WC NOTS Final Unit New -7

Wireless Communication: Unit 7 - Wireless Digital Modulation Techniques & Hardware

Prof. Suresha V, Dept. Of E&C E. K V G C E, Sullia, D.K-574 327 Page 1

UNIT-7

Wireless Digital Modulation Techniques & Hardware

Learning Objectives: Upon completion of this unit, the student should be able to

This unit deals more deeply into the air interface of wireless mobile systems.

Discuss the general characteristic of wire line and fiber-optic transmission lines.

By comparison of wire line transmission and wireless transmission it is felt that

complex coding schemes require for wireless systems to combat transmission errors.

Modern digital encoding techniques with their inherent spectrum efficiencies and their

ability to mitigate radio channel impairments.

Also presents system enhancement techniques such as antenna diversity and rake

receivers will be used to improve wireless system quality and transmission rates.

Explain the basic fundamental concepts of digital modulation technique and their

advantages

Discuss the basic principles behind the operation of ultra-wideband radio technology.

Discuss the typical GSM BSC and RBS hardware found at a modern cell site.

7. 1 Transmission characteristics of wire line

Two commonly used wire line transmission are

1. Conductor based transmission lines

2. Fiber optics transmission lines

1. Conductor based transmission lines(TL) characteristics:

TL characteristics to consider are bandwidth, susceptibility to noise and frequency

response.

These channels are more reliable channel than the typical wireless radio channel.

These lines are frequency dependent, i.e

o At low frequencies current flows within the conductors with no radiation.

o Higher frequencies, the current flow takes place near the conductor surface

o At radio frequencies and higher, the transmission line acts as the structure

that guides electromagnetic waves.

It acts like low pass filters, there signal attenuations increases with frequency.

It provides differing levels of bandwidth maximum transmission rate and reliability.

Common types of wire line are unshielded and shielded twisted pair (UTP and STP).

Some applications are used in local-loop connection to the telephone central office,

LAN connectivity, and broad band cable TV service, etc..



2. Fiber optics transmission lines characteristics:

It is highly used dielectric wire line transmission media.

It carries the signal in the form of light

Basic principle of transmission is based on total internal reflection.

It consists of three layers (core, cladding and outer jacket) and made of glass or

plastics materials.

Advantages are offers very high B.W, Low noise, Safe and secure, Support for High

data rates (Gbps).BER is extremely low. Low cost easy to install and maintain. Etc.

7. 2. Characteristics of the Air interface

Less reliable channel than the typical wire line channel.

Wireless signal means EM signal called Radio wave signals.

Radio wave signals propagation are frequency depended.

Wave propagation below 2 MHz called ground waves tend to travel on curvature of

the earth surface and lose strength fairly rapidly as the distance it travels.

Wave propagation between 2 and 30 MHz propagate as sky waves. Bouncing back

from the ionosphere layers.

Above 30 MHz tend to travel in straight-line or rays, therefore limited in their

propagation by the curvature of the earth.

EM propagation depends on antenna size and penetration of the structures.

Wave propagation effects at UHF and above are Reflection, Scattering, Diffraction

and Other factors

Wave propagation takes Multipath propagation during non- line off sight(NLOS)

between the transmitter and receiver

For Indoor and outdoor propagation examples shown in Figure 7-1 and Figure 7-2

Fig 7.1: Typical outdoor propagation case



Fig 7.2: Typical indoor propagation case

Note: Multipath propagation and multipath fading is common in wireless communication

Path loss models for various coverage areas***

(July-2014-5M, July-2013-10M, July-2011-8M)

Path-loss models are used to predict the average received signal strength at receiver for

given transmitted power at a distance d.

Types of Path loss model

1. Free space model

2. Two-ray model

3. Okumura model

4. Okumura-Hata model. Etc

1. Free space propagation model***:

This model is used to predict received signal strength when the transmitter and receiver

have a clear line-of-sight path between them. Examples

o Satellite communication

o Microwave line-of-sight radio link

The received signal power at distance d (Friis free space equation)

Where Pt : transmitted power, d : T-R separation distance (m)

Pr : Received power, Gt: transmitter antenna gain , : =c/f

Gr : receiver antenna gain

Limitation: It does not give accurate result when applied to mobile radio environments.

