CDMA & GSM

GMSK Continuous Phase FSK. MSK and need for filtering. GMSK and its properties.

GSMGSM Speech Coding GSM Channel Coding Error Detecting Codes Convolutional Coding / Decoding Interleaving / De-interleavingCiphering / Deciphering Modulation / Demodulation

WHAT IS CDMA ?

CDMA stands for CODE DIVISION MULTIPLE ACCESS

 It is a digital cellular technology that uses spread-spectrum techniques.

Unlike competing systems, such as GSM, that use TDMA, CDMA does not assign a specific frequency to each user. Instead, every channel uses the full available spectrum. 

HOW IT WORKS ?

Encode sender0 vector0=(1,–1),

data0=(1,0,1,1)=(v,–v,v,v)

encode0=vector0.data0 Encode0=(1,–1).(1,–

1,1,1) encode0=((1,–1),(–1,1),

(1,–1),(1,–1)) signal0=(1,–1,–1,1,1,–

1,1,–1)

Encode sender1 vector1=(1,1),

data1=(0,0,1,1)=(–v,–v,v,v)

encode0=vector0.data0 encode1=(1,1).(–1,–

1,1,1) encode1=((–1,–1),(–1,–

1),(1,1),(1,1)) signal1=(–1,–1,–1,–

1,1,1,1,1)

Because signal0 and signal1 are transmitted at the same time into the air, they add to produce the raw signal:(1,–1,–1,1,1,–1,1,–1) + (–1,–1,–1,–1,1,1,1,1) = (0,–2,–2,0,2,0,2,0)

This raw signal is called an interference pattern .The receiver then extracts an intelligible signal for any known sender by combining the sender's code with the interference pattern, the receiver combines it with the codes of the senders. The following table explains how this works and shows that the signals do not interfere with one another:

Decode sender0

• vector0=(1,–1), pattern=(0,–2,–2,0,2,0,2,0)

• decode0=pattern.vector0• decode0=((0,–2),

(–2,0),(2,0),(2,0)).(1,–1)• decode0=((0+2),

(–2+0),(2+0),(2+0))• data0=(2,–2,2,2), meaning

(1,0,1,1)

Decode sender1

• vector1=(1,1), pattern=(0,–2,–2,0,2,0,2,0)

• decode1=pattern.vector1• decode1=((0,–2),

(–2,0),(2,0),(2,0)).(1,1)• decode1=((0–2),

(–2+0),(2+0),(2+0))• data1=(–2,–2,2,2),

meaning (0,0,1,1)

Further, after decoding, all values greater than 0 are interpreted as 1 while all values less than zero are interpreted as 0. For example, after decoding, data0 is (2,–2,2,2), but the receiver interprets this as (1,0,1,1).

Advantages of CDMA over other techniques

Efficient Practical utilization of Fixed Frequency Spectrum

Flexible Allocation of Resources Privacy protection in Spread Spectrum

CDMA due to anti-jamming capabilities of PN sequences

Gaussian Filtered Multiple Shift Keying (GMSK)

Gaussian minimum shift keying or GMSK is a continuous-phase frequency-shift keying modulation scheme.

It is similar to standard minimum-shift keying (MSK)

An important application of GMSK is GSM.

Non-Continuous & Continuous Phase FSK

Continuous Phase Frequency Shift Keying

Carrier frequencies for logic 0 & 1 are synchronized with input binary rate so that the transition between 0 to 1 (or 1 to 0) does not cause an abrupt phase discontinuity.

This is achieved by selecting carrier frequencies which are integral multiples of one-half the bit rate.

Minimum Shift Keying MSK is a typical continuous phase FSK

(CPFSK) method where the frequency changes occur at the carrier zero crossings.

MSK is unique due to the relationship between the frequencies used for logic 0 and 1.

The difference between the frequencies is always ½ the bit rate.

This is the minimum frequency spacing that allows the 2 carriers to be coherently orthogonal.

Minimum Shift Keying

The baseband modulation starts with a bitstream of 0s and 1s and a bit-clock.

The baseband signal is generated by using NRZ encoding.

This signal is then frequency modulated to produce the complete MSK signal.

Example of MSK 1200 bits/sec baseband MSK data signal Frequency spacing = 600Hz

Modulation Index for MSK

Modulation index, m = Δf x Twhere,Δf = |flogic 1 – flogic 0| = ½(bit rate)

T = 1/bit rateTherefore, m = ½

Advantages of MSK Since the carrier signals are

orthogonal and at minimal distance, the spectrum can be more compact.

The detection scheme can take advantage of the orthogonal characteristics.

Due to smooth transition between the frequencies, probability of error while detection is reduced.

