Transmission Problem

7/31/2019 Transmission Problem

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TIME DISPERSION It is another problem

relating to multiple paths

to the Rx antenna ofeither an MS or BTS.However, in contrast toRayleigh fading, thereflected signal comesfrom an object far away

from the Rx antenna.Time dispersion causesInter SymbolInterference(ISI) whereconsecutive symbolsinterfere with each other

making it difficult for thereceiver to determinewhich symbol is thecorrect one. Where thesequence 1,0 is sent fromthe BTS.

1

1

1

1

0

0

0

1


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TIME ALIGNMENT Each MS on a call is allocated a time slot

on a TDMA frame. This is an amount of

time during which the MS transmits

information to the BTS. The informationmust also arrive at the BTS within the

time slot. The time alignment problem

occurs when part of the information

transmitted by an MS does not arrive

during the next time slot, and may

interfere with information from another

MS using that other time slot. Time

alignment is caused by a large distance

between the MS and the BTS.

Effectively, the signal can not travel over

the large distance within the given time.

For Example, an MS is close to a BTS

and has been allocated time slot 3(TS3). During the call, the MS moves away

from the BTS causing the information

sent from the BTS to arrive at the MS

later and later. The answer from the MS

also arrives later at the BTS. If nothing is

done, the delay becomes so long that

the transmission from the ms in time slot3 overlaps with the information which the

BTS

A Close to

BTS

Far away from

TS0 TS1 TS2

TDMA FRAME


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COMBINED SIGNAL LOSS Each of the problem described above

occur independently of each other.However, in most calls some of theseproblems may occur at the same time.

An illustration of what the signalstrength may look like at the MS Rxantenna when moving away from theBTS Tx antenna. The problem of pathloss, shadowing and Rayleigh fadingare present for this transmission path.The signal strength as a global meanvalue decreases with the distance (path

loss) and finally results in a lostconnection. Around this global mean,show variations are present due toshadowing effects and fast variationsare present due to Rayleigh fading. Thelowest signal strength value requiredfor a specified output is called receiversensitivity level. To detect the

information sent from Tx antenna, Xwatts must be received. If the signalfalls below X, the information will belost and the call may be dropped. Toensure that no information is lost, theglobal mean value must be as manydB above the receiver sensitivity levelas the strongest(deepest) fading dip

give rise to. This fading margin is thedifference between the global meanvalue and the receiver sensitivity.

Fadi

ng

Mar

gin

Rx Signal

Strength

PATH LOSS

Rayleigh FadingLog(Distance

)

Signa

l

Level)

(dB)


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RATE(BER) The BER difference between optical and wireless transmission is in the order

of 107 1010 , value down to 10-3 are not uncommon.

Further more, errors are likely to appear in burst(lightening, interference of

other devices etc.) RADIO SWITCH OVERTIMES Most radio equipments in Half duplex :

It does not make sense to hand and receive on the some frequency at thesame time.

Building Blocks of receivers can be recycled in the transmitter reducingcostve to stabilize..

Tx and Rx functions are not interleaved and usually employ differentfrequencies.

To switch the radio from reception to transmission, the internal circuit .

SIGNAL LOCK-ON If radio signal starts receiving a signal it locks itself on the signal(e.g., using

PLLs)

If there is another signal interfering after the lock-on receiver circuits may stillreceive the 1stsignal(Signal Carriers) are unlikely to be on exact somefrequency.

More of a phenomenon a problem, although it can cause some strangeeffects.


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problems This section describes some solutions to the problem described in

previous sections. Although many of these do not entirely solve all

problem on the radio transmission path, they do play an important part in

maintaining call quality for as long as possible.

CHANNEL CODING In digital transmission, the quality of the transmitted signal is often

expressed in terms of how many of the received bits are incorrect. This is

called Bit Error Rate(BER). BER defines the percentage of the totalnumber of received bits which are incorrectly detected.

Transmitted Bits 1 1 0 1 0 0 0 1 1 0

Received Bits 1 0 0 1 0 0 1 0 1 0

Errors 3/10 =

30%BER

This percentage should be as low as possible. It is not possible to reduce

the percentage to zero because the transmission path is constantly

changing. This means that there must be an allowance for a certain

amount of errors and at the same time an ability to restore the in

formation, or at least detect errors so the incorrect information bits arenot inter reted as correct. This is es eciall im ortant durin


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G In reality, bit errors often occur in sequence, as caused by long fading

dips affecting several consecutive bits. Channel coding is most

effective in detecting and correcting signal errors and short errorsequence. It is not suitable for handling longer sequence of bit errors.

For this reason, a process called interleaving is used to separate

consecutive bits of a message so that these are transmitted in a non-

consecutive way. For example, a message block may consist of four

bits(1234). If four message blocks must be transmitted, and one is lostin transmission, without interleaving there is a 25% BER overall, but a

100% BER for that lost message block. It is not possible to recover

from this.

