SUMMER 13 EXAMINATION

Transcription

1 MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) (ISO/IEC Certified) Subject Code: SUMMER 13 EXAMINATION Model Answer Important Instructions to examiners: 1) The answers should be examined by key words and not as word-to-word as given in the model answer scheme. 2) The model answer and the answer written by candidate may vary but the examiner may try to assess the understanding level of the candidate. 3) The language errors such as grammatical, spelling errors should not be given more Importance (Not applicable for subject English and Communication Skills). 4) While assessing figures, examiner may give credit for principal components indicated in the figure. The figures drawn by candidate and model answer may vary. The examiner may give credit for any equivalent figure drawn. 5) Credits may be given step wise for numerical problems. In some cases, the assumed constant values may vary and there may be some difference in the candidate s answers and model answer. 6) In case of some questions credit may be given by judgment on part of examiner of relevant answer based on candidate s understanding. 7) For programming language papers, credit may be given to any other program based on equivalent concept. 1

2 1. a) Attempt any THREE of the following: (12) i) Explain MAHO in detail. Ans MAHO -- 4M In today s 2G systems, handoff decisions are mobile assisted. In mobile assisted handoff (MAHO) every mobile station measures the received power from surrounding base stations and continually reports the results of these measurements to the serving base station. A handoff is initiated, when the power received from the base station of a neighboring cell begins to exceed the power received from the current base station by a certain level or for a certain period of time. In MAHO method call handed over between base stations is much faster than first generation analog systems.as handoff measurements are made by each mobile.msc no longer constantly monitors signal strengths. MAHO is particularly suited for microcellular environments where handoffs are more frequent. During the course of a call, if a mobile moves from one cellular system to a different cellular system controlled by a different MSC, an intersystem handoff becomes necessary. An MSC engages in an intersystem handoff when a mobile signal becomes weak in a given cell and the MSC cannot find another cell within its system to which it can transfer the call in progress. Q( ii) Draw block diagram and explain the operation of transmitter unit of mobile handset. Ans. Diagram 2M ; Explanation 2M 2

3 It is a low power unit operating in the frequency range of 824 to 845 MHz. Carrier is obtained from frequency synthesizer and applied to Phase Modulator along with the amplified voice signal. Modulator output is converted using a mixer to final transmitter frequency. Mixer output is applied to Class C amplifier. The final amplifier stage is to design to supply 3W to antenna. The automatic power control circuit controls the o/p power of the transmitter automatically, with the help of power output detector & DC amplifier. Transmitter output is fed to duplexer. Carrier input for the phase modulator & the local oscillator frequency signal for mixer are produced by frequency synthesizer. Transmitter output power controlled by cell site & MTSO. Receiver picks up the special control signals & sends to APC that sets transmitter o/p power Level. (iii) Describe various SS7 services. Ans There are 3 types of services : a) Touchstar 2 M b) 800 services 1 M c) Alternate Billing Service & Line Information Database (ADB/LIDB) 1 M Touchstar It is also known as CLASS. It is a group of switch Controlled Services that provides its users with certain call management capabilities. Provides services such as call return, call forwarding, repeat dialing, call block, call tracing & caller ID. 800 services Introduced by bell System to provide toll-free access to the calling party to the services & database which is offered by private parties. Cost associated with the processing of calls is paid by the service subscriber. 3

