CNW 609 Gsm Part1 [Compatibility Mode]

7/31/2019 CNW 609 Gsm Part1 [Compatibility Mode]

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WIRELESS COMMUNICATION

SYSTEMS

LECTURE NOTES ON2G (GSM,IS-95)

2.5G (GPRS, EDGE).

Week 3(Background T1: 5-7) Part 2

2G (GSM)


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Abbreviations

PDN- Packet data network

PSTN -(Public switched telephone network)

ISDN -(Integrated services digital network)[Digital

Voice/data/video over PSTN)

PLMN -Public land Mobile network(U.S)

CNW-609-Wireless-Communication-Systems

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GSM-Introduction

GSM System Architecture

GSM Sub Systems

GSM Interfaces

GSM Functional Architecture

GSM Basic Channel Structure

Agenda


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What is GSM ?

Global System for Mobile (GSM) is a second generation cellular standard

developed to cater voice services and data delivery using digital modulation.

GSM is a cellular network, which means that mobile phones connect to it by

searching for cells in the immediate vicinity.

GSM Frequency Bands


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Brief Specifications

Frequency 1850Mhz to 1900Mhz(MS to BS)

Duplex-distance


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GSM System Architecture6


HLR and VLR are data-bases ofusers held by mobile-operators.

VLR is for roaming users HLR is for permanent usersFull View

Simplified View


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GSM Sub Systems A GSM network is divided into three

sub networks: the radio access network, the core

network,the management network.

These sub-networks are called

subsystems in the GSM standard.

The respective three subsystems are

called as : Base Station Subsystem (BSS), the

Network Switching Subsystem (NSS)

and the Operation (and Maintenance)

Subsystem (O(M)SS).

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GSM Interfaces8


Full View Simplified View


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GSM Interfaces

(The Air/Radio interface/Um Interface) The Radio Interface (MS to BTS).

The Um radio interface (between MS and base transceiver stations [BTS])(Most important)

Addresses the demanding characteristics of the radio environment. The physical layer interfaces to the data link layer and radio resource

management sublayer in the MS and BS and to other functional units in the MS

and network subsystem (which includes the BSS and MSC) for supporting traffic

channels. The physical interface comprises a set of physical channels accessible through

FDMA and TDMA


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GSM Interfaces(Abis Interface) Abis Interface (BTS to BSC)

The interconnection between the BTS and the BSC is through a standardinterface.

Abis (most Abis interfaces are vendor specific).

The primary functions carried over this interface are Traffic channel transmission, terrestrial channel management, and radio channel

management. This interface supports two types of communications links:

Traffic channels at 64 kbps carrying speech or user data for a full- or half-rate

radio traffic channel.

Signaling channels at 16 kbps carrying information for BSC-BTS and BSC-MSC

signaling


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GSM Interfaces(A Interface) A Interface (BSC to MSC)

The A interface allows interconnection between the BSS radio base subsystemand the MSC.

The physical layer of the A interface is a 2-Mbps standard Consultative

Committee on Telephone and Telegraph (CCITT) digital connection.


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GSM Functional Architecture


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Operation

administration and

Maintenance

Operator

Communication

Management

User

Mobility

Management

Radio Resource

Management

Transmission Management

Call Control

Handles Mobility Security HLR,VLR

Ensures stable radio connection andhandover process

Physical Layer(Coding,modulation,

channel multiplexing etc)


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GSM Protocol Architecture


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Mapping Of GSM ontoOSI14



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The Air/Radio Interface

(Radio System Description)-Basic Channel Structure

GSM is a multicarrier TDMA system, i.e. it employees a combination of FDMA

and TDMA for multiple access.

The physical channels are defined here by a TDMA scheme. On top of the physical channels, a series of logical channels are defined, which

are transmitted in the time slots of the physical channels.

Logical channels perform a multiplicity of functions, such as payload transport,

signalling, broadcast of general system information, synchronization an channelassignment.

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GSM Basic Channel Structure Is based on a multi-carrier, time-

division multiple access and

frequency division duplex,MC/TDMA/FDD

The carrier spacing is 200 kHz

allowing, a guard band of 200 kHz

at each end of the sub-bands. Each radio frequency is time divided

into TDMA frames of 4.615 ms.

