GMSK with GSM

GSM What ?

GSM: Global System for Mobile Communications. Formerly: Group Special Mobile

When ? 1982: GSM created to set standard. 1988: Industrial development started. 1991: First system deployed.

Why ? Higher digital voice quality. Low cost alternatives for making calls and sms

Architecture Diagram :

MS: Mobile equipment

Handset + SIM +

SIM:Subscriber Identification Module

Base Transceiver Station:

Time and frequency synchronizing

Decoding, decrypting

Uplink channel measurements

Base station Controller :

Connect between the mobile and the MSC.

Time and frequency synchronization

Power management

BSS consists of three element

BTS ( Base transceiver station)

BSC ( Base station controller )

TC ( Tran coder)

Network Switching Subsystem (NSS) NSS typically consists of following

elements MSC (Mobile switching center)

HLR (Home location register )

VLR ( visitor location register )

AUC (Authentication center )

EIR ( Equipment identity register)

GMSC ( Gate way mobile switching center )

HLR:Function

1. Subscriber identity

2. Subscriber supplementary services

3. Subscriber location information

4. Subscriber authentication information

VLR : It is a temporary database .

Stores only information of subscriber those are in that

MSC.

Every MSC has its own VLR

When a subscriber roams into a new MSC service area, the

VLR connected to that MSC requests information about

the subscriber from the subscriber's HLR.

EIR : Data base used to store mobile identity

Help us blocking calls from stolen MS

AUC is a database connected to the HLR

Function is to authenticate the subscribers attempting to use a network.

AUC:

MSK modulation is one type of FSK modulation, but it occupies even less bandwidth than FSK modulation so the bandwidth is interrupted by very little noise. The fact that noise is low means that it has a high carrier-to-noise ratio and good receiver sensitivity which gives it a long range with stable communication. The modulation index of sub-carrier MSK modulation is MSK modulation, so it’s m = 0.5.

GMSK modulation is based on MSK, which is itself a form of continuous-phase frequency-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: Minimum shifting keying .

MSK gives us half cycle sinusoidal wave for a symbol

Gaussian Minimum Shift Keying.

Continuous phase digital frequency modulation

Modulation index h=1/2

Gaussian Frequency Shaping Filter

GMSK = MSK + Gaussian filter

Characterized by the value of BT T = bit duration

B = 3dB Bandwidth of the shaping filter

BT = 0.3 for GSM Bandwidth Time

BT = 0.5 for DECT (Digital Enhanced Cordless Telecommunications)

Data rate: 270.8 kbps

GMSK:

Spectral density of MSK and GMSK signals

Signal using MSK modulation

Voltage control oscillator

GMSK generator

Generating GMSK using a Gaussian filter and VCO

GMSK BIT ERROR RATE Bit error probability for GMSK is

GMSK PROPERTIES Improved spectral efficiency.

Power Spectral Density.

Reduced main lobe over MSK

Requires more power to transmit data than many

comparable modulation schemes.

Self synchronizing capability

Constant envelope over entire B.W

Good BER performance

APPLICATION Most widely used in the Global System for Mobile

Communications (GSM).

Used for CDPD (cellular digital packet data)overlay

network.

Used for GPRS system

ADVANTAGES DISADVANTAGES

High spectral efficiency.

Reducing sideband power.

Excellent power efficiency due to constant envelope.

Good choice for voice modulation.

Higher power level than QPSK.

Requiring more complex channel equalization algorithms such as an adaptive equalizer at the receiver.

Inter symbol interference