Identifying an interferer by the characteristics of its signal

In the last of a three-part series on interference in wireless systems, we look at all the different types of potential interfering signals and how to identify them.

Armed with the understanding of causes and sources of interference and an appreciation of how receiver design can fail to reject the wide spectrum of possible interfering signals, the field technician now faces the task of running down the interferer. With the many sources listed below, please note that the frequency and power levels relate to the US and will differ for the Asia-Pacific market.

Broadcast stations

Analog TV

TV stations are powerful transmitters, all completely legal and licensed, but often overpowering for co-located lower-power communications systems. UHF TV stations operating in the 450-850 MHz band can be licensed for 50,000 W and transmit equally in all directions. Channel spacing in the US is 6 MHz. Video modulates the main carrier, while TV audio is contained in an FM-modulated 'sub-carrier' spaced 4.5 MHz away (6-7 MHz in Europe). The distinctive 4.5 MHz spacing of the sub-carrier helps identify a TV signal as a possible interferer.

TV transmitters cause signal interference in two ways. In their tower vicinity, the TV carrier power can drive affected receivers into overload. But the US Federal Communications Commission (FCC) specifications also allow harmonics up to -30 dBc (50 W on a 50 kW station). Such harmonic signals can easily swamp communications channels which are assigned to those doubled or tripled harmonic frequencies.

Digital TV

The newest video transmission technology authorised by the FCC is digital video. By sophisticated digitisation of the video signal, it is possible to pack four separate channels of video into the same channel bandwidth previously allocated to one analog TV channel. Harmonics of the carrier can likewise be a problem with these digital power transmitters.

FM broadcast

FM broadcast stations also feature very high transmit powers in the 87.7 to 107.9 MHz band. FM broadcasters can choose quite high power transmitters, and if co-located on mountain tops with communication systems, they cause the same overload as TV stations. The broadcast FM spectrum profile is relatively flat with sloping sides depending on the particular audio content being broadcast at the time. Music tends to have a wide flat top, while silence exhibits a narrower pattern. Generally, the more complex the audio, the wider the spectrum of the modulated signal.

AM broadcast

AM broadcast channels are spaced every 10 kHz from 500 to 1500 kHz. AM modulation sidebands on the Anritsu MS2711B analyser display, for example, show evident amplitude jumps with voice, and less activity with music. By using the AM demodulator function, the user can determine if the signal is a broadcast AM station. The frequency band of AM broadcast is a long way from wireless radio, so the interfering mode is usually the out-of-band overload effects in co-located environments.

Traditional communications systems

FM mobile

Before the emergence of the personal mobile wireless (cellular) phones in the 1990s, public safety applications, such as police, fire and forest service, used narrowband FM technology. These applications still exist in the 50, 150 and 450 MHz FM bands. Typical mobile FM transmitters emit 5 to 150 W while their permanent base stations often transmit at 150 W with an omni-directional footprint. The spectrum profile of narrowband FM spans about 5 kHz.

AM aircraft communications

Using the VHF frequencies in the 118-136 MHz region, authorities allocated 25 kHz-wide channels for a higher voice quality AM for aircraft communications. Being exceedingly mobile, aircraft interferers are also difficult to pin down since any one aircraft is only in the area for tens of seconds. But again, their ground transmitters can be a constant source of relatively high signal powers. The spectrum profile again reflects the voice nature of this application.

Paging systems

Simple paging systems typically use a frequency-shift-keyed (FSK) modulation format which exhibits a spectrum profile with two separated peaks, each representing one of the two frequencies which shift according to the digital 'one' or 'zero' being transmitted. As more complex data, such as an alphanumeric message, is transmitted, the space between the two peaks fills in.

Amateur radio (Ham radio)

Scattered throughout the frequency spectrum are a number of allocated frequency bands dedicated to 'Ham' radio operators. While their transmitters largely use AM modulation, they are also authorised to run experimental transmissions in other formats. Their emitted powers can be quite high since they intend to transmit to others around the earth.

Hams often use large, steerable directional arrays of HF antennas to increase their directional power, so their interfering power can be quite high. Further, these transmitters are mostly found in residential areas where wireless base stations are located.

If the affected receiver is not well filtered at its input and is in the boresight direction of the Ham transmitter, there is a possibility of interference. Ham transmitters can contain harmonics which extend into wireless bands.

Wireless communication applications

The wireless technology and cellular explosion has filled the allocated frequency spectrum with millions of mobile phones and base stations. One hardly has to mention that all those little transmitters essentially saturate their assigned spectrum.

Analog cellular
AMPS/NAMPS	Adv Mobile Phone Syst	MS 824 - 849 BS 869 - 894
Digital cellular
TDMA	IS-54/136	MS 824 - 849 BS 869 - 894
CDMA	IS-95	MS 824 - 849 BS 869 - 894
GSM	Global Syst Mobile Comm	MS 880 - 915 BS 925 - 960
DCS 1800	Dig Comm Syst	MS 1710 - 1785 BS 1805 - 1880
Personal communications systems
PCS-TDMA	Based on IS-136	MS 1850 - 1910 BS 1930 - 1990
PCS-CDMA	Based on IS-95	MS 1850 - 1910 BS 1930 - 1990
PCS-1900	Based on GSM	MS 1850 - 1910 BS 1930 - 1990
Digital cordless phones
DECT	Dig enhanced	1880 - 1900
Note: The bands shown are the US transmitter frequency allocations for Base Station (BS) or Mobile Station (MS).

Table 1: Some frequency allocations for typical wireless applications.

Table 1 serves as a starting point to pinpoint conflicting signals. It lists some popular formats, but does not list all US or global applications.