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Example 1 :**(Dec-210-10M)

What is the received power in dBm for a signal in free space with a transmitting power of

1W, frequency of 1900 MHz, and distance from the receiver of 1000 meters if the

transmitting antenna and receiving antenna both use dipole antennas with gains of

approximately 1.6? What is the path loss in dB?

Solutions:

o Given Pt = 1 W, f = 1900MHz, d = 1000 mts, Gt = Gr = 1.6, Path Loss PL in db = ?

Pr in dBm = ?

o Use Friis equation

Pr = Pt Gt Gr 2 /(4)2 d2

where = c/f = 3 108 / 1900 106

= 0. 15789 mts

o Pr in watts = 1 1.6 1.6 (0.15789)2/ (4)2 ( 1000)2 = 4.042 10-10 W or 4042nW

o Pr in dBW = 10 log (0. 4042 10-9) = - 93.934 dB

o Pr in dBm = 10 log (0. 4042 10-9 103 ) = - 63.934 dBm.

o Path Loss PL in watt = Pt Pr = 1 - 0. 4042 10-9 1W

o Path Loss PL in dB = 10 log(1) = 0 db

o Path Loss PL in dBm = 10 log(1 103) = + 30dBm

Example 2**( July-2011-8M)

Find the received power at distance of 1km for a transmitting 900MHz.Assume transmitting

and receiving antenna gains as zero dB.

Solutions:

o Given Pt is not given, assume = 1 W, f= 900MHz, d = 1km, Gt = Gr = 0 dB or 1 W

Find Pr =? & Path Loss PL in db and dBm =?

o Use Friis equation

Pr = Pt Gt Gr 2 /(4)2 d2

where = c/f = 3 108 / 900 106

= 1/3 mts

o Pr in watts = 1 11 (1/3)2/ (4)2 ( 1000)2 = 7.036 10-10 W or 0.7036 nW

o Pr in dBW = 10 log (0.7036 10-9) = - 91.52 dB

o Pr in dBm = 10 log (0.7036 10-9 103 ) = - 61.526 dBm.

o Path Loss PL in watt = Pt Pr = 1 - 0.7036 10-9 1W

o Path Loss PL in dB = 10 log(1) = 0 db

o Path Loss PL in dBm = 10 log(1 103) = + 30dBm



2. Two-Ray model***: (Jan-2015-7M)

It is also called Ground Reflection Model

It is the simple approximation model for a land mobile outdoor environment.

It is reasonably accurate model for predicting large scale signal strength over distance of

several kilometers

In this Model that considers both the direct (LOS) path and a ground reflected path

between transmitter and the receiver.

The equation that approximates 2- ray model

Where ht and hr are the height of the transmitting and receiving antennas.

Analysis: For d >> ht hr

Low angle of incidence allows the earth to act as a reflector .The reflected signal is 180

out of phase. Pr 1/d4

EM wave undergoes an attenuation of -6 db every time the distance it travels doubles.

The approximation equation for path loss using two ray model can be written as

Path Loss = 40 log d (10 log Gt + 10 log Gr + 20 log ht+ 20 log hr)

2. Okumura Model:

This model is one of the most widely used models for signal prediction in urban areas.

Wholly based on measured data - no analytical explanation

It is the simplest & best for in terms of path loss accuracy in cluttered mobile

environment

Common standard deviations between predicted & measured path loss 10dB to 14dB

Useful for

o Frequencies ranging from 150 MHz-1920 MHz

o Frequencies can be extrapolated to 3GHz

o Distances from 1km to 100km

o Base station antenna heights from 30m-1000m

Okumura developed a set of curves in urban areas with quasi-smooth terrain.

This model is fairly good in urban and suburban areas, but not as good in rural areas.

Disadvantage with this model is its slow response to rapid changes in terrain.



3. Hata Model:

It is an empirical formulation of the graphical path loss data provided by Okumura and is

valid from 150 MHz to 1500 MHz.

This model has been proven to be accurate and is used by computer simulation tools.

Hata presented the urban area propagation loss as standard formula and supplied

correction equations for application to other situations.

The predictions of the Hata model compare very closely with the original Okumura

model, as long as d exceeds 1 km.

Hata model is well suited for large cell mobile systems, but not PCS which have cell size

on the order of 1 km radius.

