The use of multiple antennas based on Multiple-Input Multiple-Output (MIMO) techniques is the key feature of the IEEE802.11n/ac standard for wireless local area networking, which in theory provides a higher data transfer rate than the earlier 802.11a/b/g versions of the standard. The availability of MIMO techniques has a big impact on the decisions of engineers who are designing Wi-Fi® modules: they need to decide whether to implement the 802.11n/ac technologies, and how much improvement in performance they will offer in practice. This paper outlines the engineering issues which affect the choice of 802.11 technology.
Since the IEEE first ratified the 802.11 Wi-Fi standard in 1997, it has gone through a process of evolution. The 802.11b version was the first widely accepted wireless-networking standard, and was followed by 802.11a, 802.11g, 802.11n and most recently, 802.11ac.
The 802.11b standard uses the 2.4GHz ISM frequency band, which allows for a maximum throughput of just 11Mbits/s. The newer 802.11a variant of the standard operates in the less crowded 5GHz band, and achieves higher throughput of up to 54Mbits/s. Combining the best of 802.11a and 802.11b, 802.11g supports bandwidth up to 54Mbits/s, and uses the 2.4GHz frequency band for longer range than 5GHz offers.
The 802.11n standard was intended to provide improvements over 802.11g, increasing data throughput by using multiple Wi-Fi signal channels and antennas rather than the one that previous versions of the standard had used. Finally, the latest 802.11ac version of the standard offers backward compatibility to 802.11b/g/n and supports a data rate higher than 200Mbits/s, as shown in Table 1.
|Max. Throughput Mbps||54||11||54||100+||200+|
|Indoor Range (Ft)||115||125||125||230||115|
|Outdoor Range (Ft)||330||460||460||820||330|
|Frequency (GHz)||5||2.4||2.4||2.4 and 5||5|
|Bandwidth (GHz)||20||20||20||20, 40||20, 40, 80, 160 (optional)|
|Max. MIMO Spatial Streams||NA||NA||NA||4||8|
Table 1: Summary of main classes of 802.11 standards
Higher throughput and extended range
Wi-Fi modules based on the 802.11a/b/g variants of the standard are best suited to applications which require low cost, low throughput, low bandwidth and low power consumption. Specifically, the strong points of 802.11b are its low cost and high signal integrity, since it is benefits from high sensitivity at low data rates. 802.11a solutions are typically more expensive than those using 802.11b, because the components required to work at up to 5.8GHz cost more. By contrast, 802.11g solutions, which operate in the 2.4GHz band, benefit from the use of cheaper components, but are more vulnerable to interference caused by other wireless devices operating in the same band.
In truth, the popularity of 802.11a/b/g-only modules is now declining: that is because demand is growing for Wi-Fi modules based on newer versions of the 802.11 standard which provide more robust performance with higher data throughput.
For instance, the highest data rate achievable with 802.11n is 600Mbits/s, against the 54Mbits/s peak speed of 802.11a/b/g. This extra speed, however, comes with a requirement for higher-cost antenna sub- systems and multiple paralleled RF signal-processing blocks to support the multiple antennas. An 802.11n module operating at the highest possible data rate will use four antennas, and double-bandwidth channels of 40MHz instead of 20MHz. It can also achieve an extra 40% improvement by tweaking the OFDM modulation and coding rate.
Another advantage of multiple antennas is to extend the radio’s range: by weighing Wi-Fi signals on each transmitting antenna separately, a high- gain antenna beam may be created. The antenna array can also help to cancel out adjacent interference sources.
Examples of Wi-Fi solutions benefiting from MIMO antenna techniques are shown in Table 2.
|Manufacturer||Series Designation||Standard||MIMO Spatial Streams|
Table 2: Typical Wi-Fi solutions featuring MIMO functionality
Challenges in 802.11 wireless communications
Over a typical 802.11g link, packets are transmitted with 17.5dBm of power, and the sensitivity of a receiver can be as low as -76dBm for a Packet Error Rate (PER) of less than 10%. This is a more than billion-fold loss (93.5dB) of power over the 802.11 wireless channels. The weakening of the signal between transmitter and receiver comes from several effects. They are path loss, shadowing and multipath, as shown in Figure 3.
