1. The Cellular Concept: System Design Issues

2. Cellular Systems: Basic Concepts (1) High Capacity is achieved by limiting the coverage of each base station to a small geographic region called cell. Same frequencies/timeslots/codes are reused by spatially separated base stations. A switching technique called handoff enables a call to proceed uninterrupted when one user moves from one cell to another Resolved problem of limited radio spectrum.

3. Neighboring base stations are assigned different group of channels so as to minimize the interference. By systematically spacing base stations and the channel groups may be reused as many number of times as necessary. As demand increases, the number of base stations may be increased thereby providing additional capacity. Cellular Systems: Basic Concepts (2)

4. MSC Mobile Switching Center PSTN Public Switched Telephone Network Bass Station Cellular Telephone Systems:

5. Forward and Reverse Channels Forward Voice Channel (FVC): Used for voice transmission from BS to MS Reverse Voice Channel (RVC): used for voice transmission from MS to BS. Forward Control Channel (FCC): Used for initiating a call from BS to MS. Reverse Control Channel (RCC): Used for initiating a call from MS to BS. The FCC and RCC are also called setup channel

6. A cell phone, when turned on, (though not yet engaged in a call) scans the group of FCC to determine the one with the strongest signal. It monitors that channel until it drops below a usable threshold. It then scans for another channel which is the strongest. Control channels are defined and standardized over the entire area of service. Typically the control channels use up to 5% of the total number of channels. Anatomy of a cellular Call

7. A call to Mobile User (1) The MSC dispatches the request to all base stations. The mobile Identification Number (MIN) is broadcast as a paging message over all FCC throughout the service area. The MS receives the paging message from the BS it is monitoring. It responds by identifying itself over the RCC. The BS conveys the handshakes to the MSC. The MSC instruct the BS to move to an unused voice channel.

8. The BS signals the MS to change over to an unused FVC and RVC. An data message (called alert) is transmitted over the FVC to instruct the mobile to ring! All of these sequence of events occur in just a few seconds, are not noticeable to the user. While the call is in progress, the MSC adjusts the transmitted power in order to maintain the call quality A call to Mobile User (2)

9. A call initiation request is sent to the RCC Along with this, the MS transmits its MIN, Electronic Serial Number (ESN) and the phone number of the called party. The MS also transmits the Station Class Mark (SCM) which indicates the maximum transmitter power level for the particular user. The BS forwards the data to the MSC, which validates the data and makes connection to the called party through PSTN A call FROM Mobile User (1)

10. Fixed telephone network runs wires to every household Suppose we give every household their own allocation of radio spectrum for analog speech of 4KHz BW 12.5 million households X 4kHz =50GHz! Clearly impractical! –No other services possible using radio transmission –Most of the spectrum unused most of the time Frequency Reuse: The Need

11. Frequency Reuse (2) Cellular radio systems rely on intelligent allocation and reuse of channels throughout the coverage area. Each base station is allocated a group of radio channels to be used within the small geographic area of its cell. Neighboring cells are assigned different channel groups. By limiting the coverage area to within the boundary of the cell, the channel groups may be reused to cover different cells. Keep interference levels within tolerable limits. Frequency reuse or frequency planning seven groups of channel from A to G footprint of a cell - actual radio coverage omni-directional antenna v.s. directional antenna

12. By design of antennas, the coverage area is limited within the cell, and the same group of frequencies is reused to cover another cell separated by a large enough distance to keep co- channel interference within limits. The design procedure of allocating channel groups for all of the cellular BS within a system is called Frequency Reuse or frequency Planning Frequency Reuse (3)

13. Frequency reuse (4) Consider a cellular system which has a total of S duplex channels. Each cell is allocated a group of k channels,. The S channels are divided among N cells. The total number of available radio channels The N cells which use the complete set of channels is called cluster. The cluster can be repeated M times within the system. The total number of channels, C, is used as a measure of capacity The capacity is directly proportional to the number of replication M. The cluster size, N, is typically equal to 4, 7, or 12. Small N is desirable to maximize capacity. The frequency reuse factor is given by

14. Hexagonal cells are conceptual. For most theoretical treatment, hexagonal model of cells is universally adopted because: Hexagons are geometric shape that approximates a circle (for omni-directional radiation) exactly six equidistance neighbors the lines joining the centers of any cell and each of its neighbors are separated by multiples of 60 degrees. Using a hexagon geometry, fewest number of cells can cover the entire geographical region The Cell Shape (1)

15. The geometry of Hexagons Only certain cluster sizes and cell layout are possible. The number of cells per cluster, N, can only have values which satisfy Co-channel neighbors of a particular cell, ex, i=3 and j=2.