4. Multipath and Doppler Effects

o Multipath: It is the propagation phenomenon that results in radio signals reaching the

receiving antenna by two or more paths. It is due to atmospheric ducting, Ionosphere

reflection and refraction. Reflection from water bodies and terrestrial objects such as

mountains and buildings. The effects of multipath include constructive and

destructive interference, and phase shifting of the signal. Destructive interference causes

fading. Path loss models do not address the real time fluctuation in RSS at the Receiver.

Where the magnitudes of the signals arriving by the various paths have a distribution

known as the Rayleigh distribution, this is known as Rayleigh fading. Shown in fig 7.3

Figure: 7.3 Typical Rayleigh fading for mobile radio in the UHF range

o Doppler Effect: There is a change in the frequency due to move in transmitter or

receiver called Doppler Effect. It is due to The Doppler effect and rapidly changing

multipath propagation due to the motion of the mobile itself. Multipath delay spread

leads to both time dispersion and frequency selective fading in the received signal.

Doppler Effect leads to frequency dispersion and time selective fading. Typically both

fading effects modeled as Rayleigh fading, which is shown in figure 7.3



Parameters affecting signal transmission on wireless channel** ( July-11-8M)

o Free space loss

o Transmission Band Width

o Refraction, Diffraction, Reflection.

o Aperture medium coupling loss.

o Absorption

o Radio frequency interference

o Electrical interference

o Environmental factors

o Path loss is also influenced by terrain contours, environment (urban or rural,

vegetation and foliage), propagation medium (dry or moist air),

o The distance between the transmitter and the receiver, and the height and location of

antennas.

7. 3 Wireless Telecommunications Coding Techniques

1. Introduction:

o Wireless radio channel is most unreliable and random characteristics channels.

Hence it is necessary to make the signal more robust before it transmitting through

wireless channels.

o At transmitter increase the transmitted signals immunity to radio channel noise and

other channel impairments like fading and multipath spread.

o In digitally based systems, need techniques correspond to an attempt to realize a

reduction in bit errors and frame errors.

o The best strategy is to employ some form of error detection and correction codes to

reduce the required number of requests for retransmission by the system.

2. Error detection and correction coding

o Errors in wireless systems tend to occur in bursts.

o These codes designed for wireless systems tend to denote a technique that codes the

transmitted bits in a way that attempts to control the overall bit error rate.

o The type of coding used is dependent upon the maximum bit error rate that can be

tolerated.

o Different codes are used to enhance the transmission of packet data over wireless

systems.

1. Block codes

2. Convolutional codes

3. Turbo codes

4. Speech coders



1. Block codes:

It is used to determine whether an error has occurred during data transmission.

Block codes used to correct errors that might have occurred during transmission are

known as Forward Error Correction (FEC) codes.

In block coding, divide message into blocks, each of k bits, called datawords and add r

redundant bits to each block to make the length n = k + r. The resulting n-bit blocks

are called codeword (See figure below).

Additional bits r may be generated through a matrix or Polynomial generator (eg.CRC

code) and added to the original block of bits to form a codeword that will be eventually

transmitted by a system.

Depending upon the type of coding level employed these schemes can both detect and

correct limited numbers of errors.

To transmit voice over a GSM traffic channel a limited number of parity bits are added to

a block of 50bits.

To transmit a message over the control channel, GSM takes a block of 184bits and adds

40 parity check bits to generate a 224bit code word.

2. Convolutional codes:**(July-2013-6M)

Convolutional codes are applied in applications that require good performance with low

implementation complexity. They operate on code streams (not in blocks)

It map information to code bits sequentially by convolving a sequence of information

bits with generator sequences k bits are input, n bits are output. k & n are very small

(usually k=1-3, n=2-6)

Input depends not only on current set of k input bits, but also on past input.

The number of bits which input depends on is called the "constraint length" K.

The ratio of input bits to output bits from the encoder is known as the code rate R of the

encoder.

In cdma2000 system a convolutional encoder with R=1/3 and K=9 is used.

In practice, the use of convolutional encoders provides better FEC capabilities than

available from block codes.



Figure below shows in block diagram form an implementation of a convolutional

encoder (with K=9 and R=1/2) specified for use in cdma2000

Block v/s Convolutional Codes

o Block codes take k input bits and produce n output bits, where k and n are large

o There is no data dependency between blocks. Useful for data communications

o Convolutional codes take a small number of input bits and produce a small number

of output bits each time period

o Data passes through convolutional codes in a continuous stream .Useful for low-

latency communications

3. Turbo codes: :**( July-2013-6M) Turbo encoders are a modified form of combined convolutional encoders that can be

used to create a new class of enhanced error correction codes.

