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MIMO can help in video delivery

Posted: 01 Jul 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Gil Epshtein? Metalink? mimo? video delivery? video?

Past attempts to deliver video over wireless networks failed for two key reasons. First, the effective throughput and coverage supported by legacy wireless technologies were simply inadequate. Second, these networks can't support video's high sensitivity to delays, jitter and loss.

The maximum effective throughput of an 802.11a/g system is about 25Mbps over a very short range and in an interference-free environment. The average HDTV stream consumes about 20Mbps with bursts that can exceed 25Mbps. When one considers that in many households, multiple TV programs are being viewed simultaneously, the throughput of 802.11a/g just won't work.

The emerging IEEE 802.11n multiple-input, multiple-output (MIMO)-based standard addresses key issues that have hindered video-over-wireless so far: increased throughput and extended reach, along with enhanced QoS. These features provide the foundation for reliable video delivery over wireless networks.

The following are points to remember when designing video-over-wireless systems:

  • Plan to cope with time-varying characteristics of the wireless channel. Although higher effective throughput is not the remedy for all issues related to wireless video delivery, it is a major step toward a robust solution. Higher throughput yields much better immunity to interference, while simultaneously providing the means to handle degraded link conditions. Excess bandwidth can also be traded for extended reach and lower power consumption. The most promising way to deliver this throughput is through MIMO and channel-bonding techniques. The latter approach extends bandwidth by bonding two adjacent 20MHz channels into a single 40MHz channel, more than doubling the bandwidth, since the guardband between the bonded channels can also be removed.

  • Focus on effective throughput. Higher physical-layer throughput is a must for coping with high bandwidth demand, but by itself is not enough. That's because traditional WLAN overhead consumes too much bandwidth that the remaining effective throughput for the application is insufficient. The answer is an aggregation mechanism that eliminates the overhead linked to each packet and replaces it with a common overhead for a group of packets. This increases MAC efficiency, which boosts total effective throughput.

  • Lower the interference probability. Unlike closed-wire networks with a finite but known bandwidth and well-defined number of users, WLANs are exposed to external impairments. Video-over-wireless systems must use a low-interference frequency band with high channel availability to avoid or cope with interference from other equipment operating in the same frequency range. Unlike the crowded 2.4GHz band, the 5GHz band is a low-interference band with high channel availability, which is mandatory for implementing the channel-bonding techniques described earlier.
  • Don't underestimate your bandwidth requirements. Systems have failed because of the false belief that 802.11a/g systems' 25Mbps theoretical rate is enough to deliver a single 20Mbps HDTV stream. It's easy to ignore considerations related to bandwidth variations in the wireless channel, the effects of distance and environment, and the need to support video-stream peak rates and personal video recorder functionalityincluding pause, instant replay, rewind and fast-forward.

  • To restore the synchronous nature of video streams and enable time stamps, the receiver must compensate for the jitter introduced by the network. This requires clocks to be synchronized at both ends.

  • Consider jitter buffer size, not only because of cost, but because a jitter buffer introduces constant latency into the stream. While it's possible to use a large jitter buffer in stored-playback applications, interactive and live streams are very sensitive to the increased latency associated with this approach.

  • Plan to support multimedia. Video isn't merely an "add-on" to existing wireless voice and data networks. The mixed support of voice, video and data poses new challenges related to packet prioritization and admission control.

  • Plan for a real home environment. What works in the lab or showroom might fail in the presence of furniture, walls, doors and people moving around. MIMO solves these coverage challenges because it can support a multipath environment and non-line-of-sight links.

- Gil Epshtein
Sr. Product Manager, Metalink Corp.

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