Each TDMA frame is subdivided into

eight full slots.

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Each of these slots can be

assigned to a full-rate (FR) traffic

channel (TCH), two half-rate (HR)TCHs or one of the control

channels.

A slot is equal to one timeslot

(TSL) on one frequency. The time and frequency structure

is displayed here.


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FDMA-TDMA Structure of GSM


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GSM utilizes a combination FDMA and TDMA on the Air-interface. That results in a two-dimensional channel structure.

TDMA frames are grouped into two types of multiframes:

26-frame multiframe (4.615ms x 26 = 120 ms) comprising of 26 TDMA

frames. 51-frame multiframe (4.615ms x 51 @ 235.4 ms) comprising 51 TDMA

frames.


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Significance of TDMA-Over FDMA In a pure FDMA system, one specific frequency is allocated for every user during a call.

That quickly leads to overload situations in cases of high demand.

GSM Considers this and uses TDMA over FDMA


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TDMA Channels on Multiple

Carrier Frequencies

TDMA on Frequency Hopping

scheme


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The FDMA-TDMA Scheme in GSM In TDMA the individual mobile stations are cyclically assigned a frequency for

exclusive use only for the duration of a time slot, which obviously requires frame

synchronization between transmitter. and receiver. The whole system bandwidth for a time slot is not assigned to one station, but the

system frequency range is subdivided into sub-bands, and TDMA is used for

multiple access to each sub-band.

The sub-bands are known as carrier frequencies, and the mobile systems using thistechnique are designated as multicarrier systems (not to be confused with

multicarrier modulation).

GSM employs such a combination of FDMA and TDMA; it is a multicarrier TDMA

system.


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The FDMA-TDMA Scheme in GSM The available frequency range is divided into frequency channels of 200 kHz bandwidth each

(with guard bands between to ease filtering),

Each of these frequency channels contains eight TDMA conversation channels. The sequence of time slots assigned to a mobile station represents the physical channels of a

TDMA system.

In each time slot, the mobile station transmits a data burst.

The period assigned to a time slot for a mobile station thus also determines the number of

TDMA channels on a carrier frequency. The time slots of one period are combined into a so-called TDMA frame.

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Advantage over pure FDMA Here in TDMA system, each user sends an

impulse like signal only periodically While a user in a FDMA system sends the signal

permanently.

A signal is sent once per TDMA frame(f1). This allows TDMA to simultaneously serve seven

other channels on the same frequency (with full

rate configuration) and 16 in half rateconfiguration.

This manifests the major advantage of

TDMA over FDMA(f2).

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GSM Physical Channels Modulation, and Multiplexing are important.

The modulation technique used on the radio channel is Gaussian Minimum Shift

Keying (GMSK).


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The data di arrives at the modulator with a bit rate of 1625/6 = 270.83 kbit/s

(Gross data rate)

This modulation rate is determined by the standard is equal to 1/T =

1625/6103

symbols/s (i.e., approximately 270.833 103

symbols/s)


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Multiplexing

(Multiple access and Duplexing) GSM uses a combination of FDMA and TDMA for multiple access.

Two frequency bands 45 MHz apart have been reserved for GSM operation 890915 MHz for transmission from the MS, i.e. uplink,. 935960 MHz for transmission from the base station, i.e. downlink.

Each of these bands of 25 MHz width is divided into 124 single carrier channels of

200 kHz width.

This variant of FDMA is also called Multi-Carrier (MC).

In each of the uplink/downlink bands there remains a guard band of 200 kHz.

Eac h R.F Channel is uniquely numbered.

A pair of channels with the same number form a duplex channel with 45 Mhz

duplex distance.

Each of the 200 kHz channels is divided into eight time slots and thus carries eightTDMA channels. The eight time slots together form a TDMA frame.

Each cell can have upto 15 carrier pairs(channels)

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Carrier Frequency Duplexing and

TDMA Frames


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Frame Relation25



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Uplink and Downlink The TDMA frames of the uplink are transmitted with a delay of three time slots with

regard to the downlink. A MS uses the same time slots in the uplink as in the downlink, i.e. the time slots with the same

number (TN).

