Chapter 8 Appendix 1: Basic Knowledge about Interference
8.3 The source and features of Interference
8.3.2 External Interference
I. Other Communication Systems
The interference does not exist in normal situations. It exists in those countries with improper allocation of spectrum. You need to pay attention to the interference only when the frequency in band 1 and band 2 is used in the same area. If a country for band 1 uses the frequency in band 2, interference appears, which are destined to appear.
Therefore this interference must be known upon network construction with known influence.
1) Interference to WCDMA system by PHS system
z Basic features of domestic PHS system
According to the RCR STD-28 standard, the carrier bandwidth of PHS is 300 kHz, with a frame per 5ms. A frame is divided to 8 timeslots, with the timeslot structure shown in Figure 8-1.
Figure 8-1 Frame structure of PHS system
Note:
The uplink and downlink protection interval is 4.7us.
According to related rules and RCR STD-28 standard, the PHS frequency range is 1900.1–1915.0 MHz. In some place, the frequency may reach 1918 MHz, which is beyond the range.
The PHS system uses continuous dynamic channel selection as an important advantage. The base transceiver station (BTS) automatically measures the interference within the working frequency band and automatically select the channel with minimum inference for talk. When the interference to the serving channel is so strong that continuous work is impossible, the BTS reselects new channel for talk or even enable the MS to handover to another BTS. Therefore the continuous dynamic channel selection is of high utilization of spectrum.
The PHS BTS transmits signals that are selected one from four and receives four-path combined signals with maximum ratio.
z Technical parameters of PHS system Table 8-1 Technical parameters of PHS system
BTS transmit power
(peak) 4000 mW (average: 500 mW) 80 mW (average:
10 mW)
Antenna gain 9 dBi 0 dBi (body loss)
Diversity gain Transmit: 3 dBi; receive: 9 dBi 0 dBi
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<2.5 μW/MHz out of 1893.5–1919.6 MHz
Serving bandwidth 288 kHz 288 kHz
Receiver sensitivity < 16 dBμ (–97 dBm, 1 * 10 – 2)
z Basis features of PHS interference
¾ Time feature
It is related to PHS traffic and is in pulse signals. It marks the users' behavior. The RTWP might increase for about 20s. No interference is in half of a 5ms period. The timeslot feature of interference meets the timeslot feature of PHS.
¾ Frequency feature
The interference is strong in low-frequency band but weak in high-frequency band. Great burst RTWP in large quantity increases. The interference occurs frequently and becomes stronger between 1920 MHz and 1930 MHz. When the frequency changes to be a higher frequency, the interference occurs less frequently and becomes weak. The PHS interference signals are noise signals.
¾ Space feature
The PHS BTS nearby transmits weak interference signals. The BTS 10–50 meter far transmit strong interference signals. If the heights of two antennas are close (the further one antenna from another is, the weaker the interference between the two antennas are due to the difference of antenna heights), the BTS which the angle between WCDMA antenna direction and PHS BTS direction is smaller than 60° transmits strong signals.
¾ Polarization feature
The interference signals are vertical polarization signals. The signals transmitted on the two antennas are similar.
¾ Modulation feature
The interference is non-modulation feature noise.
¾ Pulse feature (short-term feature)
The occurrence of interference is closely related to the occurrence of PHS signals. No interference exists in half of the time.
¾ Strength feature
The interference strength changes sharply from the maximum RTWP according to the statistics to the noise level.
¾ Affected range feature
The Interference affects one or two sectors of BTS.
z RTWP statistics feature of PHS
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¾ The impact on WCDMA NodeBs by PHS is closely related to the number of PHS interfered users. The RTWP varies with the number of users.
Affected by PHS system, in the statistics RTWP, the RTWP is completely related to traffic and user behaviors. The RTWP keeps increasing for about 20s.
¾ According to scanned RTWP diagram, with PHS interference, the scanner RTWP diagram is related to the number of PHS users. When there are many PHS users using a frequency, the RTWP increases sharply.
Therefore the statistics data is sawtooth.
¾ The RTWP of WCDMA fluctuates sharply in the working period of PHS, and high RTWP peaks appear in 1920–1930 MHz. It is similar in other RX bands of WCDMA, but the peaks occur less probably. Therefore, WCDMA NodeBs interfered with by PHS have RTWP peaks.
¾ The BTS interfered by PHS has relevant main and diversity in terms of RTWP.
z Impact on system by PHS interference
¾ PHS has less impact on system sensitivity than on RTWP.
¾ The interference can be ignored if its impact on RTWP is within 5 dB.
¾ PHS BTSs have little impact on WCDMA, especially when the PHS antenna and WCDMA antenna are far away or near.
¾ Knowing the location of PHS sites upon WCDMA site construction effectively helps to reduce PHS interference.
z Elimination of PHS interference
You can see by connecting multiple MSs to PHS system that the RTWP strength is related to access of the MS.
To eliminate PHS interference, move the WCDMA antenna to PHS antenna as close as possible. The height of WCDMA antenna should be higher or lower than PHS.
2) Interference between GSM and WCDMA
The interference between GSM and WCDMA is usually too weak to be considered.
One exception is the interference between WCDMA 2100 MHz and GSM 1900 MHz, which cannot coexist. Instead, you can use WCDMA 1900 MHz or WCDMA 1700 MHz. Another exception is the interference between WCDMA 1800 MHz and GSM 1900 MHz, which also cannot coexist. Instead, you can use WCDMA 1900 MHz or WCDMA 1700 MHz. If a country using GSM1900 MHz constructs a network of WCDMA 2100 MHz, the following problems occur:
z The signals transmitting by GSM network of 1900 MHz interferes with the receiving by WCDMA network, so you must use a narrowband transmission filter. This is easy. A 1900 MHz BTS uses a narrowband filter. If it uses a non-narrowband filter, a filter needs to be added on the top of the BTS.
z WCDMA UEs transmit signals that interfere the signals from GSM MSs. When a WCDMA UE is transmitting signals, the GSM MSs nearby cannot make a call.