As the radiated signal spreads out from a transmitting antenna over a wider area in the air, its power drops as fast as the square of the distance when the signal propagates. This is called path loss. Outdoor obstacles (such as tall buildings, trees and pools) and indoor walls can also noticeably attenuate the Wi-Fi signal due to the shadowing effect.
The most problematic kind of fading for 802.11 signals is multipath fading. This results in constructive and destructive interference, and phase shifting of the signal. At 2.4GHz and 5GHz, RF signals bounce off ground surfaces and walls when they propagate indoors. This scattering causes many copies of the signal to travel along many different paths. If the signals arrived at the receiver at the same time and out of phase, this could cause fast signal fading. Because multipath effects depend on the phases of signals, they are strongly frequency-selective.
For this reason, 802.11 uses special modulation techniques called Orthogonal Frequency Division Multiplexing (OFDM) in the physical layers, to mitigate the fading effect if only a single antenna is used.
In 802.11a, each 20MHz channel band is partitioned into 52 sub- carriers shown in Figure 4, in such a way that each subcarrier can be thought of as its own narrowband channel. As different subcarriers will experience different fading effects, it is very likely that some narrowband channels drop out very fast but some less affected, can continue to provide good performance.
|802.11a OFDM PHY Layer Parameters|
|Pilot Subcarriers||4 (BPSK)|
|Modulation||BPSK, QPSK, 16QAM, 64QAM|
|Coding Rate||1/2, 2/3, 3/4|
MIMO technology in 802.11
In IEEE802.11n and 802.11ac, MIMO using multiple antennas has become an essential element of wireless communication systems. The use of more than one antenna provides extra independently faded paths, which can be exploited to improve the tolerance of a Wi-Fi link to fading. There is also an increase in antenna array gain from using multiple antennas. These factors combine to enhance the data rate and extend the range. There are three basic types of MIMO techniques supported in 802.11:
• Spatial diversity
• Spatial multiplexing
The exact implementation of beam-forming was not standardised until 802.11ac, so only spatial-diversity and spatial-multiplexing techniques are widely used in 802.11. Spatial diversity can improve the Signal-to-Noise Ratio (SNR). Spatial diversity is best suited for applications with a low SNR. Spatial multiplexing can provide more spatial streams and is better suited to use in applications which enjoy a high SNR.
In Single-Input Single-Output (SISO) wireless systems, according to Shannon’s Law, the upper boundary to the maximum data rate, C, of a link can be determined by the available bandwidth, B, and the signal-to- noise ratio, β, of the link.
The effect on the maximum data rate of the spatial-diversity and spatial- multiplexing techniques is shown in Figure 5.
|MIMO Method||Maximum Data Rate (bps)|
|Spatial Diversity (1 x N or N x 1)||Blog2 (1 + Nβ)|
|Spatial Diversity (M x N)||Blog2 (1 + MNβ)|
|Multiplexing (M x N)||NS Blog2 (1 + β), NS = min (M, N)|
Fig. 5: Maximum throughput for spatial diversity and spatial multiplexing. M x N represents M antennas and N antennas at the transmitter and receiver respectively.
Spatial diversity uses multiple antennas to increase range and improve link reliability by transmitting or receiving redundant streams of information in parallel along the different spatial paths between the Transmit and Receive antennas. Diversity techniques can be applied at the transmitter and/or on the receiver side.
Transmit diversity techniques
As shown in Figure 6(a), two Transmit antennas are sending signals to one Receive antenna. This is a 2×1 system. The antenna at the receiver receives two copies of signals which are modified by a gain of hij. hij is a complex number which represents both signal attenuation and phase shift.