It is constructed from two systematic, recursive, convolutional encoders connected in

parallel with an interleaver preceding the input to the second convolutional encoder.

The output bit steams of the two convolutional encoders are multiplexed together and

repeated to form the final code symbols.

For cdma2000, Rate 1/2, 1/3, 1/4 and 1/5 turbo encoders are employed instead of

convolutional encoders for various higher-bit transfer rates and radio configurations.

4. Speech coders:

The speech coders used for both GSM and CDMA wireless system.

Speech coder take 20-msec segments and process it into lower-bit-rate digitally encoded

speech in preparation for its transmission over the air interface

Two broad classifications of speech coders:

1. Waveform coders: Example PCM at the 64kbps data rate.

2. Vocoders: QCELP encoder used in IS-95 CDMA or the RPE-LTP encoder used in GSM

In GSM systems, speech may be transmitted at Full rate, Half rate, Enhanced full rate

In CDMA systems, the speech coders may operate at either 9.6 or 14.4 kbps.



Block Interleaving***(Jan-2015-6M, July-2014-5M, Dec-2012-8M, , July-2011-5M)

It is a technique used by mobile wireless systems to combat the effects of bit errors

introduced during transmission of frames.

The basic idea here is that the error control code used by the system may be able to

correct one bit error out of a block of 8 bits. However, it is not able to correct a burst of

say six errors within the 8 bit block.

If the bits of the block can be interleaved with the bits from other blocks, then the burst

of six errors can be spread out over six other blocks and the ECC can correct each of the

single bit errors in each of the six blocks.

Figure 7.5 depicts this process for several noise bursts.

Figure 7.5 Typical block interleaving scheme

Examples of coding and interleaving

o A block diagram of the GSM channel encoding system is shown by figure below

o The coding process consists of the following steps as indicated by figure 7.6.

o The Coding process consists of following steps:

o The 260 bits delivered by the full-rate coder are divided into

182 bits of class 1 (protected bits) and

78 bits of class 2 (unprotected bits).

o The 50 most important bits of class 1(class 1a bits) are protected by 3 parity bits as

shown in the second row of figure 7.6



Figure 7.6: Detail steps of GSM channel encoding for voice traffic

o The 78 class 2 bits are separated from the class 1a, 1b, and CRC bits.

o These Class 1 bits are now partitioned and reordered as shown in row three of the figure

and applied to an R = 1/2 convolutional encoder.

o The outputs of the bits from the encoder are combined with the 78 Class 2 bits to yield a

456-bit packet.

The interleaving process consists of following steps:

o The 456 coded bits are now interleaved over eight half subframes of 57 bits as shown by

Figure 7-7.

o Each group of 57 bits goes into a half subframe of a normal traffic burst.

o Another level of interleaving occurs as the user data is prepared to be transmitted over

the air interface.

Figure 7.7: GSM interleaving of encoded voice data



o The user's 456-bit, 20-msec frame consisting of eight subframes is interleaved with

other user's data over a sequence of normal traffic bursts.

o Figure below depicts this process. If a severe fade occurs, its effect will be spread out

over the traffic of several users.

o At the receiver, a deinterleaving process must be performed to reorder the incoming

bursts of user traffic.

7.4 Digital Modulation Techniques:

Suitable modulation Techniques are used for wireless communication, since wireless

channels are more random, noisy and lot of B.W scarcity.

Spectral efficient modulation schemes are required to meet the required data rates.

Basic modulation schemes like ASK, FSK, PSK ,MSK are not sufficient to meet the

required design goal, Hence modern modulation scheme like n-PSK, n-QAM, OFDM etc

are consider.

1. Quadrature Phase Shift Keying (QPSK):**(July-2014-10M)

Quadrature Phase Shift Keying (QPSK) is a form of Phase Shift Keying in which two bits

(Called dibits) are modulated at once; selecting one of four possible carrier phase shifts

(/4, 3 /4, 5 /4, 7 /4).

QPSK perform by changing the phase of the In-phase (I) carrier from 0 to 180 and the

Quadrature-phase (Q) carrier between 90 and 270.

