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As much a part of the infrastructure for the next great evolution of mobile products that builds towards distributing high-bandwidth multimedia services over the existing digital cellphone network, power management considerations are as important as ever.
While bandwidth is the most critical resource in the wired portion of the network, power consumption is the most critical resource for the customer. Overall, far more power is consumed in the radio base station at the end of the chain than the total power consumed in the rest of the content server and distribution network. The mobile video devices also consume significant power, and the impact on the user experience is much higher because the power is supplied by batteries, which generally limit the play time of the personal video device.
In the first part of this two-part article, we'll look at power consumption in the mobile multimedia delivery chain and the design approaches for reducing power in those sections of the chain that are the major power consumers. In part two, we'll focus on optimizing power consumption in the peripheral systems.
Power consumption in the video distribution chain
Figure 1 shows the end-to-end video distribution chain. It consists of:
•Servers—typically clusters of blade servers, each blade containing one or more processors and mass storage. The video content is accessed from here, unless cached in a downstream node. A cluster has a switch fabric that connects the blades to some number of I/O modules that support high-bandwidth communication standards (10G Ethernet, etc.).
•Routers—the high-bandwidth wired portion of the network. Core routers pass information to and from other routers. Edge routers make connections between core routers and interfaces to media consumers, such as cellphone radio base stations, cable modem head ends, and DSL multiplexers.
•Radio base station—provides the stationary end of the wireless connection. It's the largest power consumer in the video delivery chain.
•Mobile multimedia device—the mobile end of the wireless connection. Important power consumers are the radio transceivers (cellphone, Wi-Fi, Bluetooth), display controller, display backlight, audio subsystem, and hard disk drive.
(Click on Image to Enlarge)
Fig. 1: Video distribution chain
Power demands vary widely among blade servers, in large part because they hit different design points in terms of computational bandwidth, power consumption, and storage capacity. The lowest-power designs use flash memory, but typical designs use hard disks. The IBM HS20, for instance, is a typical blade server that provides up to 40 Gbps aggregate I/O bandwidth and consumes 180 watts.
The bandwidth required to support high-quality video is about 1.5 Mbps. However, mobile video devices must stay within bandwidth limits acceptable to the wireless network. The iPhone, for instance, uses the Low-Complexity H.264 Baseline Profile with AAC-LC audio, which requires 160 kbps to maintain a 30 Hz frame rate on a 480-by-640 display. This is within the speed of AT&T's GSM/EDGE network, as upgraded in recent months by their Fine Edge program, reportedly to speeds of 200 kbps or more.
At 40 Gbps, the HS20 blade server has about 200,000 times the I/O bandwidth required to support one stream of iPhone video. Thus the stream's share of the blade's power may be estimated at 0.9 milliwatts. This analysis ignores the power consumed in the switch fabric, I/O modules, facility air conditioning, and the inefficiencies that result in less than 100 percent bandwidth utilization. But those do not have a significant effect on the total power required to support a mobile video session. Indeed, the power consumed in by some of the major components and stages in the video distribution chain are measured in watts rather than milliwatts.
A similar situation exists at the router level. A session may pass through several core routers before reaching the router on the destination edge of the network. Juniper Network's 1440 router, for instance, has an aggregate bandwidth of 40 Gbps, the same as IBM's HS20 blade, but the router bandwidth must be counted twice because the session goes in one port and out another. The total power (including switch fabric and I/O modules) is 2,400 watts. If we divide that by, say, 100,000, we see the power to support one mobile video session is 2.4 milliwatts per router. Only a session with at least five hops would exceed 10 milliwatts in the routers.
On the other hand, Nokia's Flexi EDGE Base Station in a typical configuration with 12 transceiver (TRX) units burns 1,000 watts. In all GSM systems, the TRX units use time-domain multiplexed access, with eight timeslots per frame. A full-rate audio channel uses one timeslot per frame, but the GSM/EDGE extensions allow a video session to use more than one timeslot, if the capacity is available. Typical systems allow four timeslots to be used for downloading video, which results in a theoretical maximum data rate of 236.8 kbps.
Recent upgrades to AT&T's GSM/EDGE network support data rates reportedly above 200 kbps, which is near this theoretical limit. This operation occupies half of the capacity of the TRX unit, so its share of the base station power is 50 percent of 1000 watts divided by 12, or about 42 watts. This dominates the power consumption of the entire fixed (non-mobile) portion of the video delivery chain.
The popularity of viewing video over networks is climbing rapidly, and so is the power required to make it possible. According to MSN, the recent Live Earth concerts generated 9 million video streams, breaking previous records for on-line viewing. ABI Research estimates that there were one million video services subscribers in 2006, growing to 250 million by 2010. As mobile video devices such as the iPhone become commonplace, an increasing share of that video will be carried over wireless networks.
A 2006 study from Nielsen Media Research estimated that Americans watch an average of 4 hours and 39 minutes of television daily. If we assume that 250 million viewers will spend that much time viewing mobile video, that's (4.65)(365.25)(250M) = 425 billion hours per year. At 42 watts per hour, that's (42)(425B) = 17,850 gigawatt-hours per year in the non-mobile portion of the network. A typical nuclear reactor has a power output of about 1 gigawatt, or about (24) (365.25) = 8,766 gigawatt-hours per year, so mobile video in the U.S. can be expected to consume the power output of about two nuclear reactors in the near future. Any technologies which can reduce this energy consumption have a large potential pay-off both in terms of direct costs and environmental impact.
Saving power in radio base stations
The radio base station (RBS) consumes most of the power in the non-mobile portion of the video distribution chain. It thus represents the greatest opportunity to conserve power. The techniques include:
•Reduced antenna feed length—by moving the transmitter closer to the antenna, we can reduce losses in the antenna. The difference in RF power losses between a ground-based transmitter and a tower-mounted unit can be a factor of two.
•Unused TRX unit shutdown—because the number of TRX units is designed to handle maximum traffic conditions, under normal conditions there will be unused units. Only recently have RBS designs implemented shutdown modes for these unused units.
•Unused timeslot power control—the most popular standard, GSM, is a time-domain multiplexed-access system with eight timeslots per TRX unit. If any timeslot is not used, the power to the transmitter can be throttled down during that timeslot.
•Digitization directly from the IF—by digitizing the received signal at the first IF, we can eliminate an entire down-conversion stage and its associated power consumption. One example is an A/D converter which can operate at the IF frequency. National's 12-bit ADC12C170, for instance, with parallel CMOS outputs and 12-bit ADC12V170 with dual data rate and parallel LVDS outputs offer 170 MSPS operation with a full-power bandwidth of 1.1 GHz for WiMAX and 3G wireless communications applications. The ADC14V155 provides 14-bit resolution and dual data rate, parallel LVDS outputs, also at 1.1 GHz bandwidth.
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