With transmit diversity in 802.11, a Wi-Fi transmitter operating via multiple antennas, such as an access point, is capable of transmitting signals which have been modulated with identical information to one receiver. To improve the reliability of the link, the transmitter simply switches and selects the best Transmit antenna, that is, the one which can send the signal to the receiver with the highest SNR.
The other transmit diversity technique with better performance is called pre-coding. This is a pre-processing technique. Instead of selecting only the best antenna, the transmitter encodes and weighs all the signals across the transmitting antennas in such a way that the received signals can add up constructively at the receiver. Of course the transmitter must know the Channel State Information (CSI) beforehand in order to select the best Transmit antenna and to perform the pre-coding.
Consider the pre-coding process in a 2×1 diversity arrangement as illustrated in Figure 7, assuming that the CSI represented by matrix He, has been estimated and known. If the estimated matrix, He, is very close to the real matrix, H, and the transmitted signals, S, are pre-coded with estimated channel gains, the received signals at the receiver can be easily detected and recovered in a Gaussian noise environment, and the signal- processing work at the receiver can be significantly reduced.
Another transmit diversity method, called space-time codes, is also very popular. In comparison to the pre-coding method, space-time codes are less effective in performance when more than two antennas are used at the transmitter.
Receive diversity techniques
Figure 6(b) is a 1×2 receive diversity system. As with transmit diversity systems, the simplest way of using the multiple receiver antennas is just to use the antenna with the highest SNR. One disadvantage of this method is that it wastes the power of the signal received by the unused antenna. A better method, then, is called Maximal-Ratio Combining (MRC). The principle of MRC is to use and combine the two received signals constructively. As is shown in Figure 8, the two received signals, A∠Θ1 and B∠Θ2, are processed at the receiver and scaled in amplitude and delayed in phase until they are added in phase.
In this way, both of the received signals are used and not wasted, but this comes at the cost of increased hardware complexity and power consumption at the receiver. This technique can be used with two or more antennas.
Spatial multiplexing, as shown in Figure 6c, takes advantage of two independent spatial paths to transmit and receive two independent streams of information at the same time over the same frequencies. The two receiving antennas decode the signals and combine them constructively. While diversity techniques simply boost the SNR, spatial multiplexing can increase the maximum data rate significantly, in accordance with Shannon’s Law.
If CSI is not known beforehand, the simplest spatial-multiplexing method is to transmit spatial streams with equal power and modulate each stream at the same rate. In this case, the maximum data rate is not optimal. If the transmitter knows the CSI and can track channel variations, maximum data rates can be improved by allocating most of the Transmit power to the best Wi-Fi channels at the cost of weakening the rest. In theory, this method can get close to the maximum data rate by selecting modulation and coding rates separately for each spatial stream. In practice, 802.11 uses an unequal modulation rate but a uniform coding rate across streams; and it is very complex and expensive to maintain and allocate different power levels across different Transmit antennas. This will definitely limit the extent to which pre-coded MIMO can be optimised.
Number of antennas: the more the better?
The discussion above has shown that using multiple antennas in 802.11 can help to boost the data rate and increase the range of a Wi-Fi link. Over a wide range of SNR values, the data rate always improves with an increase in the number of antennas according to Shannon’s Law. So, is it always better to use as many antennas as possible in a Wi-Fi design?
The answer is no. The great performance improvement in terms of both throughput and range always calls for the implementation of more advanced RF antenna systems and more complex signal-processing units, and the use of a more powerful microcontroller. Such a design will occupy a larger board area, use faster RF switches, require additional phase shifters, and call for more costly power amplifiers: this means that a Wi-Fi MIMO systems will be markedly more expensive. In addition, power consumption will be much higher than that of a single-antenna Wi-Fi module, because the MCU is awake for much longer periods.
So the choice between SISO and MIMO is always a matter of trading-off between performance and cost. Today, Wi-Fi modules do not contain more than four antennas: the small extra gain in performance of using a fifth antenna would not be worth the large extra cost. In fact, most Wi-Fi MIMO modules have either a 2×2 or 3×3 configuration: this provides the best balance of space, cost and power consumption.