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802.11n MIMO – promises and challenges


 

How does 2x2 MIMO work in a cabled environment? Given the use of identical cables, aren’t the channels also correlated?
In order for spatial multiplexing to work you need at least 2x2 MIMO and uncorrelated channels between the Veriwave test system and the AP. It would seem that in a cabled (conducted) test set-up, the propagation delay, phase shift and amplitude would all be identical across the 2 coaxial cables between the Veriwave test system and the DUT.

However, an uncorrelated channel matrix CAN be realized with a cabled setup even though the propagation delay (equivalent to phase shift) and amplitude are identical across the cables. This can be explained in two ways.

From a strictly mathematical standpoint, an uncorrelated MIMO channel is realized when the rank of the channel matrix is greater than or equal to the number of antennas. The rank of a channel matrix, in turn, corresponds to the number of non-zero eigenvalues. (A simplified way of looking at it is that an eigenvalue of a channel matrix represents a particular “mode” of propagation – each mode of propagation can be excited by a different RF stream without affecting the other modes – thus if you have more modes, you can support more RF streams.)

For two isolated cables, the channel matrix (transfer function, for linear signal theory purists) is a simple unitary matrix:

  • | 1 0 |
  • | 0 1 |

which has a rank of 2. Thus the you should be able to set up two independent RF streams. This is 2x2 MIMO. Hence there should be no difficulty obtaining a MIMO channel in a cabled environment.

From a rather more empirical perspective, a system with two separate isolated cables can obviously send two parallel and independent RF signals down these two cables. The 2x2 MIMO transmitter has two RF chains and will drive each cable with some linear superposition of two analog data streams (Signal1 and Signal2), as follows:

  • Cable1 RF = TX1signal = a * Signal1 + b * Signal2
  • Cable2 RF = TX2signal = b * Signal1 + a * Signal2

At the receiver, the two received signals (TX1signal, TX2signal) can be separated back into the original constituents (Signal1 and Signal2) quite easily, because the coefficients a and b are fixed, and can be figured out during processing of the pilot tones in the preamble. Then the two source analog signals Signal1 and Signal2 can be extracted for the actual data portion of the frame by a process similar to solving simultaneous equations.

What’s key here is that there is no mixing up or combining of the two sets of signals traveling down Cable1 and Cable2. Thus the signals traveling down the cables are in fact uncorrelated – whatever happens to TX1signal, for example, does not happen to TX2signal, and vice versa. In fact the signal at RX antenna port 1 is purely TX1signal, and that at RX antenna port 2 is purely TX2signal, so they are completely separated. Signal1 and Signal2 would be extracted with excellent signal-to-noise ratio, and you would realize pretty much the optimal data rate.

However, if you now take away the cables and replace them with closely spaced antennas, and there are no scatterers in the environment, then TX1signal and TX2signal would be mixed up (linearly combined) at both of the RX antennas. The signal levels and weighting factors would be nearly equal (because the TX antennas are close to each other and the RX antennas are close to each other) and it would not be possible to separate the signals. 2x2 MIMO is not possible.

In order to once again separate the signal in this situation, it is necessary to have scatterers in the environment. To take a simplified view, signals that are “bounced off the scatterers” will show small changes in phase an amplitude between the two transmit antennas because of the different propagation path lengths and angles. When you have enough scattering going on, you can compare the phases and amplitudes of the signals “bounced off the scatterers” with each other and decorrelate the TX1signal and TX2signal. Having separated the signals you are once again in a position to extract the data streams using the same process as in the cabled case.

Note that the channel emulator built into every VeriWave 802.11n MIMO test interface allows you to achieve non-ideal channels (with varying degrees of correlation between TX1signal and TX2signal, similar to a real environment) and test the ability of the AP’s MIMO receiver to recover Signal1 and Signal2 even when there is some partial mixing-up. However, that is a story for another FAQ.

