PhD Viva: Pre-Bunched Free Electron Maser

1. PhD Viva: Pre-Bunched Free Electron Maser Cellular Radio Communication EKT443 School of Computer and Communication Engineering Universiti Malaysia Perlis

2. Cellular Systems Base stations (towers) provide radio access between mobile users and MSC. PSTN Mobile Switching Center

3. Basic entities in Cellular Systems • Mobile Stations • Transceiver • Antenna • Control circuitry • Moves at pedestrian or vehicle speed • Base Stations • Several transmitters and receivers • Tower that supports several transmitting and receiving antennas • Link between all mobile users and the MSC • Mobile Switching Centre (MSC) • Connects mobiles to Public Switching Telephone Network (PSTN) • Coordinates activities of all BS • Controls billing and system maintenance functions

4. Cellular Systems • Geographic region broken into “non-overlapping” cells • Each served by its own base station • Band of frequencies allocated to each cell • Cells set up such that antennas of all neighbours are equidistant (hexagon) pattern • Users within a cell communicate with the cell BS • As users move between cells, calls go through “hand-off” when switching to new cell BS

5. Why are cells required? • Original mobile voice networks used transmitter with large power to cover very large area. • Capacity severely limited by available bandwidth • Example: Bell mobile system in New York City in 1970s could support a maximum of 12 simultaneous calls over 100 miles2 • Spectrum limited, hence could not increase capacity by adding new channels • Cellular concept was born

6. Cellular System – Why? B G C A F D E • Limited Spectrum • The cellular concept solved the problem by replacing a single, high power transmitter (large cell) with many low power transmitters (small cells). Each providing coverage to only a small portion of the service area.

7. Cellular concept • Idea: Replace high power transmitters with several low power transmitters to create smaller cells • Multiple cells over a geographic area • Each cell assigned a set of frequencies • Neighbouring cells assigned different group of frequencies to reduce adjacent cell interference • Enables spatial reuse • Increase capacity by increasing number of transmitters and decreasing transmit power • Enables fix bandwidth to serve arbitrarily large number of subscribers

8. Cellular Network Organization • Multiple low power transmitters • 100w or less • Area divided into cells • Each with own antenna • Each with own range of frequencies • Served by base station • Transmitter, receiver, control unit • Adjacent cells on different frequencies to avoid crosstalk

9. Shape of Cells • Square • Width d cell has four neighbours at distance d and four at distance d • Better if all adjacent antennas equidistant • Simplifies choosing and switching to new antenna • Hexagon • Provides equidistant antennas • Radius defined as radius of circum-circle • Distance from center to vertex equals length of side • Distance between centers of cells radius R is R • Not always precise hexagons • Topographical limitations • Local signal propagation conditions • Location of antennas

11. Cellular concept • Problem: Not all mobile calls can be completed within a single cell • Handoff developed • Expensive to build a system with thousands of cells • Large-radius cells evolve into small radius cells over time • When too much traffic in a cell, the cell splits • Central ideas of cellular systems • Small coverage areas (cells) • Frequency reuse • Handoff • Cell splitting to increase capacity

12. Frequency Reuse • Frequency reuse refers to the use of radio channels on same carrier frequency • A set of cells each operate on different channels in a group to form a cluster • A cluster is repeated as many times as necessary to cover wide area

13. Characterizing Frequency Reuse • D = minimum distance between centers of cells that use the same band of frequencies (called co-channels) • R = radius of a cell • d = distance between centers of adjacent cells (d = R) • N = number of cells in repetitious pattern • Reuse factor • Each cell in pattern uses unique band of frequencies • Hexagonal cell pattern, following values of N possible •  N = I2 + J2 + (I x J), I, J = 0, 1, 2, 3, … •  Possible values of N are 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, … • D/R= • D/d =

14. Handoff • When a mobile moves into a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base station. • Handoff operation • identifying a new base station • re-allocating the voice and control channels with the new base station. • Handoff Threshold • Minimum usable signal for acceptable voice quality (-90dBm to -100dBm) • Handoff margin cannot be too large or too small. • If is too large, unnecessary handoffs burden the MSC • If is too small, there may be insufficient time to complete handoff before a call is lost.

16. Handoff must ensure that the drop in the measured signal is not due to momentary fading and that the mobile is actually moving away from the serving base station. • Running average measurement of signal strength should be optimized so that unnecessary handoffs are avoided. • Depends on the speed at which the vehicle is moving. • Steep short term average -> the hand off should be made quickly • The speed can be estimated from the statistics of the received short-term fading signal at the base station • Dwell time: the time over which a call may be maintained within a cell without handoff. • Dwell time depends on • propagation • interference • distance • speed

17. Handoff measurement • In first generation analog cellular systems, signal strength measurements are made by the base station and supervised by the MSC. • In second generation systems (TDMA), handoff decisions are mobile assisted, called mobile assisted handoff (MAHO) • Intersystem handoff: If a mobile moves from one cellular system to a different cellular system controlled by a different MSC. • Handoff requests is much important than handling a new call.

