Digital Mobile Cellular Communication

A Typical Mobile Cellular Network

BTS

Digital Cellular Concept

• In a cellular system each transmitter, called a base stn, covers a certain area, a cell. Cell radii can vary from tens of meters in buildings, hundreds of meters in cities, up to tens of kms in the country side. The shape of the cells are never perfect circles or hexagons, but dependent on the environment, weather conditions and on system load.

• Typical systems using this approach are mobile telecommunication systems where a mobile station (MS) within the cell around a base station communicates with this base station and vice versa

Cont…

• The concept was first developed & deployed for military applications to provide improved reception in a highly interference-prone environment and to provide a high level of security against eavesdropping on the radio path and against unauthorized access.

Cells

• A cell may be defined as an area of radio coverage from one BTS antenna system. It is the smallest building block in a mobile network and is the reason why mobile networks are often referred to as cellular networks. Typically, cells are represented graphically by hexagons

Honey Comb Pattern

Cont…

• The border between the coverage area of two cells is the set of points at which the signal strength from both antennas is the same. In reality, the environment will determine this line, but for simplicity, it is represented as a straight line.

• If six BTS’s are placed around an original BTS, the coverage area-that is, the cell-takes on a hexagonal shape

Types of Cells

• There are two main types of Cell:• Omni Directional-An Omni-directional cell is served by a BTS

with antenna which transmits equally in all directions (360) degrees

• Sector Cell-A sector cell is the area of coverage from an antenna, which transmits, in a given direction only. .For example, this may be equal to 120 degrees or 180 degrees of an equivalent omni-directional cell. One BTS can serve one of these sector cells with a collection of BTS’s at a site serving more than one, leading to terms such as two-sectored sites and more commonly, three sectored sites

• Omni-directional cells are used to gain coverage, whereas sector cells are used to gain capacity

What is the Principle of Cellular Systems?.

• The principle involves-To divide a large geographic service area into cells with diameter from <500 m to 50 Km

• Each of which is allocated a number of radio frequency (RF) channels

• Transmitters in each adjacent cell operate on different frequencies to avoid interference

• Since, transmit power and antenna height in each cell are relatively low, cells that are sufficiently far apart can reuse the same set of frequencies with out causing co-channel interference

• The theoretical coverage range and capacity of a cellular system are therefore unlimited

• As the demand for service grows, additional cells can be added, as traffic grows in an given area, cells can be split to accommodate the additional traffic

Cell Footprint

– Coverage contour should be circular. However it is impractical because it provides ambiguous areas with either multiple or no coverage.

– Due to economic reasons, the hexagon has been chosen due to its maximum area coverage.

– Hence, a conventional cellular layout is often defined by a uniform grid of regular hexagons.

Cell Footprint

Functions of Different Sub-Systems

• The BTS consists of all radio equipment,• i.e.,• antennas,• signal processing,• amplifiers necessary for radio transmissions• .A BTS can form a radio cell or using sector antennas

several cells & is connected to MS via the Um Interface (ISDN U interface for mobile use) and to BSC via the Abis interface.

• Um interface contains all the mechanism necessary for wireless transmission (TDMA,FDMA etc)

BSC

• Base Station Controller-BSC basically manages the B T S’s. It reserves the radio frequencies, handles the handover from one BTS to another with in the BSS, and performs paging of the MS.BSC also multiplexes the radio channels onto the fixed network connections at the A interface

• Mobile Services Switching Center (MSC)-MSC is a high performance digital ISDN switches. They set up connections to other M S C’s & to the B SC’s via the A interface and form the fixed back borne network of a GSM system. An MSC manages several B S C’s in a geographical region. A gateway MSC (GMSC) has additional connections to other fixed networks such as PSTN and ISDN. An MSC handles all signaling needed for connection set up, connection release and handover connections to other mSC’s.SS7 is used as the standard signaling system.SS7 covers all

MSC (Cont)

• Aspects of control signaling for digital networks (reliable routing & delivery of control messages, establishing and monitoring calls).

• An MSC also performs all functions needed for supplementary services such as call forwarding, multi-party calls etc.,

• HLR-(Home location Register) It is the most important data base in a GSM system as it stores all user relevant information. It contains static info such as the ISDN number (MSISDN), sub-scribed services (e.g. ,call forwarding, roaming restrictions, G PRS) IMSI. Also it contains dynamic info such as current location area LA of the MS,MSRN & current VLR and MSC. As soon as the MS leaves its current LA, the information is up dated in the HLR.HLR also supports charging & accounting

• VLR-Visitor Location Register The VLR with each MSC is a dynamic data base which stores all important information needed for the MS users currently in the LA that is associated to the MSC

SIM

Functions

It is a memory device which stores information such as:-

• Subs identification Number

• Networks details

• Countries where subscriber is entitled to service.

• Privacy Keys.

NSS

• NSS handles switching of GSM calls between external NW and the BSC’s

• Also responsible for managing and providing external access to several customer data bases.

• MSC’s is the hub of NSS and controls the traffic among all of the BSC’s

• In NSS there are 3 different data bases called HLR, VLR & AUC

Cont..

AUC is a strongly protected database which handles the authentication and encryption keys for every single subscriber in the HLR and VLR.EIR-EIR is in the AUC, which identifies stolen or fraudulently altered phones that transmit identity data that does not match with information contained in either the HLR or VLR

OSS

It supports one or several operation maintenance centres (OMC) which are used to monitor and maintain the performance of each MS, BS, BSC & MSC with in a GSM system. OSS has three main functions:-

• To maintain all telecommunications hardware and NW operations with a particular market.

• Manage all charging and billing procedures.• Manage all mobile equipment in the system.

OSS (Cont..)

• OMC in GSM system is dedicated for the above tasks and has provision for adjusting all base station parameters and billing procedures a well a for providing system operators with the ability to determine the performance and integrity of each piece of subscriber equipment in the system.

Mobile Services

Teleservices or Telephony Services:

A Teleservice utilises the capabilities of a Bearer Service to transport data, defining which capabilities are required and how they should be set up.

Cont…

Voice Calls:

The most basic Teleservice supported by GSM is telephony. This includes Full-rate speech at 13 Kbps and emergency calls, where the nearest emergency service provider is notified by dialing three digits. A very basic example of emergency service is 911 service available in USA.

Cont..