4 Service is offered under two plans : 800-NXX plan : The first 6 digits of an 800 call are used to select the interexchange carrier (IXC). 800 database plan : The call is looked up in the database to determine the appropriate carrier & routing information. Alternate Billing Service & Line Information Database (ADB/LIDB) These services use the CCS network to enable the calling party to bill a call to a personal number (third party number, calling card, or collect etc.) from any number. (iv) Describe and explain the Radio Spectrum used for IMT Ans -- 4M OR Any correct Explanation should be considered The International Telecommunication Union (ITU) made a request for proposals for Radio Transmission Technologies (RTT) for international mobile telecommunications (IMT) 2000 program (ITU, 2002). IMT-2000 formerly called future public land mobile telecommunication system that (FPLMTS), tried to establish a common worldwide communication system that allowed for terminal and user mobility, supporting the idea of universal several recommendations for FPLMTS systems, e.g. network architectures for FPLMTS (M.817), requirements for radio interface(s) for FPLMTS (M.1034), or Framework for services supported by FPLMTS (M.816). The number 2000 in IMT-2000 should indicate the start of the system (year 2000+x) and the spectrum used (around 2000 MHz). IMT-2000 includes different environments such as indoor use, vehicles, satellites and pedestrians. The world radio conference (WRC)1992 identified and MHz as the frequency bands that should be available worldwide for the new IMT-2000 systems (Recommendation ITU-R-M.1036). Within these bands, two times 30MHz have been reserved for mobile satellite services (MSS). In addition to the original frequency allocations, the world radio conference (WRC) allocated new terrestrial IMT-2000 bands in the range of MHz, MHz and MHz in

5 b) Attempt any ONE of the following: (6) i) State the range of frequency, access method and modulation type used in CT 2 and DECT. Ans 3 M for each Sr.No TYPE Frequency Access Method Modulation type 1. CT MHz FDMA GFSK 2 DECT MHz TDMA GFSK ii) Draw block diagram and explain the operation of control unit in mobile. Ans Diagram-- 3M; Explanation 3M The control unit consists of a handset with speaker & microphone, Keypad. Control unit is operated by a separate microprocessor which drives the LCD display & other indicators. Microprocessor implements all the manual control functions. Numbers & other important information is stored in microprocessor memory. The transmitter, receiver, frequency synthesizer & logic section are put in one block. Handset consists of earphone & loud speaker to receive & transmit voice. 5

6 Demodulated signal coming from receiver is amplified by an audio amplifier & applied to loud speaker. Electrical equivalent of voice signal is produced by the microphone & amplified by audio amplifier. Signal acts as modulating signal & applied to modulator. Q2. Attempt any FOUR of the following: (16) a) Define and explain the following: i) Forward Channel ii) Cell Ans a) i) Forward Channel -- 2M Radio channel used for transmission of information from base station to mobile. The channels used for voice transmission from the base station to the mobile are called forward voice channels (FVC). The channel responsible for initiating the mobile calls are called as forward control channels (FCC) ii) Cell -- 2M Each cellular base station is allocated a group of radio channels to be used within a small geographic area called a cell. Base stations in adjacent cells are assigned channel groups which contain completely different channels than neighboring cells. The base station antennas are designed to achieve the desired coverage within the particular cell. By limiting the coverage area to within the boundaries of a cell, the same group of channels may be used to cover different cells that are separated from one another by distances large enough to keep interference levels within tolerable limits. The hexagonal cell shape is conceptual and is a simplistic model of the radio coverage for each 6

7 Base station. It has been universally adopted since the hexagon permits easy and manageable analysis of a cellular system. b) Draw the GSM architecture and elaborate the functions of VLR & HLR. Ans b) Diagram 2M Functions 1.Home Location Registers (HLR) 1M Permanent database about mobile subscribers in a large service area (generally one per GSM network operator) Database contains subscriber & location information Database contains IMSI (International Mobile Subscriber Identity), prepaid/postpaid, roaming restrictions, supplementary services Each Subscriber assigned IMSI to identify home user 7