18. Practical Handoff Consideration • Different type of users • High speed users need frequent handoff during a call. • Low speed users may never need a handoff during a call. • Microcells to provide capacity, the MSC can become burdened if high speed users are constantly being passed between very small cells. • Minimize handoff intervention • handle the simultaneous traffic of high speed and low speed users. • Large and small cells can be located at a single location (umbrella cell) • different antenna height • different power level • Cell dragging problem: pedestrian users provide a very strong signal to the base station • The user may travel deep within a neighboring cell

20. Handoff for first generation analog cellular systems • 10 secs handoff time • is in the order of 6 dB to 12 dB • Handoff for second generation cellular systems, e.g., GSM • 1 to 2 seconds handoff time • mobile assists handoff • is in the order of 0 dB to 6 dB • Handoff decisions based on signal strength, co-channel interference, and adjacent channel interference. • IS-95 CDMA spread spectrum cellular system • Mobiles share the channel in every cell. • No physical change of channel during handoff • MSC decides the base station with the best receiving signal as the service station

21. Interference and System Capacity • Source of interference • Another mobile in the same cell • A call in progress in the neighboring cell • Other BS operating in the same frequency band • Noncellular system leaks energy into the cellular frequency band • Two types of interference • Adjacent channel interference • Co-channel interference • To reduce co-channel interference, co-channel cell must be separated by a minimum distance. • When the size of the cell is approximately the same • co-channel interference is independent of the transmitted power • co-channel interference is a function of • R: Radius of the cell • D: distance to the center of the nearest co-channel cell

22. Co Channel Interference • Increasing the ratio Q=D/R, the interference is reduced. • Q is called the co-channel reuse ratio

24. Adjacent Channel Interference

25. Adjacent channel interference: interference from adjacent in frequency to the desired signal. • Imperfect receiver filters allow nearby frequencies to leak into the passband • Performance degrade seriously due to near-far effect. • Adjacent channel interference can be minimized through careful filtering and channel assignment. • Keep the frequency separation between each channel in a given cell as large as possible • A channel separation greater than six is needed to bring the adjacent channel interference to an acceptable level.

26. Improving Capacity in Cellular Network • Add new channels • Not all channels used to start with • Frequency borrowing • Taken from adjacent cells by congested cells • Or assign frequencies dynamically • Cell splitting • Non-uniform distribution of topography and traffic • Smaller cells in high use areas • Original cells 6.5 – 13 km • 1.5 km limit in general • More frequent handoff • More base stations

27. Cell Sectoring • Cell divided into wedge shaped sectors • 3 – 6 sectors per cell • Each with own channel set • Subsets of cell’s channels • Directional antennas • Sectorization relies on antenna placement and directivity to reduce co-channel interference. Beams are kept within either a 60° or a 120° sector.

28. Microcell • Micro cells can be introduced to alleviate capacity problems caused by “hotspots”. • By clever channel assignment, the reuse factor is unchanged. As for cell splitting, there will occur interference problems when macro and micro cells must co-exist. • Microcells • Reduced power • Good for city streets, along roads and inside large buildings

29. Cellular Traffic • The basic consideration in the design of a cellular system is the sizing of the system. Sizing has two components to be considered. • Coverage area • Traffic handling capability • After the system is sized, channels are assigned to cells using the assignment schemes mentioned before.

30. Terminology in traffic • Trunking: exploits the statistical characteristics of the users calling behaviour. Any efficient communication system relies on trunking to accommodate a large number of users with a limited number of channels. • Grade of service (GoS) : A user is allocated a channel on a per call basis. GoS is a measure of the ability of a user to access a trunked system during the busiest hour. It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before). • Trunking theory : is used to determine the number of channels required to service a certain offered traffic at a specific GoS. • Call holding time (H) : the average duration of a call. • Request rate (λ) : average number of call requests per-unit time.

31. Traffic flow or intensity A • Measured in Erlang, which is defined as the call minute per minute. • Offered traffic for a single user is given as Au= λ ⋅H λ  average number of call request H  duration of a call • For a system containing U user, total offered traffic A = U⋅ Au • Exercise : There are 3000 calls per hour in a cell, each lasting an average of 1.76 min. Offered traffic A = (3000/60)(1.76) = 88 Erlangs

32. If the offered traffic exceeds the maximum possible carried traffic, blocking occurs. There are two different strategies to be used. • Blocked calls cleared • Blocked calls delayed • Trunking efficiency : is defined as the carried traffic intensity in Erlangs per channel, which is a value between zero and one. It is a function of the number of channels per cell and the specific GoS parameters. • Call arrival process: it is widely accepted that calls have a Poisson arrival.