Videotext and Facsmile:Another group of teleservices includes Videotext access, Teletex transmission, Facsimile alternate speech and facsimile Group 3,Automatic facsimile Group 3 etc.

Cont…

Short Text Messages:

SMS (Short Messaging Service) service is a text messaging which allow you to send and receive text messages on your GSM Mobile phone. Services available from many of the world's GSM networks today - in addition to simple user generated text message services - include news, sport, financial, language and location based services, as well as many early examples of mobile commerce such as stocks and share prices, mobile banking facilities and leisure booking services.

Bearer Services or Data Services

Using your GSM phone to receive and send data is the essential building block leading to widespread mobile Internet access and mobile data transfer. GSM currently has a data transfer rate of 9.6k. New developments that will push up data transfer rates for GSM users are HSCSD (high speed circuit switched data) and GPRS (general packet radio service) are now available.

Supplementary Services

Supplementary services are provided on top of teleservices or bearer services, and include features such as :-

* Multiparty Service or conferencing* Call Waiting

* Call Hold

* Call Forwarding

Cont..

• Call Barring • Number Identification • Calling Line Identification Presentation• Calling Line Identification Restriction • Connected Line Identification Presentation • Connected Line Identification Restriction • Malicious Call Identification

Cont…

• Advice of Charge (AoC)

• Closed User Groups (CUGs)

• Unstructured supplementary services data (USSD)

Handoff

The final obstacle in the development of the cellular network involved in the problem of connectivity when a mobile subscriber traveled from one cell to another during a call.

As adjacent areas do not use the same radio channels, a call must either be dropped or transferred from one radio channel to another when a user crosses the line between adjacent cells. Because dropping the call is unacceptable, the process of handoff was created.

Handoff occurs when the mobile telephone network automatically transfers a call from a radio channel to another radio channel as mobile crosses adjacent cells.

Cont..

During a call, two parties are on one voice channel. When the mobile unit moves out of the coverage area of a given cell site, the reception becomes weak. At this point, the cell site in use requests a handoff

. The system switches the call to a stronger-frequency channel in a new site without interrupting the call or alerting the user. The call continues as long as the user is talking, and the user does not notice the handoff at all.

Handoff between Adjacent Cells

Handoff

Handoff consists of 4 stages:-

1. initiation

2. resource reservation

3. execution

4. completion

Soft HandoffA flawless handoff (i.e no perceivable interruption of service) is called a soft handoff and normally takes approximately 200 ms, which is imperceptible to voice telephone users; although the delay may be disruptive when transmitting data

. With a soft handoff, a mobile unit establishes contact with a new

base station before giving up its current radio channel by transmitting coded speech signals to two base stations simultaneously. Both base stations send their received signals to the MSC , which estimates the quality of the two signals and determines when the transfer should occur.

A complementary process occurs in the opposite direction. A soft hand off requires that the two base stations operate synchronously with one another. Soft handoff is used in CDMA

Hard Handoff

A connection that is momentarily broken during the cell-to-cell transfer is called a hard handoff. A hard handoff is a break-before-make process. With a hard handoff, the mobile unit breaks its connection with one base station before establishing voice communications with a new base station. Hard handoff generally occur when a mobile unit is passing between disjointed systems with different frequency assignments, air interface characteristics, or technologies. Hard handoff is used in GSM systems.

Clusters

• Groups of frequencies can be placed together into patterns of cells called clusters. A cluster is a group of cells in which all available frequencies have been used once and only once

• Since the same frequencies can be used in neighboring clusters, interference may become a problem. Therefore, the frequency reuse distance must be kept as large as possible. However, to maximize capacity the frequency re-use distance should be kept as low as possible

• The re-use patterns recommended for GSM are the 4/12 and the 3/9 pattern.4/12 means that there are four three-sector sites supporting twelve cells using twelve frequency groups

A Seven Cell Cluster

Frequency Reuse

As only a small number of radio channel frequencies were available for mobile systems, engineers had to find a way to reuse radio channels in order to carry more than one conversation at a time.

The solution the industry adopted was called frequency planning or frequency reuse. Frequency reuse was implemented by restructuring the mobile telephone system architecture into the cellular concept.

Cont..

The concept of frequency reuse is based on assigning to each cell a group of radio channels used within a small geographic area. Cells are assigned a group of channels that is completely different from neighboring cells.

The coverage area of cells are called the footprint. The footprint is limited by a boundary so that the same group of channels can be used in different cells that are far enough away from each other so that their frequencies do not interfere.

Frequency Reuse

Frequency reuse is a technique of reusing frequencies and channels within a communications system to improve capacity and spectral efficiency. Frequency reuse is one of the fundamental concepts on which commercial wireless systems are based that involves the partitioning of an RF radiating area (cell) into segments of a cell.

One segment of the cell uses a frequency that is far enough away from the frequency in the bordering segment that it does not provide interference problems.

Frequency re-use in mobile cellular systems means that each cell has a frequency that is far enough away from the frequency in the bordering cell that it does not provide interference problems.

The same frequency is used at least two cells apart from each other. This practice enables cellular providers to have many times more customers for a given site license.

Frequency Reuse

Cont..

Cells with the same number have the same set of frequencies. Here, because the number of available frequencies is 7, the frequency reuse factor is 1/7. That is, each cell is using 1/7 of available cellular channels.

Cell Splitting

Unfortunately, economic considerations made the concept of creating full systems with many small areas impractical.

To overcome this difficulty, system operators developed the idea of cell splitting. As a service area becomes full of users, this approach is used to split a single area into smaller ones

In this way, urban centers can be split into ax many area as necessary in order to provide acceptable service levels in heavy-traffic regions, while larger, less expensive cells can be used to cover remote rural regions.

Cell Splitting

Major Propagation Mechanisms

• There are major propagation differences between fixed & wireless communications:-

• Drn of propagation

• Behavior Predictions

• Range

• Quality of Signal

sender

transmission

Distance

detectionde

Detction

interference

Ranges for Transmission, Detection & Interference (Propagation Mechanism)

• Transmission Range:-Within certain radius of transmitter transmission is possible, i.e., a receiver receives the signals with an error rate low enough to be able to communicate and can also act as sender

• Detection Range:-With in a second radius, detection of the transmission is possible, i.e., the transmitted power is large enough to differ from back ground noise. However, the error rate is too high to establish communication

• Interference Range:-With in a third even larger radius, the txmitter may interfere with other transmission by adding to the background noise. A Rx will not be able to detect the signals, but the signals may disturb other signals

Cont

The three spheres earlier referred to represent a polygons with their shape being time and frequency dependent. Terrain will also play a major role in propagation.

