a) Describe the basic cellular system. (2M Diagram & 2 M Explanation)

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1 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 judgement 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. Q.1A.) Solve any three of the following a) Describe the basic cellular system. Answer:- (2M Diagram & 2 M Explanation) OR 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 transreceiver 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 1

2 Telephone switching office (MTSO): The Switching center is one component which co-ordinates the routing of calls in a large service area. An MTSO controls channel assignment, call processing, call setup & call termination which includes signaling switching, supervision & allocating radiofrequency channels. The MTSO provides a centralized 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) Draw the block diagram of mobile unit & explain each block Answer:-. (2 M Diagram & 2 M Explanation) OR 2

3 (Relevant explanation of above block diagram should be given marks) Mobile unit:- The RF selection contents the transmitter and receiver CKT including mixers local oscillators or frequency. Synthesizers for channel selection. The receiver LNA and the transmitter power ampere. The base band selection contains the Vocoder with its A / D and D/ A converts plus a DSP chip that handles many processing functions typically performed by analog ckt in older synthesizers. An embedded control handles all the digital control and signaling, hand offs and connection and identification, operation that take place transparent to the subscriber. It also takes care of running the display and keyboard and all user functions such as number storage, auto dialing and caller ID. This has led to some interesting architectures especially, in the receiver section. Although super heterodyne design is still used, several variations have emerged as dominant. These are the down conversion and very low - IF designs. c) Describe the functional Architecture of IMT-2000 Answer. (2M Diagram & 2M Explanation) 3

4 OR Functional architecture of IMT 2000 :- IMT 2000 expected to support a number of different radio operating environments covering indoor picocells with very high overall system capacity all the way through large outdoor terrestrial cells and satellite. The broad classes of functions that need to be supported by an IMT system are shown in the fig. above. It is seen that separation of call and collection controls functions and is desirable for supporting multimedia and advanced services in IMT In this architecture the connection and call control function are integrated rather than separated. It illustrates one possible assignment of functional entities to physical entities and identifies relationship between these collection and function that need to be supported by the physical interface specification. 4

5 d) Describe the GSM system architecture with neat block diagram. Answer : (2 M Diagram & 2M Explanation) BTS BTS BSC HLR VLR AUC BTS PSTN MS MSC ISDN BTS BTS BSC DATA NETWORKS MS BTS BASE STATION SUBSYSTEM OMC NETWORK SWITCHING SYSTEM PUBLIC NETWORK MS: MOBILE STATION BTS: BASE TRANSWERCIVER STATION BSC: BASE STATION CONTROLLER MSC: MOBILE SWITCHING CENTER z VLR: VISITOR LOCATION REGISTER HLR: HOME LOCATION REGISTER Mobile Station The mobile station (MS) consists of the mobile equipment (the terminal) and a smart card called the Subscriber Identity Module (SIM). The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI). The SIM card contains the International Mobile Subscriber Identity (IMSI) used to identify the subscriber to the system, a secret key for authentication, and other information. Base Station Subsystem: The Base Station Subsystem is composed of two parts: 1. The Base Transreceiver Station (BTS) and 2. The Base Station Controller (BSC). These communicate across the standardized Ab is interface, allowing (as in the rest of the system) operation between components made by different suppliers. The Base Transreceiver Station houses the radio transreceivers that define a cell and handles the radio-link protocols with the Mobile Station. The Base Station Controller manages the radio resources for one or more BTSs. It handles radiochannel setup, frequency hopping, and handovers, as described below. The BSC is the connection between the mobile station and the Mobile service Switching Center (MSC). Network Subsystem: The central component of the Network Subsystem is the Mobile services Switching Center (MSC). It acts like a normal switching node of the PSTN or ISDN, and additionally provides all the functionality needed to handle a mobile subscriber, such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber. 5

6 These services are provided in conjunction with several functional entities, which together form the Network Subsystem. The MSC provides the connection to the fixed networks (such as the PSTN or ISDN). Signaling between functional entities in the Network Subsystem uses Signaling System Number 7 (SS7), used for trunk signaling in ISDN and widely used in current public networks. The Home Location Register (HLR) and Visitor Location Register (VLR), together with the MSC, provide the call-routing and roaming capabilities of GSM. Q.1B.) Solve any one of the following. (6 Marks) a) With the help of neat sketch explain the forward & reverse link channel structure used in IS- 95. Answer : (1M Diagram & 2 M Explanation for forward & reverse link structure) 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. It provides a phase reference for coherence demodulation. It provides each mobile with an answer 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. Forward Traffic channel: Information on the forward traffic channel includes the primary traffic (voice and data) secondary traffic (data) and signaling. 6

