New approaches to an old problem

Dec 1, 2007 12:00 PM, By Dave Plank

Standards limitations also must be taken into account. When initiating the handoff of a device from an outside cell to a femtocell, for example, only a limited number of cell sites — typically 16 — are scanned and measured; in a crowded environment, there may be well more than 16 sites in range. Also, in a system where the outside and femtocell networks use the same frequency band, the cells may interfere with each other as mobile units increase their transmit power to the femtocell. There also is the issue of coverage area — femtocells on different floors can interfere with other users. In an open system, where the femtocells are not managed, such a scenario can quickly spiral out of control, with users on the wrong cells and cells interfering with each other. In a closed system, the units are spectrum-managed, with a remote controller constantly adjusting the power of different cells to maintain network performance.

There are several partial solutions to this problem, but the best way to prevent interference is to use a different frequency for the femtocell coverage, particularly in CDMA deployments. Less-foolproof solutions include using fixed-power options for handsets, which prevent the mobile unit from increasing its power and causing interference.

Backhaul is another problem. Each femtocell requires backhaul connectivity, but the best location of a cell for backhaul may not be the best location for coverage, Scheinert said. And because femtocells may cause interference to outside signals, they often operate on very low power — typically in the 1-10 mW range.

If a building is small enough to be covered by the weak femtocell signal, there's no problem. But buildings of several thousand square feet — or campuses with multiple buildings — may require high-output power that interferes with outside cells. And an in-building dominant signal can be difficult to achieve, with some construction materials significantly reducing signal strengths. That means high data rates may be available in only some parts of the building.

Combining base stations with a DAS, Scheinert said, can provide the best performance with the least interference. DAS can enable higher loads of femtocells, with improvements in base stations perhaps eventually allowing more capacity than outside cellular services. And because DAS separates the base station and the antennas, coverage can be tailored to individual buildings.

Another path to femtocell deployment was chosen by Meru Networks of Sunnyvale, Calif., in its efforts to bring FMC to Osaka Gas Co., Japan's second-largest utility. In what the company calls the largest FMC deployment in the world, Osaka Gas first deployed a pervasive WLAN infrastructure capable of supporting multiple data and voice applications.

The quality of wireless voice-over-IP (VoIP) service was of particular concern for the company, said Racha Ahlawat, vice president of strategic marketing for Meru. As a utility, the company cannot allow delays in times of emergency. Ahlawat said Meru's system allows wireless VoIP in large environments with guaranteed service quality without any proprietary extensions to the client's network. Meru's system also treats all physical access points as a single, virtual access point, making the question of handoffs irrelevant, Ahlawat said. Because the devices recognize only one access point, voice clients have seamless roaming with no loss in quality or dropped calls, she said.

Another improvement in the Osaka Gas system is simplified, single-channel operation of access points, which according to Meru eliminates the need for complicated RF site surveys and allows operators to deploy networks as easily as they might 802.11 or other wireless data networks. In addition, contention management and load balancing inherent in the network eliminate the bandwidth-allocation demands common to high-density deployments, as well as many interference issues.

The Osaka Gas installation began in May 2005 and was fully deployed by the end of March 2006. The network uses 800 access points with 72 controllers and serves more than 5000 handsets, Ahlawat said. Besides the wireless phones, the company still has wired phones at 17 of its 49 offices for emergency communications. Ahlawat said Osaka Gas expects total annual savings of about $4 million from its wireless VoIP solution.

In addition to the cost savings, the Osaka Gas network seems to deliver many of the other efficiencies touted by femtocell boosters: Employees can answer extension calls anywhere, anytime, as if they were at their own desks. And critical documents stored on the company's central server are now accessible anywhere, allowing employees to deliver information to their customers at a moment's notice.

Whether deployed alone, Wi-Fi style, or as part of a hybrid femtocell/DAS system, femto networking seems to be ready to serve the demands of even exacting enterprise customers. The long-ballyhooed FMC era may finally be emerging.