You Make the Call

As you continue to monitor your network with an eye toward boosting capacity and maximizing call quality, noise in the form of co-channel interference can be one of your greatest headaches. Too much interference will stand in the way of high completion rates, successful handoffs, top-notch voice quality and successful data transmission, all of which are essential to customer satisfaction and reduced churn rates. Keeping a check on co-channel interference is imperative. You should be well equipped to measure it and combat it.

Co-channel interference occurs when two or more sites or subscriber units are operating on the same channel and interfere with each other. Many carriers employ frequency reuse, a technique through which the same frequencies are used in some other portion of the network to offer greater capacity to the subscriber population. A greater spatial separation is of benefit here -- the farther away the two channels are, the less interference. But the constraints of achieving maximum capacity often require carriers to reuse frequencies in densely populated metropolitan areas as close as 10 miles apart.

HOW NOISY IS IT? In cellular radio engineering, co-channel interference is normally quantified as the ratio of a wanted signal to an unwanted signal plus noise over a defined bandwidth. The extra noise that may appear in a channel's bandwidth could be RF noise from another source, such as a system noise, broadband interference, adjacent interference or natural noise. The total co-channel interference is thus represented by C/I+N, where C is the carrier, or the wanted signal; I is the interference from the co-channel; and N is the in-band interference consisting of the latter-mentioned sources.

S/N or C/I is measured as a decibel quantity and indicates the comparative level of the two. As an example, a C/I of 25dB would mean the interference is 25dB lower than the wanted signal. For example, suppose a signal of 10mW is measured and an interfering signal level of 0.5mW prevails, the C/I+N can be calculated as:

10 log(10) 10/0.5 = 13dB

GIVE & TAKE The quintessential rule of C/I+N is: With what level can you live? With vendor input and RF- engineering recommendations, you should derive a value that provides a realistically sized cell, thus determining the spacing between cells. You should make every effort to maintain this value as a bare minimum.

A safe working value will provide good voice quality at the outer edges of properly sized cells. Should your chosen value produce smaller cells, then your system build-out and expansion costs will be high.

Conversely, a lower C/I+N will produce larger cells, but at the expense of signal quality and reliability.

A typical test gear setup for quantifying co-channel interference consists of a scanning receiver capable of working with your technology, a GPS receiver and laptop computer. You should periodically drive test individual sites and your entire market to ensure that the co-channel interference level is below your established minimum. A drive test is particularly valuable before assigning new frequencies to a site, as the true level of a potential co-channel interferer can be quantified before you put the new frequency on-air. Software supplied with your test and measurement equipment will read GPS location and carrier signal-strength data, which can be plotted back at base, providing your RF engineers an instant perspective of the prevailing conditions.

COMBATING CO-CHANNEL INTERFERENCE Where frequencies are at a premium and capacity is demanding, combating interference will be challenging, as subscriber growth demands more channels and hence more reuse. Some carriers using high sites in dense areas or reusing the same frequencies on the edge of a lake have experienced great difficulties. High sites normally give an "as-good-as-line-of-sight" condition to co-users some distance away. Similarly, in many cases, if water separates two co-users, they may as well be neighboring sites.

A good, solid frequency plan is cardinal rule number 1 and is also a fine theory. In practice, you should perform a site-by-site evaluation of your system to determine what action to take to resolve the interference issue. Antenna downtilting can reduce the size of a cell's reach in one direction and may reduce the C/I+N by a few decibels or more. In dense systems, you should lower high sites, particularly in the market's core. Lowering sites will allow for greater reuse. You can address any resulting coverage holes by adding new sites using the new frequencies made available from your revised frequency plan.

Another popular alternative is reducing the effective radiated power (ERP) of a site. Decreasing the site power certainly will reduce interference to some extent, but it also reduces the site's coverage, in some cases negatively affecting customer satisfaction. Each site's exact coverage can be measured using the test equipment mentioned above, and again with a site-by-site evaluation performed to ascertain optimum coverage needs.

UP & DOWN Co-channel interference can, of course, be both uplink- and downlink-related and is sometimes unavoidable on the uplink. Users situated in high office buildings or perched on high ground have greater visibility to the market than a user on the street, and hence they have the potential to interfere with other portable users' channels.

A high-power mobile device will talk farther than a low-power portable, so a market supporting various classes of wireless devices runs the risk of a high-power mobile at a site possibly conflicting with a lower-power mobile at the co-use site. You must consider this when you are on the hunt for uplink-related problems.

Omnidirectional sites broadcast their channels in all directions, whereas sectored sites can broadcast fewer channels in three or more directions. This means that the same frequencies can be reused much closer together using sectorized sites than they can be with an omnidirectional site.

Co-channel interference is all about effective management of spectrum, your most precious resource. Careful planning and high-quality engineering equipment operated by motivated, high-caliber RF engineers is vital to your wireless operation.

For information on co-channel interference, consult the following: Cellular and PCS: The Big Picture, Harte, Prokup and Levine, McGraw-Hill; Cellular System Design and Optimization, Smith and Gervelis, McGraw-Hill.