17. Axes U, V intersect at 60 degree Unit scale is distance between cell centers If cell radius to point of Hexagon is is R, then 2Rcos30=1, or To find the distance of a point P(u,v) from the origin : X=u cos 30, y=v+usin30 The geometry of Hexagons

18. Using this equation, to locate co-channel cells, we start from a reference cell and move i hexagon along the U- axis then j hexagons along the V-axis Hence the distance between co-channel cells in adjacent cluster is given by: The number of cells in a cluster, N, is given by Hence, the possible values of N are 1, 3, 4, 7, 12, …. The geometry of Hexagons

19. Examples

20. Examples (1) A spectrum of 30MHz is allocated to a wireless FDD cellular system which uses two 25KHz simplex channels to provide full duplex voice and control channels, compute the number of channels available per cell if a system uses (i) four-cell reuse, (ii) seven-cell reuse, (iii) 12-cell reuse. If 1MHz of the allocated spectrum is dedicated to control channels, determine an equitable distribution of control channels and voice channels in each of the three systems.

21. Examples (2) prove that for a hexagonal geometry, the co- channel reuse ratio is given by,where

22. The coverage area of a cellular system is 2000sq km with each cell having a radii of 5km, and there are a total of 1000 radio channels available for handling the traffic. a)Calculate the system capacity for 7-cell reuse. b)If N=4, how many times the cluster has to be replicated in order to approximately cover the entire cellular area? Calculate the system capacity for the given case. c)Does decreasing the cluster size increase the system capacity? explain Examples (3)

23. Assignment Strategy Depends on the interference tolerable Spacing same-band cells three cells apart results in less interference than spacing them two cells apart. This tolerable interference is called signal-to- interference ratio SIR, often called the carrier-to- interference ratio CIR. SIR is defined to be the ratio of the desired average signal power at a receiver to the total average interference power.

24. Channel Assignment Strategies Frequency reuse scheme –increases capacity –minimize interference Channel assignment strategy –fixed channel assignment –dynamic channel assignment Fixed channel assignment –each cell is allocated a predetermined set of voice channel –any new call attempt can only be served by the unused channels –the call will be blocked if all channels in that cell are occupied Dynamic channel assignment –channels are not allocated to cells permanently. –allocate channels based on request. –reduce the likelihood of blocking, increase capacity. –MSC needs to collect real time data on channel occupancy, traffic distribution and RSSI of all channels on a continuous basis.

25. Handoff Strategies (1) 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.

26. Handoff Strategies (2)

27. 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 Handoff Strategies (3)

28. 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. Handoff Strategies (4)

29. 29 Handoff Decision –signal level decreases due to signal fading → don’t handoff mobile moving away from base station → handoff –must monitor received signal strength over a period of time → moving average –time allowed to complete handoff depends on mobile speed large negative received signal strength (RSS) slope → high speed → quick handoff –statistics of the fading signal are important to making appropriate handoff decisions → Chapters 4 and 5 Handoff Strategies (5)

30. 30 1st Generation Cellular (Analog FM → AMPS) –Received signal strength (RSS) of RVC measured at base station & monitored by MSC –A spare Rx in base station (locator Rx) monitors RSS of RVC's in neighboring cells Tells Mobile Switching Center about these mobiles and their channels –Locator Rx can see if signal to this base station is significantly better than to the host base station –MSC monitors RSS from all base stations & decides on handoff Handoff Strategies (6)

31. 31 2nd Generation Cellular w/ digital TDMA (GSM, IS- 136) –Mobile Assisted HandOffs (MAHO) important advancement The mobile measures the RSS of the FCC’s from adjacent base stations & reports back to serving base station if Rx power from new base station > Rx power from serving (current) base station by pre-determined margin for a long enough time period → handoff initiated by MSC Handoff Strategies (7)

32. 32 –MSC no longer monitors RSS of all channels reduces computational load considerably enables much more rapid and efficient handoffs imperceptible to user Handoff Strategies (8)

33. 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 Handoff Strategies (9)

34. Handoff Strategies (10)

35. 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 Handoff Strategies (11)

36. Soft Handoff CDMA spread spectrum cellular system provides a unique handoff capability. Unlike chanellized wireless systems that assign different radio channel during a handoff (called hard handoff), the spread spectrum MS share the same channel in every cell. The term handoff implies that a different BS handles the radio communication task. The ability to select between the instantaneous received signals from different BSs is called soft handoff.