7 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. b) Draw & describe the transmitter system in cellular communication with block diagram. Answer:- (3 M Diagram & 3 M Explanation). It is a Low-Power FM unit operating in Frequency range of 825 MHz. Channel 1 is MHz, Channel 2 is MHz & So on. The carrier furnished by frequency synthesizer is phase-modulated by the voice signal. 7

8 The modulator output is translated up to the final transmitter frequency by a mixer whose second input also comes from frequency synthesizer. The mixer output is fed to class c & or class E power amplifier stages where output signal is developed. The final ampere stage is designed to supply about 600 mw to the antenna. The APC ckt can introduce power attenuation in steps of 4db to 28db. This signal is used in APC ckt & is transmitted back to the cell site, permitting the M TSO to know the current power level. The transmitter output is fed to a duplexer ckt or isolator that allows the transmitter & receiver to share the same antenna. Since cellular telephone units use full duplex operation, the transmitter & receiver operates simultaneously. The duplexer consists of two very sharp BPF, one for the transmitter & one for the receiver. The transmitter output passes through this filter to the antenna. Q.2) Solve any four of the following. a) Draw frequency reuse pattern with cluster size 7 & 12. Answer: - (2 marks for each pattern) For cluster size 7: For cluster size 12: 8

9 b) List various features & services of IS-95. Answer:- Features & services of IS-95 (Any Eight 4M) 1. Diversity 2. Power control 3. Soft handoff 4. IS-95 system capacity 5. Soft capacity 6. Quality of service 7. Economics 8. Variable Rate Vocoder Offering high speed coding and reducing background noise and system 9. Interference based on the detection of the voice activity. 10. Coding Technique for Enhancing the privacy and security. 11. Roaming c) Describe the mobile call Terminations Answer:-. (2M Diagram & 2M 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 HLR passes the MSRN to the GMSC. 9

10 6. Using the MSRN, the GMSC routes the call to the serving MSC. 7. The MSC interrogates the VLR for the current location area identity (LAI) for the MS. 8. The VLR provides the current location for the MS. 9. The MSC pages MS via the appropriate BSS. The MS responds to the page and sets up the necessary signaling links. 10. When the BSS has established the necessary radio links, the MSC is informed and the call is delivered to the MS. 12. When the MS Answer the call, the connection is completed to the calling PSTN user. The VLR passes the MSRN to the HLR d) Draw & describe the network architecture of UMTS Answer:- (2 marks Diagram & 2 marks Explanation) The UMTS is a system that is capable of providing a variety of mobile services to a wide range of global mobile communication standards. To handle a mixed range of traffic, a mixed cell layout, which would consist of macrocells overlaid on microcells & picocells is one of the plans being considered. This type of network distributes the traffic with the local traffic operating on the microcells & picocell, while the highly mobile traffic is operated on macrocell, thus reducing the number of handoffs required for fast moving traffic. OR 10

11 It is observed from the fig. that the macro cells cover the spots not covered by other cells & also provide redundancy in certain areas. Thus macro cells will also be able to avoid the failures of the overlapped cells. e) Define the following terms w.r.t. mobile communication. Base station Forward Channel Control Channel Voice Channel Answer:- (1M each) Base station: - A fixed station in a mobile radio system used for radio communication with stations. Base stations are located at the center or on the edge of a coverage region & consist of radio channel & Transmitter and Receiver antennas mounted on a tower. Forward Channel: - Radio channels used for Transmission of information from the BASE STATION to the MOBILE. Control Channel :- Radio channels used for Transmission of call setup, call request, call Initiation & other beacon or control purpose. Voice Channel: Radio channels used for Transmission & reception of voice messages. 11

