1 Principle of Mobile Communication

1.1 Introduction

In Telecom network conventionally each user is connected to the Telephone exchange individually. This dedicated pair starts from MDF, where it is connected to the appropriate Equipment point and ends at the customer premises Telephone. (With flexibility at cabinet/pillar/ distribution points DPs)

The connectivity from exchange to customer premises is called “Access Network or Local Loop”, and mostly comprises of underground cable from exchange up to DP’s and insulated copper wires (Drop Wires)later on This type of Access Network does not require separate Authentication of customer before extending services. Whenever the cable capacity has reached the maximum additional cable is laid to augment the capacity. Even though there are advantages in introducing wireless connectivity in Subscriber’s loop, we have to tackle certain issues Viz,

1. Duplexing methodology.2. Multiple Access methods.3. cellular principle or reuse concept.4. Techniques to cope with “mobile” environment.

Duplexing Methodology:Duplexing is the technique by which the send and receive paths are

separated over the medium, since transmission entities (modulator, amplifiers, demodulators) are involved.

There are two types of duplexing. Frequency Division Duplexing FDD Time Division Duplexing TDD

Frequency Division Duplexing FDD Different Frequencies are used for send and receive paths and hence there

will be a forward band and reverse band. Duplexer is needed if simultaneous

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transmission (send) and reception (receive) methodology is adopted .Frequency separation between forward band and reverse band is constant

Time Division Duplexing (TDD) TDD uses different time slots for transmission and reception paths. Single

radio frequency can be used in both the directions instead of two as in FDD. No duplexer is required. Only a fast switching synthesizer, RF filter path and fast antenna switch are needed. It increases the battery life of mobile phones.

GSM and CDMA systems use Frequency Division Duplexing and corDECT uses Time Division Duplexing.

Multiple Access methodology:The technique of dynamically sharing the finite limited radio spectrum by

multiple users is called Multiple Access Technique. By adopting multiple access techniques all users can not get the services simultaneously and some amount of blocking is introduced by the system. This is known as GOS (Grade of Service).

Generally there are three different types of multiple access technologies. They are

Frequency Division Multiple Access (FDMA) Time Division Multiple Access (TDMA) Code Division multiple Access (CDMA)

Frequency Division Multiple Access (FDMA):FDMA is a familiar method of allocating bandwidth, where a base station is

allowed to transmit on one or more number of preassigned carrier frequencies and a mobile unit transmits on corresponding reverse channels. No other base station within range of the mobile will be transmitting on the same forward channel, and no other mobile within range of the base station should be transmitting on the same reverse channel. Both the base and the mobile usually transmit continuously during a conversation, and fully occupy their assigned forward and reverse channels. No other conversation can take place on these channels until the first conversation is completed.

FDMA Analogy

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It may be easier to visualize FDMA by imagining a cocktail party where two people wish to converse with each other. Then everyone in the room must be silent except for the speaker. The speaker may talk as long as they wish, and when they finish someone else may start speaking, but again only one at a time. New speakers must wait (or find another party) for the current speaker to finish before starting. Everyone in the room can hear and understand the speaker, unless they are too far away or the speaker's voice is too soft. If the intended listener is close enough, the speaker may decide to whisper. Conversely, if the listener is too far away, the speaker may have to shout. Since no one else should be talking, this presents no problem. If someone talks out of turn, the listener will probably be confused and not be able to understand either speaker.

Features Of Frequency Division Multiple Access (FDMA) No Precise coordination in time domain is necessary in FDMA System. It is well suited for narrow band analog systems. Guard spacing between channels causes wastage of frequency resource.

Otherwise good modulation techniques are to be employed to avoid such guard spacing.

The transmission is simultaneous and continuous and hence duplexers are needed. Continuous transmission leads to shortening of battery life.

