INTERFERENCE PATTERN IN MOBILE COMMUNICATIONSy

Two types of interference patterns are existing in mobile communications. They are:1)Co-channel Interference. 2)Adjacent Channel Interference.

INTRODUCTION TO CO-CHANNEL INTERFERENCEy Co-channel cells: cells that use the same set of

frequencies in a coverage area . y Cells of same number are co-channel cells.2 1 1 4 1 2 3 2 3 4 1 3 4 3 2 1 4 1 2

ADJACENT CHANNEL INTERFERENCE

y Adjacent channel cells: cells that use different

set of frequencies in a coverage area . y Cells of different numbers are adjacent 2 channel cells.1 1 4 1 2 3 2 3 3 2 3 4 4 1 1 4 1 2

REAL TIME CO-CHANNEL INTERFERENCEy Co-channel interference occurs in two places. They are

1)Cell Site, 2)Mobile Station. y In real time applications, for the reduction of cochannel interference C / I >18 db.

ANTENNASy An Antenna is a device that can radiate and receive EM

signal.

ACTIVE ANTENNAS PASSIVE y Antennas can carry RF signal & Power signal. y Functionality of an antenna depends on two factors. 1)frequency of its design. 2)Direction of the signal.

y VSWR:- Volta e Sta

It s t e

Wave Ratio eas re e t of leaka e of RF si

i

al.

leaka e = 1/ atc . OL RIZ IO :Its efi e as t e orie tation of RF si nal in t e open air . olarization can e one in 2 ays. ey are: 1)Horizontal olarization, 2)Vertical olarization.

TYPES OF ANTENNASS1. 2. 3. 4. 5. 6. 7.

orn Antenna. ens Antenna. Smart Antenna. Polaroid Antenna. Parabolic Antenna. Yagi-Uda Antenna. Artificial dielectric lens Antenna.

DESIGN OF ANTENNA SYSTEMy Ta ing N 7,in it the co-channel reduction factor is q 4.6 a co-

D+R

D

D -R

D+R

D

D -R

Worst case of interference cells is shown above.

y Effective Aperture :Its defined as the ratio of maximum received power to that of the power density of the incident wave. The ratio of effective area Ae to the maximum effective area Ae(max) is nown as effectiveness ratio .

Capture area Effective area + loss area + Scattering area y Antenna Bandwidth :Antenna Bandwidth is defined as the range of frequencies in which the antenna performance meets a specified standards. It s the bandwidth in which gain , front-tobac ratio, standing wave ratio is maintained below the selected value.

y Front-to-bac ratio :The front-to-bac ratio is defined as the ratio of power radiated in the desired direction to that of the power radiated in opposite direction. If FTB is higher, then the system is within better operation. The tuning conditions and the electrical length of antenna has to be designed properly. y Polarization :Polarization of an electromagnetic wave is defined as the wave radiated or received by the antenna in a particular direction. It describes above the electrical vector quantity E . The electric vector E and magnetc vector are perpendicular to each other.

The antenna is said to be either vertically or horizontally polarized. y Antenna input impedence :The antenna input impedance or self impedance .its also nown as feed-point or driving-point impedance. In case if the antenna is lossless , then the terminal impedance will bee equal to thee self-impedance of the antenna. The input impedance of antenna is a function of frequency. y Types of fading :There are two types of fading. a)Small scale fading : The small scale fading are mainly characterized by rapid amplitude fluctuations and deep fades of less wavelength

The small scale fading is due to signal fades,caused by multiple path and reflections with respect to mobile movement. b) arge scale fading :The large scale fading is generated by shadowing effects which is due to changes in both the nature of surroundings and the terrain profile involved. The large scale fading is log-normally spreaded with particular standard deviation value of approximately 10 dB in urban areas.