12 Q3 Attempt any four. a) Describe the near end far end interference in cellular system (16 Marks) Answer: (4M for correct explanation) If motor vehicle in a given cell is moving, then some mobile phone units are close to the cell site and some are not. The close in mobile phone unit has a strong signal which causes adjacent-channel interference In this situation, near end far end interference can occur only at the reception point in the cell site If a separation of five channel bandwidth 95B) is needed for two adjacent channels in a cell in order to avoid the near end far end interference, it is then implied that a minimum separation of 5B is required between each adjacent channel used with one cell The adjacent channel interference can also occur between two systems in a duopoly market system In this situation, adjacent channel interference can occur at both the cell site and the mobile phone unit For instance, mobile phone unit A can be located at the boundary of its own home cell A in the system A but very close to cell B of system B The other situation would occur if mobile phone unit B was at the boundary of cell B of system B but very close to cell A of system A. The same interference will be introduced at cell B and mobile unit B The two causes of near end far end interference are as below Interference caused on the set up channels Interference caused on the voice channels The adjacent channel interference can be minimized through careful filtering and channel assignments Since each cell is given only a fraction of the available channels a cell need not be assigned channels which are all adjacent in frequency By keeping the frequency separation between each channel in a given cell as large as possible the adjacent channel interference may be reduced considerably Some channel of allocation schemes also prevent a secondary source of adjacent channel interference by avoiding the use of adjacent channels in neighboring cell sites In practice, the base station receivers are preceded by high Q cavity filter in order to reject adjacent channel interference The adjacent channel interference can be reduced by the frequency assignment 12

13 b) Describe the WLL technology and also write its application Answer: (2M diagram, 2M explanation) WLL stands for Wireless Local Loop. Microwave wireless links can be used to create a wireless local loop such as shown in figure below. 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 13

14 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. c) Describe the microcell zone concept Answer: (2 Marks--- Diagram 2 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. 14

15 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 localized. 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 d) List the basic features and coding methods in CDMA Answer:- The various types of codes and coding methods used in CDMA system are as under: Coding methods: (2M) Auto correlation codes- a) Barker code b) Pseudo- noise codes Orthogonal codes Walsh codes Scrambling codes Channelization codes Carrier modulation codes Features: (any 4 feature, 2M) It is wide band DS-CDMA system W-CDMA assures backward compatibility with the 2G GSM. Packet data rate Mbps Channel Bandwidth- 5MHz Provides high capacity upgrade path for GSM Frame structure- 16 slots per frame Network structure and bit level packaging of GSM data is retained Computers, entertainment devices, and telephone may all share same wireless network and be connected to internet anytime anywhere. Each channel will be able to support between 100 to 350 voice channels. Uses Direct Sequence Spread Spectrum (DSSS) with chip rate of 16Mcps. Provides Videoconferencing and Virtual Home Entertainment (VHE) 15

16 e) Write a short note on the evolution of radio mobile communications Answer:- (4M for Correct Explanation) Mobile wireless (or radio) communication has started after of electromagnetic (or radio) waves by Hertz Mobile radio communication systems using simplex channel introduced in 1920 for emergency services in USA First public mobile radio system in USA introduced in 1946 The cellular concept was developed by ATT in 1947 and commercial tests were done in 1962 In 1970, ATT proposed AMPS. In 1979 worlds first cellular system was implemented by Nippon Telephone. The analog cellular mobile systems fall in the category of 1st generation mobile system Besides frequency reuse, digital signal processing, integrated circuits, increased battery life lead to exponential growth in mobile communication. The first digital system cellular system was introduced in 1990 by ETSI for GSM. This was considered to be second generation mobile communication. The 3rd generation digital cellular system called PDC was developed by Japan for commercial uses In late 1991, DSDC system hardware was installed in USA, it improved the capacity by 3 times of that provide by AMPS A cellular system based on CDMA was developed by Qualcomm and standardized by TIA. With the rapidly increasing penetration of laptop computers which are primarily used by mobile users to access internet services. Mobile IP allows IP nodes to change physical location without having to change IP address there by offering nomadicity to internet users IMT-2000 standard is being developed to set stage for 3rd generation mobile communication system. It will consolidate different wireless environment and will also ensure global mobility in terms of global seamless roaming and delivery of services ETSI has also developed 3rd generation mobile communication system call UMTS which belongs to IMT-2000 system Q-4 Attempt any three (12 Marks) a) Describe the messages and call processing in GSM system Answer: (1 mark Diagram 16