The VeriWave WLAN Capacity Calculator shows that throughput of a WLAN system will rise as the number of clients increases. How can this be possible?
The VeriWave theoretical maximum capacity calculator computes the theoretical maximum medium capacity of a compliant 802.11 BSS comprising one or more active STAs (stations – i.e., APs and/or clients). The value output by the calculator is the upper bound on what can be achieved. A real network that is compliant to the 802.11 standard will always achieve less than this upper bound, though in the 1-STA or 2-STA case it is possible to come very close.

The calculator assumes a completely ideal scenario where all active STAs implement perfect deference and have perfect random access mechanisms, such that no collisions ever occur. In such a situation, the active STAs will effectively subdivide the backoff period among themselves, and as a consequence the medium capacity will rise asymptotically as the number of clients increases. (The limit as the number of clients tends to infinity is a zero backoff time for all the clients taken as a whole, though of course each individual client will be 802.11-compliant and will have a random backoff time distributed in the range 0 – CWmin.) Thus for one STA accessing the medium, the full random backoff is inserted after each access (average backoff = CWmin/2); for 2 STAs, they split the random backoff between themselves (average backoff = CWmin/4); and so on.

Of course, a real 802.11 system cannot have perfect deference and perfect random access in this manner, and hence an actual WLAN shows a different behavior. Typically the medium capacity rises as the number of active STAs goes from 1 to 3, and then starts falling rapidly. By the time 25 STAs are active, the realizable medium capacity falls to about 10% of the capacity with a single STA. If interference or hidden-node issues are present, then the realizable medium capacity falls even further.

Further, note that this applies only to upstream traffic, i.e., when many clients are talking to a single AP. In the upstream case there are many active STAs. When there is a single AP that is talking to a number of clients (i.e., downstream traffic), the situation is quite different. At this point, there is only one STA that is active. Hence the subdivision of backoff will not occur (the single active STA must adhere to the 802.11 protocol, after all) and the medium capacity will be exactly the same regardless of the number of clients that the AP is talking to. Therefore, in the downstream case only the 1-client case as calculated by the spreadsheet should be used.

Will pre-802.11n products be software upgradeable to the final 80211n standard?
In the enterprise area, quite possibly the pre-ratification products will be upgradeable to the final standard. This is because of the timeline and not any special actions on the vendors' part.

The upcoming 802.11n D2.1 draft is widely regarded as likely to be very stable and not many changes of substance will be needed before it goes out to sponsor ballot and eventually gets ratified.

What is Spatial Multiplexing?
Spatial multiplexing is the default mode for N x M MIMO operation in 802.11n, where N and M are both greater than 1. Thus spatial multiplexing is used for 2 x 2 MIMO, 2 x 3 MIMO, 4 x 4 MIMO, etc. In spatial multiplexing, a single high-speed stream of digital data are split and encoded into N parallel but slower-speed streams, and transmitted on N antennas. The M receive antennas (where M >= N) receive these parallel streams, decode them, and combine them back into a single high-speed data stream.

What is STBC?
STBC stands for Space-Time Block Coding. STBC is an optional alternative to spatial multiplexing that is more complicated and expensive but gets better performance, particularly for systems where the number of receive antennas is less than the number of transmit antennas. A typical case is that of an AP transmitting to a handset or handheld client; the AP of course can support multiple antennas and transmitters but the client may only have room (and battery power) for a single receiver. In this case, STBC can be used to significantly increase the signal to noise ratio (SNR) at the receiver, thereby improving the range.

What is Transmit Beam Forming (TBF)?
The 802.11n PHY specification allows beamforming to be optionally performed when the number of transmit antennas exceeds the number of spatial streams, or when the channel between the receiver and transmitter is known accurately enough by the transmitter to permit it to send most of the signal energy in directions that will benefit the receiver. Beamforming requires a knowledge of the channel, which is obtained implicitly (by analyzing the HT-LTF portions of frame preambles received from the far end) or explicitly (by using sounding packets). In either case, once the channel matrix is known, the transmitter adjusts the output power sent to the various transmit antennas in such a way as to form distinct beams or lobes that maximize the power directed towards the receiver.