Kinds of Diversityy The two basic inds of diversity are termed as

a) Microscopic Diversity :To avoid deep fading in the signal received under smaller distances can be minimized by this technique. It can prevent small signal fades in case of less antenna separations, if two antennas are used.Smaller signal fades rapidly. b) Macroscopic Diversity :Method of diversity used to reduce large scale signal fades is termed as macroscopic diversity. This is highly useful at the base station receiver end. y Important diversity techniques :-

The important diversity techniques are discussed in the following ways:1) Space diversity technique :y Its also called as antenna diversity scheme. y Rayleigh fading is present as there is no direct path between transmitter and receiver. y Space diversity technique can be used at both base station and mobile units. space diversity combining schemes :a) selective diversity :Branches having strongest received signal is selected. Demodulators are used and gain is adjusted to give mean signal to noise ratio. Finally the best signal pocessing good signal strength is processed.

b)Feedbac Diversity technique:Its also nown as scanning diversity. ere n signals are scanned in a proper sequence and the signal which is above threshold value is selected. Scanning is done . Its easier to implement than other methods. c)Maximal combining ratio technique:All branch signals are combined coherently with their weighting coefficients. Signal is co-phased before addition process. utput signal to noise ratio will be equal to the sum of each individual signal to noise ratio.

d)Equal gain combiner technique :ere all diversity branches are coherently added with a same weighting factor. It also co-phases each diversity branch and adds up. It provide an equal gain factor. ne of the demerit is it degrades the value of SNR by 0.5 dB when it combines two branches. y Polarization Diversity :In polarization diversity both horizontal andd vertical polarizations are involved. Two uncorrelated fading signals would appear at receiver because of different fading variations in polarizations and reflection coefficient values.

y Time Diversity :-

Information is transmitted repeatedly at specific time spacings that would exceed the coherence time of mobile channel. Time diversity technique is well suited for spread spectrum CDMA system, in which RAKE receiver is used for reception. y Frequency Diversity :In this method ,information is transmitted on many carrier frequencies. ere frequencies are separated by more than that of coherence bandwidth. Its applied in microwave fields where S S lin s many carrier channels propagation is used. in frequency division multiplex mode.

Directional diversityy The received signals would arrive from different

incident angles due to reflection, diffraction or scattered signals around the mobile terminal. y By using selective directive antennas, the faded signals can be received. y It is suitable to apply in mobile terminal end where limited directions of signals at base stations is lin ed.

Path diversityy In path diversity method the signals are coherently

combined i.e., both direct and delayed signal components are combined. y This method is also called implicit diversity. y The advantages: -- no extra power required. -- no extra antennas required --no extra frequency spectrum required. y This technique is effective at antennas receiver end.

of diversity is received. y If e=1, then, no diversity is obtained. y For two branch diversity technique the correlation coefficient should be atleast 0.7 in the cellsite such that antenna separation requirement is met. y Advantages of using diversity techniques: -- at the cellsite: at cellsite diversity reciever the transmitter would experience a 7 db reduction in power level. The amount of interference is reduced from mobile transmitter t0 cellsite reciever. -- at the mobile unit: for the case of no diversity the signal i.e., below the threshold level is 10% and it would be 1%. If diversity technique is applied.

y If the correlation coefficient is zero an optimum result

Non cochannel interference

The cells of same letters(A,A) ,(B,B) etc.are cochannel cells.on the other hand the neighbouring cells of A(B,C .G) are nown as adjacent cells. If the interference is generated due to non cochannel cells,it is called non-cochannel interference.

Subjective and objective tests: In cellular mobile system the voice quality is degraded as follows. a.multipath environment factors. b.speed of the vehicle.(mobile unit) i)Subjective test: The subjective test considers atleast 75% of the mobile subscribers receive good signal quality for a given c/n ratio. ii)Objective test: The objective test at baseband level is done for both voice signal and data.the data can be tested by few factors namely bit error rate(BER),word error rate(WER). The BER is independent on speed but WER is dependent on vehicular speed. Sensitivity of the reciever should be high. Reciever carrier level should be high. ow distortion has to be maintained. Diversity receiver has to be used for reducing fading effects.

SINAD measurement: The signal to noise ratio and distortion ratio is applied for measuring 2 main factors. 1.quality of communication signal at baseband level. 2.FM sensitivity at cellular mobile reciever.

The high signal level can be measured by the ratio of signal to noise using SINAD meter as shown below.

In the upper line of the SINAD meter signal is measured and at the bottom line noise is measured. If the noise level is less than distortion then SINAD meter acts as distortion meter.