37. 37 A mobile may move into a different system controlled by a different MSC –Called an intersystem handoff –What issues would be involved here? Prioritizing Handoffs –Issue: Perceived Grade of Service (GOS) – service quality as viewed by users “quality” in terms of dropped or blocked calls (not voice quality) assign higher priority to handoff vs. new call request a dropped call is more aggravating than an occasional blocked call

38. 38 Guard Channels –% of total available cell channels exclusively set aside for handoff requests –makes fewer channels available for new call requests –a good strategy is dynamic channel allocation (not fixed) adjust number of guard channels as needed by demand so channels are not wasted in cells with low traffic

39. 39 Queuing Handoff Requests –use time delay between handoff threshold and minimum useable signal level to place a blocked handoff request in queue –a handoff request can "keep trying" during that time period, instead of having a single block/no block decision –prioritize requests (based on mobile speed) and handoff as needed –calls will still be dropped if time period expires

40. Interference and System Capacity Sources of interference –another mobile in the same cell –a call in progress in the neighboring cell –other base stations operating in the same frequency band –noncellular system leaks energy into the cellular frequency band Two major cellular interference –co-channel interference –adjacent channel interference

41. Co-channel Interference and System Capacity Frequency reuse - there are several cells that use the same set of frequencies –co-channel cells –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 Increasing the ratio Q=D/R, the interference is reduced. Q is called the co-channel reuse ratio

42. For a hexagonal geometry A small value of Q provides large capacity A large value of Q improves the transmission quality - smaller level of co-channel interference A tradeoff must be made between these two objectives

43. Let be the number of co-channel interfering cells. The signal-to- interference ratio (SIR) for a mobile receiver can be expressed as S: the desired signal power : interference power caused by the ith interfering co-channel cell base station The average received power at a distance d from the transmitting antenna is approximated by or n is the path loss exponent which ranges between 2 and 4. close-in reference point

44. When the transmission power of each base station is equal, SIR for a mobile can be approximated as Consider only the first layer of interfering cells Example: AMPS requires that SIR be greater than 18dB –N should be at least 6.49 for n=4. –Minimum cluster size is 7

45. For hexagonal geometry with 7-cell cluster, with the mobile unit being at the cell boundary, the signal-to-interference ratio for the worst case can be approximated as

46. Adjacent Channel Interference 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.

47. 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.

48. Power Control for Reducing Interference Ensure each mobile transmits the smallest power necessary to maintain a good quality link on the reverse channel –long battery life –increase SIR –solve the near-far problem

49. Improving Capacity in Cellular Systems Methods for improving capacity in cellular systems –Cell Splitting: subdividing a congested cell into smaller cells. –Sectoring: directional antennas to control the interference and frequency reuse. –Coverage zone : Distributing the coverage of a cell and extends the cell boundary to hard-to-reach place.

50. Cell Splitting Split congested cell into smaller cells. –Preserve frequency reuse plan. –Reduce transmission power. microcell Reduce R to R/2

51. Illustration of cell splitting within a 3 km by 3 km square

52. Transmission power reduction from to Examining the receiving power at the new and old cell boundary If n = 4 and set the received power equal to each other The transmit power must be reduced by 12 dB in order to fill in the original coverage area. Problem: if only part of the cells are splited –Different cell sizes will exist simultaneously Handoff issues - high speed and low speed traffic can be simultaneously accommodated

53. Sectoring Decrease the co-channel interference and keep the cell radius R unchanged –Replacing single omni-directional antenna by several directional antennas –Radiating within a specified sector

54. Interference Reduction position of the mobile interference cells

55. Microcell Zone Concept Antennas are placed at the outer edges of the cell Any channel may be assigned to any zone by the base station Mobile is served by the zone with the strongest signal. Handoff within a cell –No channel re- assignment –Switch the channel to a different zone site Reduce interference –Low power transmitters are employed

56. First Generation- Frequency Assignment First-generation analog mobile system in the US, the Advanced Mobile Phone System (AMPS) system 25 MHz of spectrum is made available for each direction of transmission in the 800–900 MHz radio band.

57. The FCC, in 1981 assigned the –824–849 MHz band to uplink or reverse channel communication, mobile to base station (BS); –the 869–894 MHz range was assigned to downlink or forward communication, BS to mobile. –Frequency modulation (FM) was the modulation type adopted for these first-generation systems, –25 MHz band in each direction broken into 30 kHz-wide signal channels, each accommodating one call. There are thus 832 analog signal channels made available in each direction.

58. AMPS Issues Roaming: The capability to receive service while in another service provider's domain is called roaming. Network to network interoperation (messaging) are essential to support roaming. The IS-41 standard has provided the technical solution to roaming between networks implemented by different equipment manufacturers.