Need for filtering Using an unfiltered binary data stream to

modulate an RF carrier will produce an RF spectrum of considerable bandwidth.

Filtering reduces the energy of the side-lobes.

Gaussian Filtering

The premodulation LPF should have the following properties:-

Narrow bandwidth and sharp cutoff. No overshoot and minimum rise and fall

time for a step input. Minimum group delay.

Gaussian LPF satisfies all the above described characteristics.

Gaussian Filtered Multiple Shift Keying (GMSK)

GMSK implemented by Frequency Shift Keying modulation with FM-VCO.

Gaussian Filtered Multiple Shift Keying (GMSK)

GMSK Properties

Constant envelope. Improved spectral efficiency. Reduced main lobe over MSK. Narrow bandwidth and sharp cut-off. Requires more power to transmit

data than many comparable modulation schemes.

Global System for Mobile communications (GSM)

INTRODUCTION

digital cellular communications system

It was designed to be compatible with ISDN systems

services provided by GSM are a subset of the standard ISDN services

(speech is the most basic).

functional architecture of a GSM system

divided into the Mobile Station(MS), the Base Station (BS), and the Network Subsystem (NS).

The MS is carried by the subscriber the BS subsystem controls the radio link with

the MS the NS performs the switching of calls

between the mobile and other xed or mobile network users as

well as mobility management.

Radio Transmission Aspects

For the GSM-900 system1 , two frequency bands have been made available

890 - 915 MHz for the uplink (direction MS to BS)

935 - 960 MHz for the downlink (direction BS to MS).

The 25 MHz bands are then divided into 124 pairs of frequency duplex channels with 200 kHz carrier spacing using Frequency Division Multiple Access (FDMA).

One or more carrier frequencies are assigned to individual Base Station (BS) and a technique known as Time Division Multiple Access (TDMA)

is used to split this 200 kHz radio channel into 8 time slots (which creates 8 logicalchannels).

A logical channel is therefore dened by its frequency and the TDMA frame time slot number. By employing eight time slots, each channel transmits the digitized speech in a series of short bursts: a GSM terminal is only ever transmitting for one eighth of the time.

8-slot TDMA together with the 248 physical half-duplex channels corresponds to a total of 1984 logical half-duplex channels.

This corresponds to roughly 283 (1984 / 7)logical half-duplex channels per cell.

Seven sets of frequencies are sucient to cover an arbitrarily large area, providing that the repeat-distance d is larger than twice the maximum radius r covered by each transmitter.

Each of the frequency channels is segmented into 8 time slots of length 0.577 ms

The 8 time slots makes up a TDMA frame of length 4.615 ms (120/26 ms).

The recurrence of one particular time slot every 4.615 ms makes up one basic channel.

The GSM system

traffic channels (used for user data)

control channels(reserved for network management messages)

only the Trac Channel/Full-Rate Speech (TCH/FS) used to carry speech at 13 kbps

TRAFFIC CHANNELS TCHs for the uplink and downlink are

separated in time by 3 burst periods a group of 26 TDMA frames The length of a 26-frame multiframe is 120

ms, which is how the length of a burst period is dened (120 ms / 26 frames / 8 burst periods per frame).

Out of the 26 frames, 24 are used for trac, one is used for the Slow Associated Control Channel (SACCH) and one is currently unused

Data are transmitted in bursts which are placed within the time slots. The trans-mission bit rate is 271 kb/s (bit period 3.79 microseconds).

(bit period 3.79 microseconds). The burst is the transmission

quantum of GSM

From Speech to Radio Waves

The GSM Speech Coding

The GSM Speech Coding The full rate speech codec in GSM is described as

Regular Pulse Excitation with Long Term Prediction (GSM 06.10 RPE-LTP).

the encoder divides the speech into short-term predictable parts, long-term predictable part and the remaining residual pulse.

it encodes that pulse and parameters for the two predictors

The decoder reconstructs the speech by passing the residual

pulse rst through the long-term prediction lter, and then through the short-term predictor,

The GSM Channel Coding Channel coding introduces redundancy

into the data ow in order to allow the detection or even the correction of bit errors introduced during the transmission

The speech coding algorithm produces a speech block of 260 bits every 20 ms (i.e.bit rate 13 kbit/s).

In the decoder, these speech blocks are decoded and converted to 13 bit uniformly coded speech samples.

The 260 bits of the speech block are classied into two groups.

The 78 Class II bits are considered of less importance and are unprotected

The 182 Class I bits are split into 50 Class Ia bits and 132 Class Ib bits

Class Ia bits are rst protected by 3 parity bits for error detection

Class Ib bits are then added together with 4 tail bits

before applying the convolutional code with rater = 1/2 and constraint length K = 5.

