Improving System Performance

Now that you have finished deploying your CDMA system, your thoughts should be focused on how to get the most "bang for your buck" at an operations level. You've reached the stage of the game in which system optimization is critical, as it significantly affects both performance and capacity of your CDMA system. By fine-tuning your soft handoff zones, controlling pilot pollution, tweaking your neighbor list and minimizing noise, you can boost system performance while maximizing capacity.

One of the biggest drains on system capacity is having too large a soft handoff zone. Soft handoff refers to a state in which the mobile device is in communication with multiple base stations at the same time. A link is established with a target cell before breaking the link with the cell currently in use. In the case of CDMA, you gain several advantages by employing soft handoff including fewer dropped calls, less portable transmit power and less interference.

But too much of a good thing can hurt you. If the soft handoff zone is too large, the system will have less capacity because two base stations are carrying the same call during soft handoff. (See Figure 1 on page 76.)

Soft handoff is a compromise between system performance and capacity. The ideal amount of soft handoff is approximately 25% to 40%. The need for minimized overlap between sectors is one of the reasons CDMA operators are deploying 65-degree and 90-degree horizontal beam-width antennas used in AMPS cellular. As an operator, you must assure that the T_ADD, T_DROP and T_TDROP parameters are set correctly for optimum performance.

Pilot Pollution Another method you can use during optimization is to minimize areas suffering pilot pollution. Pilot signals act as beacons to notify potential users of the existence of a CDMA base station. Portable devices use the pilots for power strength comparison, which is essential for the process of a handoff.

The pilot signal is the strongest channel, comprising 20% of the total radiated power in a CDMA signal. Each CDMA sector sends its own pilot signal, but each is on the same frequency. If a portable device is in a location where numerous pilots are received with relatively equal signal strength, the result is pilot pollution.

Pilot pollution can cause dropped calls and decreased capacity. To avoid pilot pollution, you can use antenna downtilt, azimuth rotation and careful horizontal beamwidth selection. Some carriers in core urban areas have been using 33-degree sector antennas to minimize pilot pollution, as shown in Figure 2.

Tweak Thy Neighbor Another method of optimizing CDMA systems, which often is ignored, is tweaking the neighbor list. An operator needs to avoid a handoff to a sector that may provide good coverage only temporarily, such as a high site as illustrated in Figure 3.

If the portable hands off to the wrong site, the call can be dropped and capacity wasted. By tweaking the neighbor list, the portable device will be instructed as to which neighbors to search first for a good handoff.

A final method of optimization is to ensure that your antenna system is minimizing the addition of noise and interference into your CDMA system, as those factors can limit your system's capacity significantly. Antenna inter-modulation (IM) is one form of interference.

You must make sure that your system antennas are designed robustly enough that their IM performance does not decrease with time. Some antennas that are designed with multiple cables and solder joints are susceptible to deterioration in IM performance once the antennas are placed in an outdoor environment. You also must weigh the pros and cons of using polarization diversity in a PCS system.

As you leave one cell site and enter a new one, you want to experience a nice, graceful handoff as you move into the next cell. However, unless your system is properly optimized, the benefits of the soft handoff are lost, making a call ping pong from cell to cell.

"Soft handoffs are dicier than analog," said Bill Rau, product manager, cellular test equipment, Motorola Communications Test Equipment.

Measuring the signal in analog is more straightforward because tools exist to measure the signal. However, carriers need to add the proper tools designed especially for soft handoff to their war chests so they can distinguish among real and potentially interfering cells.

A number of things can interfere with one cell's signal and perhaps not with that of the adjacent cell. Terrain factors such as foliage or canyons can provide interference and in effect fool cells.

Brian Joyce, key account manager with Safco, agreed that additional optimization areas should be considered for successful soft handoffs. For example, he said Qualcomm developed a report in which you can analyze the percentage of time a call is spent in a 2- or 3-way handoff vs. a singly served cell. An appropriate range exists that balances quality and capacity vs. power requirements. However, if that balance isn't maintained, the imbalance "chews up" system resources trying to deal with the handoff process.