This is used to indicate the four states of a 2-bit binary code. Each state of these carriers

is referred to as a Symbol.

Figure 8.12 shows the Truth table and constellation diagram for 4-PSK (QPSK). Typical

generic QPSK transmitter shown in figure 7.8

Advantages of QPSK: Spectra efficient modulation techniques. Increased data rate with

same Band Width, since symbol time remains constant and only the number of encoded

bits per symbol increases.



Figure 7.8 is the Typical generic QPSK Transmitter

2. Quadrature Amplitude modulation (n-QAM):

It encodes information in both the phase and amplitude of the transmitted signal.

64-QAM is capable of encoding 6 bits per transmitted symbol or therefore achieving

a bandwidth efficiency of six times.

For pass band modulation schemes, as the value of level of modulation n increases

and the C/I ratio for the channel remains constant, bit error rate will predictably

increase.

64-QAM is not yet used for any commercial wireless systems due to its unacceptable

bit error rate.

It is however specified for use in 5-ghz band for wireless LANs (IEEE 802.11a) and

also for wireless MANs (IEEE 802.16).



3. Digital Frequency Modulation

IG cellular system use FM to provide voice service over 30 KHz channel.

The 2G digital GSM standard use Gaussian minimum shift keying (GMSK) (a form of FSK).

GMSK mitigate adjacent channel interference by reducing the side lobe power of the

transmitted RF signals.

Early GSM is a FDMA-based wireless system with 200-khz-wide channel.

Depending upon the type of digital traffic sent over the radio link, Gaussian filters with

different bandwidth characteristics perform better than others.

GMSK is a popular air interface modulation scheme for 2G wireless radio systems.

4. Digital Phase Modulation

Here signal is encoded in the phase of the transmitted RF signal.

Quadrature PSK or QPSK (n=4) encodes 2 bits per transmitted symbol.

Further enhancements to basic QPSK modulation are possible yielding several QPSK

variants. They are

o Offset QPSK or OQPSK: It reduce fluctuations in the modulated signal amplitude and

the amount of possible phase shift between different symbols. QPSK is used by IS-95

CDMA for the modulation of the forward channels and OQPSK is used for the

modulation of the CDMA reverse channels. CDMA2000 also uses these same basic

modulation schemes but adds 8-PSK and 16-QAM.

o /4-QPSK: This form of QPSK restricts the phase shift between different symbols to

either /4 or 3/4. Figure 8-14 shows the constellation diagram of the possible

symbols of /4-QPSK.

o The above diagram consists of two QPSK constellations overlaid on one another with

a phase shift. It can be seen from the diagram, the transition from one symbol to

another (indicated by the dotted lines) never goes through zero amplitude.

o Therefore, /4-QPSK, like OQPSK, also reduces signal amplitude fluctuations

significantly and thus reduces the magnitude of possible side lobe regeneration.

o /4-QPSK performs better than OQPSK in the presence of multipath spreading and

fading.



5. Orthogonal Frequency Division Multiplexing (OFDM)***

OFDM is a form of multi-carrier, multi-symbol, multi-rate FDM in which the user gets to

use all the FDM channels.

It is a specialized FDM; in which all the carrier signals are orthogonal to each other.

This technique gaining in popularity was chosen for the IEEE 802.11a WLAN.

OFDM Implementation:

o Here instead of attempting N symbols per second over a single forward carrier

link, M carriers (the multicarriers) are used to transmit N/M symbols per second,

which ends up yielding the same data transfer rate, N.

o The frequency spacing between each carrier is chosen to satisfy the orthogonality

criteria.

o For each carrier, a multisymbol digital modulation scheme is used to transmit

more than 1 bit per symbol time. Typically, some form of n-PSK or n-QAM would

be used for this purpose.

Another feature of an OFDM system provides rate adaptation based on C/I ratio.

OFDM advantages

o Can easily adapt to severe channel conditions without complex time-domain

equalization.

o Robust against narrow-band co-channel interference.

o Robust against intersymbol interference (ISI) and fading caused by multipath

propagation.

o High spectral efficiency as compared to conventional modulation schemes, spread

spectrum, etc.

o Efficient implementation using Fast Fourier Transform (FFT).

o Low sensitivity to time synchronization errors.

o Tuned sub-channel receiver filters are not required (unlike conventional FDM).