59. IS-41 IS-41, also known as ANSI-41, is a mobile, cellular telecommunications system standard to support mobility management by enabling the networking of switches. ANSI-41 is the standard now approved for use as the network-side companion to the wireless-side AMPS (analog), IS-136 (Digital AMPS), cdmaOne, and CDMA2000 networks

60. AMPS channels Each call uses a dedicated forward channel paired with a dedicated reverse channel at a 45 MHz offset. Some of the channel pairs (21 of them) are used for control purposes in the AMPS environment. Analog frequency modulation (FM) with 8 kHz deviation is used in the traffic channels, which convey voice conversations. Binary frequency shift keying (FSK) at 10 used in the control channels used for signaling. AMPS is an analog FM system. AMPS channels are insecure AMPS channels can suffer from interference Analog signals suffering from multipath fading cannot be corrected. AMPS radio resource management is based on signal strength, which can only be measured indirectly via supervisory audio tones or SAT.

61. AMPS-issues AMPS cellular operation consists of call origination and call termination procedures, supported by radio resource management and mobility management functions. AMPS was designed as a voice-only system, Data transmission on AMPS systems is based on this circuit-switched mode of operation

62. AMPS call management When an AMPS mobile station powers up, it searches through up to 21 predefined control channels. These control channels are physically similar to AMPS traffic channels but they are used-for control purposes only. Each cell utilizes a forward control channel to continuously broadcast information needed by the mobile station for registration. This information includes the system identification or SID of the MSC, which allows the mobile to know whether it is roaming.

63. Mobile originates a call via the reverse control channel in the cell the mobile is currently located in. The mobile transmits its request (which includes information about the subscriber such as the MIN and ESN) and "listens" on the forward channel for its subsequent channel assignment. The base station forwards the request to the MSC. After validating the mobile via the HLR and the VLR, the MSC selects a traffic channel pair for the mobile. If no channels are available, the MSC simply rejects the request. HLR- Home Location Register, a central database that contains details of each mobile phone subscriber VLR-The Visitor Location Register contains all subscriber data required for call handling and mobility management for mobile subscribers currently located in the area controlled by the VLR.

64. If the MSC grants a channel for the subscriber, it must then connect the call through to the destination. This is done via standard telephony procedures. MSC simply appears to be a private branch exchange (PBX) to the PSTN. The channel grant message is relayed to the mobile via the forward control channel. The mobile then tunes its transmitter and receiver to the assigned traffic channel pair for the duration of the call. Call and power control from this point forward are handled in-band on the AMPS traffic channel assigned to the mobile.

65. Call issues In communicating with the network, the mobile provides two identifiers for registration, call control and validation. The first of these identifiers is the mobile identification number or MIN, which is the programmed handset phone number used to call the subscriber. This programmed identifier is associated with the subscriber and is stored in erasable non-volatile memory in the handset. The second identifier is the electronic serial number or ESN, which is a manufactured characteristic of the mobile unit. This identifier is (in theory) permanent and associated with the physical equipment. It is 32 bits in length, with the first 8 bits identifying the manufacturer. Both the MIN and the ESN are transmitted unencrypted by both the mobile and the network. Recently the cellular industry has instituted a subscriber-entered personal identification number or PIN as an escalation in the war on cellular fraud.

66. Second- and third-generation –digital systems use TDMA (time-division multiple access) and CDMA (code-division –multiple access) techniques over the same 25 MHz frequency bands in each direction to –provide increased capacity, hence allowing more simultaneous calls to be transmitted. –The above is made possible by cellular concept which adopted, is to frequency reuse - similar channels in different cells, increasing the capacity.

67. Mobile stations are now expanding their types of gadgets. From traditional cell phones to PDAs, Laptops, GPS, Palm tops etc., many more gadgets can now be called as mobile stations which travel in between cells. Since the inputs, outputs and the identifying modules differ in each type of mobile stations, will the same Design for Handoff would work fine or how are these scenarios handled. The main design strategies of handoff are –Signal strength –Cell structures

68. What are different system structures of a cell? Macro, micro, macro/micro overlays

69. What happens if a user, after completion of his communication wants to disconnect and at the same time there is a handoff to other cell? (Will the purposeful termination detection be done by the first cell or the other cell to which the signal has been handed off?) The Serving BST handles it.Hand off is not a one time process, so either of the BSTs handle the mobile stations call. At a particular time the station which is having more signal strength will handle the call. Even the termination is being handled by either of the base stations.

70. In types of handoff, there is “break before make” i.e. hard handoff & “make before break” that is soft handoff. So, which one of the two type is better and under which circumstances for each scenario? If possible, explain with example. Soft handoff is advantageous over hard handoff because the mobile does not lose contact with the system during handoff execution, also unnecessary call terminations doesn’t occur. No one likes unnecessary call terminations. Hard handoff is advantageous when system performance is not effected even if the mobile system has to reconnect to the BST.