Owing to the shift of three time slots, a MS does not have to send at the same time as it receives,

and therefore does not need a duplex unit.

In addition to the separation into uplink and downlink bands FDD with a distance of

45 MHz the GSM access procedure contains a TDD component. Thus the MS does not need its own high-frequency duplexing unit.

Each time slot of a TDMA frame lasts for a duration of 156.25 bit periods and, if

used, contains a data burst. The time slot lasts 15/26 ms = 576.9 s; so a frame takes 4.615 ms.

The same result is also obtained from the GMSK procedure, which realizes a gross datatransmission rate of 270.83 kbit/s per carrier frequency.

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Channel Usage Channel 1 and 124 will not normally be used (guard band of 200 KHz) in

order to protect services using adjacent spectrum bands.

These 124 possible carriers are defined for the uplink (Fu) and downlink (Fd)as follows:

Fu(n) = 890.2 MHz + 0.2(n-1) MHz (1< n < 124)

Fd(n) = 925.2 MHz + 0.2(n-1) MHz (1


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GSM Time Slots A GSM Multiframe is the basic unit, and is 120 ms

long.

There are 26 Frames in each Multiframe, witheach Frame being 4.61538 ms long (120 ms/26).

Within each Frame are 8 Timeslots at 576.92 s

per Timeslot (577 s in round numbers).

Finally, there are 156.25 Bits per Timeslot, each

Bit being 3.69231 s long.

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Bursts The nature of TDMA transmission is that

radio energy is emitted in a pulsed

manner rather than continuously. Mobile stations and BTSs send bursts

periodically.

The actual data transmission happens

during the time period represented in the

Figure as a horizontal line.

This time period is 148 bits, or 542.8 s

long.

Because GMSK (at least in theory) does

not contain an amplitude modulatedsignal, the effective transmission power is

constant over the entire transmission

period.

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The burst in the power-over-time

In total a burst has a window of 577

s or 156.25 bit before the next

time slot starts.

This means the power level has to bereduced by 70 db after 577 s .

This applies to both up and down

link and determines the maximum

bits a MS can send and receive.


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More on Bursts The net bit rate is only 114 bits per burst, not 156.25.

This reduced number of bits results from the mapping of a physical burst to

a logical burst. The physical burst needs bits for administrative purposes that reduce the space

available for signaling or user data.

Each burst always begins with tail bits, which are necessary to synchronize

the recipient.

Tail bits are, (except for the access burst) always coded as 000. The tail bits are followed by 148 data bits, which differ in format for the

various burst types.

Each burst is terminated by another set of tail bits and the guard period.

This guard period is required for the sender to physically reduce thetransmission power.

The guard period is particularly long for the access burst, to allow mobile

stations that are far from a BTS and hence experience propagation delays

to also access the BTS.


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Details of Bursts31



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Classification of Bursts


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33 The GSM Frame In GSM, every impulse on frequency is called a burst.

Every bursts corresponds to a Time slot(TS).

Eight bursts or TSs numbered from 0 to 7 form a TDMA frame. Every TDMA frame is assigned a fixed number, which repeats itself in a

time period of 3 hours, 28 minutes, 53 seconds 760 milliseconds.

This time period is referred to as hyper frame.

Multi frame and super frame are layers of hierarchy that lie between thebasic TDMA frame and the hyper frame.

26-frame multi frame is used to carry traffic channels and their associated

control channels.

51-frame multi frame is exclusively used for control channels.

A super frame contains 26 51 or 51 26 multi frames.


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34 Frame Hierarchy


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Frames and Slots


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Physical and Logical Channel. Physical channels are all the available TSs of a BTS.

Every TS corresponds to a physical channel.

Two types of channels are present The half-rate channel and the full-rate channel.

A BTS with 6 carriers, as shown in the previous figure, has 48 (8 times 6) physical

channels (in full rate configuration).

Logical channels are piggybacked on the physical channels.

A logic channel refers to specific type of information carried by the physicalchannel.

Logical channels are, so to speak, laid over the grid of physical channels.

Each logical channel performs a specific task.

Logical channels are divided into two categories Traffic channels and signalling (control) channels.


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Traffic Channel


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Traffic channel(TCH) Used for the transmission of user payload data (speech, data).