What is Cyclic-Delay Diversity (CDD)?
In addition to actively forming beams towards the receiver, the 802.11n draft standard also includes a scheme for preventing unintentional beamforming. This can occur if the data being transmitted down the various spatial streams inadvertently forms correlated patterns, i.e., similar data sequences that are synchronized to each other; for example, a binary sequence such as '10101010...' will split among the antennas such that the signals emitted from all the antennas are phase-aligned. In a situation where the transmitted signals from multiple antennas are coherent in amplitude and phase, the radiation pattern will form beams. This is much like the manner in which antenna arrays obtain their directive characteristics by feeding multiple antennas with phase-shifted copies of the same signal. Unlike intentional beamforming, however, the pattern of lobes and nulls may not be oriented in such a way as to maximize the effect at the receiver, and thus unintentional beamforming can be detrimental to the system. To avoid unintentional beamforming, the IEEE 802.11n draft standard uses a process known as Cyclic Delay Diversity (CDD), which basically just offsets each spatial stream by a different constant, non-coherent delay. The offset considerably lowers the likelihood of correlated signals being transmitted by two or more antennas. This, in conjunction with a pseudorandom scrambler run over the transmitted data bits, ensures that the likelihood of two spatial streams correlating is very low.

What is Maximal Ratio Combining (MRC)?
MRC allows for combining diversity (as opposed to the switching diversity used in traditional 802.11a/b/g systems) and is principally used in schemes such as 2 x 3 MIMO, where the number of receive antennas exceeds the number of transmit antennas. The number of spatial streams is limited to the smaller of the number of active transmit antennas and the number of active receive antennas. If there are more receive antennas than transmit antennas, then the extra receive antennas can be employed to provide MRC diversity.

In 2 x 3 MRC diversity, for example, the extra receive antenna is used to improve the signal to noise ratio (SNR) on the link. The 3 received and demodulated signal streams are constructively combined (added in an optimal ratio) down to 2 streams that have a correspondingly higher SNR. The SNR improvement translates to an increase in range and/or reduction in bit error rate for a given PHY rate. Unlike spatial multiplexing, the maximum possible PHY bit rate does not change. (Of course, if the SNR is low then the resulting high error rate may force the system to rate-adapt down to a lower PHY bit rate. In this situation, the increased SNR available from MRC diversity may in fact permit a higher PHY bit rate and thus increased throughput.)

How does the 802.11n backward compatibility "mixed mode" mechanism operate? What are the tradeoffs from a deployment point of view? Should enterprises avoid the use of mixed mode?
In mixed mode operation the 802.11a/g preamble is prefixed to the 802.11n preamble. The 802.11a/g preamble can be decoded by legacy devices and can be used to instruct them to keep off the air while the (undecodable) 802.11n packet is being transmitted. The deployment tradeoffs are similar to that of running a mixed 802.11b/g network: the use of the 802.11a/g preamble introduces a lot of overhead and slows down the overall network. Also in mixed mode you can't really use the 40 MHz option and this further reduces the available BW. Enterprises cannot avoid the use of mixed mode unless they are lucky enough to have no installed base, or rich enough to afford a forklift upgrade.

SPECTRUM SELECTION: Should enterprises deploy 802.11n in 5GHz or 2.4GHz spectrum? What are the best practices? What are the tradeoffs?
The choice of 5 GHz vs 2.4 GHz is very dependent on the goals of the enterprise. If they can afford a larger number of APs and a denser deployment then 5 GHz is obviously the way to go, you have many more channels and there are less interference issues. Otherwise 2.4 GHz is used. Note that it is not easy to find cheap 5 GHz clients these days, so many enterprises are forced to deploy 2.4 GHz whether they like it or not.

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.

Will 802.11n have any impact on security (e.g., IDS systems?)
It is unlikely that 802.11n will have an impact on security at all. In fact the increased interference resistance of 802.11n may make it slightly harder to mount simplistic DoS attacks such as are possible with 802.11a/g today. Also by the time 802.11n becomes mainstream in the enterprise it will be coupled with MFP (Management Frame Protection) making link-layer DoS attacks even less practical. Of course other forms of DOS attacks can still be carried out.