The coverage of an area is affected by path loss, shadowing & multi path fading

Also radio transmission has to contend with atmospheric, obstacles such as buildings, mountains

Moving transmitters & receivers etc.,

Path Loss

• Free space radio signals propagate as light• They follow a straight line (LOS)• Even if no matter exists between a sender & receiver the

signals still experience free space loss. Received power Pr α 1/d2

• The emitted signal from a sender is with certain energy• The signal travels away from the sender at the speed of

light as a wave with spherical shape and the surface area S = 4ii d2

• Pr depends on the wavelength, antenna gains, air, rain, fog. dust particles etc.,

Path Loss (Cont)

• Path loss or attenuation influences long distances & is basically the attenuation loss in free space.

ø1ø2 (

(

30-100 m

2km or further

Direct Path

Reflected path

Here C α R-4 = α R-4 , where C=received carrier power, R=distance measured from tx to rx, α =constant

C2/C1=(R 2 /R1) -4 the eqn can be expressed in decibels as

C (in dB)=C2-C1(in db)

=10 logC2/C1=40 log R1/R2,where R2=2R1,

C=-40dbwhen

This 40 db/dec is the general rule for the mobile radio environment

Additional Signal Propagation effects

• Blocking or shadowing-This is due to large obstacles in the path of radio signals. Even small obstacles like a simple wall, a truck on the street or tree can block the signal

• Reflection of the Signal-It occurs when a direct or refracted wave strikes a surface and what energy that is not absorbed by the struck medium is reflected back into space. The energy intensity of the reflected wave is dependent on the angle of striking (angle of incident), size of the object & the wave length of the signal

• Reflected signal is not as strong as direct signal

• Reflection helps in transmission where no– exists ?.R

• Refraction-This is due to variation in velocity of EM wave due to different density of medium through which it travels (it is equal to c only in--?.)

• The LOS of radio waves is bent towards the earth (Because the earth is a dense medium and the density of the atmosphere is closer to ground)

Scattering & Diffraction

• Scattering-When radio waves impinges on a rough surface, reflected energy is spread out in all directions due to scattering. At times, due to this effect even when there is no LOS, the RSL in a mobile environment is often stronger than what is predicted by reflection and diffraction. Scattering also is due to the size of the object is in the order of wavelength or less. An incoming signal is scattered into several weaker outgoing signals

• Diffraction-Radio waves will be deflected at an edge and propagate in different directions

• The results of scattering & diffraction are of varying signals depending on the location of the receiver. Effects like attenuation, scattering, diffraction, refraction are all happens simultaneously and are frequency and time dependent

Multi Path Propagation

• Due to the finite speed of light, signals traveling along different paths with different lengths arrive at the receiver at different times. This effect ( caused by multi path propagation) is called delay spread: the original signal is spread due to different delays of parts of the signal.

• Typical values for delay spread are appx 3 micro seconds in cities and upto 12 micro seconds can be observed. GSM can tolerate up to 16 micro seconds.

• Such delay spread can cause ISI (energy intended for one symbol will spill over to the adjacent symbol)

• ISI limits the BW of radio channel. Due to this interference, the signals of different symbols can cancel each other leading to misinterpretations at the receiver and causing transmission errors

Fading

• Signal Levels in Wireless propagation undergoes continuous variations

• These rapid signal fluctuation of the signal is known as fading

• There are two types of fading in wireless communications:-

• Small scale fading

• Log Normal fading

• Due to the different paths taken by signals they have different phases and may cancels each other. The receiver has to constantly adopt to the varying channel characteristics, by changing the parameters of the equalizer. However if these changes are too fast, such as driving on a high way through a city, the receiver can not adapt fast enough and the error rate of transmission increases

• Log Normal fading-Small scale signal variation when averaged out is called the local mean and is expressed in log scale of power, and is called log –normal fading. It could also called long term fading ?.

Diversity

• It is another technique used to compensate for fading channel impairments, and is usually implemented by using two or more receiving antennas. As with an equalizer, diversity improves the quality of a wireless communication link without altering the common air interface, and without increasing the transmitted power and bandwidth. Diversity is usually employed to reduce the depth and duration of the fades experienced by a receiver in a local are which are due to motion.

Cont..It is often used at both base station and mobile receivers. The most common diversity technique is called spatial diversity where by multiple antennas are strategically spaced and connected to a common receiving system.

While one antenna sees a signal null, one of the other antennas may see a signal peak, and the receiver is able to select the antenna with the best signal at any time.

Other diversity techniques include antenna poliarization diversity, frequency diversity and time diversity (Rake receiver in CDMA Systems)

Cont

• Diversity exploits the random nature of radio propagation by finding independent ( or at least highly uncorrelated) signal paths for communication. In all applications, diversity decisions are made by the receiver, and are unknown to the transmitter

• The diversity concept is that if one radio path undergoes a deep fade, another independent path may have a strong signal. By having more than one path to select from, both the inst instaneous and average SNRs at the receiver may be improved, often by as much as 20 db to 30 db

• For small scale fading, two antennas separated by a fraction of a meter, one may receive a null while the other receives a strong signal. By selecting the best signal at all times, a receiver can mitigate small-scale fading effects (this is called antenna diversity or space diversity)

Counter Measures

• There are several counter measures used to combat fading in wireless communication systems:-

• Space Diversity• Frequency Diversity• Polarization Diversity• Hybrid Diversity • Direct Sequence Spreading• Equalizers• Interference Cancellers

Frequency Diversity

Frequency diversity is simply modulating two different RF carrier frequencies with the same IF intelligence, then transmitting both RF signals to a given destination. At the destination, both carriers are demodulated, and the one that yields the better-quality IF signal is selected.

Frequency diversity

Space Diversity

With space diversity, the output of a transmitter is fed to two or more antennas that are physically separated by an appreciable number of wavelengths. Similarly, at the receiving end, there may be more than one antenna providing the input signal to the receiver. If multiple receiving antennas are used, they must also be separated by an appreciable number of wavelengths.

Space Diversity

Cont..

When space diversity is used, it is important that the electrical distance from a transmitter to each of its antennas and to a receiver from each of its antennas is an equal multiple of wavelengths long. This is to ensure that when two or more signals of the same frequency arrive at the input to a receiver, they are in phase and additive.