Disadvantages of OFDM

o Sensitive to Doppler shift.

o Sensitive to frequency synchronization problems.

o High peak-to-average-power ratio (PAPR), requiring linear transmitter circuitry,

which suffers from poor power efficiency.

o Loss of efficiency caused by cyclic prefix/guard interval.



SPREAD SPECTRUM MODULATION TECHNIQUES: (Dec-2012-8M,Dec-210-10M)

o This modulation technique widely used for wireless systems. It is implemented as

some variation of CDMA & 3G GSM/NA-TDMA wireless and WLAN system

o Main advantages of spread spectrum are the ability to overlay with already

deployed radio services.

o Other advantages are extremely good anti-interference characteristics, high

wireless mobile system capacity, and robust and reliable transmission over radio

links in urban and indoor environments that are susceptible to intense selective

multipath conditions.

o There are two basic ways of implementing spread spectrum transmission:

1. Frequency Hopping Spread Spectrum (FHSS)

2. Direct Sequence Spread Spectrum (DSSS).

1. Frequency Hopping Spread Spectrum (FHSS): It consists of a system that changes the

center frequency of transmission on a periodic basis in a pseudorandom sequence. Here

data are transmitted through number of different carrier frequencies hops. All the carrier

frequency hops independent from one another. For the system to work both the transmitter

and receiver must have prior knowledge of the hopping sequence. Figure 8-15 shows an

example of a FHSS system.

As the transmitter implements the hopping sequence the effective signal bandwidth

increases to include all of the utilized carrier frequencies. The use of FHSS does not provide

any improvement in a noise-free environment.

2. Direct Sequence Spread Spectrum (DSSS): Here a spreading code is applied to the

.baseband data stream at the transmitter and the same spreading code is applied to the

received signal to perform demodulation. The number of chips per second now

determines the basic bandwidth of the transmitted signal. DSSS systems improved

noise immunity provided by the increased signal bandwidth. Special orthogonal Walsh

codes are used as part of the spreading process. Walsh code property is increase the

system capacity in a limited amount of frequency spectrum.



Ultra-Wide Band (UWB) Radio Technology:***

UWB radios are extremely wideband radios with very high potential data rates.

This technology is extremely suited for the short range applications, typically 1-10mts.

These systems are able to provide high data rates of 100-500Mbps.

It uses extremely narrow pulses with a fraction of nanoseconds.

These systems can operate either at baseband or at a carrier frequency in the 3.6 to 10.1

GHz range.

Modulation used is PPM.

Application of UWB radio technology are

o Imaging systems

o Vehicular radar

o Measurement and positioning systems

o High data rates wireless PAN

o Future advanced intelligent wireless area networks and wireless sensor networks

DIVERSITY TECHNIQUES***( July-2011-7M)

Basic principle: Diversity is achieved by creating several independent paths between the

transmitter and receiver

Each path fades independently, hence, there is a low chance they fade together

Receiver combines the received signal for the several paths using some method

Diversity is used in all wireless mobile communication systems

Major obstacles to be solved by diversity techniques are:

o Multipath fading: signal is scattered among several paths, each path has a different

time delay.

o Interference: ISI in case of channels with memory + multi-user interference

Types of Diversity techniques:

1. Frequency diversity

2. Time diversity

3. Space diversity

4. Polarization diversity

5. Multipath diversity, etc

1. Space diversity: (Jan-2015-7M)

It is used to improve the mobile wireless system performance.

It is achieved by using multiple transmit and multiple receive antennas with a minimum

separation of /2 between neighboring antennas.

Multiple Tx: Split power over several Tx antennas. More antennas = more power split



Multiple Rx: Collect signal by several Rx antennas. More antennas = more collected

power.

If use directional antennas (typically) larger separation is required. Differently polarized

antennas can also be used. Figure 7.8 shows several practical implementation

Figure 7.8: Space and Polarization diversity antenna scheme

From the above figure, both space and polarization diversity can be used by the

appropriate position of the antenna units. The antenna feed multiple receivers with

strongest received signal being used by the system.

Polarization diversity is used to counter the change in EM signal polarization that can be

induced by the environment during reflection, scattering and so on.

Smart Antennas:

It is one of the 3G specifications.

This technique to improve system performance makes use of phased array or beam

steering antenna system.

Beam steering antenna can use narrow pencil beam patterns to communicate with a

subset of the users within the cell.

