4G Digest 5.21

4G Digest

March 10th, 2010 - Volume 5, Issue 21

The Benefits of 2Tx in Mobile WiMAX Systems
By Craig Miller, Vice President Marketing & Business Development, Sequans

The WiMAX market is growing steadily and WiMAX is delivering on its promise of serving people around the world with high-speed wireless services, in both developed and emerging markets. As operators grow to serve more and more people, the efficiency and performance of WiMAX networks is becoming more and more important. One of the most important efficiency-increasing capabilities of WiMAX systems is two-by-two MIMO technology, where the air interface supports two channels in the downlink (from the base station to the subscriber device) and two channels in the uplink (from the subscriber device to the base station). However, while WiMAX implementations leverage the two downlink channels, most do not fully leverage the two uplink channels, thereby missing a key opportunity to improve network performance.

Typical WiMAX networks are limited by the uplink because the base stations have higher transmit power coupled with two downlink MIMO streams, while the mobile devices operate at lower output power with only a single uplink stream. This imbalance can be as much as 6 dB, which means most subscriber devices have only one fourth of the ‘reach’ of the base station they are trying to connect to. Improving the uplink performance by utilizing both uplink channels can offset the downlink/uplink imbalance and have a powerful effect on a WiMAX network’s coverage and capacity. To fully exploit the two antennas that are on every WiMAX device, you must transmit on both antennas simultaneously.

Simple, single Tx, switched diversity transmit schemes that switch between antennas are limited because they can only transmit on one antenna at a time, which may salvage only ½ to 1 dB of the imbalance, but transmitting on both uplink channels simultaneously (2Tx) provides immediate combining gains, enables advanced 2Tx closed loop transmit diversity (2T-CLD) and, in the future, support for true uplink MIMO. 2Tx capability can improve cell coverage so much that operators can significantly reduce their site density, reducing capital expense by almost half.

With 2Tx, WiMAX devices can instantly realize a 3dB gain by combining the output power of the two transmitters. And by adding closed loop diversity, further diversity gains of up to 2.5 dB can be realized for a tremendous 5.5 dB overall gain, regaining nearly all of the typical 6 dB imbalance. Using information from the downlink, 2T-CLD dynamically selects the best antenna for every sub-carrier, on a frame-by-frame and tone-by-tone basis, which always results in the best possible diversity selection for a given channel environment, even one that is changing rapidly. 2Tx with CLD delivers link budget gains well beyond primitive single-Tx solutions and guarantees the best possible uplink performance. Gains can be used to improve cell edge and network entry performance as well as increase throughput.

Network operators do not need to upgrade existing base stations to realize the benefits of 2Tx technology. 2T-CLD technology is 100 percent compatible with existing base stations, giving operators the benefits of increased coverage and lower infrastructure costs without any incremental capital expense. Implementing 2T-CLD in mobile devices delivers a 26 percent improvement in coverage and a 37 percent reduction in the number of required base stations. And 2Tx technology is future proof. Operators and base station vendors can introduce true 2x2 MIMO by adding STC (space time coding) at the base station, which delivers a 37 percent improvement in coverage and a 47 percent reduction in the number of required base stations, with no hardware changes required in the subscriber devices that have 2Tx support.

2Tx delivers a tremendous opportunity to improve operator economics and end user satisfaction, making it one of the most important WiMAX innovations available today.