Spectral Reuse: The Big Squeeze - Technology Information

Telecommunications, April, 2001 by Tom Johnson

Providers could get up to eight times the capacity in every cell.

Bandwidth is scarce. Everyone wants it, but not everyone can get it, particularly when dealing with fixed wireless broadband. However, analysts agree that the fixed wireless broadband market is on the verge of rapid growth, as most recently evidenced by major carriers--including Sprint, BellSouth and WorldCom--announcing their strategies in this area. With more and more bandwidth for wireless broadband being auctioned by the FCC, and fixed wireless establishing its viability among U.S. carriers, it is only a matter of time until the technology becomes a viable option for broadband delivery to the enterprise and residential markets.

More demand, however, exposes the fact that spectrum is limited. As they deploy fixed wireless strategies, service providers need to ensure that they squeeze as many end users as possible into their licensed spectrum, thereby maximizing revenue. Spectral reuse is enabling providers to deliver up to eight times more capacity in every cell, which allows them to increase the total number of subscribers and generate significantly greater revenue.

Spectral reuse involves limiting the geographic area within a cell over which the signal is transmitted so that the same frequency channels can be used again in another sector (see Figure 1). This approach results in a multiplication of the effective capacity of spectrum in a given area. Spectral reuse can generate up to 24 independent, 15-degree sectors, resulting in the most efficient spectrum use.

Because spectral reuse provides the flexibility to deploy and illuminate each sector independently, service providers are able to radiate only in the specific directions in which subscribers are located. This capability for flexible deployment:

* Gives service providers the ability to maximize return on invested capital;

* Provides a fully scalable wireless broadband infrastructure;

* Enables greater aggregate data throughput;

* Substantially reduces the adjacent sector and cell interference common to traditional antennas.

Planning Deployment

Initially, the significant elements of the RF planning process include the areas of coverage, potential transmit sites, range and spectrum available. However, as planning advances to the more detailed design phase, anticipated demand drives the architecture toward either greater sectorization or high data rate technology to meet the demand.

While promising technologies offer some hope for higher data rates, near-term solutions must rely on current technology. This reliance places emphasis on the architecture, since an improperly designed or deployed system will quickly reach capacity. Expansion of available capacity can be achieved through greater sectorization if additional spectrum is made available or if the existing spectrum can be utilized more efficiently.

The Supercell

Spectral reuse technology will typically be used for fixed wireless networks in the 2.5-GHz range in the United States and the 3.5-GHz range internationally. The technology is deployed using the "supercell" and the "multicell," both of which capitalize on spectral reuse's potential for greater sectorization. The supercell--which has been applied effectively worldwide in point-to-multipoint systems--consists of 24 sectors, with each sector supporting a downlink and an uplink. Supercell channels use three frequencies for the downlink and three for the uplink. These frequencies are reused eight times to form 24 independent, full-duplex access channels.

The channel width is variable, ranging from 2 MHz to 8 MHz depending on spectrum owned and the desired/allowed spectrum partitioning. Splitting the cell into 24 sectors results in a 15-degree azimuth sector angle. This 24-sector supercell is typically used as a single cell and has been successfully deployed to provide service over a 20-mile cell radius.

For this type of installation, the system is placed on a high tower or terrain that provides unobstructed line of sight to potential customers. Typical performance at this range is [l0.sup.-8] BER and 99.9 percent availability. The frequency use of 8X enables a high spectral efficiency.

The Multicell

Each multicell also consists of 24 sectors, with each supporting a downlink and an uplink. Within a cell, eight distinct channel types are used in each direction and can be fulfilled by either eight different frequencies or four frequencies with two polarizations. This article focuses on the deployment of the four frequencies with both polarizations.

With two polarizations, these frequencies are reused six times in a cell to form 24 independent full-duplex access channels. The same frequencies are reused throughout the network of cells and are the only ones necessary, resulting in a network requirement of only four frequencies.

As is the case with the supercell, the channel width is variable, ranging from 2 MHz to 8 MHz depending on spectrum owned and the desired/allowed partitioning. Again, splitting the cell into 24 sectors results in a 15-degree azimuth sector angle. The multicell is typically used in cellular fashion and can be deployed in extended formations with each cell adjacent to the next. This deployment can be extended as far as desired.