and eventually, spectrum from 4 GHz to 10 GHz.
Unfortunately, the mobile industry doesn't understand the implications of MIMO or beamforming, so they are asking for the wrong thing. They are asking for more spectrum near their current bands (below 2.1 GHz) or lower, e.g. in former TV bands below 700 MHz. This is all wrong.
Until recently you could say: lower frequencies "work better" meaning they go farther. But this was a technology limitation, not something in the physics. From a physical point of view, 5 GHz photons pass through the atmosphere just as well as 700 MHz photons. Today, MIMO and beamforming are correcting some of these historic technology limitations and changing everything.
- MIMO not only delivers significant performance improvement, it makes 5 GHz spectrum as useful as lower frequency spectrum for all sorts of applications.
- Beamforming delivers the next performance increment (20x and up), and it's actually easier to implement at higher frequencies.
Addressing mobile data demand
There is no doubt data consumption is growing more rapidly than mobile operators' data capacity (see slides 10 and 11 of my presentation at 4GWE). Unfortunately, US cellular operators are pressing for more low frequency radio spectrum and they are being heard in Washington. In his keynote at this year's Consumer Electronics Show, FCC Chairman Julius Genachowski said an impending shortage of wireless spectrum in the U.S. will dampen future economic growth unless action is taken to fix the problem.
However, the short term issue is investment dollars not spectrum and the long term solution is new technology at much higher frequencies (above 4 GHz) rather than more spectrum for existing technology near existing bands.
Why 5 GHz spectrum is more useful than TV frequencies
Everyone know TV signals go long distances but remember than TV broadcasters use from 100 Kilowatts to 5 Megawatts ERP. That's a million to a hundred million times more signal than your mobile handset puts out. No wonder it covers a lot of distance.There's also an equation most wireless engineers use in one form or another to calculate free space path loss which says higher frequencies have more loss. But this equation encapsulates two factors: the true path loss and the size of the antenna. It assumes a 1/2 wavelength antenna. Higher frequencies have shorter wavelengths, so it assumes the antenna gets smaller as the frequency goes up. Smaller antenna, less signal. With comparable antenna apertures, path loss in the atmosphere is the same from below 50 MHz to nearly 10 GHz. Thus in open air, 5 GHz photons go just as far as 500 MHz (US channel 19) photons or indeed photons for Channel 2 or Channel 50.
Multi-path and MIMO
The reason people have had trouble with higher frequencies for the past 100+ years is "multi-path" propagation. As a signal radiates from a source, some of it goes directly to the receiving antenna but some of it goes in other directions where it may be reflected or refracted by objects it encounters. When reflected signals also reach the receiving antenna, they are slightly delayed because they traveled a slightly longer distance. In the days of over-the-air analog TV, we saw these delayed signals as "ghosts" or shadows around images on our TV screens. For digital data transmission, multi-path contributes to the "noise" in the signal-to-noise ratio. The historic problem with higher frequencies is their shorter wavelengths made it easier for them to be reflected and refracted by man-made objects like buildings, window frames and even closely spaced double pane glass surfaces.
But with MIMO, all this changes. With MIMO's multiple antennas and multiple radio front ends, it's possible to separately decode and make use of the multi-path signals. Now "multi-path" is not only removed as a source of "noise," it adds additional signal and helps carry more data.
A beamformer uses signal processing to control the phase and relative amplitude of the signal at each of a group of independent antenna elements. Radiation from multiple antenna elements causes a pattern of constructive and destructive interference in the resulting wavefront. This can produce a tight beam just like one from a highly directional antenna. But with beamforming, the antenna beam can be steered in software on a packet-by-packet basis.
With eight antenna elements spaced 1/2 wavelength apart (total 3.5 wavelengths), you can create a beam like this:
Highly directional beams significantly extend the usable range of a wireless system. And since the beam is computed with digital signal processing software, it can be steered to different directions in microseconds. What's more, the benefits of beamforming apply both while transmitting and while receiving. Either way, the beamformer accentuates the signal in the desired direction while surpressing signals to/from other directions.
But what about wavelength? To obtain this narrow beam, the outer antenna elements are 3.5 wavelengths apart. At 5.8 GHz, that's less than 7.5" so the whole antenna array easily fits in a ceiling mounted access point just 8" x 3" by 2". At 700 MHz, that degree of beamforming still requires 3.5 wavelengths, but wavelengths are longer so now we need 5 feet of separation — something that may fit on a cell tower, but is difficult for a microcell and impossible for a femtocell. For beamforming, higher frequencies are an advantage.
More spectrum at higher frequencies
Finally, there's a lot more spectrum potentially available at higher frequencies and, today, in the 5GHz band, there is over 555 MHz of license-exempt spectrum already available for applications like Wi-Fi. That's more spectrum than Verizon Wireless, AT&T Wireless, Sprint PCS and T-Mobile USA have, combined!
Wi-Fi blazes the trail
The Wi-Fi market place is vastly more diverse than the 3G/4G mobile operator market place, which means many new technologies show up in Wi-Fi years before they are deployed in mobile networks. That is certainly true of so-called 4G modulation (OFDM) which was deployed for Wi-Fi with 802.11a (1999) and 802.11g (2003), years ahead of WiMAX (2005) or LTE (2010).
The 802.11n specification includes MIMO and optional beamforming and silicon technology is appearing to support 4x4 MIMO with beamforming. MIMO products (2x2) have been shipping since 2007 and 4x4 MIMO in consumer products expected in the next six months. Meanwhile, many players are scrambling to deliver 11n options, including beamforming. Early systems are already deployed.
Mobile operators, please pay attention
The technology benefits of MIMO and beamforming apply to the mobile phone industry, it will just take a few years for the deliberate pace of the industry to catch up. Meanwhile, mobile phone operators should be tracking real world measurements of Wi-Fi performance to understand what spectrum they will really need five to eight years hence.