Pushing 802.11n Wi-Fi Technology So It Can Deliver Multiple Simultaneous Streams
Consumers expect the same quality from their wireless technology as they do from wired Ethernet, and to meet these expectations, service providers are demanding that 802.11n Wi-Fi technology deliver as many as four HD video streams, simultaneously, at more than 100 Mbps data rates, anywhere in the home, with near-zero error rates. Achieving these performance levels requires a critical combination of technologies in order to overcome signal impairments and dead zones that are typical in the home. The latest 4x4 MIMO antenna architectures deliver on these expectations, by supporting four unequally modulated spatial streams while also using LDPC and dynamic beamforming technology to optimize performance, reach and reliability. This approach enables service operators to support multiple high-quality HD streams with full 1080p+ video resolution, and distribute them anywhere in the home.
Bandwidth requirements depend on the amount of compressed video that is needed per HDTV, and how many TVs and wireless game consoles there are. It is reasonable to assume a 30Mbps video-encoding rate is needed per TV and about 60Mbps for wireless gaming, understanding that some HD compressed sources may have much higher peak data rate than the average data rate. These rates are likely to be required across as many as three or four HDTVs as well as a variety of gaming gear. The industry needs to plan ahead so it’s not necessary to re-configure service offerings every few years. This may mean it’s necessary to allocate a sustained 120 Mbps of compressed HD video rates in the downstream direction from a variety sources such as STBs, RGWs and NAS boxes. Even greater peak data rates may be required when planning ahead for total home network capacity.
Delivering on consumer expectations for wireless-HD video performance and reliability requires the right MIMO architecture plus enhancements that optimize connection strength and reliability for consumer entertainment and the viewing experience. One of the key considerations is how many antennas to use for a given application. A wireless MIMO channel is a multipath system, which means multiple reflections create many paths between all of the antennas. On the transmit side, any single antenna can transmit signals which can then be deflected and diffracted into many radio wave branches as each signal moves forward. On the receive side, any single antenna can be the recipient of many of these radio wave branches, in which case each antenna can be viewed as an independent “observer” that performs independent sampling, improving signal-to-noise ratio (SNR).
A wireless channel can be classified in terms of channel rank, or how many independent paths it can support between transmit and receive antennas. The higher the channel rank (meaning more independent paths), the more the antennas can make use of these independent observation and sampling opportunities that improve SNR.
As the number of spatial streams increases, we have the opportunity to assign at least one antenna per spatial stream. Each spatial stream can carry a large amount of data—in the 802.11n protocol, the maximum amount is 150Mbps in a 40 MHz bandwidth. As one would expect, not every spatial stream can carry 150 Mbps, and not every spatial stream has equal capacity or is truly independent. As such, sometimes the use of extra antennas may not yield additional throughput. Figure 1 shows how two spatial streams are multiplexed over an array of 4 x 4 antennas in order to utilize the capacity offered with only two available independent paths. The use of extra antennas (i.e., two antennas per spatial stream) benefits the signal reliability by an average factor of two and in some cases even more. At the receiver, reliability can be improved by employing an algorithm known as Maximal Ratio Combining (MRC), which optimizes the received SNR by enabling all four antennas to be used to recover the two spatial streams.
FIGURE 1 • 4x4 MIMO with multiplexing of two spatial streams
In order to achieve the maximum raw data rate of 150 Mbps per spatial stream, sufficient SNR must be available. Adequate SNR enables the receiver to decode the incoming signal, where each spatial stream has a 64 quadrature amplitude modulation (QAM) index. Support of a lower modulation index is required (i.e., 16QAM or QPSK) when the available SNR at the receiver is lower. On-time packet delivery is very important for IPTV reliability. The User Datagram Protocol/Transmission Control Protocol (UDP/TCP) PER must be very low—generally from .01 percent to .001 percent. This contrasts with web surfing, in which the UDP/TCP is set to about 1 percent PER. It is the combination all of these SNR and PER factors that determines what kind of data rate is possible per spatial stream for a given application.