They do not carry any control information of Layer 3. Communication over a TCH can be circuit-switched or packet-switched.

In the circuit-switched case, the TCH provides a transparent data connection or a

connection that is specially treated according to the carried service.(Telephony)

In packet switched TCH carries user data of ODI Layer 2 and 3.

A TCH may either be fully used (full-rate TCH, TCH/F) or be split into two half-rate channels (half-rate TCH, TCH/H).

They can be designated as Mobile B (Bm) or Lower rate (Lm) channel.


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Signaling Channel Control and management of a GSM cellular network demands a very high

signaling effort.

Even in the absence of active connection, signaling information (for example,location update information) is permanently transmitted over the air

interface.

The GSM signaling channels offer a continuous, packet-oriented signaling

service to MSs in order to enable them to send and receive messages at anytime over the air interface to the BTS.

GSM signaling channels are also called Dm channels (mobile D channel).

They are further divided into Broadcast Channel (BCH), Common Control

Channel (CCCH) and Dedicated Control Channel (DCCH)


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Channel Classifications in GSM


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Following ISDN terminology, the GSM traffic channels are alsodesignated as Bm channel (mobile B channel) or Lm channel(lower-rate mobile channel, with half the bit rate).


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Logical Channels in GSM


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Signaling Channels in GSM


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What Modulation Technique is used?

GSM uses the modulation technique of Gaussian

minimum shift keying (GMSK). GMSK comes with a narrow frequency spectrum

and theoretically no amplitude modulation (AM)

part.


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More functions at the Physical Layer

We saw the basic function of

physical layer at the air interface.

Several additional functions areperformed at the physical layer

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Transmitter Side Physical Layer Functions

Ciphering.

Modifies the contents of these blocks through a secret code known only by the mobile

station and the base station. Burst formatting.

Adds synchronisation and equalisation information to the ciphereddata.

Modulation. Transforms the binary signal into an analogue signal at the right frequency. Thereby the

signal can be transmitted as radio waves.


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Receiver Side Physical Layer Functions

Demodulation.

Transforms the radio signal received at the antenna into a binary signal. Today most

demodulators also deliver an estimated probability of correctness for each bit. This extrainformation is referred to as soft decision or soft information.

Deciphering.

Modifies the bits by reversing the ciphering code.

De-interleaving.

Puts the bits of the different bursts back in order to rebuild the original code words. Channel decoding.

Tries to reconstruct the source information from the output of the demodulator, using the

added coding bits to detect or correct possible errors, caused between the coding and

the decoding.

Source decoding. Converts the digitally decoded source information into an analogue signal to produce the

speech.


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Power Levels

The highest is 20 watts (43 dBm) and the lowest is 800mW (29 dBm).

As mobiles may transmit for only one-eighth of the time (i.e. for their allocated

slot, which is one of eight), the average power is one-eighth of the maximum. To reduce the levels of transmitted power and hence the levels of interference,

mobiles are (en)able(d) to step the power down in increments of 2 dB from the

maximum to a minimum of 13 dBm (20 mW).

The mobile station measures the signal strength or signal quality (based on the

bit error rate), and passes the information to the BTS and hence to the BSC,

which ultimately decides if and when the power level should be changed.


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A point to be noted

A further power-saving and interference-reducing facility is the discontinuous

transmission (DTx) capability that is incorporated within the specification.

We know that at times there are (long) pauses in speech such as when theperson using the mobile is listening and during these periods there is no need

to transmit a signal.

In fact, it is found that a person speaks for less than 40 per cent of the time

during a normal telephone conversation.

The most important element of DTx is the Voice Activity Detector, This correctly distinguish between voice and noise inputs

If a voice signal is misinterpreted as noise, the transmitter is turned off and an

effect known as clipping results. (This annoys any person).

If noise is misinterpreted as a voice signal too often, the efficiency of DTx isdramatically decreased.


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Solution?

The system adds background or comfort noise when the transmitter is turned

off, because complete silence can be very disconcerting for the listener.

Accordingly, this is added as appropriate. The noise is controlled by the

Silence Indication Descriptor (SID).


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GSM Mobile Set Block Diagram


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Documents

CNW 609 Gsm Part1 [Compatibility Mode]