Power Control | Voice Mobility with Wi-Fi

Power control, also known as transmit power control (TPC), is the ability of the client or the access point to adjust its transmit power levels for the conditions. Power control comes in two flavors with two different purposes, both of which can help and hurt a voice mobility network. The first, most common flavor of power control is vested in the client. This TPC exists to allow the client to increase its battery life. When the client is within close range to the access point, transmitting at the highest power level and data rate may not be necessary to achieve a similar level of voice performance. Especially as the data rates approach 54Mbps for 802.11a and 802.11g, or higher for 802.1 In, the preamble and per-packet backoff overhead becomes in line with the over-the-air resource usage of the voice data payload itself. For example, the payload of a voice packet at the higher data rates reduces to around 20 microseconds, on par with the preamble length for those data rates. In these scenarios, it makes sense for the client to back off on its power levels and turn off portions of the radio concerned with the more processing-intensive data rates, to extend battery life while in a call. To do this, the client will just directly reduce its transmit power levels, as a part of its power saving strategy. This mechanism can be used for good effect within the network, as long as the client is able to react to an increase in upstream data loss rates quickly enough to restore power levels should the client have turned power levels down too low for the range, or if increasing noise begins to permeate the channel.

The other TPC is vested within the network. Microcell networks, specifically, use access point TPC to reduce the amount of co-channel interference without having to relocate or disable access points. By reducing power levels, cell sizes in every direction are reduced, keeping in line with the goals of microcell. Reducing co-channel interference is necessary within microcell networks, to allow a better isolation of cells from fluctuations in their neighboring cells, especially those related to the density of mobile clients.

Network TPC has some side effects, however, that must be taken into account for voice mobility deployments. The greatest side effect is the lack of predictability of coverage patterns for the access points. This can have a strong effect on the quality of voice, because voice is more sensitive than data to weak coverage, and areas where voice performs poorly can come and go with the changing power levels, of both the access point the client is associated to and of the neighbors. Unfortunately, power levels in microcell networks usually fluctuate on the order of a few seconds or a few minutes, especially when clients are associated, as the network tries to adapt its coverage area to avoid causing the increase in packet rate and traffic caused by the clients from affecting neighboring cells. Site surveys, which are performed to determine the coverage levels of the network, are always snapshots in time and cannot take TPC into account. However, the TPC variation is necessary for proper microcell operation, and unfortunately needs to happen when phones are associated and in calls. Therefore, it can cause a strong network-induced variation in call quality. It is imperative, in microcell deployments, for the coverage and call quality to be continuously monitored, to ensure that the TPC algorithms are behaving properly. Follow the manufacturer's recommendations, as you may find in Voice over WLAN design guides, to ensure that problems can be detected and handled accordingly.