17 Mobile call origination in GSM: 1 mark explanation The MS sends the dialed number indicating service requested to the MSC(via BSS) 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. If the call is allowed, the MSC routes the call to GMSC. The GMSC routes the call to the local exchange of called user. The LE alerts (applies ringing) the called terminal. Answer back (ring back tone) from the called terminal to LE. Answer back signal is routed back to the MS through the serving MSC which also completes the speech path to the MS. OR 1 mark Diagram 17

18 Mobile call termination: 1 mark Explanation The PSTN user dials the MSISDN of the called user in GSM. The LE routes the call to the GMSC of the called GSM user. The GMSC uses the dialed MSISDN to determine the serving HLR for the GSM user and interrogates it to obtain the required routing number. 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. The VLR passes the MSRN to the HLR. The HLR passes the MSRN to the GMSC. Using the MSRN, the GMSC routes the call to the serving MSC. The MSC interrogates the VLR for the current location area identity (LAI) for the MS. The VLR provides the current location for the MS. The MSC pages MS via the appropriate BSS. The MS responds to the page and sets up the necessary signaling links. When the BSS has established the necessary radio links, the MSC is informed an the call is delivered to the MS. 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 contain the following: An application message header Mandatory fixed parameters Mandatory variable parameters Remaining length Optical variable parameters b) Describe the architecture of SS Answer:- (Diagram 2M, Explanation 2M) The message transfer part (MTP) provides OSI level protocol model as level-1 data service, level-2 link service and level-3 network service The full level-3 service is provided by the signaling connection control part (SCCP) The SCCP provides an enhanced addressing capability that may be considered as level 3+ or close to level-4 18

19 The layer 4-6 in the OSI model don t exist in SS7 protocol model The transaction capability application part (TCAP) level and operations maintenance and administration (OMAP) part level are considered the same as the Application level-7 in the OSI The application service element ASE is at the same level as the OMAP The TCAP includes protocol and services to perform remote operations The primary use of TCAP in these networks is for invoicing remote procedures in supporting IN service like, 800 services The OMAP provides the application protocols and procedures to monitor coordinate and control all the network resources which make communication based on SS7 possible The ASE is for the MTP routing verification test (MRVT) which uses the connectionless services of TCAP The MRVT is an important function on OMAP c) How and why is the variable spread factor used in CDMA 2000 Answer:- (4 Marks for Correct Explanation) In case of CDMA 2000 the spread factor can vary from 4 to 256 depending upon the data rates. The use of interleaving and variable spread factor enables support of the data rates up to 386kbps When high data rate signals are to be transmitted then multi carrier transmission can be used We can use N carriers where N=1, 3, 6, 9 or 12 in CDMA Similarly chipping rates of Nx M-chips-per-sec can be used 19

20 In CDMA 2000 there are three carriers of 1.25 MHz each for N=3 and all carriers have a separate code for each channel Any relevant answer should be given full marks d) What is the necessity of logical channel in GSM system Answer:- (4 marks for Correct explanation) In practice, a multimode terminal used by a third generation (3G) mobile communication system network will have to scan for suitable frequency band or channel, identify application radio and standard and select from among the set or available services If it develops at a very large number of frequency bands need to be scanned and the many standards need to be searched registering such a roaming multimode terminal by means of systematic scanning procedure will become very inefficient, tending to degrade quality of service for the users perspective This problem can be alleviated by using common physical or logical broadcast channel called the global radio control channel Therefore there is a necessity of logical channel in GSM system to scan a single frequency or a small range of frequency and thereby find the required information on available network or standards and services Q.4 b) Attempt any one: (6 Marks) a) Describe the processing units in CDMA-2000 system. How do these units differ from those in a CDMA-one system? Answer: (Diagram 3M, Explanation 2M, difference from CDMA-one 1M) Diagram: OR 20

