Is 802.11n too power-hungry for handsets?

Most 802.11n access points draw more power than Power over Ethernet (PoE) can supply, while 802.11a/b/g access points work comfortably with PoE. So 802.11n must be more power consumptive than 11g, right?

The answer is yes, but when you delve into the reasons why you may discover that an 802.11n handset can still have comparable, or better battery life than an 802.11g one.

The big power drain for 802.11n is MIMO, for two reasons. First, MIMO demands a separate radio transmitter for each of its channels. In the Farpoint white paper linked above, testing was done with six transmitters – 3 at 2.4 GHz, 3 at 5GHz. The 11n specification allows up to 4 MIMO channels, and Wi-Fi certification requires at least two. Each of these transmitters burns as much power as the single (or dual in the case of an a/g AP) transmitter in an 11a or 11g access point. A second increase in power demand by 11n comes from the increased processing load not just because of the increased number of channels, and not just because of the increased data throughput, but also because each individual MIMO stream places a heavier processing load than a single 11a or 11g stream.

But the Wi-Fi Alliance (WFA) has waived the MIMO requirements for handsets, allowing 802.11n certification for single-radio devices. So none of these increases in power dissipation needs to apply to handsets.

Single-channel 802.11n still requires more processing than single channel 802.11g, because of advanced features like STBC and LDPC, but STBC and LDPC are amenable to hardware implementation (which reduces their power demand), and these and other advanced features of 802.11n improve “rate at range,” meaning that the transmitter is active for shorter times, and can transmit at lower power.

The net is that Redpine Signals, a pioneer of 11n for handsets, claims that a handset using the Redpine 11n chip actually has better battery life than it would with a competitor’s 11g chip.

Wi-Fi state of the art is a rapidly moving target, and over the past 12 months there have been startling improvements in power efficiency. I have written here about the new Atheros chip, for example. So if the latest 11g handset chips are more power efficient than 11n competitors, it is more a function of their recency than their adherence to 11g.

The benefit of 5 GHz operation is compelling for Voice over Wi-Fi, and it will be hard for handset vendors to promote the decade-old 802.11a over 802.11n. 802.11n is already the Wi-Fi flavor of choice for access points and PC clients, and it soon will be for handsets, too. How soon? It’s hard to say. So far the only chip vendors to announce 11n for handsets are TI, Redpine Signals and Conexant, and Conexant exited the handset Wi-Fi business just two months after it announced this chip. No phone is yet shipping with 802.11n although TI said it was sampling its WiLink 6.0 with 11n in February 2007. The Wi-Fi alliance has not yet published its Handheld profile for 802.11n certification. On the other hand, ABI research in September 2006 predicted that the majority of the 300 million Wi-Fi enabled handsets to ship in 2011 will support 802.11n.

If 802.11n handset shipments fall short of this prediction, it won’t be because of battery life considerations.

2 Replies to “Is 802.11n too power-hungry for handsets?”

  1. I wonder if anyone will read this comment considering how old the article is, but the STBC/LDPC comment is inacurate : those two option are optional, and I haven’t seen yet a device that support them

  2. Actually, STBC is in practically all the current mobile Wi-Fi chips – Atheros, Redpine, Qualcomm, Marvell, Broadcom and so on. LDPC is not so common. But if you Google “Atheros LDPC mobile” you see that the AR6003 family has both.

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