Fear of Interference

Techniques to increase capacity also may increase co-channel interference.

Although interference problems in wireless networks can come from outside sources, most engineers first must focus on the interference their own network design creates, co-channel interference.

In AMPS, GSM and IS-136 networks, calls are assigned specific frequencies. In GSM and IS-136, calls also are assigned specific timeslots within those frequencies. Design engineers are careful to plan which frequencies can be used at which base stations, the idea being to keep an adequate distance between the reuse to ensure they won't interfere with each other. This is co-channel interference.

Traditionally, design engineers have used the “pattern of 7” to lay out which sites could use which frequencies. In this pattern, frequencies are reused only two base stations away. With an overall fixed amount of available spectrum, this reuse pattern then becomes the limiting factor for capacity. Lower the reuse pattern, and you can increase capacity, but you also increase the possibility of co-channel interference.

Infrastructure vendors obviously realize the importance of mitigating co-channel interference to increase capacity. A basic feature is alternating channel assignments. If a set of frequencies simply were assigned to specific sectors, as requests for channels came in, the base station always would start with the first frequency and work its way up. That first frequency almost always would be up and would have the most use on it. If the co-channel site had those same frequencies assigned in the same way, they both would be transmitting on the same frequencies all the time. By alternating the assignments — in other words, one sector assigns from the top of the list down while the other starts at the bottom and works up — as long as all of the channels are not used, co-channel interference can be limited.

Dynamic Channel Allocation

Most infrastructure vendors also have implemented a dynamic channel-allocation feature in their networks. Essentially, this feature allows a set of frequencies to be assigned to base stations and allows the network to choose which frequency channels to use for each call across a cluster of sites. Without this feature, a set of frequencies would be assigned to each sector, but all might not be used simultaneously, depending on demand.

With dynamic channel allocation, the network is able to determine what channels are free in a specific cluster of sites at the time of the call and assign the channels dynamically as needed. This can help design engineers use co-channels closer together in many cases. Related to this is the concept of frequency hopping, in which, during the process of a call, multiple channels are used to spread the transmissions among many channels, again with the idea of negating the effects of interference on any given channel.

The first step in troubleshooting co-channel problems is to identify that you have a co-channel problem in the first place. Such interference would manifest itself in the form of RF losses and handoff failures, in addition to poor call quality. In digital systems, there also will be an increase in the bit error rate (BER).

Network design can cause two specific types of interference — co-channel, as described above, when the same channel is used too close together, and adjacent channel, when the adjacent channel is used too close together. Adjacent-channel interference is essentially the same as co-channel, except that the interferer generally must be higher in power than the signal of interest to cause problems, and, of course, the interfering signal is in the adjacent channel as opposed to the same channel.

There is a variety of opinions on when co-channel interferers begin to cause problems. Generally, in IS-136 systems, most people choose 17dB below the signal of interest as a good threshold. Drive-test gear available today can measure the power from specific base stations at any given location. In this way, an engineer can measure the power of both base stations and determine whether there is a co-channel problem. The obvious solutions are changing frequency assignments; however, antenna re-orientations, power and parameter changes also can be used.

Pilot Pollution

In IS-95 and 3G CDMA-based wireless formats, co-channel interference is inherent to the network design at a couple of levels. Each user is assigned a code instead of a specific frequency. In this way, all of the users actually transmit in the same frequency channel. A complex power-control system in these formats helps ensure that any one user is not overpowering another user.

CDMA networks are designed so a set amount of users still can maintain a quality uplink and downlink in a cell's coverage, even though all of the power for all of the users is at the same frequency. Any extra energy in the band — for example, energy from the adjacent cells that are transmitting at the same frequency — can cause problems. This is mitigated with the soft-handoff concept: a mobile can receive signals from separate base stations simultaneously, combining them to actually create a more robust connection.

Too much of anything is never a good thing. If more than four base stations reach a mobile with enough power, the mobile will not be able to receive all of them, and the excess energy only will add to the noise floor, thus lowering the signal-to-noise ratio, which dictates the BER — or call quality. In CDMA circles, this is called pilot pollution rather than co-channel interference.

Pilot pollution generally is the result of putting too many cell sites too close together at too high a transmit power. In CDMA network design, capacity is directly related to how close the cell sites are to each other. Like the 2G TDMA systems discussed previously, design engineers must move cell sites closer together to gain capacity. Again, the trade-off is the potential for pilot pollution.

Pilot pollution can be identified with today's drive-test gear fairly easily, specifically with the use of an independent PN scanner. This scanner can look at the power in all of the PN offsets, the specific code offset that identifies base stations in CDMA networks. Via post processing, these tools then can show locations where the condition exists. Often in CDMA, the solution is to lower pilot power, although antenna re-orientations are also common solutions.

Uplink Issues

Pilot pollution is related specifically to the downlink in today's CDMA systems. The uplink also has concerns that are related to co-channel use. Depending on the cell radius, only a certain number of wireless phones will be able to transmit in the same frequency channel before the base station will begin to have trouble receiving the signals. Even though the complex power-control systems in CDMA allow all of the handsets to transmit at a power level such that at the base station they are all equal, as the base station is receiving the signal from one phone, all of the other phones' energy adds to the noise of the received S/N ratio.

At some point, this noise rise will be so high that the S/N ratio will not be high enough to support receiving the call adequately. Many consider this noise rise the capacity-limiting factor of a cell. This is one of the main reasons for the recent proliferation of tools designed to make the handsets transmit at lower powers, yet with high call quality.

For example, the superconductor-based filter and low-noise amplifier system can lower the handset transmit powers by as much as 3dB to 5dB by increasing the base station's sensitivity. In this way, more phones can be established on a call before the noise level increases too much, and the uplink capacity is increased.

As carriers try to squeeze more capacity out of 2G TDMA and CDMA systems, the drive will be to deploy more sites closer together. This means co-channel interference, whether on the uplink or the downlink, may surface. Understanding this up-front in the network-design phase, whether in a brand new digital overlay or simply a capacity offload deployment, is the easiest way to head off problems later. Troubleshooting the condition, however, is not terribly difficult with the tools available today, assuming over-burdened performance engineers have the time available to perform such tasks.

Miceli (amiceli@suptech.com) is Superconductor Technologies national director of carrier sales and author of Wireless Technician's Handbook.