Time Division Multiple Access (TDMA)TDMA is a more efficient, but more complicated way of using FDMA

channels. In a TDMA system each channel is split up into time segments, and a transmitter is given exclusive use of one or more channels only during a particular time period. A conversation, then, takes place during the time slots to which each transmitter (base and mobile) is assigned. TDMA requires a master time reference to synchronize all transmitters and receivers.

TDMA AnalogyIn TDMA, everyone in the room agrees to watch a clock on the wall, and

speak only during a particular time. Each person wishing to talk is given a set period of time, and each person listening must know what that time period will be. For example, everyone may agree on time slots with duration of ten seconds. Speaker number one may talk for ten seconds starting from the top of the minute. The listener who wishes to hear this speaker must also be made aware of the schedule, and be ready to listen at the top of the minute. Speaker number two may speak only from ten seconds after the minute until twenty seconds after. As with FDMA, only one person at a time may speak, but each speaker's time is now limited and many persons may take their turn. If someone in the room cannot see

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the clock, they will not be able to speak and will have great difficulty understanding the speakers.

Features of TDMA There can be only one carrier in the medium at any time, if a simple TDMA

scheme is followed. Transmission is in bursts and hence is well suited for digital communication.

Since the transmission is in bursts, Battery life is extended.

Transmission rate is very high compared to analog FDMA systems.

Precise synchronization is necessary.

Guard time between slots is also necessary Time and Frequency Division Multiple AccessBoth methods of FDMA and TDMA are combined to achieve higher

capacity in practical systems. A channel gets a certain frequency band for a certain amount of time. The Best example for such system is GSM.

Code Division Multiple Access (CDMA)CDMA is fundamentally different than TDMA and FDMA. Where FDMA

and TDMA transmit a strong signal in a narrow frequency band, CDMA transmits a relatively weak signal across a wide frequency band. Using a technique called direct sequence spread spectrum, the data to be transmitted are combined with a pseudo-noise code (a pre-determined binary sequence that appears random) and transmitted broadband. CDMA under Interim Standard 95 uses a bandwidth of 1.25 MHz.

The pseudo-noise code (PN code) is a series of binary "chips" that are much shorter in duration than the data bits. Since the chips appear to be in a random pattern, and there are many chips per data bit (in IS-95 there are 128 chips for each data bit), the modulated result appears to normal (FDMA) receivers as background noise.

A spread spectrum receiver with a different PN code will not be able to recover that signal, and if the PN codes were chosen incorrectly, will hear nothing but noise. This relative immunity to interference, whether from outside sources or other spread spectrum transmitters, gives CDMA systems the ability to pack many users into the same frequency space at the same time. It also gives a measure of

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security to each signal, since each user will have a different PN code. CDMA also does not require different base station radios for each user - the same radio may serve multiple users with just a change in PN code.

CDMA AnalogyIn CDMA, the speaker and the listener have agreed beforehand to use a

language that no one else at the party understands. Many speakers may talk at a CDMA party, each using a different language, and it is relatively easy for the listener to hear and understand the speaker as long as there aren't too many speakers talking at the same time. As more and more speakers start talking, the noise level in the room goes up and it becomes harder and harder for the listener to make out what their speaker is saying. If a speaker begins to shout, in order for their listener to hear better, it raises the noise level even more

Comparison of Multiple Access Techniques:

Coverage Offered by CDMA System is more compared to TDMA and FDMA systems.Capacity of CDMA System is more compared to TDMA and FDMA systems.

Duplexing and Multiple Access Techniques in use:

No Name of System Multiple Access Duplexing

1 GSM FDMA-TDMA FDD

2 CDMA CDMA FDD

3 Cor-DECT TDMA-multi carrier

TDD

Cellular Concepts:

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Even though multiple access techniques allowed multiple users to share the medium simultaneously, due to constraints in providing resources, an amount of blocking will exist. The amount of blocking is called “Grade Of Services”(GOS). Based on GOS and resource availability (no. of carriers/no. of timeslots/both) the traffic handling capacity of the system is calculated. If this total traffic is divided by traffic per subscriber, we get number of subscribers supported by the system. For these purposes Erlang B table (Blocking calls cleared) is useful particularly in FDMA-TDMA.

