Some Important Questions for Interview point of view.

1.Q What is GSM Tech. Principle behind this?

Ans: - GSM (Global System for Mobile Communication.) It is a digital cellular mobile

communication system. This system meets the criteria as follows.

1. Spectrum efficiency. 2. International Roaming. 3. Low mobile and base station cost. 4. Good Subjective Voice Quality. 5. Compatibility with other systems such as ISDN. 6. Ability to support new services.

MS Mobile Station

BTS Base Transceiver Station

BSC Base Station Controller

BSS Base Station Sub-system

MSC Mobile services Switching Center

HLR Home Location Register

VLR Visitor Location Register

AuC Authentication Center

EIR Equipment Identity Register

OMC Operations and Maintenance Center

NMC Network Management Center

ADM Administration Center

Mobile Station The mobile station comes in a number of different forms, ranging from the traditional car-

mounted phone operating at 20W, through transportables operating at 8W and 5W, to the

increasingly popular hand portable units which typically radiate less than 2W. A fifth class for

hand portables operating at 0.8W has been specified for Micro Cellular versions of the

network.

One of the main factors governing the hand portable size and weight is the battery pack.

Several features of the system are designed to allow this either to be smaller or to give a

substantially longer life between charges. Chief among these is Discontinuous Receive (DRX).

This allows the mobile to synchronize its listening period to a known paging cycle of the

network. This can typically reduce the standby power requirements by 90%.

6.3.2.2. The Radio Sub-system

When the mobile user initiates a call, his equipment will search for a local base station. A base

station sub-system (BSS) comprises a base station controller (BSC) and several base

transceiver stations (BTS), each of which provides a radio cell of one or more channels. The

BTS is responsible for providing layers 1 and 2 of the radio interface, that is, an error-corrected

data path. Each BTS has at least one of its radio channels assigned to carry control signals in

addition to traffic. The BSC is responsible for the management of the radio resource within a

region. Its main functions are to allocate and control traffic channels, control frequency

hopping, undertake handovers (except to cells outside its region) and provide radio

performance measurements. Once the mobile has accessed, and synchronized with, a BTS the

BSC will allocate it a dedicated bi-directional signaling channel and will set up a route to the

Mobile services Switching Center (MSC).

6.3.2.3. The Switching Sub-system

The MSC routes traffic and signaling within the network and interworks with other networks.

It comprises a trunk ISDN exchange with additional functionality and interfaces to support the

mobile application. When a mobile requests access to the system it has to supply its IMSI

(International Mobile Subscriber Identity). This is a unique number which will allow the

system to initiate a process to confirm that the subscriber is allowed to access it. This process is

called authentication. Before it can do this, however, it has to find where the subscriber is

based. Every subscriber is allocated to a home network, associated with an MSC within that

network. This is achieved by making an entry in the Home Location Register (HLR), which

contains information about the services the subscriber is allowed. The HLR also contains a

unique authentication key and associated challenge/response generators.

6.3.2.4. Mobility Management and Security

Whenever a mobile is switched on, and at intervals thereafter, it will register with the system;

this allows its location in the network to be established and its location area to be updated in

the HLR. A location area is a geographically defined group of cells. On first registering, the

local MSC will use the IMSI to interrogate the subscriber's HLR and will add the subscriber

data to its associated Visitor Location Register (VLR). The VLR now contains the address of

the subscriber's HLR and the authentication request is routed back through the HLR to the

subscriber's Authentication Center (AuC). This generates a challenge/response pair which is

used by the local network to challenge the mobile. In addition, some operators also plan to

check the mobile equipment against an Equipment Identity Register (EIR), in order to control

stolen, fraudulent or faulty equipment.

The authentication process is very powerful and is based on advanced cryptographic principles.

It especially protects the network operators from fraudulent use of their services. It does not

however protect the user from eavesdropping. The TDMA nature of GSM coupled with its

frequency hopping facility will make it very difficult for an eavesdropper to lock onto the

correct signal however and thus there is a much higher degree of inherent security in the

system than is found in today's analogue systems. Nevertheless for users who need assurance

of a secure transmission, GSM offers encryption over the air interface. This is based on a

public key encryption principle and provides very high security.

6.3.2.5. Call Set-up

Once the user and his equipment are accepted by the network, the mobile must define the type

of service it requires (voice, data, supplementary services etc.) and the destination number. At

this point a traffic channel with the relevant capacity will be allocated and the MSC will route

the call to the destination. Note that the network may delay assigning the traffic channel until

the connection is made with the called number. This is known as off-air call set-up, and it can

reduce the radio channel occupancy of any one call thus increasing the system traffic capacity.

GSM Recievers.