33. Capacity S = no of duplex channels available K = no of channel in one cell N = no of cell/cluster M = no of cluster in a given system C = total no of duplex channel available in a cellular system (capacity) C = M * K * N = M * S Example: For K = 100, N = 7, calculate system capacity for M = 6 and M = 4 • C = 6 * 100 * 7 = 4200 channels • C = 4 * 100 * 7 = 2800 channels

34. Example 1 If total of 44 MHz of bandwidth is allocated to a particular FDD cellular radio system which uses two 25kHz simplex channels to provide full duplex voice and control channel, calculate the number of channels available per cell, k for • 3 cell reuse • 7 cell reuse • 12 cell reuse

35. Formula (Cellular Traffic) i) Total number of channel per cell (NC) NC = (Allocated spectrum) / (channel BW x Frequency reuse factor) unit = channel / cell ii) No.of cell in the service area = Total coverage area / area of the cell. Unit = cell Traffic intensity of each cell can be found from table or Erlang B chart. Depend on NC and GOS Traffic capacity = # of cell x traffic intensity /cell ( Erlang ) Iii) Total no.of user (U) = total traffic (A) / traffic per user ( Au) Iv) number of call that can be made at any time = NC x no.of cell

36. Example 2 How many users can be supported for 0.5% blocking probability for the following number of trunked channels in a blocked calls cleared system. • 1 • 5 • 10 • 20 • 100 Assume each user generates 0.1 Erlangs of traffic

37. Example 3 A city has an area of 3000 sq.km and is covered by a cellular system using 7 cell per cluster. The area of a cell is 100 sq.km. The cellular system is allocated total bandwidth of 40 MHz of spectrum with full duplex channel bandwidth of 200 KHz. For the GOS of 2 % and the offered traffic per user is 0.03 Erlangs, calculate; • The number of cell in the city • The number of channels per cell • Traffic intensity of each cell • Traffic intensity for the city • The total number of users that can be served in the city • The number of mobiles per channel • Number of call that can be made at any time in the city

38. Erlang B Trunking GOS

39. Erlang B

40. Mobile Communication Channel • Voice • Forward (FVC) : BS to Mobile • Reverse (RVC): Mobile to BS • Control • Forward (FCC) • Reverse (RCC) • Involved in setting up call and moving it to unused voice channel • Call initiation and service request • Control channels monitored by mobiles when no call in progress • FCCs continually broadcast traffic requests for mobiles in system

41. Base Station - Mobile Network Forward Voice Channel ReverseVoice Channel Forward Control Channel Reverse Control Channel RVC FVC RCC FCC

42. Anatomy of a cellular call • Mobile phone turns on • Mobile phone scans forward control channels • Determine FCC with strongest signal • Stay with this FCC until signal drops below a certain level • Again scan the FCCs to find with strongest signal • Control channels ~ 5% of total channels • Control channels standardized • Mobile phone knows which channels to scan no matter where the mobile phone is located within a country or region

43. Receiving a call • Step 1:MSC receives phone call from PSTN • Step 2: MSC tells all BS in cellular system to broadcast the mobile identification number (MIN) = mobile phone number • Step 3: MIN broadcast as a paging message over all FCCs • Step 4: Mobile receives paging message on FCC it is currently monitoring, and responds by identifying itself to BS over the RCC • Step 5: BS informs MSC of successful handshake • Step 6: MSC assigns unused voice channel within the cell of the specified BS to this call • Step 7: BS informs mobile of which forward and reverse channels it has been assigned. Alert message sent over FVC instructing mobile phone to ring. Call setup is complete.

44. Receiving a call IncomingTelephone Call to Mobile X Base Stations 2, 6 Step 1 Mobile Switching Center 5 4 3, 7 Mobile X PSTN

45. Placing a call • Step 1: Call initiation request sent on RCC • MIN • ESN (Electronic serial number) • Telephone number of intended party • Station Class Mark (SCM): max transmitter power level of user • Step 2: BS sends data to MSC • Step 3: • MSC makes connection to intended party through PSTN • MSC assigns unused voice channel within the cell of the specified BS to this call • BS informs mobile of which forward and reverse channels it has been assigned • Call setup is complete

46. Placing a call Mobile Switching Center 3 2 PSTN Telephone Call Placed by Mobile X 1