21 Explanation: CDMA 2000 is a technology for the evolution of IS-95/IS-95A to 3G services. The CDMA 2000 cellular technology provides a seamless and evolutionary high data rate upgrade path for current users of 2G. The processing units in CDMA2000 is used for lower control messages and channels for data synchronization, pilot and traffic baseband transmission and use of mux unit, long code and mask, PNa, PNi and Walsh code In IS95 the data channel is divided into I and Q components after chipping with Walsh codes Data channel is divided before scrambling In CDMA2000, the mux/iq first divides the signal into the I and Q from other user traffic and user data channels. The pilot and synchronization channels are multiplexed and passed through PN short code spreaders and base band filters There are three sets of chips after multiplexing when the chipping rate is 3 times MChips/s Each chip has two components I and Q. After the PN short code encoding with I and Q pilots and based band filtering of I and Q components, a balanced QPSK modulator modulates the signal for downlink and a dual channel QPSK modulator modulates for uplink Mobile station (MS): This is the mobile phone unit with the user. The MS terminates the radio path on the user side and enables the user to gain access to service from the network. Base station (BS): the MS terminates the radio path and connects to the MSC. Base Transceiver Station (BTS): BTS consists of one or more transceivers placed at a single location and terminate the radio path on the network side. Base Station Controller (BSC): The MSC is the control and management system for one or more BTS. The BSC exchanges messages with both the BTS and MSC. 21

22 Mobile Switching Centre (MSC): The MSC is an automatic system that interfaces the user traffic from wireless network with the wire line network or other wireless networks. MSC provides radio contact to a call. Authentication, Authorization and Accounting (AAA): This unit in the processing unit is responsible for verification of the user and billing. The system and end users are connected with IP network through cloud. Difference from CDMA-one System: The CDMA 2000 employs multicarrier operation in which user is allowed to use 1,3,6, or 9 of the cdmaone channels to support a more reliable voice and variety of data channels. The system can use QPSK, 8-PSK or 16-QAM modulation on a single 1.25MHz channel. The uplink is more flexible using TDMA as well as CDMA to divide a channel into 240 time slots, each of which can be allotted to a different mobile user. It can support up to twice as many voice users as the CDMA-one system. The CDMA 2000 system used rapidly adaptable baseband signaling rates and PN code rates for each mobile user through incremental redundancy and multi0-level keying. The main difference in IS95 processing unit is that the data channel is divided in I and Q components after scrambling with Walsh codes. While in CDMA2000 they are first divided and then passed through PN short code spreaders. b) Draw and explain the block diagram of cellular receiver system in mobile communication. Answer: (Block diagram 3M, Explanation 3M) Block Diagram: 22

23 RF Amplifier: RF amplifier boosts the level of the received cell site signal. The receiver frequency range is to MHz. There are 666 receive channels spaced 30 KHz apart. First Mixer and IF amplifier: The first mixer translates the incoming signal down to a first IF of 82.2 MHz or 45MHz. The LO signal is derived from the frequency synthesizer. The LO frequency sets the receive channel. The signal then passes through IF amplifier for amplification. Second mixer and IF amplifier: This mixer is driven by a crystal controlled local oscillator. The second IF is usually either 10.7MHz or 455 KHz. FM Demodulator: The signal is then demodulated, de-emphasized, filtered, and amplified before being applied to the output speaker in the handset. The output of the demodulator is also fed to other filter circuit that select out the control audio tones and digital control data stream sent by the base station to set and control both the transmitter and receiver. DC amplifier and RSSI signal: The demodulator output is also filtered into a DC level whose amplitude is proportional to the strength of the received signal. This is the received signal strength indicator (RSSI) signal that is sent back to the base station so that MTSO (MSC) can monitor the received signal from the cell and make decision about switching to another cell. 23

24 Q.5. Attempt any two: 16 Marks a) Describe how the improving coverage and capacity in cellular system and also list the techniques for improving the capacity of cellular system. Answer: (Note: If student draw the diagram of Cell splitting, Cell sectoring, microcell zone concept, frequency reuse then 2 M for diagram 2 M for explanation should be given) Description: Cell splitting is the process of subdividing congested cell into smaller cells, each with its own base station and a corresponding reduction in antenna height and transmitter power. Cell splitting increases capacity of a cellular system since its increases the number of times that channels are reused. Cell sectoring increases SIR using directional antennas, then capacity improvement is achieved by reducing the number of cells in a cluster, thus increasing frequency reuse. Often wireless operator needs to provide dedicated coverage for hard-to-reach areas. Such as within buildings, or in valleys or tunnels. Radio transmitters, known as repeaters are often used to provide such range (coverage) extension capabilities. In microcell zone concept each of three (or possibly more) zone sites are connected to a single base station and share the same radio equipment. Decreased co-channel interference improves the signal quality and also leads to increase in capacity without the degradation in trunking efficiency caused by sectoring. 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 distance large enough to keep interference levels within tolerable limits and capacity of system can be increases is called a frequency reuse or frequency planning. Techniques for improving capacity: Cell splitting Cell sectoring Microcell-zone concept Frequency Reuse b) Write a note on following components: a) Mobile station b) Forward channel c) Base Station controller And also state the advantages of cellular mobile services. 24