1. GSM 900 in Europe and Asia Pacific.With 890 MHZ 915 MHZ Uplink and 935 MHZ 960 MHZ Downlink Frequencies. There are 124 carriers per channel and carrier width is 200 KHZ, Bandwidth 25 MHZ, Wavelength 33cm. and Channel

separation 20 MHZ. ( Freq MHz = 890 + 0.2 * n ) where 1 n 124

2. GSM 1800 in Europe, Asia Pacific and Australia. With 1710 1785 MHZ Uplink and 1805 1880 MHZ Downlink. The carrier width is 200 khz , Band width 75 Mhz, and channel Separation is 20 Mhz. There are 375 carriers per channel.

Freq. MHz = 1710 + 0.2 * (n 512) , where 512 n 885

3. GSM 1900 US , Canada, Latin America and Africa.With 1850 1910 MHZ Uplink and 1930 1990 MHZ Downlink. There are 300 Careers per channel, 60 MHZ Band width, Channel Separation is 20 MHZ.

Modulation Used in GSM 900 is GMSK (Gaussian Minimum Shift Keying)

Principle Behind this is the Frequency Reuse. A Geographical area is divided into several

hexagonal cells. Each cell has some specific radius and having a set of frequencies. The

frequencies allotted to each cell in such a way that after some distance these frequencies can

again be reuse by other cells without interfering each other.

2.Q How many channels used in GSM .Explain each ?

Ans: - Data burst for traffic

Data burst for control

Two types of channels Physical and Logical.

Physical channel is combination of Time slot and Carrier freq. One RF channel supports eight

physical channels in time slots 0, 1, 2, ----7.

A logical channel carries information of specific type.

Traffic channel (TCH) carries digitally encoded user speech or data and have same function

in both forward and reverse link.

Control channel carries signaling and synchronizing commands between BS and MS. Certain

type of control channels defined for forward and reverse link.

TCH Traffic Channel Full rate and Half rate.

When transmitted as full rate user data is contained within 1 time slot per frame. 22.8 Kbits/ps.

When transmitted as half rate user data is mapped onto same time slot but in alternate frames.

11.4 Kbits/ps.

Four types of control channels.

1. Broadcast Control Channels. 2. Associated control Channels. 3. Dedicated Control Channels. 4. Common Control Channels.

Broadcast Channels: - operates on forward link and transmit data on first time slot. It

Contains.

1. SCH (Synchronization Channel) it is used to identify the serving BS and allowing each mobile to frame synchronize with the BS. The frame no. is sent with the BSIC

during SCH burst. And also 6 bit BSIC.

2. FCCH (Frequency Correction Channel) The FCCH allows each MS to synchronize its internal freq. with exact freq. of the BS.

3. BCCH (Broadcast Control Channel) It carries informations such as cell and network identity. It also broadcast a list of channels that are currently in use within

a cell.

4. CBCH (Cell Broadcast Channel) Used to transmit short alphanumeric text msg. to all MS within a cell.

Common Control Channels (CCCH): - CCCH helps to establish the call from the MS. Three

different types of CCCH are defined.

1. The Paging Channel (PCH). It is used to alert the MS of an incoming call. 2. The Random Access Channel (RACH). Is used by MS to access the network. 3. The Access Grant Channel (AGCH). Is used by the Base Station to inform the MS

that which channel it should use.

Dedicated Control Channels (DCCH): - These channels are used for message exchange

between several mobiles or a mobile and network. Two types of DCCH are there.

1. Stand Alone Dedicated Control Channels (SDCCH). Authentication, Registration, Location area updation, SMS etc. needed for setting up a TCH.

2. Slow Associated Control Channels (SACCH).

Associated Control Channels: - Associated with the TCH.

1. Slow Associated Control Channel (SACCH). Associated with TCH, Channel quality, Signal power level.

2. Fast Associated Control Channel (FACCH). Uses time slots from TCH, Handover info.

4.Q What is Timing Advance ?

Ans.

Timing Advance

The timing of the bursts transmissions is very important. Mobiles are at different distances

from the base stations. Their delay depends, consequently, on their distance. The aim of the

timing advance is that the signals coming from the different mobile stations arrive to the base

station at the right time. The base station measures the timing delay of the mobile stations. If

the bursts corresponding to a mobile station arrive too late and overlap with other bursts, the

base station tells, this mobile, to advance the transmission of its bursts.

1 TA = 554m.

Calculation is given below.

Timing Advance:

T T (bit) = (2d) c Where T= Timing Advance

C = vel.of light 310^8 m /ms

T (bit) = 1 270.833 (Raw bit rate per carrier is 270.833 Kbps. Each carrier is shared by 8 users in TDMA Fashion.

There for bit rate for one user or one time slot is 1 / 270.833 Kbps ).

Now d = T ((T (bit) c) 2)

= T ((1 270.833) 3 10^5) 2)

Now after calc. d= T 554 m

TA is from 0 to 63.

5.Q What type of modulation used in GSM ?

Ans.

Digital Modulation

Figure 4: GMSK modulator

The modulation chosen for the GSM system is the Gaussian Minimum Shift Keying (GMSK).

