Could directional Wi-Fi solve the capacity problem?
Directional aerials could mean more channels per access point.
As WLANs grow in size, their complexity increases even faster, and so do the problems of coverage and scalability. There are 13 different Wi-Fi channels, but they overlap, meaning that only three or four access points (APs) can coexist in the same airspace before they start interfering with each other - and interference means less range and capacity.
To add to the problems, a WLAN access point (AP) operates like an old-fashioned Ethernet hub, with its bandwidth shared among the local users. The net result is that each can only give reasonable bandwidth to perhaps 10 users. While adding another AP on a different channel would give more capacity, it makes it even harder to avoid interference with neighbouring cells.
One way to solve the problem could be the emerging MIMO (multiple input multiple output) standard, which can increase range and throughput by using multiple antennae at both AP and client. Software on the client compares the different received signals to deconvolve the multiple datastreams that were transmitted.
The snag is that it requires new MIMO hardware at both ends, as well as new software. Other developers are therefore looking for ways to improve 802.11 via the infrastructure side alone, so they can use existing client hardware and software. One such is Texan start-up Bandspeed, which is now working on the second generation of its SDMA, or spatial division multiple access, technology.
Six transmitters in every AP
SDMA is a fancy way of saying that you divide the network up by pointing different transmitters in different directions. Bandspeed's access points (APs) include six transmittters, along with sophisticated antennae to ensure each only covers a 60 degree angle with minimal overlap (3dB or less is claimed), and software that intelligently switches network channels and adjusts transmit power so adjacent sectors do not interfere.
Alternatively, the box can transmit over 120 degree sectors with two channels available in each. This allows one to be used as the backhaul between APs, for example. Sectorisation also means reflected signals can be used, giving greater coverage, and the technology is ETSI-compliant.
Directional aerials of this kind are also used in cellular phone systems and are attracting interest for wireless broadband too (for example in ArrayComm's iBurst technology, also offered by Kyocera), not least because they can provide directional range and capacity. In Bandspeed's case, they allow it to pack the capacity of six APs into one box.
However, Bandspeed's VP of marketing Blaine Kohl stresses that the important part is not the aerials, but the logic that controls them. "We have a multichannel baseband and MAC chip which interfaces to external radios," she says. "We are a silicon company, not an antenna company. We use six Atheros NICs in the reference design.
"The WLAN market breaks into three buckets - SoHo, hotspots and enterprise," she adds. "Most people think of SoHo. Most of the press attention has been to public access, but people are realising that's hard to make money from. The model has changed from paid-for access to providing it for free to your customers. That's a different world from what people thought.
The enterprise is conservative
"The enterprise market seems obvious but IT is very conservative and worried about security and management. The security issue is largely resolved with WPA and WPA2 thought."
She acknowledges that because larger companies have been slow to pick up on wireless LANs, specially designed enterprise-class Wi-Fi products have not been a great success. "People have been trying to differentiate themselves with features, but now they're starting to understand what the real problems are," she says.
"Now they're struggling with RF manageability - they quickly find out it's not scaleable, and with more than three APs in one space they will hear each other. People don't realise how much RF is out there - it's everywhere. Even in large public spaces like airports it is becoming a problem, it's a market that's built out of rogue networks.
"Then there are capacity issues as more users come on board. Today the AP is a hub that everyone shares. Most say you can get six to 10 clients per AP, then you can have three APs per space. So the wireless market in the enterprise is not late, it's early."
The other emerging problem is clients roaming between APs, which is a particular issue for voice over WLAN, as phones tend to move around a lot more than PCs and because voice calls are more sensitive to breaks in reception.
According to Kohl, SDMA deals with these issues because the multiple segments give more capacity per AP, plus beamforming allows them to cover ground more efficiently. She adds that if security is the issue, the directional nature of the aerials can be used to minimise the amount of radio that's broadcast outside the building.
She says that Bandspeed already has a few networks up and running on its Gypsy SDMA evaluation units, the challenge now is to get manufacturers to adopt its reference designs and get the technology into mass production.