Microwave Engineering Europe - November 2007 - (Page 28)

28 WIRELESS VIDEO Transporting video over wireless networks By Gil Epshtein, Director of Product Marketing, MetaLink ver the last ﬁve years, wireless data networks and the applications that access them have made our lives more convenient and provided us with precious ﬂexibility. Wireless technologies have become so entrenched in our everyday life that they have now permeated into the home entertainment market. While consumers used to watch DVDs or video clips in front of a stationary TV at a single location in the house, they now want to share and access their growing pool of DVRs, mobile devices, laptops, and other entertainment resources anywhere in the home, at any time. This presents a challenge for today’s developers, which is to provide seamless networking for multimedia and entertainment convergence. Consumers want their wireless network and applications to be just as reliable as if they were wired. Unlike transporting general data over wireless networks, however, video applications can’t tolerate bandwidth ﬂuctuations, so the challenge is signiﬁcantly more rigorous. There are many, many considerations to evaluate when pushing video content through a wireless network. Earlier wireless LAN technologies simply weren’t up to the task, and the industry responded with the most recent IEEE 802.11n speciﬁcation. Even this high-performance WLAN standard, however, isn’t enough for transporting video, and requires supplementary technologies to provide the kind of high- O quality entertainment experience that consumers demand. A number of critical factors contribute to satisfactory performance as perceived by the consumer. These include bandwidth, latency, breadth of coverage, and quality of service (QoS). Bandwidth is particularly important and is optimized through the use of MIMO (multiple-input multipleoutput) and channel-bonding techniques. These techniques also contribute to QoS, because higher throughput improves immunity to interferences and makes it easier to handle degraded link conditions. In addition, any excess bandwidth can be traded for longer reach and better power efﬁciency – the more bandwidth, the better. Not enough throughput At the same time, though, raw bandwidth from higher PHY throughput isn’t enough. What’s needed is higher effective bandwidth at satisfactory levels for the given application, and this requires the additional step of substantially increasing media access controller (MAC) efﬁciency. This can be achieved using an aggregation mechanism that eliminates the overhead linked to each packet and replaces it with a common overhead. Aggregate exchange sequences are enabled with a protocol that acknowledges aggregated MAC protocol data units (AMPDUs). As a result, there’s a single block acknowledgement (Block ACK) instead of multiple ACK signals, and there’s no need to initiate a new transfer for every MPDU. The result is a MAC efﬁciency of 70 percent as compared with typical 50 percent MAC efﬁciency ratings for IEEE 802.11a/b/g, as shown in Table 1. Another key consideration is how far the network can reach; the gold standard is whole-house coverage. Although users can tolerate “dead spots” and limited coverage when using home data networks, dead spots and limited coverage aren’t acceptable for wireless entertainment. Today’s centralized multimedia storage devices are expected to serve as the source for all multimedia content, no matter where in the home it’s being viewed or heard. This means that, unlike data networks, the bit rate can’t drop with increased distance from the access point. In addition, the use of forward error correction (FEC) schemes extends the reach that’s possible at any given data rate. For instance, 3 dB of coding gain derived from using the low-density parity check (LDPC) code translates into about 20 percent improvement in range. Alternatively, the additional gain can be used to increase throughput (using higher constellation) or to increase the robustness and immunity to interference, as shown in Figure 1. Whole-home coverage at optimal video performance is so critical to wireless entertainment that a key new mandatory test for all wireless entertainment networks should be a dropped-packet performance across the full reach of the typical home environment. Consider QoS The ﬁnal consideration for an optimal wireless entertainment experience is IEEE 802.11a achieves a 25-Mbit/s effective throughput at less than 9 meters (10 feet), which barely covers the living room of a typical home. Bandwidthenhancing techniques in the 5-GHz band enable a reach of more than 30 meters (100 feet) at the same rate, with full coverage. 802.11n MIMO 2x3 CB Max PHY throughput Typical application throughput Max PHY throughput @ 30 meter 300 150 60 802.11g/a 54 25 20 802.11b 11 5.5 4 Microwave Engineering Europe ● November 2007 ● www.mwee.com 028_029_030_MWEE.indd 28 25/10/07 14:09:01 http://www.mwee.com

Table of Contents Feed for the Digital Edition of Microwave Engineering Europe - November 2007

Microwave Engineering Europe - November 2007 Contents News Comment Metamaterials: Metamaterials Tackle Communications Wavelengths Microwave Components — EM tools: Microwave Component Design Easier With New EM and EDA Tools Cover Feature: RF Testing for OFDMA in LTE Base-Stations Startup Eyes Battery-Free Wireless Sensor Nets High-speed ADC Technology Paves the Way for Software Defined Radios Planning a WiMAX network: Maximising the ROI by Using Advanced Optimisation Tools Transporting Video Over Wireless Networks Ultrawideband Under the Gun Specifying the Proper SAW Filter Products Product Feature: RF Test Solution Supports Emerging 4x4 MIMO as Well as Multiple Commercial Standards Calendar

Microwave Engineering Europe - November 2007

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