Why Cellular?Assuming 30mE traffic per subscriber, sub density of 30 per sq.km, and GOS 1%

Radius Area (KM2) Subs Total Traffic RF Channels1 3.14 100 3.0E 83 28.03 900 27E 3810 3.14 10000 300E 360

Providing 360 RF channels for 10,000 subscribers in an area of 314 sq.km on a single base station is not feasible and if still either the area of coverage or sub density increases, the system cannot function at all for want of bandwidth.Hence the solution is dividing the service area into small units, called cell, with base stations radiating with low power, and limited number of carriers required as per traffic. The same carriers are again reused at a different cell, which is geographically separated. (Frequency Reuse)In case of CDMA it appears that there is no limitation for simultaneous calls but practically there is a limit to CDMA capacity. And it is essentially the amount of interference a CDMA receiver can tolerate. As more and more units transmit, the amount of noise a receiver sees goes up, since all signals not using the receiver's specific PN code appear as noise. At some point there is so much noise that the receiver can no longer hear the transmitter. Boosting the transmitter power won't help overall, since it increases the noise for all the other receivers, who would in turn tell their transmitters to boost power, and the situation remains. In a nutshell, if a unit near a base station is transmitting with too much power, signals from units far from the base station will be lost in the noise.

Hence cellular concept is applicable even in the case of CDMA where code used for identification of cell/sector is reused.

Advantages of Cellular Principle Base stations can transmit at low power compared to a single high power transmitter. It requires less RF bandwidth to cover a given area. Frequency reuse gives good spectrum efficiency. (FDMA-TDMA)

Disadvantage of cellular principle Reuse introduces interference.

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Established calls should be handed over to next cell to avoid dropping of calls when the customer is in mobility.

Mobile Environment:BTS is connected to Mobile or Fixed Wireless Terminal by air

Interface. This connectivity differs from our earlier UHF/Microwave which is purely Line of Sight (LOS) system. In mobile communication due to the mobility of the user from the BTS LOS to BTS may exist or may not exist. The radio wave is subject to attenuation, reflection, Doppler shift and interference from other transmitter. These effects cause loss of signal strength and distortion which will impact the quality of voice or data. To cope with the harsh conditions, any mobile technology makes use of an efficient and protective signal processing. Proper cellular design must ensure that sufficient radio coverage is provided in the area.

Types of signal strength variations The signal strength variation for mobile is due to different types of

signal strength fading. There are two types of signal strength variations Macroscopic Variations Due to the terrain contour between BTS and MS

The fading effect is caused by shadowing and diffraction (bending) of radio waves.

Microscopic variations. Due to multipath, Short-term or Rayleigh fading. As the MS moves, radio waves from many different paths will be received.

Macroscopic Variations Macroscopic Variations can be modeled as the addition of two

components that make up the path loss between mobile and base station. The first component is the deterministic component (L) that adds loss to the signal strength as the distance(R) increases between base and mobile. This component can be written as

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L=1/Rn

Where n = typically 4. The other macroscopic component is a Log normal random variable which

takes into account the effects of shadow fading caused by variations in terrain and other obstructions in the radio path.

Local mean value of path loss=deterministic component +log normal random variable Microscopic Variations

Microscopic Variations or Rayleigh Fading occur as the mobile moves over short distances compared to the distance between mobile and base. These short term variations are caused by signal scattering in the vicinity of the mobile unit e.g. by hill, building or traffic. The result is that not one but many different paths are followed between transmitter and receiver (Multipath Propagation). The reflected wave will be altered in both phase and amplitude. The signal may effectively disappear if the reflected wave is 180 degree out of phase with the direct path signal. The partial out of phase relationships among multiple received signal produce smaller reduction in received signal strength.