Figure 7.9 depicts of a smart antenna system.

Figure 7.9: depiction of a 3G smart antenna system

The narrow beam shown in figure 7.9 may be pointed the users always their moving

direction through the use of sophisticated antenna technology. It will increase the

system performance



RAKE Receiver:*** :**(July-2013-4M, Dec-2012-4M,Dec-210-6M)

It is a radio receiver designed to counter the effects of multipath fading.

It recognizing that multiple signals will arrive at a receiver over the mobile radio

channel.

These receivers isolating the signal paths at the receiver.

It using several "sub-receivers" called fingers, that is, several correlators each

assigned to a different multipath component.

Each finger independently decodes a single multipath component; at a later stage the

contribution of all fingers are combined in order to make the most use of the

different transmission characteristics of each transmission path.

This could very well result in higher S/I ratio in a multipath environment.

The rake receiver is so named because it reminds the function of a garden rake, each

finger collecting symbol energy similarly to how tines on a rake collect leaves.

Figure 7.10 for a block diagram of the structure of a typical RAKE receiver used for

CDMA

Figure 7.10: RAKE receiver block diagram

Few RAKE taps possess the ability to dynamically adjust the taps (move the rake fingers) in response to a search algorithm used to locate multipath components.

These smart receivers standard diversity combining techniques to provide a more reliable receiver output and therefore improve system performance.

There are potential problems with this type of receiver that are tied to the multipath delay and spread introduced to the radio link.

The multipath components that can be resolved have a time dependence that is proportional to the inverse of the system chip rate and the system-tolerated multipath spread is proportional to the inverse of the symbol time.

For the IS-95 CDMA system, using a chip rate of 1.2288 Mcps allows the resolution of multipath components of the order of approximately 1/1.2288 Mcps or 800 ns by the RAKE receiver.

Typical multipath spreads for outdoor is tens of microseconds and for indoor is few ns. In an indoor environment the CDMA RAKE receivers would not able to resolve multipath

components.



7.8 Typical GSM System Hardware:

This section mainly deals with actual hardware implementation of Base Station

System (BSS) of GSM system, which includes

1. Base Station Controller(BSC )

2. Radio Base Station (RBS).

1. Base Station Controller(BSC ) of GSM system: It includes

o Typical GSM BSC block diagram

o Specific BSC parts

o BSC Radio Network Operations

The typical block diagram of BSC with major subsystems as shown in figure 7.11

Figure 7.11: Typical GSM BSC block diagram

Specific BSC parts which perform the function of interfacing the RBS to the MSC and

PDN, They are

1. MUX And Group Switch Unit: It can provide interconnections to the MSC, PDN, or RBSs. A

leased T1 carrier circuits connects the MSC to the BSC and from the BSC to the RBS with

rate of 64 kbps PCM voice signals and call control (LAPD) information messages. The T1

signal carrying twenty-four DSO, 64 kbps voice signals must be demultiplexed at the BSC

to provide to the group switch. Once the voice signals from the PSTN have been

transcoded, they are multiplexed together and forwarded to the proper RBS over T1

facilities at a much lower bit rate.



2. Group Switch: It is used to cross-connect 64-kbps timeslots, placing a call onto the

correct timeslot on the correct communications link to the correct RBS. The

subrate switch is able to switch at submultiples of 64 kbps (i.e., n x 8 kbps).

3. Transcoder Rate Adaptation Unit: It performs the translation of 64 kbps PCM into

digitally encoded (vocoded) speech at of 13 kbps (full rate) toward the RBS and

reverses the process toward the MSC.

Full-rate speech: 64 kbps PCM signal is converted to 13 kbps to which 3 kbps of

overhead is added to bring the total to 16 Enhanced full-rate speech.

Half-rate transcoders decode and encode between 64 and 6.5 kbps, with 1.5

kbps added to yield a rate of 8 kbps.

Full-rate and half-rate data calls are rate adapted so that 14.4 kbps becomes

16 kbps and 4.8 kbps becomes 8 kbps.

The new GSM Adaptive Multi-Rate (AMR) codec defines multiple voice

encoding rates (from 4.75 to 12.2 kbps) depending upon the channel

conditions

4. Packet control Unit(PDU): It resides in the BSC and provides the interface between

the serving support node (SGSN) of the GPRS PLM network and the RBSs for the

transmission of data rate of 16kbps over the air interface.