Polarization Diversity

With polarization diversity, a single RF carrier is propagated with two different electromagnetic polarizations (vertical and horizontal). Electromagnetic waves of different polarizations do not necessarily experience the same transmission impairments. Polarization diversity is generally used in conjunction with space diversity. One transmit/receive antenna pair is vertically polarized, and the other is horizontally polarized. It is also possible to use frequency, space, and polarization diversity simultaneously.

Receiver Diversity

Receiver diversity is using more than one receiver for a single radio-frequency channel. With frequency diversity, it is necessary to also use receiver diversity because each transmitted frequency requires its own receiver. However, sometimes two receivers are used for a single transmitted frequency.

Hybrid Diversity

Hybrid diversity is a somewhat specialized form of diversity that consists of a standard frequency-diversity path where the two transmitter/receiver pairs at one end of the path are separated from each other and connected to different antennas that are vertically separated as in space diversity. This arrangement provides a space-diversity effect in both directions-in one direction because the receivers are vertically spaced. This arrangement combines the operational advantages of frequency diversity with the improved diversity protection of space diversity. Hybrid diversity has the disadvantage, however, of requiring two radio frequencies to obtain one working channel.

Direct Sequence Spreading

• The inherent path diversity of a direct sequence spread spectrum system will reduce the effect of long-term envelope variation due to shadowing. This will result in a network with more homogeneous QoS, reduces hand-over and improve capacity

Radio Propagation in Mobile Environment

In reality, hexagons are extremely simplified models of radio coverage patterns because radio propagation is highly dependent on terrain and other factors. The problems of path loss, shadowing and multipath fading all affect the coverage of an area. For example, time dispersion is a problem caused by the reception of radio signals, which are reflected off far away objects. The carrier-to-reflection (C/R) ratio is defined as the ratio between the direct signal © and the reflected signal (R).

Cont..

Also, due to the problem of time alignment the maximum distance an MS can be from a BTS is 35 km. This is the maximum radius of a GSM cell. In areas where large coverage with small capacity is required, it is possible to allocate two consecutive TDMA time slots to one subscriber on a call. This enables a maximum distance from the BTS of 70km.

Improvement of Received Signal Quality

• Equalization, diversity and channel coding are three techniques which can be used independently or in tandem to improve received signal quality and link performance over small-scale times and distances in a hostile mobile radio environment.

Cont..• Equalization-Compensates for inter symbol interference

(ISI) created by multi path with in time dispersive channels. If the modulation bandwidth exceeds the coherence bandwidth of the radio channel, ISI occurs and modulation pulses are spread in time into adjacent symbols.

• An equalizer with in a receiver compensates for the average range of expected channel amplitude and delay characteristics. Equalizers must be adaptive since the channel is generally unknown and time varying.

• In radio channels, a variety of adaptive equalizers can be used to cancel interference while providing diversity. Since the mobile fading channel is random and time varying, equalizers must track the time varying characteristic of the mobile channel and thus are called adaptive equalizers

Channel Coding• Channel coding improves the small-scale link

performance by adding redundant data bits in the transmitted messages so that if an instantaneous fade occurs in the channel, the data may still be recovered at the receiver.

Channel Coding (Cont..)

At the base band portion of the transmitter, a channel coder maps the user’s digital message sequence into another specific code sequence containing a greater number of bits than originally contained in the message. The coded message is then modulated for transmission in the wireless channel

• Channel coding is used by the receiver to detect or correct some (or all) of the errors introduced by the channel in a particular sequence of message bits. iAs decoding is performed after the demodulation portion of the receiver, coding can be considered to be post detection technique. The added coding bits lower the raw data transmissions rate through the channel( that is, coding expands the occupied bandwidth for a particular message data rate). Channel codes used are-block codes, convolution codes and turbo codes.

Channel Coding (Cont..)

• Equalization, diversity and channel coding are used to improve radio link performance ( i.e., to minimize the instantaneous bit error rate) but cost & complexity and effectiveness of each technique varies widely in practical wireless communication systems

Modulation

• Modulation is the process of encoding information from a message source in a manner suitable for transmission. Process involves translating a base band signal (source) to a band pass signal to a higher frequency than the base band frequency

• Demodulation is the process of extracting the baseband message from the carrier so that it may be processed and interpreted by the intended receiver

• In mobile environment the first generation mobile systems where using analog systems such as AM,FM & Phase Modulation

Modulation Techniques to Counter Fading

• Three basic modulation schemes can be derived from

• g (t) = At cos (2iift+øt) there are three parameters:-

• Amplitude, frequency & phase which may be varied in accordance with data or another modulating signal

• For digital modulation,0 & 1 are translated into an analog signal (base band signal)

• In wireless networks, however, digital transmission cannot be used. Here the binary bit-stream has to be translated into an analog signal first. The basic translation methods are-ASK,FSK &PSK

•

Digital Modulation

• Modern mobile communication systems use digital modulation techniques as it has become cost effective due to VLSI & DSP

• The advantage of Digital Modulation are:-• Greater noise immunity• Easier multiplexing of various forms of information• Greater security• Facility for digital error –control codes• Improved system performance due to source coding,

• encryption, equalization• Software friendly

Digital Modulation-Important Factors • Low-bit error rate at low received S/N ratio• Can perform well in multi path & fading condition• Minimum band width requirement• Easy & cost effective to implement• Performance of modulation schemes are measured in terms of its

power efficiency and bandwidth efficiency• Power efficiency-It is the ability of a modulation technique to

preserve the fidelity of the digital message at low power levels• Bandwidth efficiency- is the ability of a modulation scheme to

accommodate data with in a limited band width• For mob environment, the modulation scheme should have high

sensitivity to detection of timing jitter, caused by time varying effects of channels

• Spectral Shaping is done through base band or IF processing & pulse shaping is done to reduce ISI effects

USE of Modulation

• For ease of radiation

• For efficient transmission

• For multiplexing

• For frequency assignment

• To improve signal to noise ratio

Digital Modulation Techniques

• Digital Modulation Techniques could be broadly classified as linear and non linear

• Linear Modulation Technique-The amplitude of the transmitted signal, s (t), varies linearly with the modulating digital signal, m(t). Linear modulation techniques are band width efficient and hence are very useful for wireless communication systems where there is an increasing demand to accommodate more and more users within a limited spectrum.