Figure 4 illustrates a GMSK modulator.

Q. What is handover? Explain it.

Ans.

Handover

The user movements can produce the need to change the channel or cell, especially when the

quality of the communication is decreasing. This procedure of changing the resources is called

handover. Four different types of handovers can be distinguished:

Handover of channels in the same cell.

Handover of cells controlled by the same BSC.

Handover of cells belonging to the same MSC but controlled by different BSCs.

Handover of cells controlled by different MSCs.

Handovers are mainly controlled by the MSC. However in order to avoid unnecessary

signaling information, the first two types of handovers are managed by the concerned BSC (in

this case, the MSC is only notified of the handover).

The mobile station is the active participant in this procedure. In order to perform the handover,

the mobile station controls continuously its own signal strength and the signal strength of the

neighboring cells. The list of cells that must be monitored by the mobile station is given by the

base station. The power measurements allow deciding which the best cell is, in order to

maintain the quality of the communication link. Two basic algorithms are used for the

handover:

The minimum acceptable performance algorithm. When the quality of the transmission

decreases (i.e. the signal is deteriorated), the power level of the mobile is increased.

This is done until the increase of the power level has no effect on the quality of the

signal. When this happens, a handover is performed.

The power budget algorithm. This algorithm performs a handover, instead of continuously increasing the power level, in order to obtain a good communication

quality.

Decibell Relation:

db and dbm :-

1W = 30 dbm

2W = 33 dbm

dbm = 10 * log ( Pwr in Watts * 1000 ) OR 10 * log (power in Watts) / 1 mW

dbi and dbd : -

1 dbd = 2.14 dbi

dbi = dbd 2.14

Grade of Service GoS :

How much traffic can one cell carry? That depends on the

Number of traffic channels available and the acceptable

Probability that the system is congested, the so called Grade of

Service (GoS).

Key Performance Indicator ( KPI )

D1 ( Droop 1 ) or SD Droop < 1%

D2 ( Droop 2 ) or TCH Drop < 1.5%

SD Blocking < 0.5%

TCH Blocking < 0.5%

Congestion on SDCCH < 0.5%

Congestion on TCH < 1.5%

HOSR (Handover Success Rate) > 95%

TCH ASSR (TCH Assignment Success Rate) > 97%

CSSR (Call Setup Success Rate) > 98%

Setup Time = 3.5 Sec.

Availability = 99.9 %

CCR (Call Completion Rate) or CSR (Call Success Rate) > 96%

Received Signal at MS and Path Loss:

= BTS (EIRP) BTS to MS Path Loss + Donner Antenna Gain (G1) Feeder Loss + Serving Antenna Gain (G2) Avg. Fading Margin.

Where as Path Loss (db) = 20 log ( 4 d f / c ) Where ( d = Distance between antennas of BTS and MS.)

MS sensitivity = -102 dbm

BTS sensitivity = -104 dbm

Diversity Gain at BTS = 3.5 dbi

Antenna Gain at MS = 0.0 dbi

Slant Polarization Loss = 1.5 db

MS o/p Power = 2W or up to 0.8 W

EIRP = 53.7 db

Transmitted Power at BTS = 41 to 45 db

Duplex Loss at BTS = 0.8 db

Feeder loss and Jumper Loss at BTS = 3.00 db

Rayleigh fade margin without hopping = 3.4 db

Interference margin = 3.00 db

Car Loss = 6.00 db

Body Loss = 3.00 db

Dense urban loss = 6.00 db

Erlang Traffic Theory :

Assuming that one cell has two carriers, corresponding typically to 2x8-2=14 traffic

channels (two physical channels are needed for signaling) and a GoS of 2% is

acceptable, the traffic that can be offered is A=8.20 E. See the table in Figure 3-1.

This number is interesting if an estimate on the average traffic per subscriber can be

obtained. Studies show that the average traffic per subscriber during the busy hour is

typically 15-20 or in special cases 40 - 50 mE. (this can correspond to e.g. one call,

lasting 54-72 seconds, per hour). Dividing the traffic that one cell can offer, Acell=8.20

E, by the traffic per subscriber, here chosen as Asub=0.025 E, the number of subscribers

one cell can support is derived as 8.20/0.025 = 328 subscribers.

When half rate is used it will theoretically double the number of available traffic channels. In

practice, however, live networks will most likely consist of a mixture between half rate mobiles

and full rate mobiles.

Half rate will affect the SDCCH dimensioning since more Signalling will be required when the

number of TCHs is increased. An important dimensioning factor is therefore the half rate

penetration, i.e. the percentage of half rate mobiles in the network.

