Trends on wireless home-networking technologies

Trends on wireless home-networking technologies

Latest 802.11n advances optimize HD video quality for digital home entertainment networks

BY DAVID COHEN
Senior Director of Marketing
and SIGURD SCHELSTRAETE
Principal Engineer
Quantenna Communications
www.quantenna.com

Consumers demand the same quality from wireless home networks as they do from wired Ethernet, and 802.11n technology has met these expectations while typically outperforming many wired options including MOCA, HPNA, and PLC. With the ability to deliver four simultaneous high-definition (HD) video streams at more than 150-Mbit/s data rates anywhere in the home with near-zero error rates, 802.11n also provides an attractive whole-home IPTV networking backbone for companion single-room wireless technologies including emerging short-range 60-GHz solutions.

To date, the highest levels of 802.11n performance, coverage, and reliability have been achieved using a 4 x 4 MIMO architecture that supports four unequally modulated spatial streams while also implementing low-density parity check (LDPC) coding and dynamic beamforming. To further improve range and throughput, the industry is considering several other approaches, including power-setting adjustments (although this also decreases power efficiency), and implementing media access control (MAC) and physical-layer (PHY) improvements as proposed in the IEEE802.11ac specification.

Building on a solid 4 x 4 MIMO foundation

One of the key decisions that Wi-Fi system designers must make when implementing MIMO technology is how many antennas to use, or the order of MIMO. The more independent paths there are between transmit and receive antennas, also known as channel rank, the more the antennas can exploit these independent observation and sampling opportunities to improve signal-to-noise ratio (SNR).

Each independent path can carry one spatial stream and, as the number of spatial streams increases, there is the opportunity to assign at least one antenna per spatial stream. In the 802.11n protocol, each spatial stream can carry up to 150 Mbits/s in a 40-MHz bandwidth.

The use of extra antennas (that is, two antennas per spatial stream) in a 4 x 4 MIMO configuration benefits the signal reliability by an average factor of two and, in some cases, even more. In addition to committing two antennas to each of these spatial streams, systems also can allow unequal modulation for each spatial stream, further improving reliability. Systems also should support LDPC to enable efficient error correction coding for superior coverage and improved range.

Another important decision is the type of beamforming technology to use. Beamforming estimates the channel between the transmitter and the receiver so the transmitter knows how to pre-compensate on a tone-by-tone basis to optimize the received signal’s SNR. When this is done dynamically, the MIMO receiver and transmitter can collaborate to redirect beams from each of the transmitting antennas. Figure 1 shows how beamforming yields a significant SNR enhancement.

Fig 1: 4 x 4 MIMO with adaptive beamforming delivers a 12-dB advantage over 3 x 3 MIMO without beamforming.

A 4×4 MIMO architecture with four spatial streams capable of unequal modulation, combined with other 802.11n enhancements including LDPC and dynamic beamforming, delivers superior overall performance as compared to alternative solutions. This architecture also performs better than lower order MIMO systems that adjust power settings to increase range and throughput. While additional power beyond the typical legal, standards-approved +23-dBm (200-mW) level will likely increase range and throughput for most 802.11n designs, operating at these power levels can introduce significant problems. Additionally, tests confirm that, at every power level, a 4 x 4 MIMO reference design will have significantly longer range than a 3 x 3 design.

802.11ac PHY and MAC enhancements

The IEEE task group TGac began work in November 2008 to significantly increase throughput within the basic service set (BSS) to a targeted maximum multi-station (Multi-STA) rate of at least 1 Gbit/s and a maximum single link throughput of at least 500 Mbits/s. Still in draft stage, the specification is targeted for final approval in December 2013. At that time, the approved specification, to be called 802.11ac, will become an official 802.11 amendment.

Unlike existing 802.11 technologies that operate in one or both of the 2.4- and 5-GHz bands, 802.11ac operates exclusively in the 5GHz band while maintaining backwards-compatibility with 802.11n and other 802.11 technologies operating in the same band. The 802.11ac specification relies on several physical layer (PHY) improvements including increased bandwidth per channel, more spatial streams, higher-order modulation (256 quadrature amplitude modulation), and multi-user multiple input multiple output (MU-MIMO) technology.

802.11ac also supports earlier 802.11n advanced digital communication techniques including space division multiplexing, LDPC coding, shortened guard interval (short GI), space-time block coding (STBC), and explicit-feedback transmit beamforming. The MAC layer also includes a number of earlier 802.11n improvements including the larger maximum size of aggregate MAC protocol data units (MPDUs).

Additionally, 802.11ac supports 80-MHz channels as well as 20- and 40-MHz channels, plus the optional use of contiguous 160-MHz channels or noncontiguous 80+80-MHz channels. Increasing channel bandwidth to 80 MHz increases performance, as long as interference from nearby networks can be mitigated. With only four or five available 80-MHz channels, it is harder for an 80-MHz system to avoid interference from neighboring networks (which could be either 80-MHz networks or 20/40-MHz networks).

Consider an 80-MHz system occupying four 20-MHz channels, with one of the 20-MHz channels also being used by a legacy 20-MHz system (see Fig. 2a ). The system will have no way to avoid the occupied channel, and must share access to the medium with the 20-MHz system. Sharing access between the two systems halves capacity of the 80-MHz system, which cannot fall back to 40-MHz transmission since the overlap occurs in that channel.

Figure 2b shows how a 40-MHz system can avoid the occupied 20-MHz channel by channel selection. The 40-MHz system has full, unshared access to the medium. A single-stream 40-MHz system would have the same capacity as a single-stream 80-MHz system. If the same 40-MHz system supported two streams; however, its capacity would be double that of the single-stream 80-MHz system.

Fig 2: Two interference scenarios, a and b

Another important consideration for 80-MHz systems is the MIMO architecture that is chosen. While these systems can provide the same performance as a 40-MHz system by using fewer antennas, this eliminates diversity and reduces transmission strength, which causes problems for high-quality video content. Carriers require at least 120-Mbit/s sustained data rates, and near-perfect per data transfer performance to provide enough quality and reliability for demanding video entertainment and gaming applications.

Video delivery stability also requires that the number of antennas be higher than the number of spatial streams. Therefore, even 80-MHz systems must be built using multiple antennas. This reduces the cost and power advantage between a (single-stream) 80-MHz-bandwidth system and a (two-stream) 40-MHz system.

The 802.11ac channel bandwidth may cause problems when channel bandwidth is limited. Exploiting channel diversity by employing more spatial streams uses spectrum more efficiently than simply doubling the transmission’s bandwidth. A 4 x 4 system with a maximum number of spatial streams and multi-user MIMO (MU-MIMO) will most likely be required, at a minimum, in order for 802.11ac to fully realize its potential. On top of this, systems will need to use space-division multiplexing, LDPC, STBC, beamforming, multiple streams, and a variety of other PHY, MAC, and coexistence enhancements.

The industry continues to enhance high bandwidth Wi-Fi technology such as 802.11n and 802.11ac. A solid foundation has been established with 4 x 4 MIMO solutions that deliver full HDTV quality with 1080p and higher video resolution, all the time, anywhere in the home. In the future, ongoing 802.11n and 802.11ac developments are poised to deliver further enhancements in throughput, reach, and reliability. ■