• The spectral efficiency of linear modulation schemes are very good but to be used along with linear RF amplifiers which have poor efficiency

• Otherwise, use of non linear amplifiers leads to the regeneration of filtered side lobes which can cause severe adjacent channel interference and results in the loss of all the spectral efficiency gained by linear modulation

Cont

• Most popular linear modulation techniques are:-• Binary Phase Shift Keying (BPSK)-here the phase of a constant

amplitude carrier signal is switched between two values according to the two possible signals m1 &m2

• Corresponding to a binary 1 and 0 • The two phases are separated by 180 degree• The BPSK signal is equivalent to a double sideband suppressed

carrier amplitude modulated wave form, where cos (2iifct) is applied as the carrier, and the data signal m (t) is applied as the modulating waveform. Hence a BPSK signal can be generated using a balanced modulator

• Other modulation techniques are-DPSK,QPSK,ii/4 QPSK etc.,

Non Linear Modulation MethodsIn non linear modulation techniques, where the amplitude of the carrier

is constant, regardless of the variation in the modulating signal. The advantage of non linear systems are:-

Power efficient class C amplifiers can be used with out introducing degradation in the spectrum occupancy of the transmitted signal

Low-out-of-band radiation of the order of -60db-70 db can be achieved

Limiter-discriminator detection can be used, which simplifies receiver design and provides high immunity against random FM noise and signal fluctuations due to Rayleigh fading

Non-linear modulation methods take more bandwidth than linear modulation schemes.

Non-linear modulation is not suited where bandwidth efficiency is more suited than power efficiency

Modulation in a Transmitter

Here after the analog modulation the centre frequency of the analog signal to the radio carrier

Demodulation & data reconstruction in a Receiver

Here the receiver demodulates the signal into the analog base band with the help of the known carrier. For digital data reconstruction we need to know the synchronization code to extract the digital data

Amplitude Shift Keying (ASK)

The most simple digital modulation scheme. Two binary values 1 & 0 are represented by two different amplitudes. Requires low amplitude but susceptible to interference. Effects like multi-path, noise, or path loss heavily influence the amplitude. In wireless transmission constant amplitude cannot be ensured and hence not used often

Frequency shift Keying (FSK/BFSK)

Here one frequency f1 to binary 1 and other f2 to binary 0 are assigned. Oscillators switch between f1 & f2 depending upon the input. Needs large band width but much less susceptible to errors

Phase shift Keying

Here a phase shift of 180 or ii as the o follows1.This simple scheme shifting the phase by 180 degree each time value changes is also called BPSK.A simple implementation of a BPSK Modulator could multiply a frequency f with +1 if the binary data is -1 if the binary data is 0

Modulation Scheme• Minimum shift keying is a binary-digital FM modulation technique

with the modulation index of m=0.5.It has the following fundamental properties:-

• Constant envelope suitable for non linear, power-efficient amplification

• Coherent and non coherent detection capability• Spectral main lobe is 50% wider than that of QPSK signals• To retain the desirable first and second properties and to

simultaneously increase the spectral efficiency (by reducing band width of the main lobe and the spectral density of the side lobes), a pre modulation low pass filter (GLPF) is inserted into the base band processor (BBP) subsystem of the MSK modulator. The cascade of a GLPF with an FM modulator VCO with m=.5 leads to a Gaussain MSK (GMSK) modulator

• The principal advantages of GMSK are power amplifier efficiency and robust performance; BER =10-3 for C/N =30 dB, both coherent and non coherent demodulation are possible

Gaussian Minimum Shift Keying

Gaussian Minimum Shift Keying, or to give it its full title Gaussian filtered Minimum Shift Keying, GMSK, is a form of modulation used in a variety of digital radio communications systems. It has advantages of being able to carry digital modulation while still using the spectrum efficiently.

One of the problems with other forms of phase shift keying is that the sidebands extend outwards from the main carrier and these can cause interference to other radio communications systems using nearby channels.

Cont..

In view of the efficient use of the spectrum in this way, GMSK modulation has been used in a number of radio communications applications. Possibly the most widely used is the GSM cellular technology which is used worldwide and has well over 3 billion subscribers.

GMSK basics

GMSK modulation is based on MSK, which is itself a form of phase shift keying. One of the problems with standard forms of PSK is that sidebands extend out from the carrier. To overcome this, MSK and its derivative GMSK can be used. MSK and also GMSK modulation are what is known as a continuous phase scheme. Here there are no phase discontinuities because the frequency changes occur at the carrier zero crossing points. This arises as a result of the unique factor of MSK that the frequency difference between the logical one and logical zero states is always equal to half the data rate. This can be expressed in terms of the modulation index, and it is always equal to 0.5.

Cont..

Signal using MSK modulation

Cont..

A plot of the spectrum of an MSK signal shows sidebands extending well beyond a bandwidth equal to the data rate. This can be reduced by passing the modulating signal through a low pass filter prior to applying it to the carrier. The requirements for the filter are that it should have a sharp cut-off, narrow bandwidth and its impulse response should show no overshoot. The ideal filter is known as a Gaussian filter which has a Gaussian shaped response to an impulse and no ringing. In this way the basic MSK signal is converted to GMSK modulation.

Cont..

Spectral density of MSK and GMSK signals

Generating GMSK modulation

There are two main ways in which GMSK modulation can be generated. The most obvious way is to filter the modulating signal using a Gaussian filter and then apply this to a frequency modulator where the modulation index is set to 0.5. This method is very simple and straightforward but it has the drawback that the modulation index must exactly equal 0.5. In practice this analogue method is not suitable because component tolerances drift and cannot be set exactly.

Cont..

Generating GMSK using a Gaussian filter and VCO

Cont..

A second method is more widely used. Here what is known as a quadrature modulator is used. The term quadrature means that the phase of a signal is in quadrature or 90 degrees to another one. The quadrature modulator uses one signal that is said to be in-phase and another that is in quadrature to this. In view of the in-phase and quadrature elements this type of modulator is often said to be an I-Q modulator.

Cont..

Using this type of modulator the modulation index can be maintained at exactly 0.5 without the need for any settings or adjustments. This makes it much easier to use, and capable of providing the required level of performance without the need for adjustments. For demodulation the technique can be used in reverse.

Cont..