When half rate TCH capacity calculations are done it is assumed that the half rate mobiles are

evenly spread among the cells, i.e. all cells have the same half rate penetration. The TCH

capacity calculations made in this guideline are best illustrated with an example:

If for example a 2 TRX cell is used, it can accommodate 14 full rate TCHs, i.e. 14 subscribers

if one SDCCH/8 is used for Signalling. A half rate penetration of 10 % would mean that 10 %

of the 14 subscribers would be using a half rate connection, i.e. 1.4 subscribers (after been

rounded up = 2 subscribers). This would result in 13 timeslots used for full rate and 1 timeslot

used for half rate, resulting in 13 full rate TCHs and 2 half rate TCHs, i.e. 15 TCHs in total.

The TCH capacity is then calculated for 15 TCHs with an Erlang B table with appropriate

blocking figure.

Knowing the SDCCH holding times, with a given number of performances during busy hour

for every procedure, the generated SDCCH traffic per subscriber can be calculated as

follows:

For each type of procedure, multiply the number of performances per busy hour and

subscriber by the holding time of the channel. By dividing the result by 3.6, the procedures

contribution to the SDCCH load in mErlang/subscriber is achieved.

SOME IMPORTANT DEFINATIONS REGARDING ANTEENAS.

The purpose of this technical brief is to provide introductory insights into wireless antennas and their characteristics. The definitions in quotation marks are taken from IEEE Standard Definitions of Terms

for Antennas, IEEE Std 145-1983.

Antenna: "That part of a transmitting or receiving system which is designed to radiate or to receive electromagnetic waves". An antenna can also be viewed as a transitional structure (transducer) between free-space and a transmission line (such as a coaxial line). An important property of an antenna is the ability to focus and shape the radiated power in space e.g.: it enhances the power in some wanted directions and suppresses the power in other directions.

Frequency bandwidth: "The range of frequencies within which the performance of the antenna, with respect to some characteristics, conforms to a specified standard". VSWR of an antenna is the main bandwidth limiting factor. Input impedance: " The impedance presented by an antenna at its terminals". The input impedance is a complex function of frequency with real and imaginary parts. The input impedance is graphically displayed using a Smith chart.

Reflection coefficient: The ratio of the voltages corresponding to the reflected and incident waves at the antenna's input terminal (normalized to some impedance Z0). The return loss is related to the input impedance Zin and the characteristic impedance Z0 of the connecting feed line by: Gin = (Zin - Z0) / (Zin+Z0).

Voltage standing wave ratio (VSWR): The ratio of the maximum / minimum values of standing wave pattern along a transmission line to which a load is connected. VSWR value ranges from 1 (matched load) to infinity for a short or an open load. For most base station antennas the maximum acceptable value of VSWR is 1.4. VSWR is related to the reflection coefficient Gin by: VSWR= (1+|Gin|) / (1-| Gin |).

Isolation: "A measure of power transfer from one antenna to another". This is also the ratio of the power input to one antenna to the power received by the other antenna, expressed in decibel (dB). The same definition is applicable to two-port antennas such as dual-polarization antennas.

Far-field region: "That region of the field of an antenna where the angular field distribution is essentially independent of the distance from a specified point in the antenna region". The radiation pattern is measured in the far field.

Antenna polarization: "In a specified direction from an antenna and at a point in its far field, is the polarization of the (locally) plane wave which is used to represent the radiated wave at that point". "At any point in the far-field of an antenna the radiated wave can be represented by a plane wave whose electric field strength is the same as that of the wave and whose direction of propagation is in the radial direction from the antenna. As the radial distance approaches infinity, the radius of curvature of the radiated wave's phase front also approaches infinity and thus in any specified direction the wave appears locally a plane wave". In practice, polarization of the radiated energy varies with the direction from the center of the antenna so that different parts of the pattern and different side lobes sometimes have different polarization. The polarization of a radiated wave can be linear or elliptical (with circular being a special case).

Co-polarization: "That polarization which the antenna is intended to radiate".

Cross-polarization: "In a specified plane containing the reference polarization ellipse, the polarization orthogonal to a specified reference polarization". The reference polarization is usually the co-polarization.

Antenna pattern: The antenna pattern is a graphical representation in three dimensions of the radiation of the antenna as a function of angular direction. Antenna radiation performance is usually measured and recorded in two orthogonal principal planes (such as E-Plane and H-plane or vertical and horizontal planes). The pattern is usually plotted either in polar or rectangular coordinates. The

pattern of most base station antennas contains a main lobe and several minor lobes, termed side lobes. A side lobe occurring in space in the direction opposite to the main lobe is called back lobe.

Normalized pattern: Normalizing the power/field with respect to its maximum value yields a normalized power/field pattern with a maximum value of unity (or 0 dB).

Gain pattern: Normalizing the power/field to that of a reference antenna yields a gain pattern. When the reference is an isotropic antenna, the gain is expressed in dBi. When the reference is a half-wave dipole in free space, the gain is expressed in dBd.

Radiation efficiency: "The ratio of the total power radiated by an antenna to the net power accepted by the antenna from the connected transmitter".

