Not your father's radio network

Jan 1, 2008 12:00 PM, By Jay M. Jacobsmeyer, P.E.

Bandwidth management (computers)

Information technology (IT) professionals are familiar with the many tools and techniques for managing network bandwidth. However, a good reference resource for obtaining more details on bandwidth management in computer networks is How to Accelerate Your Internet, edited by Rob Flickenger, which is available for free from http://bwmo.net.

Spectrum management (radios)

The goal of spectrum management is to maximize users per MHz per square kilometer while maintaining some minimum service threshold. The service threshold may have multiple parts that apply simultaneously, such as minimum signal strength, maximum probability of call blocking, minimum subjective voice quality (e.g., DAQ = 3.4) and minimum throughput.

To illustrate good spectrum management in a packet radio network, let's use 802.11b as an example. Assume that the service area cannot be covered by a single access point (AP) because one AP cannot provide adequate radio coverage or support the expected number of simultaneous users. Given these assumptions, we face two constraints:

Spatially adjacent co-channel APs will create dead zones between cells where service will be unsatisfactory unless the network is lightly loaded.
Only three non-overlapping 802.11 channels exist in the 2.4 GHz band: Channel 1 at 2412 MHz, Channel 6 at 2437 MHz and Channel 11 at 2462 MHz.

One way to address these constraints is to borrow a concept from cellular phone networks and use an N=3 frequency reuse pattern, which is illustrated in Figure 2. With N=3 reuse, each co-channel cell is separated by three cell radii, which create enough additional path loss that co-channel interference is minimized. Because a single AP can handle only a finite number of simultaneous users (e.g., 100) and a finite amount of traffic, such a network also maximizes the number of users per square kilometer.

One principle of traffic engineering applies to both circuit-switched and packet-switched networks: All users should share one set of channels (circuit-switched) or one medium (packet-switched) for the most efficient use of the available bandwidth. IEEE 802.11 networks are no exception. A network coordination function minimizes collisions between users on the same network, but harmful interference and low throughput result when two separate networks operate in the same area on the same frequencies.

Another example is 4.9 GHz networks. In the 4.9 GHz band, the FCC authorizes channel bandwidths of 1, 5, 10 and 20 MHz. Existing IEEE 802.11 standards specify channel bandwidths of 10 and 20 MHz, but some vendors also offer 5 MHz channels. The most common channel bandwidth used at 4.9 GHz is 20 MHz because it allows the use of the 802.11a protocol and offers the highest maximum bit rate of 54 Mb/s.

Unfortunately, there is only 50 MHz authorized in the 4.9 GHz band, so there are only two and one half 20 MHz channels. This limitation creates a problem if each agency in a metropolitan area wants its own radio channel — there simply aren't enough channels. A better approach is to operate one metropolitan-wide network using 10 MHz channels. The 10 MHz channel will allow an N=3 reuse pattern with two channels left over for contingencies or the occasional point-to-point link. The 10 MHz channel results in a 3 dB improvement in sensitivity compared with the 20 MHz channel, and it is more robust in the presence of multipath-induced delay spread. The only drawback to the 10 MHz channel is that the maximum bit rate is cut in half.

Jay Jacobsmeyer is president of Pericle Communications Co., a consulting engineering firm in Colorado Springs, Colo. He holds BS and MS degrees in electrical engineering from Virginia Tech and Cornell University, respectively, and has more than 25 years of experience as a radio frequency engineer.