Block diagram of I-Q modulator used to create GMSK

Advantages of GMSK modulation

There are several advantages to the use of GMSK modulation for a radio communications system. One is obviously the improved spectral efficiency when compared to other phase shift keyed modes.A further advantage of GMSK is that it can be amplified by a non-linear amplifier and remain undistorted This is because there are no elements of the signal that are carried as amplitude variations..

GMSK Advantages

This advantage is of particular importance when using small portable transmitters, such as those required by cellular technology. Non-linear amplifiers are more efficient in terms of the DC power input from the power rails that they convert into a radio frequency signal. This means that the power consumption for a given output is much less, and this results in lower levels of battery consumption; a very important factor for cell phones

Cont..

A further advantage of GMSK modulation again arises from the fact that none of the information is carried as amplitude variations. This means that is immune to amplitude variations and therefore more resilient to noise, than some other forms of modulation, because most noise is mainly amplitude based.

GMSK highlights

GMSK modulation is a highly successful form of modulation, being used in GSM cellular technology, and as a result, its use is particularly widespread. It is also used in other radio communications applications because of its advantages in terms of spectral efficiency, resilience to noise and its ability to allow the use of efficient transmitter final amplifiers. Even though other radio communications systems utilise other forms of modulation, GMSk is an ideal choice for many applications

GSM uses combination of both FDMA & TDMA

FDMA involves the division by frequency of the (maximum) 25 MHz between into 124 carrier frequencies spaced 200 KHz apart.This carrier is then divided in time using TDMA scheme. This enables the different users of the single radio frequency channel to be allocated different time slots. They are then able to use the same RF channel with out interference. The slot is then the time that is allocated to the particular user, and the GSM burst is the transmission that is made in this time

Interference

Co-channel Interference (C/I)Cellular networks are more often limited by problems caused by interference rather than by signal strength problems. Co-channel interference is caused by the use of a frequency close to the exact same frequency. The former will interfere with the latter, leading to the terms interfering frequency (I) and carrier frequency (C).

Cont..

The GSM specification recommends that the carrier-to-interference (C/I) ratio is greater than 9 decibels (dB). However, generally up to 12 dB can be used as planning criterion.This C/I ratio is influenced by the following factors:

• The location of the MS• Local geography and type of local scatters• BTS antenna type, site elevation and

position

Adjacent channel interference (C/A)

Adjacent frequencies (A), that is frequencies shifted 200kHz from the carrier frequency (c), must be avoided in the same cell and preferably in neighboring cells also. Although adjacent frequencies are at different frequencies to the carrier frequency they can still cause interference and quality problems.

Cont..

The GSM specification states that the carrier-t-adjacent ratio (C/A) must be larger than -9dB. However it is recommends that higher than 3 dB be used as planning criterion.By planning frequency re-use in accordance with well established cell patterns, neither co-channel interference nor adjacent channel interference will cause problems, provided the cells have homogenous propagation properties for the radio waves. However, in reality cells vary in size depending on the amount of traffic they are expected to carry.

Cont..

Therefore, real cell plans must be verified by means of predictions or radio measurements to ensure that interference does not become a problem. Nevertheless, the first cell plan based on hexagons, the nominal cell plan, provides a good picture of system planning.

Rayleigh Fading

Rayleigh fading is the name given to the form of fading that is often experienced in an environment where there is a large number of reflections present. The Rayleigh fading model uses a statistical approach to analyse the propagation, and can be used in a number of environments.

Rayleigh fading basics

The Rayleigh fading model is particularly useful in scenarios where the signal may be considered to be scattered between the transmitter and receiver. In this form of scenario there is no single signal path that dominates and a statistical approach is required to the analysis of the overall nature of the radio communications channel.

Cont..

Rayleigh fading is a model that can be used to describe the form of fading that occurs when multipath propagation exists. In any terrestrial environment a radio signal will travel via a number of different paths from the transmitter to the receiver. The most obvious path is the direct, or line of sight path.

Cont..

The Rayleigh fading model can be used to analyse radio signal propagation on a statistical basis. It operates best under conditions when there is no dominant signal (e.g. direct line of sight signal), and in many instances cellular telephones being used in a dense urban environment fall into this category. Other examples where no dominant path generally exists are for ionospheric propagation where the signal reaches the receiver via a huge number of individual paths. Propagation using tropospheric ducting also exhibits the same patterns. Accordingly all these examples are ideal for the use of the Rayleigh fading or propagation model.

Simple reference model used here

Application

Transport

Network

Data Link

Physical

Medium

Data Link

Physical

Application

Transport

Network

Data Link

Physical

Data Link

Physical

Network Network

Radio

Influence of mobile communication to the layer model

– service location– new applications, multimedia– adaptive applications– congestion and flow control– quality of service– addressing, routing,

device location– hand-over– authentication– media access– multiplexing– media access control– encryption– modulation– interference– attenuation– frequency

• Application layer

• Transport layer

• Network layer

• Data link layer

• Physical layer

Overlay Networks - the global goal

regional

metropolitan area

campus-based

in-house

verticalhandover

horizontalhandover

integration of heterogeneous fixed andmobile networks with varyingtransmission characteristics

Multiple Access Schemes

• Multiple access schemes are used to enable many users to share simultaneously a finite amount of radio spectrum

• Multiple access systems specify how signals from different sources can be combined efficiently for transmission over a given radio frequency band and then separated at the destination with out mutual interference

Multiple access techniques for wireless systems

In a wireless communication system, radio resources must be provided in each cell to assure the interchange of data between the mobile terminal and the base station. Uplink is from the mobile users to the base station and downlink is from the base station to the mobile users. Each transmitting terminal employs different resources of the cell. A multiple access scheme is a method used to distinguish among different simultaneous transmissions in a cell.

Cont..

A radio resource can be a different time interval, a frequency interval or a code with a suitable power level. All these characteristics( i.e, time, frequency, code and power) univocally contribute to identify a radio resource. If the different transmissions are differentiated only for the frequency band, we have the Frequency Division Multiple Access (FDMA). Whereas, if transmissions are distinguished on the basis of time, we consider the Time Division Multiple Access (TDMA).

Cont..