25 Answer : Mobile Station: (2 Marks) A Station in cellular radio service intended for use while in motion at unspecified locations. Mobile stations may be handheld personal units (portable) or installed in vehicles (mobiles). A MS comprises of single antenna, transceiver, and microprocessor- based control circuit. Because cellular system is full duplex, the transmitter and receiver must operate with single antenna. Forward channel: (2 Marks) A radio channel used for transmission of information from the base station to mobile is called forward channel. The forward voice channel can be further divided in to Forward Voice Channel (FVC) which is used for voice transmission from base station to mobile, and Forward Control Channel (FCC) which is responsible for initiating mobile calls, and service requests, and are monitored by mobile when they do not have a call in progress. Base Station Controller (BSC): (2 Marks) The base station controller manages each of the radio channels at each cell-site, turns the radio transmitter and receiver ON and OFF, transfers data on to the control and voice channels, monitors calls and performs built-in diagnostic tests on the cell-site equipment. The BSC operates under the control of the central switching center MSC or MTSO. Advantages of cellular mobile services: (2 Marks) Instant Communications- Coast to Coast in Under a Second. Solves many interoperability problems with first responders. Bypasses the PSTN. Higher voice compression= less blocking. Priority Connect available. High and variable data rate. On the air privacy Subscriber Identity Module (SIM) c) Draw the block diagram and explain each block and also write the list of various types of SS7 services. Note: Any of the block diagram or Protocol architecture should be given marks Answer: The SS7 signaling protocol is widely used for Common Channel Signaling (CCS) between interconnected networks. 25

26 Block Diagram: (3 Marks) Explanation of each block: (3 Marks) An SS7 network is composed of service switching points (SSPs), signaling transfer points (STPs), and service control points (SCPs). Service Switching Point (SSP): The SSP gathers the analog signaling information from the local line in the network (end point) and converts the information into an SS7 message. These messages are transferred into the SS7 network to STPs. Signaling Transfer Points (STPs): The STP transfers the packet closer to its destination. When special processing of the message is required (such as rerouting a call to a call forwarding number), the STP routes the message to a SCP. Service Control Points (SCPs): The SCP is a database that can use the incoming message to determine other numbers and features that are associated with this particular call. Signaling Connection Control Part (SCCP) translation function: This is a process in the SS7 system that uses a routing tables to convert an address (usually a telephone number) into the actual destination address (forwarding telephone number) or into the address of a service control point (database) that contains the customer data needed to process a call. OR Protocol Architecture: (3 Marks) 26

27 Explanation of protocol architecture: (3 Marks) Network Service Part of SS7: The NSP provides ISDN with a highly reliable and efficient means of exchanging traffic using connectionless services. a) Signaling data link functions (MTP Level 1): This level provide an interface to the actual physical channel over which communication takes place. b) Signaling link function (MTP Level 2): It provides a reliable link for the transfer of traffic between two directly connected signaling points. c) Signaling Network Function (MTP Level 3): Provides procedure that transfer message between signaling nodes. d) Signaling Connection Control Part (SCCP): The SCCP provides enhancement to the addressing capabilities provided by the MTP. SCCP also provides the ability to address global title messages, such as 800 numbers or non-billed numbers. SS7 User Part: SS7 user part provides call control and management functions and call setup capabilities to the network. a) Integrated Service Digital Network User Part (ISUP): The ISUP provides the signaling functions for carrier and supplementary services for voice, data and video in ISDN environment. ISUP uses the MTP for transfer of messages between different exchanges. b) Transaction Capabilities Application Part (TCAP): The TCAP part in SS7 refers to the application layer which invokes the services of the SCCP and the MTP in a hierarchical format. 27