Special features of mobile technologiesAll mobile techniques incorporate some special features to overcome

the hazards created by mobile environment. The following are a few to name:1. Coding.2. Diversity techniques.3. Adaptive equalization( in case of GSM)4. Rake Receiver (in case of CDMA)

1.Coding:Coding includes

Speech coding, Convolutional coding or Forward Error Correction coding Interleaving

Speech Coding:Human speech is band limited between 300Hz to 3400Hz and undergoes

Frequency Modulation in analog systems. In digital fixed PSTN systems band limited speech is sampled at the rate of 8 KHz and each sampled is encoded into 8 bits leading to 64Kbps (PCM A-Law of encoding).Digital cellular radio cannot handle the high bit rate used for PSTN systems. Smart techniques for signal analysis and processing have been developed for reduction of the bit rate

Different mobile communication systems use different bit rates for voice encoding. The following table gives a glimpse.No. Technology Bit rate per voice chl Voice coding technique1 GSM 13Kbps RPE-LTP2 CDMA IS95A 9.6Kbps/14.4 Kbps QCELP/EVRC

3 Cor- 32Kbps ADPCM

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DECT

RPE-LTP: Regular Pulse Excited Long Term PredictionQCELP: Qualcomm Code Excited Linear PredictionEVRC: Enhanced Variable Rate CodingADPCM: Adaptive Differential Pulse Code Modulation

Forward Error Correction Coding:Sometimes this process is called Convolutional Coding or Channel

Coding. The purpose of this process is to build redundancy in the signal so that even if error occurs, the receiver will be able to recover the lost information. Several methods are available for this purpose and each mobile system uses its own choice.

Interleaving:Interleaving is a simple, but powerful, method of reducing the effects of

burst errors and recovering bits when burst errors occur. The symbols (output of Forward Error Correction Coder) from each group are interleaved in a pattern that the receiver knows. The interleaver is located at the BTS and in the phone.

2. Diversity Techniques:To cope up with the mobile environment Diversity techniques are

employed .This can be Space Diversity, Polarisation Diversity, Frequency Diversity and Time Diversity.

Space and Polarisation Diversity:It is implemented in the BTS by deploying two antennas, one for

Transmitting and receiving, the other for only receiving. Both antennas should be kept with minimal separation (10 times wave length). Space Diversity can be combined with Polarisation Diversity by making the Diversity antenna in an opposite polarization. In modern times the same antenna with dual polarized elements are available so that with single antenna, at least polarisation diversity can be achieved. Space Diversity can be implemented only when sufficient space is available in the tower for mounting the antennas.

Frequency Diversity:Signal degradation can be averted by changing the present frequency

to another in case of narrow band systems. This avoids frequency selective fading. In a narrow band system like GSM this is achieved by slowly hopping the frequency of transmission of BTS in a predetermined manner.

In case of a wide band system like CDMA signal occupies a large bandwidth and frequency diversity is inherently achieved.

Time Diversity:In all the mobile communication systems by employing interleaving

time diversity is automatically achieved.

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3. Adaptive Equalisation:The transmitter trains the receiver to adapt to the air environment by

sending a known sequence along with the data. Corrections as applied to the known sequence are applied to the data to retrieve it error free. This is used in GSM.

4. Rake Receiver:The rake receiver is multiple receivers in one. There is a rake receiver at

both the mobile and BTS. It turns what is a problem in other technologies into an advantage for CDMA. Signals sent over the air can take multi-paths resulting in degradation of signal. The rake receiver identifies the three strongest multi-path signals and combines them to produce one very strong signal. The rake receiver therefore uses multipath to reduce the power the transmitter must send.

Conclusion:Wireless means convenience. However to achieve this certain

precautionary measures are taken to overcome the bandwidth scarcity, multipath problems, etc., There are multiple access techniques to share the bandwidth amongst several users.

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