Finally, if a different code is adopted to separate simulataneous transmissions, we have the Code Division Multiple Access(CDMA). However, resources can be also differentiated by more than one of the above aspects. Hence, hybrid multiple access schemes are possible (eg: FDMA/TDMA)

SDMA - general scheme

Objectives

• Medium access in wireless environment– Motivation– SDMA/FDMA/TDMA/CDMA– Random

• Aloha• Reservation schemes • Multiple Access with CA (Collision Avoidance)

– Polling – Inhibit Sense Multiple Access

• Wireless LANs (WLAN)– IEEE 802.11 (Wi-Fi)– Overview of other WLANs

Medium access in wireless environment

– Motivation

– SDMA/FDMA/TDMA/CDMA– Random

• Aloha• Reservation schemes • Multiple Access with CA (Collision Avoidance)

– Polling– Inhibit Sense Multiple Access

Motivation• In wireless, medium is always shared• Can we use MAC protocols from fixed networks?

– Example CSMA/CD• send as soon as the medium is free, listen into the medium if a collision

occurs • Problems in wireless networks

– decreased signal strength: radio signal attenuates as it propagates through matter (path loss), proportional to the square of the distance

– interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); motors interfere as well

– multipath propagation: radio signal reflects off objects ground, arriving at destination at slightly different times

– the sender would apply CS and CD, but the collisions happen at the receiver– a sender may not “hear” the collision, i.e., CD does not work– furthermore, CS might not work if, e.g., a terminal is “hidden”

Access methods SDMA/FDMA/TDMA

• SDMA (Space Division Multiple Access)

– segment space into sectors, use directed antennas

– cell structure

• FDMA (Frequency Division Multiple Access)

– assign a certain frequency to a transmission channel between a sender and a receiver

• TDMA (Time Division Multiple Access)

– assign the fixed sending frequency to a transmission channel for a certain amount of time

• CDMA (Code Division Multiple Access)

– assign a different spreading code to each sender

• Random Access…

FDM (Frequency Division Multiplexing)• Here the frequency is sub divided into several non-

overlapping frequency bands • Each # is now allotted its own frequency band.• Senders using a certain frequency band can use this

band continuously• Guard spaces are needed to avoid frequency band

overlapping (called adjacent channel interference)• For mobile applications, communications takes place for

only a few minutes at a time .Assigning a separate frequency for each possible communication need would be waste of frequency resources & inflexible and limit the number of users

FDMA

U1 U2 U3U4

F1 F2 F3 F4 Frequency

Channel ID= frequency slot ID

FDMA - general scheme, example GSM

f

t

124

1

124

1

20 MHz

200 kHz

890.2 MHz

935.2 MHz

915 MHz

960 MHz

Here all up links use the band between 890.2 and 915 mhz, all down links use 935.2 to 960 mhz. The base stn allocates a certain frequency for up & down link to establish a duplex channel with the mobile. Up & Down link have fixed relations. If the up link frequency is fu=890 mhz+n.0.2 mhz, the dn link frq is fd=fu+45 mhz, i.e., fd=935 mhz+n.0.2 mhz for a certain channel n

FDMA (Frequency Division Multiple Access)

• The base stn selects the channel. Each channel (up & dn) has a band width of 200 khz having (124 channels per direction for 900 mhz)

• Frequency division duplex (FDD) is the process in which up & dn links use two separate frequencies for duplex communication

• Analog systems, which were the initial implementations of the cellular concept all use FDMA techniques

TDMA (Time division multiple Access)

• Compared to FDMA,TDMA offers a much more flexible scheme, it allocate certain time slots for communication

• There is no need to tune to a certain frequency, the receiver can stay at the same frequency the whole time.

• In TDMA, time slots are allotted to channels in a fixed pattern resulting in a fixed bandwidth

• The only crucial factor is accessing the reserved time slot at the right moment

• The synchronization if ensured, each MS knows its turn and no interference will happen. Each will have a fixed delay

TDMA - general scheme, example DECT

1 2 3 11 12 1 2 3 11 12

tdownlink uplink

417 µs

Here, the base stn uses one out of 12 slots for the down link, where as the mobile stn uses one out of 12 different slots for the up link. Up link & dn are separated in time. Up to 12 different mobile stations can use the same frequency with out interference. Here the pattern is repeated after every 10ms & each slot has a duration of 417microseconds.For GSM, each frequency channel (200 khz) is divided into 8 time slots. Guard bands are need to be catered between time slots& frequency channels

TDMA (cont)

Time

FrequencyF1

U1

U2

U3

T1

T2

T3

U4

Here the available spectrum is partitioned into narrow frequency bands or frequency channels ( as in FDMA ), which in turn are divided into a number of time slots. An individual user is assigned a time slot that permits access to the frequency channel for the duration of the time slot. Thus, the traffic channel in case of TDMA consists of a time slot in a periodic train that make up a frame

U5

F2

U6

U8

U9

U7

F3

U10

U11

U12

F4

GSM FDMA & TDMA• Here each band is divided into 200khz slots called

ARFCN’s ( Absolute Radio Frequency Channel Numbers)

• As well as dividing up the frequency, the ARFCN is also divided in time into 8 time slots (TS), each TS being used in turn by a different MS. The 8 TS together are known as a frame

• TCH’s (Traffic Channels) each one of them uses a particular ARFCN and Time slot. Two of the TCH’s are on the same ARFCN using different time slots,the other two are on different ARFCN’s

• The combination of a TS number and ARFCN is called a physical channel

CDMA

• CDMA – a spread spectrum technique

– used in several wireless broadcast channels (cellular, satellite, etc)

– unique “code” assigned to each user; i.e., code set partitioning

– all users share same frequency, but each user has own “chipping” sequence (i.e., spreading code) to encode data

– encoded signal = (original data) X (chipping sequence)

– decoding: inner-product of encoded signal and chipping sequence

– allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

• Disadvantages:

– higher complexity of a receiver all signals should have the same strength at a receiver (near-far problem)

• Advantages:

– all terminals can use the same frequency, no planning needed

– huge code space (e.g. 232) compared to frequency space

CDMA

• The CDMA system utilizes the spread spectrum technique, where by a spreading code ( called a pseudo-random noise or PN code) is used to allow multiple users to share a block of frequency spectrum. In CDMA cellular systems that use direct sequence ss techniques, the (digital) information from an individual user is modulated by means of a unique PN code (spreading sequence) assigned to each user.

• All the PN code-modulated signals from different users are then transmitted over the entire CDMA frequency channel (e.g.,1.23 Mhz in case of IS 95).

• At the receiving end, the desired signal is recovered by de spreading the signal with a copy of the spreading

Cont

• Sequence ( PN Code) for the individual user in the receiving co-rrelator.