E-plane: "For a linearly polarized antenna, the plane containing the electric field vector and the direction of maximum radiation". For base station antenna, the E-plane usually coincides with the vertical plane.

H-plane: "For a linearly polarized antenna, the plane containing the magnetic field vector and the direction of maximum radiation". For base station antenna, the H-plane usually coincides with the horizontal plane.

Front-to-back ratio: "The ratio of the maximum directivity of an antenna to its directivity in a specified rearward direction". Sometimes the directivity in the rearward direction is taken as the average over an angular region.

Major/main lobe: "The radiation lobe containing the direction of maximum radiation". For most practical antenna there is only one main beam.

Side lobe level: Is the ratio, in decibels (dB), of the amplitude at the peak of the main lobe to the amplitude at the peak of a side lobe.

Half-power beamwidth: In a radiation pattern cut containing the direction of the maximum of a lobe, the angle between the two directions in which the radiation intensity is one-half the maximum value". The Half-power beamwidth is also commonly referred to as the 3-dB beamwidth.

Antenna directivity: The directivity of an antenna is given by the ratio of the maximum radiation intensity (power per unit solid angle) to the average radiation intensity (averaged over a sphere). The directivity of any source, other than isotropic, is always greater than unity.

Antenna gain: The maximum gain of an antenna is simply defined as the product of the directivity by efficiency. If the efficiency is not 100 percent, the gain is less than the directivity. When the reference is a loss less isotropic antenna, the gain is expressed in dBi. When the reference is a half wave dipole antenna, the gain is expressed in dBd (1 dBd = 2.15 dBi ).

Antenna efficiency: The total antenna efficiency accounts for the following losses: (1) reflection because of mismatch between the feeding transmission line and the antenna and (2) the conductor and dielectric losses. Effective radiated power (ERP): "In a given direction, the relative gain of a transmitting antenna with respect to the maximum directivity of a half-wave dipole multiplied by the net power accepted by the antenna from the connected transmitter".

Power handling: Is the ability of an antenna to handle high power without failure. High power in antenna can cause voltage breakdown and excessive heat (due to conductor and dielectric antenna losses) which would results in an antenna failure.

Passive intermodulation (PIM): As in active devices, passive intermodulation occurs when signals at two or more frequencies mix with each other in a non-linear manner to produce spurious signals. PIM is caused by a multitude of factors present in the RF signal path. These include poor mechanical contact, presence of ferrous contents in connectors and metals, and contact between two galvanically unmatched metals. PIM spurious signal, which falls in the up link band, can degrade call quality and reduce the capacity of a wireless system.

Side lobe suppression: "Any process, action or adjustment to reduce the level of the side lobes or to reduce the degradation of the intended antenna system performance resulting from the presence of side lobes". For base station antenna, the first side lobe above the horizon is preferred to be low in order to reduce interference to adjacent cell sites. At the other hand, the side lobes below the horizon are preferred to be high for better coverage.

Null filling: Is the process to fill the null in the antenna radiation pattern to avoid blind spots in a cell site coverage.

Isotropic radiator: "A hypothetical, loss less antenna having equal radiation intensity in all direction". For based station antenna, the gain in dBi is referenced to that of an isotropic antenna (which is 0 dB).

Omnidirectional antenna: "An antenna having an essentially non-directional pattern in a given plane of the antenna and a directional pattern in any orthogonal plane". For base station antennas, the omnidirectional plane is the horizontal plane.

Directional antenna: "An antenna having the property of radiating or receiving electromagnetic waves more effectively in some directions than others".

Half-wave dipole: "A wire antenna consisting of two straight collinear conductors of equal length, separated by a small feeding gap, with each conductor approximately a quarter-wave length long".

Log-periodic antenna: "Any one of a class of antennas having a structural geometry such that its impedance and radiation characteristics repeat periodically as the logarithm of frequency".

Microstrip antenna: "An antenna which consists of a thin metallic conductor bonded to a thin grounded dielectric substrate". An example of such antennas is the microstrip patch.

Linear array: A set of radiating elements (e.g. dipole or patch) arranged along a line. Radiating elements such as dipole and patch have dimensions comparable to a wavelength. A linear array has a higher gain, than a single radiator, and its radiation pattern can be synthesized to meet various antenna performance requirements such as upper side lobe suppression and null fill. It should be noted that the gain of any antenna is proportional to its size.

Coaxial antenna: "An antenna comprised of a extension to the inner conductor of a coaxial line and a radiating sleeve which in effect is formed by folding back the outer conductor of the coaxial line".

Collinear array antenna: "A linear array of radiating elements, usually dipoles, with their axes lying in a straight line".

Adaptive (smart) antenna: "An antenna system having circuit elements associated with its radiating elements such that one or more of the antenna properties are controlled by the received signal".