This is difficult to be predicted and felt. In later stage of the network, it may be a critical problem. If the system in which GSM and WCDMA coexist is constructed by no other means, operators must be informed of this problem.
¾ Interference features
The interference sources are spreading, namely, widely and irregularly distributed in the whole area. Interferences are similar. A site with interference is not far away from other communication systems.
¾ Interference location
Check the spectrum distribution of the communication system.
Check the relative location between BTSs.
Check whether the wide-range statistics feature of RTWP is discrete.
Check the transmission filter bandwidth and spectrum features of another communication system.
¾ Solutions to interference
Use the proper spectrum according to normal method for allocating spectrum.
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1) Causes to interference with repeaters
z Repeaters are designed broadband without selection of frequencies.
z If a repeater uses an improper host NodeB or is far from the host NodeB, the transmit power becomes too high to interfere with other adjacent NodeBs.
z Repeaters are unstable, so self-excitation of repeaters occurs.
z Improper configuration of repeaters and gain causes noise to interfere with UEs.
2) Causes to problematic line amplifier
z The gain of line amplifier (LA) is improperly configured.
z LAs are unstable, so self-excitation of LAs occurs.
3) Interference features of repeaters and Las
z Long-term stable interference and burst interference.
z The interference from repeaters is a directional large-scale interference.
z The interference from LAs spreads in a round shape.
z Long-term and stable interference spectrum is systematic spectrum feature.
z Burst interference spectrum is variable tone signal.
z Locate the approximate location of the interference source according to interference features of a cell.
z A long-term stable interference is probably due to improper gain.
The burst interference is due to self-excitation of active device in two aspects:
there are abundant irregular bursts; the burst interference exists in a fairly short time; the interference with a fixed period exists for several seconds.
4) Solutions to interference
Change the way to use repeaters to ensure frequency selectiveness of repeaters and to guarantee stable and pure host link of repeaters. Adjust the gain of repeaters to proper range.
III. Microwave Transmission
The features of microwave transmission are as below:
z Long-term stable interference.
z The interference is bidirectional.
z The interference is in a large-scale range
z The spectrum is broadband spectrum.
Eliminating the interference from microwave transmission is adjusting the frequency of microwave transmission.
IV. All Power-Consuming Equipment
V. All Controlling Components (Especially Big Electric Controlling Components) The features of all controlling components are as below:
z The interference is instantaneous. Maybe the time feature of interference surge is present but unseen.
z The interference spectrum is from 0 (direct current) to several GHz.
z The interference affects a small range, usually a NodeB.
VI. Equipment with Clocks
The interference from equipment with clocks exists permanently or appears irregularly. It appears irregularly due to time frequency drift caused by unlocked frequency.
z The interference spectrum is tone or drift tone.
z The interference exists in other transmission systems.
z Locate the approximate location of interference by using the features that interference affects a cell.
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The features are as below:
z The interference from non-linear components in great electromagnetic field is due to the PIM occurring on high-power transmission equipment. It is related to high-power transmission equipment.
z The time feature of interference is related to work features of high-power transmission source.
z The interference frequency includes broadband interference frequency and frequency meeting intermodulation conditions.
z The interference affects a limited range, namely, a single cell.
VIII. Radar
The features are as below:
z The interference from radar is regular in period. It is pulse interference usually multiple periods.
z Its spectrum has features of a SINC factor.
z It affects a wide range.
z Locate the approximate location of interference by using the feature that interference affects a cell.
IX. Handset Interferer The features are as below:
z The interference from handset interferer exists for a certain period or for a long time, such as only during a daytime meeting. It features apparently in terms of time.
z The interference spectrum has broadband features.
z The handset interferer is used by government, military, hospital, and gas stations.
z The outdoor antennas that receive the interference signals lie in spherical points.
z Locate the approximate location of interference by using the feature that interference affects a cell.
X. Intermodulation Signals from Various Transmitters (Especially TV Station) The intermodulation signals from various transmitters, especially television station, features as below:
z The signals are modulated, from which the features of carrier and modulation spectrum are distinguished.
z The interference signals are stable and permanent, or exist intermittently.
z The interference affects a large range.
z Locate the approximate location of interference by using the feature that interference affects a cell.
XI. Large Equipment with Great Instant Variation of Electricity The features are as below:
z The electricity changing instantly is the precondition of generating high-frequency signals. The size is the precondition of transmission capacity.
z When the electricity changes, the interference occurs accordingly like pulse.
z The interference spectrum is broadband signals.
z The interference usually affects a single NodeB.
XII. Equipment with Feedback Channel (Self-excitation) The equipment includes:
z Signal source of various wireless systems
z Local oscillation
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z High-gain amplifier
z Repeater
z LA
z Automatic switch controlling equipment
z Sound-control equipment
z Light-control equipment
The features of the previous equipment are as below:
z The interference is random.
z The spectrum is unstable tone spectrum.
z The affected range is also random.
XIII. Discharge Equipment The features are as below:
z The features of interference are the same as the features of discharge, usually like pulse. A floodlight keeps blinking, so approximately continuous interference occurs.
z The interference spectrum is broadband spectrum.
z The interference affects a small range.