• All other signals (belong to other users), whose PN codes do not match that of the desired signal, are not de spread and as a result, are perceived as noise by the co-rrelator

• Since the signals in the case of CDMA utilize the entire allocated block of spectrum, no guard bands of any kind are necessary with in the allocated block

CDMA

u1

u2u3

un

c1

C2C3

Cn

F=F1 F4

.

.

CDMA Encode/Decode

slot 1 slot 0

d1 = -1

1 1 1 1

1- 1- 1- 1-

Zi,m= di.cmd0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutput

channel output Zi,m

sendercode

databits

slot 1 slot 0

d1 = -1d0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutputreceiver

code

receivedinput

Di = Zi,m.cmm=1

M

M

CDMA: two-sender interference

CDMA: another two-sender example (1)

• Sender A

– sends Ad = 1, key Ak = 010011 (assign: “0”= -1, “1”= +1)

– sending signal As = Ad * Ak = (-1, +1, -1, -1, +1, +1)

• Sender B

– sends Bd = 0, key Bk = 110101 (assign: “0”= -1, “1”= +1)

– sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1)

• Both signals superimpose in space – interference neglected (noise etc.)

– As + Bs = (-2, 0, 0, -2, +2, 0)

• Every receiver will receive (-2, 0, 0, -2, +2, 0)• How to know what A or B sends?

CDMA: another two-sender example (2)

• Receiver wants to receive signal from sender A

– apply key Ak bitwise (inner product)

• Ae = (-2, 0, 0, -2, +2, 0) Ak = 2 + 0 + 0 + 2 + 2 + 0 = 6

• 6/6=1, therefore, original bit was “1“”• Receiver wants to receive signal from sender B

– apply key Bk bitwise (inner product)

• Be = (-2, 0, 0, -2, +2, 0) Bk = -2 + 0 + 0 - 2 - 2 + 0 = -6,

• -6/6=-1, therfore, original bit was “0”• Orthogonal codes

– Ak = 010011 (-1, 1, -1, -1, 1, 1)

– Bk = 110101 ( 1, 1, -1, 1, -1, 1)

– Ak Bk = -1+1 +1 -1 -1 +1 = 0,

– so Ak and Bk are orthogonal and can be used at the same time

Comparison SDMA/TDMA/FDMA/CDMA

Approach SDMA TDMA FDMA CDMAIdea segment space into

cells/sectorssegment sendingtime into disjointtime-slots, demanddriven or fixedpatterns

segment thefrequency band intodisjoint sub-bands

spread the spectrumusing orthogonal codes

Terminals only one terminal canbe active in onecell/one sector

all terminals areactive for shortperiods of time onthe same frequency

every terminal has itsown frequency,uninterrupted

all terminals can be activeat the same place at thesame moment,uninterrupted

Signalseparation

cell structure, directedantennas

synchronization inthe time domain

filtering in thefrequency domain

code plus specialreceivers

Advantages very simple, increasescapacity per km²

established, fullydigital, flexible

simple, established,robust

flexible, less frequencyplanning needed, softhandover

Dis-advantages

inflexible, antennastypically fixed

guard spaceneeded (multipathpropagation),synchronizationdifficult

inflexible,frequencies are ascarce resource

complex receivers, needsmore complicated powercontrol for senders

Comment only in combinationwith TDMA, FDMA orCDMA useful

standard in fixednetworks, togetherwith FDMA/SDMAused in manymobile networks

typically combinedwith TDMA(frequency hoppingpatterns) and SDMA(frequency reuse)

still faces some problems,higher complexity,lowered expectations; willbe integrated withTDMA/FDMA

Wireless Systems & Standards

• AMPS:-• Frequency Uplink- 824-849 Mhz• Down link 869-894• Frequency Separation 45 Mhz• Channel Spacing 30 khz• Number of Channels 832 full duplex• Voice Transmission FM+/- 8 Khz• Data Transmission FSK 10 kb/s +/- 8 khz deviation• Error Protection code BCH• Mobile Tx Power 3 W nominal

Advanced Mobile Phone System (AMPS)

• Developed by Bell Lab in mid 1970• First system fielded in Chicago in 1977-78• AMPS system uses frequency modulation with 12 khz deviation for

speech (8khz for signalling)• By adopting the wider12 khz deviation for a 30 khz channel spacing,

the AMPS system provides an extended dynamic range for speech and there fore increased protection against co-channel interference

• To meet the co-channel interference objective ,the typical frequency reuse plan employed in AMPS system is either a 12 gp frequency cluster with omni drn antennas or a 7 gp cluster with three (120 degree) sectors per cell

• Further while the call is in progress, the base station transmits a low level supervisory audio tune (SAT) in the region of 6 khz

(Cont)

• Three different SAT frequencies are used by the network and are allocated to the base stations such that base stations most likely to cause interference have a different SAT from the serving base station

• The mobile continuously monitors the received SAT• If the mobile ( or the base station) detects a difference

between the received SAT and the one expected, the audio path is muted to prevent the interfering signal from being over heard. If the condition persists, the call may be aborted

GSM (European TDMA Digital Cellular Standard)

• ETSI (European Telecommunication Standards Institute) is responsible for GSM standardization/revision/enhancements to various aspect of GSM standards

• The characteristics of the initial GSM standard include the following:-• Fully digital system utilizing the 900 Mhz frequency band• TDMA over radio carriers (200 khz carrier spacing)• 8 full-rate or 16 half rate TDMA channels per carrier• user/terminal authentication for fraud control• encryption of speech and data transmission over the radio • path• Full international roaming capability• low speed data services (up to 9.6 kbs)• Compatibility with ISDN for supplementary services• Support of short message service (SMS)

Cont• GSM supports a range of services known as, bearer services, tele services,

and supplementary services • The most important service supported by GSM is telephony• Other services included are emergency calling & voice messaging• The common ISDN-like supplementary services supported by GSM include

the following:-• Call offering services- call forwarding• call restriction services- call barring• call waiting service• call hold service• multi party service-tele conferencing• calling line presentation restriction services• advice of charge service• closed user group service

CDMA Key Specifications

• Up Link Frequencies 824-849 Mhz

• Down Link Frequencies 869-894 Mhz

• Duplexing FDD

• Multiple Access Tech CDMA

• Modulation BPSK with Quad-

• rature spreading

• Carrier Separation 1.25 Mhz