Antenna Terms:

Directivity: The directivity of a transmitting antenna is defined as the ratio of the radiation

intensity flowing in a given direction to the radiation intensity averaged over all direction.

The average radiation intensity is equal to the total power radiated by the antenna divided

by 4 . If the direction is not specified, the direction of maximum radiation intensity is

usually implied. Directivity is some times refered to as directive gain.

Absolute gain: The absolute gain of a transmitting antenna in a given direction is defined

as the ratio of the radiation intensity flowing in that direction to the radiation intensity that

would be obtained if the power accepted by the antenna were radiated isotropically. If the

direction is not specified, the direction of maximum radiation intensity is usually implied.

(Absolute gain is closely related to directivity, but it takes into account the efficiency of

antenna as well as its direction characteristics. To distinquish it, the absolute gain is some

times refered to as power gain.)

Relative gain: The relative gain of a transmitting antenna in a given direction is defined as

the ratio of the absolute gain of the antenna in the given direction to the absolute gain of a

reference antenna in the same direction. The power input to the two antennas must be the

same.

Efficiency: The efficiency of a transmitting antenna is the ratio of the total radiated power

radiated by the antenna to the input power to the antenna.

Effective area (aperature): The effective area or aperature of a receiving antenna in a

given direction is defined as the ratio of the available power at the terminals of the antenna

to the radiation intensity of a plane wave incident on the antenna in the given direction. If

the direction is not specified, the direction of maximum radiation intensity is usually

implied. It can be shown, that when an isotropic area is used as a receiving antenna its

effective area is the wavelength squared divided by 4 . Thus, the gain of a receiving

antenna is the ratio of the antennas effective area to that of an isotropic antenna -- i.e.

.

Antenna factor: The ratio of the magnitude of the electric field incident upon a receiving

antenna to the voltage developed at the antenna's output connector (assuming a 50 ohm

coxial connector) is called the antenna factor. The antenna factor is clearly related to the

gain of antenna, but is often found to be the most convenience parameter for use in the

monitoring of electromagnetic emissions.

IMPORTANT PARAMETERS FOR DRIVE TEST

Drive Test

Drive Test is done to:-

Troubleshoot the customer complaint

Diagnose other problems impacting customer.

Bench mark the other network parameters.

Reduce interference in the network.

Optimise the network.

Drive Test Tool

Output of Drive Test tells you about:-

Unsuccessful calls.

Dropped calls.

Coverage

Poor received quality

Hand over indication

Hand over failure.

Failure signaling message

RF Parameters

1. RX Level -10 to 120 dbm

-35 to -75 < -65 is Good

We can receive signal on road as well as in house.

-75 to -85 Satisfactory (So-So).

We can receive signal only on road and not in house.

> -85 Bad

The signal quality is very poor.

2. RX Quality 0 to 7.

0 to 7 0 is the best.

Rx quality depends upon Bit Error Rate BER.

GPS GSM TEST

MOBILE

COMPUTER

POST PROCESSING

ANALYSIS SOFTWARE

BER Rx Q

0 0.2 0 0.2 0.4 1 0.4 0.8 2 0.8 1.6 3 1.6 3.2 4 3.2 6.4 5 6.4 12.8 6 12.8 25.6 7

Call without Hoping Rx Q should be less then 4.

Call with Hoping RxQ should be less then 5.

3. Speech Quality Index (SQI) -20 to 30.

-20 to 30 Maximum value is good.

Speech Quality is dependent on Rx Quality and C/I ratio.

4. Speech Quality

1 to 5 Maximum value is good for speech quality.

Index Quality Scale

1 Unsatisfactory (Speech not understandable) 2 Poor 3 Fair 4 Good 5 Excellent

5. Carrier to Interference (C/I) or Co-channel interference ratio.

-5 to 25 db > 15 Good

> 9 So-So.

6. Carrier to Adjacent Interference Ratio (C/A) > 23.

7. Frame Error Rate It should be 0.

8. Timing Advance TA 1 TA = 554m

T T (bit) = (2d) c Where T= Timing Advance

C = vel.of light 310^5 m /ms

T(bit) = 1 270.833

Now d = T ( ( T (bit) c ) 2 )

= T ( ( 1 270.833 ) 3 10^5 ) 2 )

Now after calc. d = T 554 m

Total TA is from 0 to 63.

Drive test before network is available for general public. The signal strength from all

BTS is measured at each geographical locations.

SD DROP

1) Bad Radio link quality (any sort of interference, if highly destructive)

2) Non availability of TCH timeslots.

3) could b a problem at NSS end.

4) hardware problem.

etc. ..