Advanced mobile phone service (AMPS)

AMPS originally operated as an analog system in the 800 MHz frequency band using 30 kHz wide channels. A variant of AMPS known as N-AMPS uses 10 kHz wide channels and consequently almost tripled channel capacity.

AMPS is still in common use throughout the Americas but is declining in the face of digital cellular standards. Wireless carriers that began service with AMPS systems have generally turned to TDMA and CDMA digital operation in the 800 MHz cellular band.

North American digital cellular (NADC, now IS -136)

This is one of the original rollouts of the new cellular technology. It was designed to utilise the existing 30 kHz channel of the AMPS cellular technology. Its spectrum profile fills the 30 kHz channel with a relatively flat-top spectrum characteristic.

Code-division-multiple-access (CDMA)

CDMA is an innovative technology that exploits the idea of interleaving hundreds of individual digitised voice signals into one fast digital data stream. That data stream, combined with a special encoding data stream, then modulates the RF carrier. The effect of this is to spread the spectrum over the entire 1.23 MHz frequency allocation for the service, and essentially explains why 798 users can fit into the one 1.23 MHz channel.

A typical CDMA spectrum profile looks like the flat-topped characteristic of Figure 1. The profile is 1.23 MHz wide with relatively sharp slopes at the band edges. If a CDMA signal is interfering with another signal, it can be identified by that 1.23 MHz wide profile.

Wideband-CDMA (W-CDMA)

This 3GPP system is the third generation wide-band version of CDMA, which utilises channels approximately 5 MHz wide. It is a sophisticated modulation system, intended for higher data rate connectivity. The 3GPP2 is a parallel project and a collaborative effort of multiple country telecommunications organisations: ARIB (Japan), TIA (North America), CWTS (China), TTA (Korea) and TTC (Japan).

Global system for mobile communications (GSM)

GSM is an international wireless standard which is used heavily outside the US. GSM is assigned two frequency bands at 900 and 1800 MHz. Each band supports 124 channels at 200 kHz spacing and each is broken into eight time slots operating in a TMDA (time-division-multiple-access) mode. The modulation type is gaussian minimum shift keyed (GMSK). See Figure 2.

Personal communications systems (PCS)

The personal communications system (PCS) is a name given to wireless communications systems in the 1800-1900 MHz frequency band. PCS was supposed to be a more comprehensive specification than the earlier cellular specification at 800 MHz. However, the only technologies that were implemented were upbanded cellular standards. Thus, the change was simply one of expanding the available spectrum by using the same signal formats at the higher 1800 MHz band. Now consumers rarely know whether their cellular phone is operating in the cellular or PCS band.

• PCS1900 - Upbanded GSM cellular
• TIA/EIA-136 - Upbanded TDMA digital cellular (ANSI-136)
• TIA/EIA-95 or IS-2000 - Upbanded CDMA digital cellular (ANSI-95, CDMAOne or CDMA2000)

All PCS systems are digital. The PCS frequency allocation in the US is three 30 MHz allocations and two 10 MHz allocations in the 1850-1990 MHz frequency band.

Unlicensed ISM data systems

Table 2 shows some popular ISM-allocated bands. In addition to myriad unlicensed applications like microwave ovens and atomic particle accelerators, they now support thousands of unlicensed data communications systems. Customers often prefer these systems because of their inexpensive nature and the ability to install them without a tedious licensing process. They are popular for point-to-point and point-to-multipoint data link applications such as ethernet bridging for intracompany data bridges. Recent Bluetooth technology promises to further fill the spectrum with close-range personal data applications.

Wireless data
Bluetooth	ISM band	2400 - 2497
Wireless LAN	IEEE 802.11b	2400 - 2484
ISM applications
ISM	Â	902 - 928
FCC Part 15.247	UNII-1	5725 - 5850 5150 - 5250
FCC Part 15.407	UNII-2 UNII-3	5250 - 5350 5725 - 5825

Table 2: Some frequency allocations for typical unlicensed ISM applications.

Wireless LANs

Wireless LAN technology in the ISM band was conceived for short-range connectivity systems. Its uses include laptop computers and data management within buildings. Both WLAN technologies, frequency-hopping (FHSS) and direct-sequence (DSSS), depend on spread spectrum technology for data modulation. These schemes trade wider bandwidth for transmission reliability. To a narrow band system, spread spectrum signals just look like random noise.

The typical FHSS system utilises a 1 MHz power spectrum which is frequency-hopped three times per second across a 75 MHz channel. A typical DSSS system utilises a constant 25 MHz wide spectrum, from a 1 W (+30 dBm) transmitter, which translates to +16 dBm per MHz.

When ISM interference from another similar system brings down an affected receiver, it is highly likely that the interferer is completely legal. It is then up to the affected to determine how to arrange other elements like antennas or perhaps modulation alternatives to solve the problem. There is no appeal to official regulators since these are unlicensed bands.

ISM microwave data links

ISM-band systems provide fast installation for applications such as ethernet bridges which connect backbone data systems with new wireless base stations without the need for digging underground cables. ISM data links at 5725-5850 MHz have considerable advantage over UNII (unlicensed national information infrastructure) systems because they are allowed higher output powers and very high gain antennas. These often give them a 48 dB interference advantage.

Conclusion

One industry trade association website shows statistics which estimate that worldwide ownership of wireless devices exceeded 800 million in 2002. Does anyone wonder that competing wireless systems will continue to interfere with each other in regions of high density installations?

Understanding the characteristics of your affected receiver's modulated signal and the effect that noise or interference has on that signal is the first step in detecting interference within your communications system. Selecting the appropriate test equipment and employing proper measurement techniques can enhance the likelihood of locating and identifying sources of interference within your system.