8 2. Visitor Location Registers (VLR) 1M Temporary database which stores IMSI & customer information for each roaming subscriber visiting the coverage area of particular MSC. It updates whenever new MS enters its area, by HLR database. It controls the mobiles roaming in its area. c) List and describe different types of Traffic channels (TCHs) provided in GSM Ans c. Full Rate 2M; Half rate 2M Traffic channels carry digitally encoded user speech or user data on both the up- and downlinks. TCH full rate or half rate Carries digitized speech or user data Full Rate:- User data is contained within one TS per frame Half rate: User data is mapped onto the same time slot,but sent in alternate frames 2 half rate users would share the same time slot but alternately transmit during during every other frame Can be on any physical channel, except TS0 of certain frequency channels (i.e. BCH freq ch) Frames of TCH data are broken every 13 th frame by SACCH or idle frames For every 26 frame the 13 th & 26 th frames consists of SACCH or idle frame respec. 26 th frame contains idle bits full rate TCH 26 th frame contains SACCH data- half rate TCH Full-Rate TCH Full-Rate Traffic Channels: Transmission Rate of 22.8kbps Full-Rate Speech Channel (TCH/FS): Carries user speech data with 13kbps With channel coding added it carries 22.8kbps. 8

9 Full-Rate Data Channel for 9600bps (TCH/F9.6): Carries user raw data with 9.6kbps. With channel coding added it carries 22.8kbps. Full-Rate Data Channel for 4800bps (TCH/F4.8): Carries user raw data with 4.8kbps. With channel coding added it carries 22.8kbps. Full-Rate Data Channel for 2400bps (TCH/F2.4): Carries user raw data with 2.4kbps. With channel coding added it carries 22.8kbps. Half-Rate TCH Half-Rate Traffic Channels: Transmission Rate of 11.4kbps Half-Rate Speech Channel (TCH/HS): Carries user speech data with 6.5kbps.With channel coding added it carries 11.4kbps. Half-Rate Data Channel for 4800bps (TCH/H4.8): Carries user raw data with 4.8kbps. With channel coding added it carries 11.4kbps. Half-Rate Data Channel for 2400bps (TCH/H2.4): Carries user raw data with 2.4kbps. With channel coding added it carries 11.4kbps. Half-Rate Data Channel for 1200bps (TCH/H1.2): Carries user raw data with 1.2kbps. With channel coding added it carries 11.4kbps. d) State important features of IS-95 Ans. Features of IS-95 Any Four 4 M 1. Diversity 2. Power control 3. Soft handoff 4. IS-95 system capacity 5. Soft capacity 6. Quality of service 7. Economics 1. Diversity The cellular system are having tendency to multipath fading and diversity methods of some are required to mitigate the effect of fading. Type of diversity in CDMA is: 9

10 Time diversity: - Provided by symbol interleaving, error detection& correction coding Frequency diversity: - provided by 1.25MHz Space (path) diversity: - Multipath signals accepted by Receiver 2. Power control For the CDMA system to work efficiently the RF power in the system need to be controlled. All the transmission from mobile must receive at base station receiver at approximately the same strength (within 1dB). To maximize the no. of users sharing a cell, only minimum RF power required for reliable communication. 3. Soft handoff The soft handoff in a CDMA system results from system capability to simultaneously deliver signal to mobile through more than one cell. 4. System capacity Key parameters that determine capacity of CDMA are as follow: a. Processing gain ratio of spreading code information data rate (W/R). b. Ratio of energy per bit to noise power(eb/no) c. Voice activity factor 5. Soft capacity CDMA based on spread spectrum concept are designed to tolerate some level of interference, with their overall capacity limited by how well this mutual interference call be controlled. This is important when call might be dropped during handoff. 6. Quality of service By using RAKE receivers and other improved signal processing techniques. Each mobile station selects the three strongest multipath signal and coherently combines them to produce an enhanced signals. 7. Economics CDMA is cost effective technology that require fewer cell site and no costly frequency reuse pattern. 10