5. System Control, Power Supply: system control provides control signals for

different events. Power supply unit energizing the functional blocks.

BSC radio network operations : It perform following functions

o It provides optimal radio resource, connection and mobility managements.

o It constantly measuring RSS of the serving cell for hand over operation and power

level control.

o Supervise the operation of a number of radio base stations that provide coverage

for a contiguous area.

o It provides the communication links to the fixed part of the wireless network

(PSTN) and the public data network (PDN)

o It is used to initially setup the radio base station parameters (channels of

operation, logical cell names handoff threshold values, etc..) or change them as

needed

o It is also used to supervise alarms issued by the radio base station to include

faults or the abnormal condition in system operation. For some faults BSC can

bring the reporting subsystem back into operation automatically, whereas other

faults require operation intervention service technician.



2. Radio Base Station (RBS):

It is typically is a self-contained unit that contain several subunits perform the

necessary operation to provide a radio link for the mobile subscriber.

Low power RBSs are use in micro or Pico cell where they are mounted to interior

walls of malls, on poles, or on the sides of buildings.

Typically block diagram of GSM RBS shown below.

RBS consists of following subsystems

a. Distribution Switch Unit

b. Radio Transceiver Units

c. RF Combining and Distribution Units

d. Power Supply Units

e. Transmitter/Receiver Units

f. Timing and Control

g. Cooling and Environmental Control Units.

a). Distribution Switch Unit (DXU):

o It is a master control unit of RBS

o It provides timing and to cross connected user data being carried on a T1/J1/E1

carrier data link from BSC with the correct RBS transreceiver and timeslot.

o It consists of timing, interface and CPU units. CPU carries resource management

function within RBS by using OMT software.

o Communication link between RBS and BSC by Abis interface. It is a T1 carrier facility

which carries 24 signals at the data rates of 64kbps.



(b). RBS Transceiver Unit:

o These units are used to broadcast and receive radio signal over radio link between

RBS and MS.

o It consists of three major sections: Transmitter and Receiver units. Signal processing,

control subsystem (see figure 7.12).

Figure: 7.12 Typical RBS Transceiver Unit

o Transreceiver: It can handle eight air timeslots and has one transmit output and two

receiver inputs for antenna diversity.

o The processing subsection: It acts as the transceiver controller. It interfaces with the

other components of the RBS system over through different signal buses. It performs

uplink and down link signal processing function such as channel coding interleaving,

encryptions, burst formatting.

o The transmitter section: It performs the digital modulation, power amplification and

power control function with typical maximum outputs in the 20 watt range.

c). RF combining and Distribution units (CDUs)

o It is used to connect several transceivers to the same antenna.

o The two most popular methods either use a device known as a hybrid to construct a

hybrid combiner.

o Hybrid combiner is a broadband device that allows two incoming transmitter signals

to be applied to it without original sources interacting with one another.

o CDU is a complex unit uses several BPFs or hybrid combiner and add other

functionality like signal divider, amplifier and isolators to protect the Transreceiver

from reflected RF wave.

o Measurement coupler that can provide accurate information about forward and

reverse power for both power control and VSWR measurements.



D).Duplex filters: It will use when same antenna performs both transmission and

reception. A typical duplex filter block diagram as shown in figure below

o It consists of two BPFs that only allow the desired signal to pass. These are also used

with tower mounted, low noise amplifier that are used to improve the receiver

sensitivity at the cell site. Typical RBS/antenna configuration will be illustrated (see

Figure below)

o

o In above fig a cell site houses a single RBS with two transceiver and only two

antennas are to be used. It is large, high power omnicell. Here tower mounted, low-

noise amplifiers are used.

o Each transceiver unit receives signals off of both antennas hence providing diversity

and it has set to suitable gain to increase the effective radiated power (ERP) of the

system.

Software Handling/Maintenance: RBSs are highly sophisticated, computer controlled,

complex transceivers.OMT software tool is used during the RBS testing, troubleshooting

and installation process.OMT software also used for updating and maintaining the RBS

internal data base, for defining RBS external alarms and during the performance of both

preventive and corrective maintenance functions on the RBS.

Prof.Suresha V. E&C Dept. KVGCE, Sullia.

Email:[email protected].

Cell No: +91 94485 24399.

Date: 10-04-2015

WC NOTS Final Unit New -7

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