What Technologies Are Included In Gsm

Published Date: 02 Nov 2017

ABSTRACT :

In this assignment GSM (Global System for Mobile communications) is discussed which the most important for mobile phones. In GSM, signaling and speech channels are digital and data communication is easy to build into the system GSM is a cellular network,and mobile phones connect to it by searching for cells in the immediate vicinity.There are five different cell sizes in a GSM network—macro, micro, pico, femto and umbrella cells. Every cell varies with the coverage area, according to the implementation environment. GSM networks works on different frequency ranges Most 2G GSM networks works in the 900 MHz or 1800 MHz bands. 900MHz GSM works with the combination of both TDMA and FDMA. It uses eight time slots, hence one carrier can support eight full rate or sixteen half rate channels. Channel separation is 200kHz with mobile transmit channels in the range 890 to 915MHz and mobile receive channels in the range 935 to 960MHz. Peak output power of the transmitters depends on the class of the mobile station and can be 0.8, 2, 5, 8, or 20 watts.GSM is based on digital cellular networks which have some advantages because it has Greater spectrum which is efficiently working.

Contents 2

What technologies are Included in GSM? 3

IMPLEMENTATION: 5

MSK (MINIMUM SHIFT-KEYING) 6

GMSK (GAUSSIAN-MINIMUM SHIFT-KEYING) 9

MATLAB IMPLEMENTATION OF GMSK : 10

RESULTS: 13

APPLICATION: 15

What technologies are Included in GSM?

GSM uses multiple access technology like FDMA/TDMA and CDMA

TDMA: With time division multiple access simultaneous conversations are supported by users transmitting in short bursts at different times or 'slots'.

FDMA: In frequency division multiple access, the total band is split into narrow frequency sub-bands and a channel is allocated exclusively to each user during the course of a call. One is used for transmission and one for reception.

CDMA: Code division multiple access allows all users, the access to all frequencies with the allocated band. A single user is extracted from the mayhem by looking for each user's individual code using a correlator. Although not selected for the current generation of mobile digital technologies, CDMA holds much promise as the future technology of choice for GSM replacement in the next century.

• GSM uses frequency division duplexing.

• Channel for uplink is from 890 - 915 MHz

• Channel for downlink is from 935 - 960 MHz

• Distance b/w the frequencies used for uplink and downlink (duplex distance) is 45 MHz

• Frequency difference between adjacent allocations in a frequency plan (channel spacing) is 200 kHz.

• Total number of frequencies are equal to 124

• Bit rate of each channel is 270.9 kbits /s

• Duration of data frame in GSM is 4.615 ms

ARCHITECTURE OF GSM NETWORK:

The GSM network can be divided into four main parts:

The Mobile Station (MS).

The Base Station Subsystem (BSS).

The Network and Switching Subsystem (NSS).

The Operation and Support Subsystem (OSS).

IMPLEMENTATION:

Modulation scheme which is used in GSM is GMSK which is based on MSK. MSK uses linear phase changes and is spectral efficient.

Block diagram of GMSK generator:

Some of the properties of the GMSK are

Improved spectral efficiency

Power Spectral Density

Reduced main lobe over MSK

Requires more power to transmit data than many comparable modulation schemes

Before the GMSK can be explained, some fundamentals of Minimum Shift Keying (MSK) must be known.

MSK (MINIMUM SHIFT-KEYING)

MSK uses changes in phase to represent 0's and 1's, but unlike most other keying

schemes we have seen in general, the pulse sent to represent a 0 or a 1, not only depends on what information is being sent, but what was previously sent.

Following is the pulse used in MSK

Where

                       

if a '1' was sent

                        

if a '0' was sent

To see how this works assume that the data being sent is 111010000, then the phase of the signal would fluctuate as seen below

In order to see the signal constellation diagram consider the following equations

which can be simplified as

Where

and

 

Thus the equations for s1 and s2 depend only on and with each taking one of two possible values. Therefore there are 4 different possibilities:

Therefore the signal constellation diagram will be

Advantages of MFSK

MSK produces a power spectrum density that falls off much faster compared to the spectrum of QPSK. While QPSK falls off at the inverse square of the frequency, MSK falls off at the inverse fourth power of the frequency. Thus MSK can operate in a smaller bandwidth compared to QPSK

GMSK (GAUSSIAN-MINIMUM SHIFT-KEYING)

Even though MSK's power spectrum density falls quite fast, it does not fall fast enough so that interference between adjacent signals in the frequency band can be avoided. To take care of the problem, the original binary signal is passed through a Gaussian shaped filter before it is modulated with MSK.

The principle parameter in designing an appropriate Gaussian filter is the time-bandwidth product WTb. Following figure shows the frequency response of different Gaussian filters. MSK has a time-bandwidth product of infinity.

As can be seen that GMSKs power spectrum drops much quicker than MSK's. Furthermore, as WTb is decreased, the roll-off is much quicker

In the GSM standard a time-bandwidth product of 0.3 was chosen as a compromise between spectral efficiency and inter symbol interference. With this value of WTb, 99% of the power spectrum is within a bandwidth of 250 kHz, and since GSM spectrum is divided into 200 kHz channels for multiple access, there is very little interference between the channels.

The speed at which GSM can transmit at, with WTb=0.3, is 271 kb/s. It cannot go faster, since that would cause inter-symbol interference.