11 The average power transmitted by CDMA mobile station average 6 to 7 mw,which is lower than required by FM and TDMA phones. Transmitting low power- battery life longer e) Describe N- AMPS mobile radio system in detail. Ans --4M To increase the capacity in large AMPS markets, Motorola developed an AMPS- like system called N- AMPS (narrowband AMPS) in N-AMPS were a useful transition technology before 2 G equipment became available. N AMPS provided three users in a 30 khz AMPS channel by using FDMA and 10 khz channels, and provided three times the capacity of AMPS. By replacing AMPS channels with three N- AMPS channels at one time, service providers were able to provide more trunked radio channels (and thus a much better grade of service) at base stations in heavily populated areas. N- AMPS used the SAT and ST signaling and blank and burst functions in exactly the same manner as AMPS, except the signaling was done by using subaudiable data streams. Since 10 khz channels are used, the FM deviation is decreased in N-AMPS. This I turn reduces the S/ (N+1) which degrades the audio quality with respect to AMPS. To counteract this, N-AMPS uses voice companding to provide a synthetic voice channel quieting. Ti counteracts this, N-AMPS, except the signaling was done by using subaudiable data streams. N-AMPS specify a 300Hz high pass audio filter for each voice channel so that supervisory and signaling data may be sent without blanking the voice. The SAT and ST signaling is sent using a continuous 200 bps NRZ data stream that is FSK modulated. SAT and ST are called as DSAT and DST in N- AMPS because they are sent digitally and repetitiously in small, predefined code blocks. There are seven different 24bit DSAT code words which may be selected by the MSC, and the DSAT code words are constantly repeated by both the base station and mobile during a call. The DST signal is simply the binary inverse of the DSAT. The seven possible DSATs and DSTs are specially designed a provide a sufficient number of alternating 0s and 1s so that dc blocking may be conveniently implemented by receivers. The voice channel signaling is done with 100bps Manchester encoded FSK data and is sent in place of DSAT when traffic must be passed on the voice channel. As with AMPS wideband signaling, there are many messages that may be passed between the base station and subscriber unit, and these are transmitted in N- AMPS using the same BCH codes as in AMPS with a predefined format of 40 bit blocks on the FVC and 48 bit blocks on the RVC. 11

12 Q3 Attempt any FOUR of the following: (16 ) a) Draw the block diagram of basic cellular system and state the function of each block. 2 Marks--diagram 2 marks-- Explanation : Explanation:- Fig shows a cellular telephone system that includes all the basic components necessary for cellular telephone communication. The radio network is defined by a set of radio frequency transreceiver located within each of the cells. The location of these radio frequency transreceivers are called base station Base station: base station serves as central control for all users within that cell. Mobile unit communicate directly with the base stations & the base stations communicate directly with a mobile Telephone switching office (MTSO):-An MTSO controls channel assignment, call processing, call setup & call termination which includes signaling switching, supervision & allocating radio-frequency 12

13 channels. The MTSO provides a centralizes administration & maintenance point for the entire network & interfaces with the public telephone network over wire line voice trunks & data links. OR Note :-Student can interpret block diagram in different way but it should consist basic blocks marks should be given for that also b) State important parameters of W-CDMA(UMTS) (4 Marks) (Any correct parameter- ½ mark each) c) Describe channel planning for wireless system. Ans : 4 Marks any correct Explanation. The process of assigning the radio channels to each base station in a cellular system is very important. These should be an appropriate frequency reuse ratio (or cluster size) and adequate separation between the adjacent co-channel cells. The wireless engineers will have to deal with some practical problems such as difficulties associated with the radio propagation and imperfect coverage regions of each cell. 13

14 The available mobile spectrum is divided into channels, which are part of an air interface standard that is used throughout a country. These channels are made up of control channels and voice channels. Typically, about 5% of entire mobile spectrum is devoted to control channels, which carry data messages that are vital for initiating, requesting call, while the remaining 95% of the spectrum is dedicated to voice channels. Since control channels are vital in call initiation and call set up, the frequency reuse strategy applied to control channels is different and generally more conservative than for voice channels. Typically, the control channels are able to handle a great deal of data such that only one control channel is needed within cell. Sectoring is used for improving the signal to interference ratio which can result as smaller cluster size. In CDMA the cluster size is N=1 and frequency planning is not nearly as difficult as for TDMA. CDMA schemes need hard hand off similar to TDMA and FDMA schemes. d) Describe 3G-TD-SCDMA with respect to spectrum utilization, bandwidth, data rate and antenna Ans : 1 Mark each parameter Spectrum utilization Bandwidth Data Rate Antenna Used Less spectrum utilization,5ms frame is used and divided in to seven time slot and assigned to user High, 1.6 MHz Up to 384 kbps of packet data rate Smart antenna 14