The resulting 378 bits are then added to the 78 unprotected Class II bits resulting in a complete coded speech frame of 456 bits

Error Detecting Codes enable assessment of the correctness of the bits

which are more sensitive to errors in the speech frame

If one of these bits are wrong, this may create a loud noise

Detecting such errors allows the corrupted block to be replaced by something less disturbing

polynomial representing the detection code for category Ia bits is G(X) = X3 +X + 1.

At the receiving side, the same operation is done and if the remainder diers, an error is detected and the audio frame is eventually discarded.

Convolutional Coding / Decoding Convolutional coding consists in transmitting the results

of convolutions of the source sequence using dierent convolution formula

The GSM convolutional code consists in adding 4 bits (set to \0") to the initial 185 bit

sequence and then applying two different convolutions: polynomials are respectively G1(X) = X4+X3+1 and G2(X) =X4 +X3+X +1. The final result is composed of twice 189 bits sequences.

The GSM convolutional coding rate per data ow is 378 bits each 20 ms, i.e.: 18.9 kb/s. However, before modulate this signal, the 78 unprotected Class II bits are added

So, the GSM bit rate per ow is 456 bits each 20 ms i.e. 22.8 kb/s.

Interleaving / De-interleaving

Interleaving is meant to decorrelate the relative positions of the bits respectively in the code words and in the modulated radio bursts.

to avoid the risk of loosing consecutive data bits.

GSM blocks of full rate speech are interleaved on 8 bursts:

the 456 bits of one block are split into 8 bursts in sub-blocks of 57 bits each.

A sub-block is dened as either the odd- or the even-numbered bits of the coded data within one burst.

Each sub-blocks of 57 bit is carried by a different burst and in a different TDMA frame

a burst contains the contribution of two successive speech blocks A and B.

bits of block A use the even positions inside the burst and bits of block B, the odd positions

De-interleaving consists in performing the reverse operation.

The major drawback of interleaving is the corresponding delay: transmission time from the first burst to the last one in a block is equal to 8 TDMA frames (i.e. about 37 ms).

Ciphering / Deciphering A protection has been introduced in GSM by

means of transmission ciphering Theciphering method does not depend on the

type of data to be transmitted (speech, user data or signaling) but is only applied to normal bursts.

Ciphering is achieved by performing an \exclusive or" operation between a pseudo-random bit sequence and 114 useful bits of a normal burst

Deciphering follows exactly the same operation.

Modulation / Demodulation

GSM uses the Gaussian Modulation Shift Keying (GMSK) with modulation index h =0:5, BT (lter bandwidth times bit period) equal to 0.3 and a modulation rate of 271(270 5/6) kbauds.

References Baier, A., Heinrich, G. and Wellens, U. \Bit

synchronization and timing sensitivityin Adaptive Viterbi Equalizers for narrowband-TDMA digital mobile radio systems", Proc. of the 38th IEEE Vehicular Technology Conference, 1988, pp. 377-384.

Forney, G.D. JR. \The Viterbi Algorithm" Proc. of the IEEE, Vol. 61, No 3, Mar.1973, pp. 268-278.

Karn, P. \Convolutional decoders for amateur packet radio", to appear at the 1995 ARRL Digital Communications Conference

Lycksell, E. \GSM System overview", Swedish Telecom Radio Internal Report, Jan.1991.

Margrave, D. \Computer Simulation of the Radio Channel Aspects of the GSMSystem", ECE 798 Research Project, see URL \http://www.utw.com/ dmargrav/gsm/ece798/paper.html", Nov. 1995.

Murota, K. and Hirade, K. \GMSK Modulation for Digital Radio Telephony", IEEE Trans. on Communications, Vol. com-29, No 7, July 1981, pp. 1044-1050.

Mouly, M. and Pautet, M-B. \The GSM System for Mobile", ISBN: 2-9507190-0-7,1992.

References (GMSK) Digital Communications-Bernard

Sklar Electronic Communications

Systems-Wayne Tomasi Practical GMSK Data Transmission-

Fred Kostedt, James C. Kemerling lEEE TRANSACTIONS ON

COMMUNICATIONS VOL. COM-29, NO. 7, JULY 1981

Digital Microwave Modulator - Bernard Darnton and John L. Bahr

The choice of a digital modulation scheme – Pritpal Singh Mudra, T. L. Singal, Rakesh Kapoor.

http://www.radio-electronics.com/info/rf-technology-design/pm-phase-modulation/what-is-gmsk-gaussian-minimum-shift-keying-tutorial.php

http://en.wikipedia.org/wiki/Minimum-shift_keying

http://en.wikipedia.org/wiki/Gaussian_filter