Is 802.11y a part of the developing 7 WiMAX? 
IEEE 802.11y is not connected with WiMAX. It is instead the extension of 802.11 technology to the 3650-3700 MHz Wireless Broadband Services band, which is a licensed radio service requiring some of the same principles as 802.11 CSMA/CA protocols.

Is 802.11n suitable for voice?
Many protocol and radio functions in 802.11n have been specifically intended to simplify the support of high-QoS protocols such as voice. In fact many of the existing 802.11e QoS features have been carried over into 802.11n and substantially improved.

Can voice handsets support full MIMO (2 radios)?
it is highly unlikely that we will see voice handsets with full MIMO capability any time soon. This is for three reasons: (a) a full MIMO radio consumes substantial power, lowering battery life significantly; (b) the multiple antennas required occupy quite a bit of space; (c) the bandwidth gains are not required by voice, which requires merely 60-70 kb/s or less per VoIP stream. Instead, we expect that once 802.11n has significantly penetrated the enterprise infrastructure, we will see voice handsets that take advantage of the 802.11n protocol functions that improve voice QoS and efficiency, without needing full MIMO support.

Is 802.11n compatible with a/b/g. If yes what are the differences in real life throughput when using a/g on its own, vs. a/g in mixed mode with 802.11n?
Compatibility functions in 802.11n are divided into two main categories: functions to ensure compatibility with legacy 802.11a/b/g devices in 20 MHz mode, and functions to ensure compatibility with 20 MHz bandwidth PHYs (including 802.11a/b/g) when operating in 40 MHz mode. Typically these compatibility functions are triggered by the AP. A full description of the compatibility features is beyond the scope of this answer; the reader is referred to subclauses 9.6, 9.13, 9.20, 11.9.8, 11.15, 11.16, and 20.3 for more information. Briefly, these compatibility features include a special mixed-mode preamble that prefixes a standard 802.11a/g preamble to the 802.11n preamble, allowing 802.11a/g devices to detect and defer to 802.11n transmissions

How many non-overlapping 40-MHz wide channels are there in the 5 GHz range?
40 MHz operation at 5 GHz involves the use of pairs of 20 MHz channels. The North American channel pairs for 40 MHz operation in the U-NII band are: 36+40, 44+48, 52+56, 60+64, 149+153, 157+161. (See Annex J of IEEE Draft 802.11n/D2.00.) These channel pairs are non-overlapping, hence there are two sets of non-overlapping channels in the U-NII low, middle and high bands.

Wouldn't it be useful to distinguish between 802.11 "mobility" and cellular "mobility," rather than as it is today where both technologies use the same word to mean entirely different things? 
Unfortunately the 802.11 usage of the words "roaming" and "mobility" are different from the cellular usage of these terms. This causes quite a bit of confusion. The 802.11 term "roaming" actually implies "handoff" in the cellular sense, for example. However, it appears that the differing usage - and the confusion - are here to stay.

Will the coverage patterns of 802.11n vary in size and consistency due to MIMO? 
The coverage pattern of 802.11a/g was limited mainly by attenuation and shadowing and hence relatively predictable. Also, 802.11a/g specified a small number of data rates and operating modes. However, the available data rates of an 802.11n MIMO link are dependent on the local scattering environment - for example, a low angular spread at the receiver results in a low probability of achieving multiple uncorrelated spatial channels - and also the number of possible data rates and modes is much larger. Thus the coverage pattern of an 802.11n AP is much more nebulous and hard to quantify. The only thing that can be positively stated is that the coverage pattern of an 802.11n AP will almost certainly be substantially larger than the coverage pattern of an 802.11a/g AP of the same power output and sensitivity.