15 e) Draw the block diagram and explain the operation of frequency synthesizer Ans:- 2Marks--Diagram 2Marks Explanation The synthesizer is used for developing all the signals used by the transmitter and receiver. It uses the PLL circuits and a mixer. The crystal oscillator provides a reference for the two PLL s The output of VCO-2 is used as the local oscillator frequency for the first mixer in the oscillalator. The outputs of the two VCOs are mixed together, to produce the transmitter output frequency. The frequency divider blocks receive the divide by numbers from the logic section. These numbers are given by the MTSO computer. The divide by numbers will set the transmit and receive channel frequencies. The two outputs produced by the frequency synthesizer are applied to the modulator box in the transmitter and the first mixer in the receiver respectively. The frequency synthesizer thus acts as a local oscillator which can produce a wide range of frequencies with high stability. 15

16 Q 4a) Attempt any three of the following (12) Ans i) List the important GSM air specifications.(any 8) (4marks) Any 8 correct specifications- 1/2 mark each. Sr.No. Parameter Specification 1. Reverse Channel Frequency MHz 2. Forward Channel Frequency MHz 3. ARFCN Number 0 to 124 and 975 to Tx/Rx Frequency Spacing 45MHz 5. Tx/Rx Time Slot Spacing 3 Time Slot 6. Modulation Data Rate kbps 7. Frame Period ms 8. User per Frame (Full Rate) 8 9. Time Slot Period 576.9µs 10. Bit Period 3.692µs 11. Modulation 0.3 GMSK 12. ARFCN Channel Spacing 200 KHz 13. Interleaving (max. delay) 40ms 14. Voice Coder Bit Rate 13.4 kbps ii) Explain message and call processing used in GSM. Ans: 1 mark Diagram 1 mark--explanation 16

17 Mobile call origination in GSM 1. The MS sends the dialed number indicating service requested to the MSC(via BSS) 2. The MSC checks from the VLR if the MS is allowed the requested service. If so, MSC asks BSS to allocate necessary resources for the call. 3. If the call is allowed, the MSC routes the call to GMSC. 4. The GMSC routes the call to the local exchange of called user. 5. The LE alerts (applies ringing) the called terminal. 6. Answer back (ring back tone) from the called terminal to LE. 7. Answer back signal is routed back to the MS through the serving MSC which also completes the speech path to the MS. 1 mark--diagram OR 1 mark Explanation Mobile call termination: 1. The PSTN user dials the MSISDN of the called user in GSM. 2. The LE routes the call to the GMSC of the called GSM user. 3. The GMSC uses the dialed MSISDN to determine the serving HLR for the GSM user and interrogates it to obtain the required routing number. 4. The HLR requests the current serving VLR for the called MS for a MSRN(MS roaming number) so that the call can be routed to the correct MSC. 5. The VLR passes the MSRN to the HLR. 17

18 6. The HLR passes the MSRN to the GMSC. 7. Using the MSRN, the GMSC routes the call to the serving MSC. 8. The MSC interrogates the VLR for the current location area identity (LAI) for the MS. 9. The VLR provides the current location for the MS. 10. The MSC pages MS via the appropriate BSS. The MS responds to the page and sets up the necessary signaling links. 11. When the BSS has established the necessary radio links, the MSC is informed an the call is delivered to the MS. 12. When the MS answers the call, the connection is completed to the calling PSTN user. Message Processing 2M OR any other correct explanation should be considered 1. Types: The types of messages to be transmitted over the reverse control channel (RCCH) are: Page response message : When the mobile station receives a page from the land station, the mobile station responds back. Origination message: The mobile station originates the call. Order confirmation message: The mobile station responds to the order from the land station. Order message: The mobile station orders the tasks which should be performed by the land station and the mobile transmission switching office (MTSO). 2. Structure : All messages contains the following: An application message header. Mandatory fixed parameters. Mandatory variable parameters. Remaining length. Optical variable parameters. iii) Describe the important features of 3G-CDMA Ans (4 marks) a) CDMA 2000 is an up gradation of 2 and2.5g CDMA technology. b) It supports much higher data rates as compared to those of 2G and 2.5G systems. c) Fundamental principle is the high speed data packet network designed for mobility using internet protocol. d) Channel bandwidth 1.25MHz per radio channel e) Up gradation ensures backward compatibility with existing CDMA. f) It has improved capabilities over W-CDMA at each cell can be introduced without changing the base station entirely. 18