Adjacent channel interferencey The ACI is the interference that generates from adjacent cells of desired cellsite. y The ACI has better control than the CCI if appropriate filters are used for filtering. y The in-band interference cannot be filtered out. y The amount of additional pathloss is

y Where d1,d2 are 2 cochannel sources to the BTS in the cellsite.

Methods to reduce ACI: y To use better design measure to reduce interference

due to naturally adjacent channels.

y Directional antenna beams can be used. y Spread spectrum technique can be applied. y To have frequency separation so that interference

due to system adjacent channels can be reduced.

y Near end far end interference:

when the vehicles are in motion some of the mobile units are closer to the cellsite while some would be distance away from cellsite. y ence mobile units which are closer to mobile unit possess stronger signal. y Thus the closer units will induce interference.y

y Near end far end ratio=(pathloss due to near end)/(pathloss due to far end).

Techniques to reduce interference: If a bandwidth separation of 5 channel bandwidth is allowed between adjacent channels in one cell then the near end far end interference can be suppressed. Cross talk: In cellular mobile radio systems a pair of frequencies is used for a channel.the call conversation on one frequency is heard through own reciever.sometimes it has major impact on call comm,but in realtime crosstal generates more interference. The cross tal is even doubled if cross tal occurs on FVC ,it will be heard on RVC.

y Near-end mobile unit:consider one mobile unit is

close to the cell and the other mobile unit is at far. if both of them calls to their landline users then nearend mobile unit will have stronger signal than other.y Now the demux c t cannot produce separation more

than 30db. The closer mobile unit will produce more interference and the mobile at far place have less interference.

Effects of coverage and interference: The adjacent channel interference can be reduced by using different techniques. They are: 1.changing antenna beamwidth. 2.decreasing the power. 3.reducing antenna height. The methodology of reducing antenna height and power level could compensate for adjacent channel interference but there will be a decrease in coverage range.

Power decrease: If the setup of antenna arrangement in the cell site is not changed and in case the power transmitted is decreased by X db then there would be decrease in power level at mobile unit by same X db level. Decrease in antenna height: In cmc,if the height of antenna is reduced there will be a reduction in the received power. The antenna height gain is equal to the difference between the old and new(reduced) antenna heights.ddecreasing of height in flat terrain is easier than in hilly region.

Channel combinery At the transmitter side a fixed tuned channel combiner

is used. y If the channel in the system if each one of them is feed to their own antenna then it is possible to eliminate the necessity of a channel combiner. y The transmitted channels has to be combined with 2 main criteria: 1) maximum signal isolation b/w the radio channels. 2)minimum insertion loss.

y The channel combiner would cause each of 16

channels to face 3db loss due to signal insertion into channel combiner.signal isolation is 17db. y There is intermodulation present at multiplexer which has to be controlled. y The intermodulation can be eliminated by using wide radio frequency bandwidth for few channels alone. y If large frequency spectrum is sacrificed the intermodulation spreading can be avoided.

Precaution measures: proper shielding has to be made when each cable is fed to the combiner units,else nonlinearity of the device would induce IM products. proper impedance matching is necessary. Under channel combiners in cellsite components the intermodulation(IM) products should be atleast 55db down from that of desired signals.

Tunable combinery The frequency agile combiner is an advanced combiner y y y y

with additional features. It can return any frequency in real time by remote control device namely microprocessor. It is essentially a waveguide resonator with a tuning bar facility. A motor ma es it to rotate and when motor starts the VSWR can be measured. The cellsite should be flexible to change their operating frequency.

Ring combinery When we want to combine two groups of channel to

give one output it is used. y It has an insertion loss of 3db. y Demerits: It reduces adjacent channel separation. power limitation problems.

y At the rxing end demux can be used to receive the intended channel groups. y The demux is nothing but a filter ban . y While splitting the channels at the demux output would produce a loss of 3db. y This is called SP IT SS . SAT TONE: The supervisory audio tone is nown as SAT tone. It does a main fuction in ma ing the cell site to decide whether to allow or terminate a call. The 3 types of SAT tones are 5970 z SAT tone. 6000 z SAT tone. 6030 z SAT tone.