With devices, which use draft n technology, can they, later, be driver/software upgraded to the actual 802.11n standard? 
This is highly dependent on the design of the chipset used in the devices. A chipset that attempts to extensively use fixed DSP technology to lower costs will be much less likely to be firmware upgradable than another chipset that implements more programmable DSP logic (possibly even performing functions such as channel estimation on a more powerful on-chip CPU). However, it is true that a chipset built to closely follow the 802.11n D2.0 draft (and including as many of the specified features as possible) is very likely to be close enough to the final 802.11n standard to permit firmware upgrades. Virtually all of the low-level functionality, including the critical 20/40 MHz interoperability requirements, are present in the D2.0 draft, and the 802.11n committee is fairly committed to maintaining this as close to the final form as possible.

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.

Does overhead remain if the legacy client deployment remains, and is it 100% added to 802.11n overhead?
Yes, the overhead of dealing with legacy clients is added to the 802.11n overhead. By definition legacy clients do not know anything about new 802.11n deployments and hence will not be able to take any action to avoid colliding with 802.11n stations. Therefore, all coexistence overhead - e.g., the higher-overhead mixed-mode preamble - must be borne by the 802.11n stations.

What exactly does pre 'n' mean?
The 802.11n standard took a while to develop into its present (fairly complete) form. This was due more to market and political forces than to technical issues. As most of the technical issues had been solved by the time the 802.11n standards development effort had got underway, various vendors decided to bring pre-standard technology to market in an attempt to gain market share and secure a commanding position before the final 802.11n standard was created and disseminated. This pre-standard development effort took place in two phases. The first phase was known as "pre-N"; vendors of 'pre-N' products tried to predict the final form of the standard and created chipsets and devices that would, in their view, most likely match that of 'real' 802.11n products. The second phase was known as "draft-N", where vendors took various (incomplete) drafts of the 802.11n standard and implemented them. As the functions and features of "pre-N" products were basically guesses, they are very likely to have interoperability issues with 'real' 802.11n products. However, as "draft-N" products used at least some initial versions of the standards documents, there is a better likelihood of such products being able to successfully interoperate with 'real' 802.11n devices. Note that there are also various forms of "draft-N" products, distinguished by the revision number of the standards draft that the vendor selected in order to begin the implementation process.

Are there any newly deployed media contention algorithms with 802.11n - Especially with multiple antennas and a higher data rate?
The media contention algorithms in 802.11n remain substantially similar to those in 802.11a/b/g - i.e., CSMA/CA, as defined by Clause 9 of IEEE Std 802.11-2007. There are some tweaks to accommodate the new 802.11n features and packet formats. There are also some efficiency improvements, such as aggregated MPDUs and MSDUs, in order to better utilize the bandwidth available. However, there are no new media contention algorithms.

How will antenna polarization affect MIMO and similar technologies in regards to smearing signals and interference? 
Most MIMO systems use the same polarization (vertical or horizontal) across all of the antennas used on a particular device. However, it is also possible to use antennas of dissimilar polarization; for example, in a 2 x 2 MIMO system, one of the antennas may be oriented horizontally and the other oriented vertically. In this case, it is possible to avoid situations where correlated multipath causes a drop in throughput for systems using identically polarized antennas; if the antennas have different polarization, then it is difficult or impossible to have a situation where different antennas see correlated multipath signals. However, this is offset by the fact that antennas of dissimilar polarization may pick up signals at widely varying levels; for example, the horizontally polarized antenna might receive signals that are 10-20 dB higher than the vertically polarized antenna. This leads to a drop in throughput because the channel as effectively seen by the antennas becomes Rician, and there is a dominant path. Therefore, the use of antennas of varying polarization on the same MIMO device is likely to be problematic. Most vendors of MIMO devices tend to avoid the use of differentially polarized antennas, for this reason.

What does 802.11n mean for the consumer electronics market? Should we expect to see more WiFi capability in devices such as digital camcorders, digital cameras, mp3 players, etc.?
One of the strongest drivers for 802.11n has been the desire to carry high-bandwidth multimedia streams using 802.11 technology. A typical compressed A/V stream may require 30 Mb/s or more of bandwidth, which is obviously beyond the "real" (i.e., sustained and guaranteed) capabilities of the existing 802.11a/g PHYs. 802.11n is therefore expected to provide many benefits in the consumer market, not only in terms of enhancing existing applications (e.g., wireless connections between A/V components) but for enabling new, unforeseen applications. A substantial number of the supporters and developers of 802.11n at the IEEE 802.11n task group are from consumer electronics vendors.