19 g) Number of users that can be supported by 3G CDMA 2000 is almost twice the users supported by 2G CDMA system. h) Also longer battery life. i) It has wide range of telecommunication services such as voice, data, multimedia, internet etc. k) It can operate in multiple radio environment such as cellular, cordless, satellite,lan etc. iv) Compare CDMA and TDMA. (any four points) Ans : (One mark for every correct comparison) 4M TDMA CDMA 1) It is multiple access technique in GSM radio system. It is multiple access technique in IS-95 radio system. 2) The receiving equipment must be able to decode the received signal by decoding the received signal and reconstitute it using the same time slot selection algorithm as was The receiving equipment must be able to decode the received signal by having the ability to replicate this pseudo-random digital sequencing. used when it was encoded and transmitted. 3) It is technology for delivering digital It is digital cellular technology that uses wireless service using time-division spread-spectrum techniques. multiplexing (TDM). 4) TDMA technology divides a radio frequency into time slots and then allocates these time slots to multiple calls. In this way, a single frequency can support multiple, simultaneous data channels. CDMA does not assign a specific frequency for each user placing or receiving a call. Individual conversations are encoded with a pseudo-random digital sequence scheme. 5) It is analog system It is digital system. 19

20 Q4. b) Attempt any ONE of the following (6) Ans: i) What is WLL? Explain with neat sketch. WLL stands for Wireless Local Loop. Microwave wireless links can be used to create a wireless local loop such as shown in figure below.3 marks--diagram 3 marks Local Loop is a network that resides between the central office (CO) and the individual homes and business in close proximity to the central office (CO) as shown in figure above. In most developed countries, copper or optical fiber cable already has been installed to residence and business. One more advantage of WLL is that we have to pay only once for that wireless equipment, after there is no additional costs involved. System WLL is based on Cellular, satellite, microcellular The WLL can greatly improve the telecommunication facilities and services in an inexpensive way. OR 20

21 3 marks Transceive WA U PST Trun Switch WLL Controlle r AM HL 3 marks Wireless Access Network Unit (WANU): Interface between underlying telephone network and wireless link. It consists of Base Transceiver Station (BTS), Radio Controller (RPCU), Access Manager (AM), Home Location Register (HLR) Wireless Access Subscriber Unit (WASU): Located at subscriber side. Translates wireless link into a traditional telephone connection ii) Draw the block diagram of receiver unit of mobile handset and state its function. Ans: 3marks 21

22 Explanation 3 marks The main function of receiver is to capture the signal of the desired frequency by filtering other adjacent frequency and demodulate signal and give to loudspeaker, apart from that mobile receiver also able to detect RSSI signal and other usable signals transmitted by BS and MSC The receiver is a dual conversion super heterodyne radio receiver. An RF amplifier boosts the level of received cell site signal. The first mixer translates the incoming signal down to a first IF of 82.2MHz. The signal passes through IF amplifier and filters to the second mixer. The second IF is usually either 10.7MHz or 455KHz The signal is then demodulated, de-emphasized, filtered and amplified before being applied to the output speaker in the hands. Q5 Attempt any TWO of the following : (16) a) With the help of neat sketches explain the microcell zone concept. State its advantages and applications. Ans : 2 Marks--- Diagram 22

