802.11n provides remarkable performance improvements in the areas of throughput, link reliability, and predictability. The move to 802.11n provides significant benefits, but most hospitals will move forward in a transition strategy. Most 802.11n solutions offer improvements in the up-link communications from the access point. Cisco ClientLink technology is unique in that it offers improvements as well as downlink communications from the access point to the client. Cisco has also added advanced signal processing into the Wi-Fi chipset. Multiple transmit antennas are used to focus transmissions in the direction of the 802.11a/g client, increasing the downlink signal to noise ratio and the data rate over range, thereby reducing coverage holes and enhancing the overall system performance. This essentially learns to optimize a way to combine the signal received from the client, and then uses that information to send packets in an optimum way back to the client.
The radio signal in a healthcare environment rarely takes the most direct or shortest path from the transmitter to receiver because walls, doors, or other structures obscure the line of site. That is why telemetry systems use diversity. 802.11n takes advantage of multipath by sendintg radio signals at the same time. 802.11n also specifies how MIMO technology can be used to improve the SNR at the receiver by using transmit beamforming. Transmit beamforming as specified in the 802.11n standard requires cooperation from both the receiver to feedback training and/or channel information about the received signal to the transmitter - so both the AP and client need to support this capability. Due to the complexity issues, the first generation of mainstream 802.11n chipsets neither the AP nor client chipsets implemented 802.11 transmit beamforming.
Cisco realized that for many networks the performance of the installed 802.11a/g client base would be a limiting factor on the network. To take advantage of capacity in the second transmit path for 802.11a/g clients, and to enhance overall network capacity by bringing 802.11a/g clients up to a higher performance level; Cisco created an innovation in transmit beamforming technology called ClientLink. ClientLink uses advanced signal processing techniques and multiple transmit paths to optimize the signal received by 802.11a/g clients in the downlink direction without requiring feedback. Since it does not require any special feedback, it works with all existing 802.11a/g clients. Another step is taken to improve performance in the downlink direction, making the client better able to hear the access point. The Wi-Fi channel is reciprocal, meaning the transmissions between access points and clients happen on the same frequency and use the same antennas. Thus, the access point can use the adjustments calculated by the MRC (maximal radio combining algorithm), to optimize the reciprocal signal transmitted back to that specific client using the APs two transmit antennas. These enhancements are made in hardware with minimal overhead on the AP platform and no performance is lost due to signal processing. ClientLink effectively enables the access point to optimize the SNR (signal to noise ratio), exactly at the position where the client is place. Improved SNR has benefits such as reducing the number or retries, provides higher data rates, and increases the overall capacity of the system. Several companies will be talking about "beamforming" in the next few months. Here are some interesting test results for review. Any questions or feedback about this would be welcome. The world of Wi-Fi with 802.11n and beamforming is going to get quite interesting in 2010.