If interleaving is used, the bits of each block may be sent in a non-

consecutive manner. If one block is lost in transmission , again there is

a 25% BER overall. However, this time the 25% is spread over the

entire set of message blocks, giving a 25% BER for each. This is more

1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4

Message Blocks

Interleaved msg

Blocks

Interleaving

1 X 3 4 1 X 3 4 1 X 3 4 1 X 3 4 ReceivedInterleaved


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DIVERSITY In telecommunication, a diversity scheme refers to a method for improving

the reliability of a message signal by utilizing two of more communicationchannels with different characteristics. Diversity plays an important role in

combating fading and co-channel interference and avoiding error bursts.It is based on the fact that individual channel experience different level offading interference. Multiple versions of the same signal may betransmitted and/or received and combined in the receiver. Alternatively, aredundant forward error correction code may be added and different partsof the message transmitted over different channels. Diversity techniquesmay exploit the multipath propagation, resulting in a diversity gain, often

measured in decibels. DIVERSITY GAIN

In wireless communication, diversity gain in the increase in signal-to-interference radio due to some diversity scheme, or how much thetransmission power can be reduced when a diversity scheme isintroduced, without a performance loss. Diversity gain is usually

expressed in decibel, and sometimes as a power ratio. An example is softhandoff gain. For selection combining N signals are received , and thestrongest signal is selected. When the N signals are independently andRayleigh distributed, the expected diversity gain has been shown to be n

k=11/K expressed as a power ratio.

DECIBEL

The decibel(dB) is a logarithmic unit of measurement that expresses themagnitude of a physical quantity (usually power or intensity) relative to a


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Cont.. When referring to measurements of power or intensity, a

ratio can be expressed in decibels by evaluating ten times

the base-10 logarithm of the ratio of the measured quantityto the reference level. Thus, XdB is calculated using the

formula.

XdB =10log10 (X/X0)

Where, X is the actual value of the quantity being measured

X0 is a specified or implied reference level, and then

XdB is the quantity expressed in units of decibels, relative X0.

If X is greater than X0 then XdB is positive.

If X is less than X0 then XdB is negative.

From the above equation we get after rearrangement

X = 10XdB/10 X0

Since a bel is equal to ten decibels, the correspondingformulae for measurin in bels X are X = Lo X/X


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The following classes of diversity schemes can be

identified:

Space Diversity

Polarization Diversity

Time Diversity

Frequency Diversity

Multi user Diversity

Antenna Diversity

Cooperation DiversityDid you know ?

One of the most expensive aspects of cellular network operation is

payment of rent for sites, e.g. hotel rooms. Great care is often taken to

ensure public support for sites. (e.g. in California some BTSs are hidden

within fiber glass palm trees.)


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SPACE DIVERSITY The signal is transferred over several different propagation

paths. In the case of wired transmission, this can be achieved by

transmitting via multiple wires. In the case of wirelesstransmission, it can be achieved by antenna diversity using

multiple transmitter antenna and multiple receiving antenna. In

the later case , a diversity combining technique is applied before

further signal processing takes place. If the antenna are at far

distance at different cellular base stations sites or WLAN accesspoints, this is called macro diversity. If the antennas are at a

distance in the order of one wavelength, this is called micro

diversity. Antenna diversity increases the received signal

strength by taking advantage of the natural properties of radio

waves. An increased received signal strength at the BTS may be

achieved by mounting two receiver antenna instead of one. If the

two Rx antenna are physically separated, the probability that

both of them are affected by a deep fading dip at the same time

is low. At 900 MHz, it is possible to gain about 3 dB with a

distance of five to six meters between the antenna. At 1800 MHz

the distance can be shortened because of its decreasedwavelen th. B choosin the best of each si nal the im act of


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POLARIZATION DIVERSITY With polarization diversity the two space diversity

antenna are replaced by one dual polarized antenna.This antenna has normal size but contains twodifferently polarized antenna arrays. The mostcommon types are vertical/horizontal arrays andarrays in 45 degree slant orientation. The two arrays

are connected to the respective Rx branches in theBTS. The two arrays can also be us Tx/Rx antennas.For most applications, the difference between thediversity gain for space diversity and polarizationdiversity is negligible, but polarization diversity

reduces the space required for antenna. The use of space diversity in the reception of mobile

radio signals is a well known technique to mitigatefading is implemented in most wireless systems oftoday. Further, in a mobile communication system,

base station antennas with a nominal 45 degree to


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TIME DIVERSITY Time Diversity is used in digital communication

systems to combat that the transmissions channelmay suffer from error bursts due to time-varying

channel conditions. The error bursts may be caused

by fading in combination with a moving receiver,

transmitter or obstacle, or by intermittentelectromagnetic interference, for example from

crosstalk in a cable, or co-channel interference from

radio transmitters. Time diversity implies that the

same data is transmitted multiple times, or aredundant error code is added. By means of bit-

interleaving, the error bursts may be spread in time.

Multiple versions of the same signal are transmitted at

different time instants. Alternatively, a redundant

forward error correction code is added and the


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FREQUENCY DIVERSITY

Frequency diversity relies on the fact that the fading is

different at different frequencies. It is sometime saidthat it is not correlated. Hence when there is a fade at

one frequency, there may not be a fade at another. To

make use of this, you simply transmit your signal on

two frequencies, perhaps 100 KHz apart. At thereceiving end, a circuit measures the signal to noise

ratio in two receivers and automatically selects which

is best at any instant in time. This works well but it is

rather inefficient to have the same informationtransmitted on two frequencies. Continuous voice

transmission with no errors is always the goal when

designing a cordless phone. Usually there are

inevitable impairments to the link budget, the solutions


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DIVERSITY Multipath transmission will occur

whenever the base station and the

handset are not both inside thesame anechoic chamber. When

the handset and base station are

within line of sight, the primary

propagation will usually be the line

of sight and secondarypropagation due to reflections will

be less significant. Reflected

propagations become more

significant if the line of sight is

obstructed. In fact, reflected

transmissions may well bedominant in a normal home

environment. Whenever there is

more than one significant wave on

a mobile receive antenna, the

receivers will be subject to varyingsignals levels as it moves around

MS

ReflectorBTS

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Transmission Problem