23 3 Marks--- Explanation The problem associated with the sectoring is the number of handoffs.this puts additional load on the switching and control link elements of the mobile system. A solution to this problem is based on microcell concept for seven cell reuse. In this scheme, all the three or more zone sites represented as Tx/Rx are connected to the same base station and share the same radio equipment. The transmission media used for connecting the zones to the base station are coaxial cable, fiber optics cable or a microwave link. So each cell consists of a base station and multiple zones. A mobile travelling within a cell, is served by the zone that has the strongest signal of all. As shown in figure, the antennas in zones are placed at the outer edge of the cell and any base station channel can be assigned to any zone by the base station. As a mobile travels from one zone to the other within a cell, it uses the same channel. this will avoid handoff. the base station will just switch the channel to the appropriate zone site. Thus a given channel is being used only in a particular zone in which the mobile is travelling. So the base station radiation is localised.this will reduce interference. The channels are distributed in space and time by all zones and are reused in the co channel cells. The microcell zone concept is very useful along highways or in the busy areas Advantages of micro zone cell concept --2 Marks It reduces co-channel interference. It maintains a particular coverage radius. It improves the signal quality and leads to increase in capacity. It reduces handoffs as compared to sectoring and cell splitting. Applications -- 1 mark Zone cell architecture are being adopted in many cellular and personal communication systems. 23

24 b) Draw the block diagram of reverse CDMA channel modulation process and explain it in detail. 4 Marks ---diagram 4 Marks --- Explanation The block diagram of reverse traffic channel modulation process is shown in the above figure. The voice information is digitized using QCELP variable rate coded. The following processes are carried out: Convolution encoder The data or digitized speech is encoded by a convolution encoder with rate 1/3.Thus tripling the effective data rate to a maximum of 28.8kbps A stronger rate convolution encoder is used to compensate for the absence of a pilot channel. For lower data rates the encoder output bits are replicated to give the 28.8kbps rate. Interleaver The data are then interleaved in 20 ms blocks to reduce the effects of errors by spreading them out. Orthogonal Modulator The data coming out of the block interleaver are grouped in units of 6 bits..each 6 bit unit serves as an index to select a row of the 64x64 Walsh matrix and that row is substituted for the input. The purpose of this encoding is to improve the reception the base station. Data Burst Randomizer. 24

25 The data from the 64 ary modulator is fed into the data burst randomizer. This is used to reduce reverse link power during a quiet period of speech by pseudorandom masking out redundant symbols produced by symbol repetition. This is achieved by data burst randomizer.. Each mobile station is assigned a unique PN spreading sequence. c) With the help of neat sketches explain the forward link and reverse link channel structure used in IS-95. Ans: Forward link Diagram-- 2 marks Explanation: 2 marks Forward channel is a communication channel used for transmission of information from BS to mobile station. Pilot channel: This channel allows a MS to acquire timing for forward CDMA channel. Provides a phase reference for coherence demodulation. It provides each mobile with a means for signal strength comparison between BS for determining when to handoff. Synchronization channel: The synchronization channel broadcasts synchronization message to the MS. The sync channel message parameters are System Identification (SID) and Network Identification (NID) Paging Channel: The paging channel is used to send control information and paging messages from BS to the mobile and paging messages from BS to the mobile. 25

26 Forward Traffic channel: Information on the forward traffic channel includes the primary traffic (voice and data) secondary traffic (data) and signaling. Reverse link channel structure: 2 marks Explanation: 2 marks Reverse channel is a communication channel used for transmission of information from MS to base station. Access channel: On access channel mobile users communicate short messages such as information registration, call origination, and responses to pages, or providing other data to cell site. When mobile user places a call, it uses the access channel to inform the serving base station. Reverse Traffic channel: This traffic channel carries the user speech or data. It is similar to the forward traffic channel and is intended to transfer dedicated user data. The traffic channels in reverse channel are unique to each mobile user. Q6 Attempt any TWO of following: (16) a) Compare the following : ( i) Hard handoff and soft handoff (4 Marks) Ans: -- 1 mark for each correct comparison 26

