The fall 2009 crop of ultimate smartphones looks more penultimate to me, with its lack of 11n. But a handset with 802.11n has come in under the wire for 2009. Not officially, but actually. Slashgear reports a hack that kicks the Wi-Fi chip in the HTC HD2 phone into 11n mode. And the first ultimate smartphone of 2010, the HTC Google Nexus One is also rumored to support 802.11n.
These are the drops before the deluge. Questions to chip suppliers have elicited mild surprise that there are still no Wi-Fi Alliance certifications for handsets with 802.11n. All the flagship chips from all the handset Wi-Fi chipmakers are 802.11n. Broadcom is already shipping volumes of its BCM4329 11n combo chip to Apple for the iTouch (and I would guess the new Apple tablet), though the 3GS still sports the older BCM4325.
Some fear that 802.11n is a relative power hog, and will flatten your battery. For example, a GSMArena report on the HD2 hack says:
There are several good reasons why Wi-Fi 802.11n hasn’t made its way into mobile phones hardware just yet. Increased power consumption is just not worth it if the speed will be limited by other factors such as under-powered CPU or slow-memory…
But is it true that 802.11n increases power consumption at a system level? In some cases it may be: the Slashgear report linked above says: “some users have reported significant increases in battery consumption when the higher-speed wireless is switched on.”
This reality appears to contradict the opinion of one of the most knowledgeable engineers in the Wi-Fi industry, Bill McFarland, CTO at Atheros, who says:
The important metric here is the energy-per-bit transferred, which is the average power consumption divided by the average data rate. This energy can be measured in nanojoules (nJ) per bit transferred, and is the metric to determine how long a battery will last while doing tasks such as VoIP, video transmissions, or file transfers.
For example, Table 1 shows that for 802.11g the data rate is 22 Mbps and the corresponding receive power-consumption average is around 140 mW. While actively receiving, the energy consumed in receiving each bit is about 6.4 nJ. On the transmit side, the energy is about 20.4 nJ per bit.
Looking at these same cases for 802.11n, the data rate has gone up by almost a factor of 10, while power consumption has gone up by only a factor of 5, or in the transmit case, not even a factor of 3.
Thus, the energy efficiency in terms of nJ per bit is greater for 802.11n.
Here is his table that illustrates that point:
The discrepancy between this theoretical superiority of 802.11n’s power efficiency, and the complaints from the field may be explained several ways. For example, the power efficiency may actually be better and the reports wrong. Or there may be some error in the particular implementation of 802.11n in the HD2 – a problem that led HTC to disable it for the initial shipments.
Either way, 2010 will be the year for 802.11n in handsets. I expect all dual-mode handset announcements in the latter part of the year to have 802.11n.
As to why it took so long, I don’t think it did, really. The chips only started shipping this year, and there is a manufacturing lag between chip and phone. I suppose a phone could have started shipping around the same time as the latest iTouch, which was September. But 3 months is not an egregious lag.
The anecdotal reports of increased consumption may come about because the experience of web surfing 9x faster is so much more pleasurable that (say) 50% more pages are visited, more than offsetting the nJ/bit gains.
The anecdotal reports may very well be correct. What is likely is that the control to power down the chipset is based on a timer, i.e., after a certain period of inactivity, power down some or all of the chipset. This timer is probably not modified in the hack enabling 11n. So, the .11 chipset power consumption increases when 11n is enabled and the time that chipset is active does not change, even though the bit rate is 10x greater. This would cause the observed shorter battery life.
Uhm ok actually, what is happening here:
22MBps at 140mW of POWER: If we assume our transmission lasts for one second we get 22MBit for 140mJ which gives us an energy consumption of 6.4nJ per bit. and a total consumption of 140mJ from the battery.
So there we are. as above.
Talking about 802.11n:
200MBps at 1W of POWER: If we assume the transmission lasts for 1 second (as above) then we get 200MBits for 1 Joule and each bit cost us 5nJ. so each bit is cheaper yes. BUT we sent 200 million of them, not 22 Million. and so our battery sucked up a massive 1 Joule of energy to send it all.
Hence: our battery died approximately 5-6x faster.
So yes, you get your data faster. And if downloading the same file should do it with less total energy consumption. BUT you will take much less time to do it on 802.11n… and your battery WILL die much faster since the average power consumption of the device WILL go up 5-6X … 🙂
Modern power management techniques involve switching off the radio between packets; it is worth powering down for just a few microseconds. The radio is only powered up while bits are being transmitted or received, so if you are thinking of the radio being on for a second, that on-time is distributed over ten times as much clock-time in an 802.11n environment as an 802.11g environment. So you are right in the sense that a second of radio-on time will consume more power with 802.11n, but for streaming a movie that lasts two hours, the 802.11n radio will only be consuming power for a tenth of the time that the 802.11g radio would, and the battery life would actually increase, according to McFarland.