. MIMO in particular provides the flexibility of creating multiple "paths" with less complex modulation schemes for better range. A 3 x 3 MIMO system running three 36-Mbit/s paths can achieve a 108-Mbit/s raw data rate with range that is limited only by the 36-Mbit/s modulation scheme.
In developing a standard that serves to improve data rates, ranges and network capacity, trade-offs for each technique or technology must be scrutinized. MIMO remains one of the leading technologies being considered thanks to its ability to increase throughput and network capacity with minimal sacrifice. MIMO allows for an N times data rate increase in speed for N transceiver paths without increasing co-channel interference sensitivity or decreasing the number of channels.
A combination of MIMO and other WLAN technology improvements discussed here is likely to form the basis for the new Task Group "n" standard through proposals submitted in 2004. With these technologies, there is a chance to exceed typical wire line speeds in the office of 100 Mbits/s. Proposals that optimize these technologies-using flexible implementations that meet the requirements of varying applications and provide maximum speeds greater than 400 Mbits/s and throughputs exceeding 200 Mbits/s-will be compelling solutions for driving the new wireless-networking market.
Vincent Vermeer and Mary Cramer are Wi-Fi technical-product managers at Agere Systems Inc. (Allentown, Pa.).
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