27 Hard Hand off The definition of a hard handover or handoff is one where an existing connection must be broken before the new one is established Hard handoff allocate different frequency of user In hard hand off a handset always communicates with one BS at any given time Hard handoff is typically used in TDMA and FDMA systems Hard hand off is not very complicated. Since the radio link between the BS and the handset is broken before it is connected in hard handoff, the link transfer may fail due to long network response time even if radio channels are available in the new BS. Soft Hand off Soft hand off is defined as a handover where a new connection is established before the old one is released. Soft hand off allocate same frequency. In soft hand off a handset may connect up to three or four radio links at the same time Soft hand off used in CDMA and some TDMA systems Soft handoff is more complicated than hard handoff On the other hand, soft handoff degrades channel availability because a handset may consume multiple radio channels (ii) Delayed hand off and queued hand off (4 Marks) Ans mark for each correct comparison Delayed hand off A Delayed hand off is a two hand off level algorithm. It provides more opportunity for a successful hand off. The MTSO always handles the handoff first and the originating calls second. If no neighboring cells are available after the second handoff level is reached, the call continues until the signal strength drops below the threshold level then the call is dropped. Lower handoffs help in handling call processing more adequately It makes the hand off occur at the proper location and eliminates possible interference in the system. Queued hand off Queued hand off is more effective than two threshold level hand-offs. The MTSO will queue the requests of hand off calls instead of rejecting them if the new cell sites are busy. With Queuing of originating calls only, the probability of blocking is reduced. It is effective when implementing a simple queue for hand off calls which reduces call drops. 27

28 Q6. b) Derive the expression for the parameter Q, called the co channel reuse ratio which is related to the cluster size N. Assume hexagonal geometry, i.e. you have to prove that Q=D/R= Ans : 8 Marks for correct derivation 28

29 Q6. c) State different GSM control channels (CCH) and explain their functions. Ans : 4 Marks---channel diagram 4 Marks Explanation 29

30 GSM control channel (CCH): 1) Broadcast channel (BCH): (a) Broadcast control channel (BCCH):The BCCH is a forward control channel that is used to broadcast information such as cell and network identity, operating characteristics of the cell (current control channel structure, channel availability and congestion). The BCCH also broadcast a list of channels that are currently in use within the cell. (b) Frequency correction channel (FCCH): The FCCH allows each subscriber unit to synchronize its internal frequency standard (local oscillator) to the exact frequency of the base station. (c)synchronization channel (SCH): SCH is used to identify the serving BS while allowing each mobile to frame synchronizes with the BS. The frame number (FN) is sent with the base station identity code (BSIC) during the SCH burst. (b) Common control channel (CCCH): (a) Paging channel (PCH): The PCH provides paging signals from the BS to all mobiles in the cell, and notifies a specific mobile of an incoming call which originates from PSTN. PCH may be used to provide cell broadcast ASCII text messages to all subscribers. (b) Random Access Channel (RACH): The RACH is a reverse link channel used by a subscriber unit to acknowledge a page from the PCH and is also used by mobiles to originate a call. (c) Access grant channel (AGCH): The AGCH is used by the BS to provide forward link communication to the mobile, and carries data which instructs the mobile to operate in a particular physical channel. (c) Dedicated control channel (DCCH): (a) Stand-alone Dedicated control channel (SDCCH): The SDCCH carries signaling data following the connection if the mobile with the BS, and just before TCH assignment issued by the BS. The SDCCH ensures that the mobile station and base station remain connected while the BS and MSC verifies subscriber unit. (b) Slow Associated Control Channel (SACCH): On the forward link the SACCH is used to send slow but regularly changing control information to the mobile such a transmit power level instruction. On the reverse link the SACCH carries information about the received signal strength. (c) Fast Associated Control Channel (FACCH): FACCH carries urgent messages and contains essentially the same type of information as SDCCH. 30