Technical Note 107: 
Cellular Packet Data EMI Considerations

The cellular industry is about to launch cellular digital packet data (CDPD). This is a digital communications system which operates in the background of normal analog voice cellular transmission. This technical note looks into some possible electromagnetic interference effects on CDPD technology implementation in existing cellular systems.

It is well known that high-powered radio frequency signals from a variety of sources such as AM and FM radio, television, and military systems can be disruptive to conventional cellular telephone operations. Over the last several years, we at LBA have become aware of numerous problems in the analog transmission phase, as well as the digital control aspects of currently used cellular technology. We have found that operation in high RF field intensity environments without adequate site shielding gives rise to such artifacts as noise and cross talk into cellular communications channels, as well as erratic operation of digital control systems*. CDPD systems may be more vulnerable than analog systems for reasons which will be discussed herein.

CDPD technology is based on the transmission of “bursty” data signals as packets of information which can be inserted at idle moments between voice call demands on cellular channels. The processing and addressing is accomplished by a dedicated computer at the mobile switching center, and transmission packets are assigned to idle carriers by a new computer added at each cell site. Each cell site is also supplied with additional data transceivers and RF sensors. Under control of this equipment, the CDPD system operates in a channel hopping mode, grabbing idle channels for a few seconds, or even milliseconds. Special control software sees that the frequency hopping transmissions are coordinated through the entire system to avoid systemic interference.

It is this combination of local computer control, fast response, and systemic interaction that may give rise to potentially devastating interference problems at unprotected cell sites in high power RF environments. Since initial CDPD equipment installation will generally be retrofits, and thus physically outboard of existing cell site equipment, they may be significantly more vulnerable to RF fields than fully integrated new systems. CDPD installation outside the existing cell site equipment racks, with numerous wires and cables penetrating the installations in these racks, further compromises what shielding may be inherent in the present installation. Furthermore, at least one popular implementation of this system will use a PS-2 class of “off-the-shelf” computer. Such consumer items are not noted for their resistance to RF interference ingress. Thus, these CDPD installations may add further vulnerability to a cell site.

High quality, efficient transmission of data will be threatened if RF “noise” contaminates data so that packets are unusable, or require excessive error correction. In the worst case, data could become so contaminated as to reduce throughput to zero. Of course, this would be the easiest case to identify and diagnose. But, how does one determine that RF contamination is burdening the system with, say, 10 percent error correction retransmissions?

From our experience in controlling interference effects in existing cellular systems, we are aware of several instances where it has been possible to document increased cell site unavailability due to digital control circuit malfunction under RF interference. The CDPD system places much greater stress on the cell site control system than present voice transmission practice. If demand for digital services materializes as forecast, transmitter control circuits could easily be exercised at a rate ten times that presently found in voice service. A cell site having RF problems in 5 percent of its operations (perhaps conservative, as at least one client had 20% without realizing a problem existed), then the sheer magnitude of errors under CDPD operation would equal one half of all of the present operations. If not detected and corrected, this could be a serious economic and operational rat hole for the operator.

*These effects and some approaches for treating them can be found in Technical Notes 101, 103, and 104.