1*1 & 1*3 Solution

In 1*1 hopping all sectors uses Same MA list and 1*3 hopping all 3 sectors uses three different

MA list

1*3 is also called discrete hopping

in which no of trx = no of MAL LIST

Handover failures other than Co-Bcch and Neighbour

1. Weak cell boundaries,

2. Congestion on target cell,

3. Bcch and TCH interference,

4. Too many neighbouring cell, and

5. incorrect handover paramerets

difference of RX full and Sub

simple definition is

RX Lev Full :- DTX is OFF

RX Lev Sub :- DTX is On

detailed

RX Lev Full:

is nothing but the Mobile transmit the measurment report(SACCH multiframe) for every

480ms. this multiframe containes 104 TDMA frames, in 104 TDMA frames 4 TDMA frames

for Decode the BSIC and remaining 100 TDMA frames for Average measurment of serving

cell and neighbouring cell

This average measurment of 100 TDMA frames are RX Lev Full

RX Lev Sub:

DTX is a discontinouse trasmission, When the mobile conversation 40% of the time either

Trasmitter or Receive is idle. When DTX is ON, DTX will switch off the Trasmitter or

Receiver when they is no speech Pulses. only few TDMA frames will trasmit, the average of

this TDMA frames is called RX Lev Sub, give you proper measurment of RX level

Sepration bw Antenna

should be minimum 3Meters.

C1 & C2

C1 is for cell selection and C2 is for celll reslection.

(when mobile is switched on, it calculates C1 first. Then after recieving neighbour cell

information, it calculates C2 based on the strongest serving cell.)

c1 is used for cell selection & parameter C1 to determine

whether a cell is suitable to camp on.

C1 depends on 4 parameters:

Received signal level (suitably averaged)

The parameter rxLevAccessMin, which is broadcast on the

BCCH, and is related to the minimum signal that the operator

wants the network to receive when being initially accessed by

an MS

The parameter msTxPwrMaxCCH, which is also broadcast on

the BCCH, and is the maximum power that an MS may use when

initially accessing the network

The maximum power of the MS.

c2 is used for reselection & encouraging MSs to select some suitable

cells in preference to others

Cell re-selection is needed if:

Path Loss criterion C1 < 0 for cell camped on, for more than 5 sec

There is DL signaling failure

The cell camped on has been barred

The is a better cell in terms of C2 criterion

HSN - MAIO

HSN (values 0-63) is basically an Algorithm that assigns frequency to the cell from block/list

of frequencies... Assignment of frequency from the list whatever HSN value is totally

random/algorithm dependent (HSN=0 being cyclic)... normally HSN assigns frequency after

each TDMA frame (4.615msec)... this hopping rate is changable...

MAIO is used as an offset from the frquency, assigned by HSN, to avoid co/adjacnt frquency

clash on the samecell/cosite cell... In b/m exmple as well we have atleast a difference of 2 in

MAIOs to avoid frquency clash...

Taking 1x1 example...

We hve a three sectored site having 1x1 implemented A,B,C... Each hving 4 TRXs, TRXs

A1,B1,C1 being BCCH TRXs

Lets say we have b/m list of frequencies in 1x1 hopping pool...

f1,f2,f3,f4,f5,f6,f7,f8,f9,f10,f11,f12,13,f14,f15,f16,f17,f18

Lets suppose HSN=17 for three cells of the site

TDMA______1____2___3

frame

freq

assgned

by

HSN_______f2___f11__f9 (suppositions)

TRX_MAIO

A2___0____f2___f11__f9

A3___6____f8___f17__f15

__________ ^ f8 is assigned which

is 6 blocks ahead of f2 because offset

(MAIO) is 6... so on...

A4___12___f14___f5__f3

B2___2____f4___f13__f11

B3___8____f10___f1__f17

B4___14___f16___f7__f5

C2___4____f6___f15__f13

C3___10___f12___f3__f1

C4___16___ f18___f9__f7

in above mentioned example hopping TRXs have hopped on to 3 frquencies during 3 TDMA

frames without any freq clash...

In 1x1 HSN is same for cells of same site... Sites in close vicinity are given different HSN

values to avoid assignment of same frequencies...

Multiple coverage and Call muting

1) Increase the HO Margin from serving cell to target cells side only

2) Reduce the power of cell to whom serving cell is atteming to HO

3) Increase the RXP for Nokia System(For Example.-95 to -110)

4) Delete the unneccessery HOs those are not required

5) Reduce the power of Overshoots.

6) Try to make single dominant.

Call Mute:

At the time of Hand over failure and Reversion, Call can be in the position of muting or one

sided.

TILTS

tilts are of two different types...

electrical and mechanical

!). Mechanical tilt:- in this you bend the antenna mechanically without any change in the

internal ckt.

The major lobe will become heart shaped in case of larger tilts. and lead to unwanted

distribution of signal in uncontrollable manner.

2).Electrical tilt:-

this is done with the help of phase shifters.