MATLAB IMPLEMENTATION OF GMSK :

clc;

clear all;

close all;

DRate = 1; % data rate is 1 bit in one second

M = 18; % no. of samples per bit

N = 36; % no. of bits for simulation [-18:18]

BT = 0.5; % Bandwidth*Period (cannot change)

T = 1/DRate; % data period, i.e. 1 bit in one second

Ts = T/M;

k = [-18:18];

alpha = sqrt(log(2))/(2*pi*BT); % alpha calculated for the gaussian filter response

h = exp(-(k*Ts).^2/(2*alpha^2*T^2))/(sqrt(2*pi)*alpha*T); % Gaussian Filter Response

% in time domain

figure;

plot(h)

title('Response of Gaussian Filter');

xlabel( 'Sample at Ts');

ylabel( 'Normalized Magnitude');

grid;

bits = [zeros(1,36) 1 zeros(1,36) 1 zeros(1,36) -1 zeros(1,36) -1 zeros(1,36) 1 zeros(1,36) 1 zeros(1,36) 1 zeros(1,36)];

% Modulation

m = filter(h,1,bits); % bits are passed through the all pole filter described by h, i.e bits

% are shaped by gaussian filter

t0=.35; % signal duration

ts=0.00135; % sampling interval

fc=200; % carrier frequency

kf=100; % Modulation index

fs=1/ts; % sampling frequency

t=[0:ts:t0]; % time vector

df=0.25; % required frequency resolution

int_m(1)=0;

for i=1:length(t)-1 % Integral of m

int_m(i+1)=int_m(i)+m(i)*ts;

end

tx_signal=cos(2*pi*fc*t+2*pi*kf*int_m); % it is frequency modulation not the phase

% modulating with the integral of the signal

x = cos(2*pi*fc*t);

y = sin(2*pi*fc*t);

figure;

subplot(3,1,1)

stem(bits(1:200))

title('Gaussian Filtered Pulse Train');

grid;

subplot(3,1,2)

plot(m(1:230))

title('Gaussian Shaped train');

xlim([0 225]);

subplot(3,1,3)

plot(tx_signal)

title('Modulated signal');

xlim([0 225]);

% Channel Equalization

channel=randn(256,1);

h = channel;

N1 = 700;

x1 = randn(N1,1);

d = filter(h,1,x1);

Ord = 256;

Lambda = 0.98;

delta = 0.001;

P = delta*eye(Ord);

w = zeros(Ord,1);

for n = Ord:N1

u = x1(n:-1:n-Ord+1);

pi = P*u;

k = Lambda + u'*pi;

K = pi/k;

e(n) = d(n) - w'*u;

w = w + K *e(n);

PPrime = K*pi';

P = (P-PPrime)/Lambda;

w_err(n) = norm(h-w);

end

figure;

subplot(3,1,1);

plot(w);

title('Channel Response');

subplot(3,1,2);

plot(h,'r');

title('Adaptive Channel Response');

rcvd_signal = conv(h,tx_signal);

subplot(3,1,3);

plot(rcvd_signal);

title('Received Signal');

eq_signal = conv(1/w,rcvd_signal);

figure;

subplot(3,1,1);

plot(eq_signal);

title('Equalizer Output');

subplot(3,1,2);

plot(eq_signal);

title('Equalizer Output');

axis([208 500 -2 2]);

subplot(3,1,3);

plot(tx_signal,'r');

title('Modulated Signal');

% Demodulation

eq_signal1 = eq_signal(200:460-1);

In = x.*eq_signal1;

Qn = y.*eq_signal1;

noiseI = awgn(In,20);

noiseQ = awgn(Qn,20);

I = In + noiseI;

Q = Qn + noiseQ;

LP = fir1(32,0.18);

yI = filter(LP,1,I);

yQ = filter(LP,1,Q);

figure;

subplot(2,1,1);

plot(yI);

title('Inphase Component');

xlim([0 256]);

subplot(2,1,2);

plot(yQ);

title('Quadrature Component');

xlim([0 256]);

Z = yI + yQ*j;

demod(1:N) = imag(Z(1:N));

demod(N+1:length(Z)) = imag(Z(N+1:length(Z)).*conj(Z(1:length(Z)-N)));

xt = -10*demod(1:N/2:length(demod))

xd = xt(4:2:length(xt))

figure;

stem(xd)

title('Demodulated Signal');

RESULTS:

APPLICATION:

GMSK is a technique applied on MSK, which is advantageous as it produces a power spectrum density that falls off much faster compared to the spectrum of QPSK. While QPSK falls off at the inverse square of the frequency, MSK falls off at the inverse fourth power of the frequency. Thus MSK can operate in a smaller bandwidth compared to QPSK. Furthermore, the MSK signals are orthogonal and minimal distance, so, the spectrum can be more compact. As a result, the detection scheme can take advantage of the orthogonal characteristics.

A primary advantage which MSK has over GMSK is low inter-symbol interference, which makes it a more reliable detection scheme as compared to GMSK. But the fundamental problem with MSK is that the spectrum has side-lobes extending well above the data rate (shown in the below figure). For wireless systems which require more efficient use of RF channel BW, it is necessary to reduce the energy of the upper side-lobes.

A solution to this problem is to use a pre-modulation filter, for which an efficient and more realistic approach is to use Gaussian Filter. By using Gaussian filter, we reduce the side lobes extension at the expense of extended time response, which increases the effect of inter-symbol interference (ISI). This trade-off is depicted in the figure below.

CONCLUSION:

From the above search and discussion i have find out that GMSK is very much efficient and have several advantages. It has faster power spectrum density. it can operate on smaller bandwidth and are orthogonal and minimal distance, so, the spectrum can be more compact. GMSK is the technique used in GSM Improved spectral efficiency and Power Spectral Density and it has Reduced main lobe over MSK and it Requires more power to transmit data than many comparable modulation schemes.