How is the range affected on 802.11n with the use of spatial multiplexing? 
The purpose of spatial multiplexing is to obtain a higher information rate (relative to a non-spatially-multiplexed situation, such as a standard 802.11g radio) with the same signal-to-noise ratio (SNR) at the receiver. However, note that 802.11n radios can trade information rate for range, and vice versa. The received SNR depends entirely on the transmitted power and the environmental path loss between transmitter and receiver. Thus we have two cases: (a) if we keep the received SNR the same between an 802.11n radio vs an 802.11g radio, the range will be the same (assuming the same aggregate transmit power for both radios) but the information rate for the 802.11n radio will be substantially higher; and (b) if we keep the information rate the same for the two radios, then the 802.11n radio will be able to use many enhancements such as STBC, TBF and MRC diversity to substantially increase range over the 802.11g radio. The most common case is likely to be something in between these two cases, leading to a somewhat higher information rate coupled with a somewhat higher range.

What is the status of 802.11n regarding the European regulation requirements (DFS, TPC...) defined in 802.11h? 
Draft 2.0 of the 802.11n standard fully comprehends the DFS and TPC functions defined in 802.11h functionality. Note that 802.11n is a complete superset of IEEE Std 802.11-2007, which includes the 802.11h amendment. 802.11n makes a few minor edits to adapt DFS channel switching to the new physical layer, but that is all.

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.

What are the number of 20/40 MHz channels available in the 2.4 and 5 GHz band, and are they the same as for 802.11a/b/g?
In the 2.4 GHz band, there are 3 non-overlapping 20 MHz channels (same as for 802.11a/b/g) but effectively only one non-overlapping 40 MHz channel. In the 5 GHz band, which is much wider, there are a total of 6 non-overlapping 40 MHz channels in the traditional U-NII bands.

Will 802.11n give any significant advantage outdoors? How different channel modeling will be for outdoor applications? 
802.11n requires a rich multipath environment to obtain its performance gains. Such an environment is relatively easy to obtain indoors, due to the density of metallic objects (wall studs, appliances, furniture, equipment, etc.). If 802.11n is used outdoors in a similarly rich multipath environment (e.g., in the middle of an urban area) then it is likely to provide similar performance gains. However, if 802.11n devices are deployed using clear, unobstructed line-of-sight paths and placed high above the surrounding metallic or RF-reflecting objects, then it is likely that the achieved performance will suffer, and will even drop back to the same level as 802.11a/g in the same situation with the same channel bandwidth. Note that some of the optional features of 802.11n (e.g. 40 MHz channelization) will still provide a substantial performance gain, and others (e.g., MRC diversity) will provide signal-to-noise ratio improvements, but spatial multiplexing will be adversely affected.

If 802.11n is LOS deployment, does it make sense for indoor deployment? 
802.11n is definitely not LOS deployment. In fact, strong, dominant LOS paths cause severe performance impact to 802.11n radios. What is ideally required for MIMO radios such as 802.11n to function well is a rich multipath environment.

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 is security enhancement in 802.11n? 
There are no significant security enhancements in 802.11n. Note that the legacy WEP and TKIP encryption methods are not allowed for 802.11n stations any more; all 802.11n compliant stations must implement and use the strong AES-CCMP security method when communicating with other 802.11n stations. A few security modifications have also been specified in order to accommodate some new frame types and formats for 802.11n, but the basic operation of AES-CCMP remains unchanged from 802.11a/b/g devices. A point to keep in mind, though is that 802.11n radios can be more resistant to interference than legacy 802.11a/b/g devices (especially when advanced options such as transmit beamforming and maximal-ratio combining diversity are used) and thus some denial of service attacks that could be used against 802.11a/g become harder with 802.11n devices.

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