El Paso upgrades analog network.

With an aging uprotected analog microwave system, increasing system reliability requirements and the need for high speed data transmission, El Paso Natural Gas Company studied, specified, designed, and is now implementing a four year digital microwave upgrade.

El Paso Natural Gas Company, a 22,000 mile pipeline network serving the Southwest and California, has owned and operated an extensive private analog microwave network between its headquarters location in El Paso, field offices and compressor stations in Texas, New Mexico and Arizona, and delivery points along its pipeline routes in several states.

When the system was first installed, data traffic consisted mainly of slow speed telemetry; compressor stations were manned and there were no computers in the field. In recent years, system reliability requirements have increased as a result of the dramatic change in the type of information being transmitted.

Since aged equipment is more susceptible to failure and spare parts are not readily available, the cost and effort of maintaining the analog system operating at a reliable level were constantly escalating.

In 1989, a comprehesive analysis revealed that besides unrealiable and costly to maintain, the analog system prevent EPNG from using services such as digital trunking between PBX systems, high speed data (56KB/s), T1 carrier (1.544 MB/s) and video teleconferencing.

An upgrade study was undertaken to address EPNG's present and future requirements for voice and data transmission. The objectives of the study were to determine the scope of the upgrade, select an economical and reliable alternative, define system configuration, determine project time tables and provide an overview of the technology to be used.

Digital microwave radio was selected over other alternatives considered, i.e. analog radio, fiber optics, etc. because digital facilities allow access to new services requiring high data rates and ensure quick and reliable connectivity. An advantage of digital radio over analog is that degradation of the signal happens much later since it is regenerated at every repeater.

The cost to carryover the four year upgrades was estimated at $19 million. Separate specifications were prepared for the frequency coordination, microwave system, antenna systems, towers, battery systems, equipment shelters and frequency coordination process.

In order to avoid impacting company operations while the transition from the analog to the digital system was taking place, an overbuild was selected. The digital microwave system will parallel the analog backbones on the 6 GHz frequency band and sidelegs on the 2 and 18 GHz bands depending on path lengths. Once the digital system is optimized and traffic cutover to it, the analog system is being decommisioned.

While studying the proposed new technology, it appeared that the rules of digital microwave radio design were to have shorter paths, larger antennas, space diversity protection, and larger battery banks.

Knowing that our systems had a good number of paths longer than 45 miles, an aggressive approach to the design and specification of the proposed digital microwave system was taken. The new system had to be implemented without having to spend great amounts of capital in the building of new repeater stations and tall towers.

The experience obtained from the operation of the analog system was used to establish an outage criteria for the digital microwave system. Since hotstandby equipment with a number of fading countermeasures was to be installed, the outage expectancy of the new backbone was set to be five times less than the analog system at the 10<6> BER threshold. This threshold was chosen since it is the point where data starts to take "hits".

At the time frequency coordination was performed, a radio manufacturer had not been selected, and "EPNG" set of "worst case" specifications was prepared using the data of the radio being considered. Frequency coordination and licensing of the backbone were completed using the "EPNG" radio. After radios were selected, the system was re-coordinated and re-licensed.

EPNG kept the responsibility of providing the engineering for the different aspects of the project.

Path engineering revealed that due to longer than desired paths and low system gains (typical of digital microwave), low fade margins were the rule.

In an effort to maintain losses to an absolute minimum, the space diversity configuration used was slightly different than the traditional. The main antenna was placed below the diversity antenna. This permitted us to have higher signal in the main path since waveguide run is shorter.