While 802.11n has gained a lot of attention as the newest and the greatest; (technical people tend to graviate to the newest and greatest!) it is probably not the best fit technology for portable medical devices, i.e. patient monitoring, etc. or any other device concerned with power consumption. More than likely 802.11n will be confined to laptop usage whereby power consumption may not be an issue. However, it should be noted that in the years ahead as SOC (Silicon on a Chip), moves forward, power consumption will be lowered. It is my opinion that the best fit for medical devices is still 802.11g and 802.11a.
What is the PoE power consumption for 802.11n devices, compared with 802.11g?
Power consumption for 802.11n devices will obviously be significantly higher than for 802.11a/g devices, which are themselves much higher than 802.11b devices. The key reasons why power consumption of 802.11a/g devices was much higher than 802.11b were (a) a more complex signal processing block consuming a lot more power, and (b) a much higher peak-to-average ratio of transmit signal resulting in a much more inefficient power amplifier (PA) and therefore a lot more power consumption in the PA to maintain the same output signal. (An 802.11g PA can take as much as half an amp at full power, to drive just 50-100 mW out.)
For 802.11n devices these issues are multiplied: there is an order of magnitude more digital signals processing in both the TX and RX chains, so a lot more power is consumed here; there is more signal integrity required in the PA chain so it will be correspondingly less efficient; and there are 2 or 3 radios needed to talk to the 2 or 3 antennas and so the power gets multiplied by that factor. Don't look for 802.11n to be coming to a low-power handset any time soon.
How much more power will be required for 802.11n?
The amount of power consumed by an 802.11n radio depends significantly on two factors: the number of transmit chains and the number of complex PHY-layer options implemented. The number of transmit chains used controls the available gain due to spatial multiplexing and beamforming (more transmit chains equals higher advantage) but each 802.11n transmit chain consumes at least as much power as that of a single 802.11a/g radio. Thus an 802.11n PHY that contains 3 transmit chains, for implementing 3 x 3 MIMO, will consume three times the power of the transmitters in an 802.11a/g PHY. Due to various factors (such as the high peak-to-average ratio of OFDM) the Power Added Efficiency, or PAE, of an OFDM transmitter is only about 20-30%. Thus a 3 x 3 MIMO 802.11n transmitter putting out 100 mW (+20 dBm) on each radio can consume more than 1 watt of DC power, as compared to 200-300 mW for 802.11g. There are few ways to reduce this power consumption, as it is constrained by physical limits. The other significant factor is the complex digital signal processing required for both the receiver and the transmitter of a MIMO radio; the amount of DSP needed is at least an order of magnitude greater than a typical 802.11a/g PHY, and even more if the complex PHY options such as MRC diversity are implemented. While technological advances (e.g., smaller chip geometries) can significantly help to reduce the space and power consumed by the 802.11n DSP functions, it is still true that 802.11n will always use substantially more power than an 802.11a/g PHY using the same chip technology. (This is in fact why 802.11n MIMO modes are less likely to be used in VoIP handsets, which must pay close attention to battery life, in the near future.)
What other exceptional benefits (in addition to throughput and range) will 802.11n bring to business users/infrastructure?
The QoS functions in 802.11n are substantially improved from those in the original 802.11e standard, and also take into account the needs of VoIP handsets. In addition, there are a number of efficiency improvements in the protocol that should allow more of the available PHY data rate to be utilized than in the legacy 802.11a/g devices, which is a benefit for enterprises that require higher infrastructure capacity. Finally, the transmit beamforming option permits lower interference levels (and also improves rejection of existing interference) which will be a benefit to enterprises that have dense AP deployments and have to cope with adjacent WLANs belonging to other entities.
Will the 802.11n device completely kill / overpower the 802.11a/b/g devices?
Draft 2.0 of the 802.11n standard contains many provisions for coexisting and interoperating with legacy devices. For example, an 802.11n AP is required to detect the presence of 802.11a/b/g systems and take specific action to avoid destroying their transmissions. Considerable effort has also been expended in ensuring that 802.11n devices in 40 MHz mode will not inadvertently interfere with 802.11a/b/g devices using 20 MHz channelization (and unaware of the presence of 40 MHz 802.11n radios). Therefore, there should be very little adverse impact on existing 802.11a/b/g devices; the key issues are more related to avoiding significant performance loss in 802.11n devices as a consequence.