The ability to increase the number of antennas per spatial stream is a major advantage of higher-order MIMO systems as compared to lower-order MIMO systems. This can be very important in home-networking models, which require two very robust spatial streams.
In addition to committing two antennas to each of these spatial streams, systems can employ other reliability-enhancing techniques. One of the most advanced techniques is to allow unequal modulation for each spatial stream (see Fig. 2). Obviously, the ability to leverage the benefits of spatial streams increases as the number of antennas is increased.
FIGURE 2 • In this 4 x 4 MIMO example, example, the 4 spatial streams are not equally modulated. Streams 1 and 3 are QPSK modulated (per subcarrier -OFDM) and streams 2 and 4 are 64QAM modulated (per subcarrier -OFDM).
Another way to improve reliability is through beamforming, which optimizes wireless connections between transmitters and receivers by estimating the channel matrix (the channel between the transmitter and the receiver) and telling the transmitter how to pre-compensate on a tone-by-tone basis to optimize SNR of the received signal. If this is done dynamically, the MIMO receiver and transmitter can work together to estimate the adverse effects of any objects that would block or deflect the beam, and mitigate and/or pre-empt those affects by re-directing the beams from each of the transmitting antennas.
Correlated MIMO channels generally possess fewer degrees of freedom relative to ideal, fully scattered channels. As the SNR decreases, the number of spatial streams also decreases and reduces the multiplexing gain of the MIMO system. Fig. 3 shows a comparison of various MIMO systems and their associated rate/reach curves for the same channel conditions.
FIGURE 3 • In this simulation of over-the-air bitrate, PER is set to about 1%. Packet retransmission is required to achieve desired reliability at the expense of delay. The 4x4 system’s throughput outperforms that of a 3x3 system by about 180% at a distance of 50 feet.
Note that as the distance increases and SNR decreases, the number of spatial streams also decreases. For a given a 4x4 system at sufficiently long distance and with enough attenuation, all versions of 4x4 MIMO systems will operate in one or two spatial streams. In all of these analyses, the simulated data rate is the maximum possible data rate under the outlined condition. Later we will discuss the outage channel capacity as we look at a none-ideal interpretation of what is going on in the channel.
The industry is now exploring opportunities to increase wireless throughput beyond was is possible using 802.11n technology. The proposed 802.11ac amendment to the IEEE 802.11 specification is currently in draft stage, with final approval targeted for December 2013. Its main goals are to achieve a maximum Multi-Station (Multi-STA) throughput of at least 1 Gbps and a maximum single link throughput of at least 500 Mbps. These higher rates are motivated by the continuing trend to transition devices and applications from fixed links to wireless links and by the emergence of new applications with ever higher throughput requirements.
Unlike existing technologies that operate in the 2.4 GHz band and 5 GHz band or both, 802.11ac operates strictly in the 5GHz band, but supports backwards compatibility with other 802.11 technologies operating in the same band (most notably 802.11n). 802.11ac relies on a number of improvements in both the MAC and Physical Layer (PHY), including Increased bandwidth per channel, an increased number of spatial streams, higher-order modulation (256 Quadrature Amplitude Modulation, or QAM), and Multi-User Multiple Input Multiple Output (MU-MIMO). 802.11ac retains a number of advanced digital communication concepts that were first introduced in 802.11n, including space division multiplexing, LDPC, shortened guard interval (short GI), Space-Time Block Coding (STBC), and explicit-feedback transmit beamforming.
It remains to be seen how 802.11ac systems will develop and emerge. In the meantime, the performance delta between today’s DBF- and LDPC-enabled 802.11n 4x4 MIMO systems and earlier 3x3 solutions continues to grow. These 4x4 solutions also continue to benefit from ongoing integration improvements that will continue to reduce product costs, much like Gigabit Ethernet (GE) physical-layer (PHY) devices did as they eclipsed earlier 10/100 Base-T market solutions. The latest generation of 4x4 MIMO solutions delivers this performance, including full HDTV quality with 1080p and higher video resolution, all the time, anywhere in the home.
Posted by Janine E. Mooney, Editor
February 15, 2012