28 c) Operation Maintenance and Administration Part (OMAP): The OMAP functions include monitoring, co-ordination and control function to ensure that trouble free communications are possible. Various types of SS7 services: (any two) (2 Marks) Touch star 800 services Alternate billing services and Line Information Database (LIDB) Q.6 Attempt any two: 16 Marks a) Draw the schematic diagram of a basic cellular telephone system and describe it. Answer: A cellular telephone system provides a wire-less connection to the PSTN user for any user location within the radio range of the system. Schematic Diagram: (4 Marks) Description: (4 Marks) Figure above shows a basic cellular system which consists of Mobile Station (MS) Base Station (BS), and Mobile Switching Centre (MSC) OR Mobile Telephone Switching Office (MTSO) 1) Mobile Station (MS): The mobile station contains a transceiver, an antenna, and control circuitry and may be mounted in a vehicle or used as a portable hand-held unit. 2) Base Station (BS): 28

29 The base stations consist of several transmitter and receiver which simultaneously handle full duplex communication and generally have towers which support several transmitting frequency and receiving antennas. The BS serves as a bridge between all mobile users and connects simultaneous mobile calls via telephone lines or microwave links to the MSC. 3) Mobile Switching Centre (MSC): The MSC co-ordinates the activities of all the base stations and connects the entire cellular system to the PSTN. A typical MSC handles 100,000 cellular subscribers and 5,000 simultaneous conversations at a time, and accommodates all billing and system maintenance functions as well. Communication between the BS and mobiles is defined by a standard Common Air Interface (CAI) that specifies four different channels. 4) Forward Voice Channel (FVC): This channel is used for voice transmission from the BS to mobile. 5) Reverse Voice Channel (RVC): This channel is used for voice transmission from mobile to BS. 6) Forward Control Channel (FCC) and Reverse Control Channel (RCC): These channels are responsible for initiating mobile calls. Control channels are often called setup channels because they are only involved in setting up a call and moving it to an unused voice channel. Control channels transmits and receives data messages that carry call initiation and service requests, and are monitored by mobiles when they do not have a call in progress. b) Describe the Co-channel interference and system capacity in mobile communication. Answer: Co-channel Interference: Diagram (any 1): (2 marks) OR 29

30 Explanation: (3 Marks) Frequency reuse implies that in a given coverage areas there are several cells that use the same set of frequency. These cells are called co-channel cells, and the interference between signals from these cells is called co-channel interference. Unlike thermal noise which can be overcome by increasing the signal-to-noise ratio (SNR), cochannel interference cannot be combated by simply increasing the carrier power of transmitter. This is because an increase in carrier transmits power increase the interference from neighboring co-channel cells. To reduce co-channel interference, co-channel cells must be physically separated by a minimum distance to provide sufficient isolation due to propagation. When the size of each cell is approximately the same and the base stations transmit the same power, the co-channel interference ratio is independent of transmitted power and becomes a function of the radius of the cell (R) and the distance between centers of the nearest co-channel cells (D). By increasing the ratio D/R, the spatial separation between co-channel cells relative to the coverage distance of a cell is increased. Thus interference is reduced from improved isolation of RF energy from the co-channel cell. Capacity of system: (3 Marks) The parameter Q is called the co-channel reuse ratio, is related with cluster size (N). For hexagonal geometry; A small value of Q provides larger capacity since the cluster size N is small. Whereas a large value of Q improves the transmission quality, due to a smaller level of cochannel interference. 30

31 A trade-off must be made between these two objectives in actual cellular design. c)list various features and services of IS-95 and also write the various services offered by a GSM system. Answer: Features of IS-95: (any three) (3 Marks) Diversity Power control Soft hand-off A 3-finger RAKE receiver is used in IS-95 to improve reception at the BS and MS. Many users of a CDMA system share the same frequency. Either TDD or FDD is used. Channel data rate is very high in IS-96 CDMA system. Services of IS-95: (any two) (2 Marks) Short Message Services (SMS) Slotted paging Over the air activation Call services Data services Services offered by GSM system: (any three) (3 Marks) 1) Telephone Services: Standard mobile telephone Mobile-originated Base-originated traffic. emergency calling Fax Videotext Tele text, SMS MMS. 2) Supplementary ISDN services: Call diversion Caller line ID Closed user group Call barring Call waiting Call hold Connected line ID Multiparty (Teleconferencing) 31

32 Call charge advice This service also include the Short Messaging Service (SMS) which allow GSM subscriber and BS to transmit alphanumeric pages of limited length (160-7 ASCII characters) while simultaneously carrying normal voice traffic. 32