The phase of the feed(voltage) to the dipoles is changed further leading to change in the

radiation pattern in a better manner

The major difference between M and E tilt is

E tilt has no back loop

Effects:-

Mechanical Tilt Causes:

Beam Peak to Tilt Below Horizon

Back Lobe to Tilt Above Horizon

At 90 No Tilt

Electrical Tilt Causes:

Beam peak to tilt below horizon

Back lobe to tilt below horizon

At 90 to tilt below horizon

All the pattern tilts

Adaptive Multi-Rate

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Adaptive Multi-Rate (AMR) is an audio data compression scheme optimized for speech

coding. AMR was adopted as the standard speech codec by 3GPP in October 1998 and is now

widely used in GSM. It uses link adaptation to select from one of eight different bit rates

based on link conditions.

The bit rates 12.2, 10.2, 7.95, 7.40, 6.70, 5.90, 5.15 and 4.75 kbit/s are based on frames which

contain 160 samples and are 20 milliseconds long. AMR uses different techniques, such as

Algebraic Code Excited Linear Prediction (ACELP), Discontinuous Transmission (DTX),

voice activity detection (VAD) and comfort noise generation (CNG). The usage of AMR

requires optimized link adaptation that selects the best codec mode to meet the local radio

channel and capacity requirements. If the radio conditions are bad, source coding is reduced

and channel coding is increased. This improves the quality and robustness of the network

connection while sacrificing some voice clarity. In the particular case of AMR this

improvement is somewhere around 4-6 dB S/N for useable communication. The new

intelligent system allows the network operator to prioritize capacity or quality per base station.

Audio data compression

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This article is about a process which reduces the data rate or file size of digital audio

signals. This should not be confused with audio level compression, which reduces the

dynamic range of audio signals.

Audio compression is a form of data compression designed to reduce the size of audio files.

Audio compression algorithms are typically referred to as audio codecs. As with other specific

forms of data compression, there exist many "lossless" and "lossy" algorithms to achieve the

compression effect.

Speech encoding

From Wikipedia, the free encyclopedia

(Redirected from Speech coding)

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Speech coding is the compression of speech (into a code) for transmission with speech

codecs that use audio signal processing and speech processing techniques.

The two most important applications using speech coding are mobile phones and internet

phones.

The techniques used in speech coding are similar to that in audio data compression and audio

coding where knowledge in psychoacoustics is used to transmit only data that is relevant to

the human auditory system. For example, in narrowband speech coding, only information in

the frequency band 400 Hz to 3500 Hz is transmitted but the reconstructed signal is still

adequate for intelligibility.

However, speech coding differs from audio coding in that there is a lot more statistical

information available about the properties of speech. In addition, some auditory information

which is relevant in audio coding can be unnecessary in the speech coding context. In speech

coding, the most important criterion is preservation of intelligibility and "pleasantness" of

speech, with a constrained amount of transmitted data.

It should be emphasised that the intelligibility of speech includes, besides the actual literal

content, also speaker identity, emotions, intonation, timbre etc. that are all important for

perfect intelligibility. The more abstract concept of pleasantness of degraded speech is a

different property than intelligibility, since it is possible that degraded speech is completely

intelligible, but subjectively annoying to the listener.

In addition, most speech applications require low coding delay, as long coding delays interfere

with speech interaction.

The A-law algorithm and the Mu-law algorithm are used in nearly all land-line long distance

telephone communications. They can be seen as a kind of speech encoding, requiring only 8

bits per sample but giving effectively 12 bits of resolution.

The most common speech coding scheme is Code-Excited Linear Predictive (CELP) coding,

which is used for example in the GSM standard. In CELP, the modelling is divided in two

stages, a linear predictive stage that models the spectral envelope and code-book based model

of the residual of the linear predictive model.

In addition to the actual speech coding of the signal, it is often necessary to use channel

coding for transmission, to avoid losses due to transmission errors. Usually, speech coding and

channel coding methods have to be chosen in pairs, with the more important bits in the speech

data stream protected by more robust channel coding, in order to get the best overall coding

results.

The Speex project is an attempt to create a free software speech coder, unemcumbered by

patent restrictions.

Major subfields:

Wide-band speech coding

o AMR-WB for

WCDMA

networks

o VMR-WB for

CDMA2000

networks

Narrow-band speech

coding

o FNBDT for military

applications

o SMV for CDMA

networks

o Full Rate, Half

Rate, EFR, AMR

for GSM networks

dB - Decibel The Decibel is a unit of comparison, in which the ratio of two power values

are expressed using a logarithmic scale usually to the base 10. Although the

dB is a unit of comparison it is sometimes useful to have an agreed reference

point. A common reference is 1mW, which is expressed as 0dBm.

Consequently 2W, the typical maximum power of a GSM handset, is rated as

33dBm.

dBc A ratio in Decibels of the sideband power of a signal, measured in a given

bandwidth at a given frequency offset from the centre frequency of the same

signal, to the total in band power of the signal

dBm A measure of power expressed in terms of its ratio (in Decibels) to one

milliwatt.

dBW A measure of power expressed in terms of its ratio (in Decibels) to one Watt.