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Wireless Cellular Networks (basics)

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1 Wireless Cellular Networks (basics)
Part 3 – GSM networks

2 History of Cellular systems
1960's: Bell Labs developed cellular concept : First field Trial for Cellular System AMPS (Advanced Mobile Phone System), Chicago 1981, Sweden, first European Systems NMT-450 (Nordic Mobile Telephone) 1985, first italian cellular system RTMS (Radio Telefono Mobile di Seconda Generazione), 450 MhZ 1990, TACS, first italian widespread systems Total Access Communication System (TACS, 900 MHZ) Second generation system: GSM in europe, D-AMPS & IS95 (CDMA) in USA, PDC Japan Digital, versus fist generation analog (frequency modulation) GSM: Specification started in 1982; EU deployment since 1992; DCS-1800 since 1994 Generation 2 ½ : GPRS, EDGE (8PSK), HSCSD Generation 3: UMTS, HSDPA  HSPA, In ITA since 2004 Generation 4: LTE (2011/2012?)

3 GSM essential components
OMC To fixed network (PSTN, ISDN, PDN) GMSC EIR AUC HLR VLR MSC BSC BTS MS Mobile Station BTS Base Transceiver Station BSC Base Station Controller MSC Mobile Switching Center GMSC Gateway MSC OMC Operation and Maintenance Center EIR Equipment Identity Register AUC Authentication Center HLR Home Location Register VLR Visitor Location Register BTS BTS BSC BTS BTS MS

4 GSM system hierarchy MSC MSC region LOCATION AREA BSC BTS
MSC: Mobile Switching Center LA: Location Area BSC: Base Station Controller BTS: Base Transceiver Station Hierarchy: MSC region  n x Location Areas  m x BSC  k x BTS

5 Mobile Station (MS) Key fact:
GSM separates user mobility from equipment mobility, by defining two distinct components Mobile Equipment The cellular telephone itself (or the vehicular telephone) Address / identifier: IMEI (International Mobile Equipment Identity) Control: Equipment Identity Register (EIR): White list, black list, gray list Stolen terminals, malfunctioning terminale Subscriber Identity Module (SIM) Fixed installed chip (plug-in SIM) or exchangeable card (SIM card) Addresses / identifiers: IMSI (International Mobile Subscriber Identity) MSISDN (Mobile Subscriber ISDN number – the phone number)

6 Base Station Sub-System
Um - Radio Interface BSS OSS BTS BSC BTS A Interface BTS A-bis Interface Base Transceiver Station (BTS) Transmitter and receiver devices, voice coding & decoding, rate adaptation for data Provides signaling channels on the radio interface Limited signal and protocol processing (error protection coding, link layer LAPDm) Base Station Controller (BSC) performs most important radio interface management functions: Radio channels allocation and deallocation; handover management; …

7 Base Transceiver Station - BTS
Um Interface (to MS) Output filter HF Transmitter TRX Digital Signal Processing Slow freq. Hopping Transmission System Abis Interface (to BSC) Input Filter HF Receiver Operation and Maintenance Functionality/clock distribution TRX radio interface functions: - GMSK modulation-demodulation - channel coding - encryption/decryption - burst formatting, interleaving - signal strength measurements - interference measurements In essence, BTS is a complex modem!

8 Base Station Controller - BSC
DB contains - state information for all BSS - BTS software DB BTS-1 BTS-2 X switch matrix From/to MSC FUNCTIONS: switch calls from MSC to correct BTS and conversely Protocol and coding conversion for traffic (voice) & signaling (GSM-specific to ISDN-specific) Manage MS mobility Enforce power control BTS-K 1 BSC may control up to 40 BTS

9 Transcoding and Rate Adaptation
BTS: collects speech traffic Deciphers and removes error protection Result: 13 kbps air-interface GSM speech-coded signal MSC: A modified ISDN switch Needs to receive ISDN-coded speech 64 kbps PCM format (A-law) Transcoding and Rate Adaptation Unit (TRAU) needed! Rationale: re-use existing ISDN switches & protocols

10 TRAU possible placements
13 kbit/s 64 kbit/s 64 kbit/s MSC On BTS BTS TRAU BSC 13 kbit/s 64 kbit/s On BSC 16 kbit/s MSC BTS BSC TRAU 64 kbit/s (4x16 sub-mux) On MSC 13 kbit/s 16 kbit/s MSC BTS BSC TRAU Why 16 kbps instead of 13? Inband signalling needed for BTS control of TRAU (TRAU needs to receive synchro & decoding information from BTS)

11 Network Switching Sub-System
Elements: Mobile Switching Center (MSC) / Gateway MSC (GMSC) Enhanced telephone switching centers (digital, ISDN) With support for user mobility, registration, handover Home Location Register (HLR ) / Authentication Center (AuC) Visitor Location Register (VLR) Equipment Identity Register (EIR) Functions: Call control User management Inter-component communication Via SS7 signalling network with suitable extensions (e.g. MAP – Mobile Application Part)

12 Location 1 MSC 1 VLR Several Location Areas MSC VLR LA-1 LA-2 LA-3
LA-n 1 MSC 1 VLR Several Location Areas

13 Location Registration (Update) (very) basic idea
3 3) Update pointer at HLR, which now knows which LAI/VLR the user is located MSC VLR BTS BSC MS HLR 2 2) Register MS ID (IMSI) into local VLR; Authenticate; receive TMSI for local paging purposes 1 MS switches ON; detects cell through BCCH carrier; Obtain Location Area Identifier (LAI) from BCCH

14 Location Registration - details
MS VLR HLR AUC BSS/MSC Loc. Upd. Request IMSI, LAI Update Loc. Area IMSI, LAI Auth. Param. Req. IMSI Auth. Info. Req. IMSI Auth. Info (Auth. Parameters) authentication Update Location IMSI, MSRN Start Ciphering Kc Activate ciphering Insert Subscrib. Data IMSI, additional data Forward new TMSI TMSI Insert Subscrib. Data ACK Locat. Upd. Accept IMSI Locat. Upd. Accept TMSI Realloc Cmd Locat. Upd. Accept TMSI Realloc ACK TMSI ACK

15 Changing MSC/VLR MSC MSC An MS always has a dedicated entry in the HLR
Public switched telephone network PSTN HLR VLR MSC VLR MSC Base Station Base Station An MS always has a dedicated entry in the HLR Plus one entry in JUST 1 VLR (related to the MSC the user is connected to)

16 Location Update: different VLR
MS VLR-new HLR VLR-old BSS/MSC Loc. Upd. Request TMSI(+ old LAI), LAI Update Loc. Area TMSI(+ old LAI), LAI determine VLR-old From old LAI Send parameters (TMSI, old LAI) IMSI response (IMSI,RAND,SRES,Kc) authentication Update Location IMSI, MSRN Cancel Location IMSI Generate New TMSI Cancel Locat. ACK Activate ciphering Start Ciphering Kc Insert Subscrib. Data IMSI, additional data Ins. subs. data ACK Locat. Upd. Accept IMSI Forward new TMSI … …

17 Call switching Gateway MSC – GMSC
Needed, as fixed network switches are not mobile capable!! GMSC task: query HLR for current MS location (if fixed network switches were able to query HLR, direct connection with local MSC would be available) X X X X X GMSC MSC MSC HLR MSC PLMN Public Land Mobile Network

18 Notation A call involves two “Parties” Calling Party (caller)
user generating the call Called Party (callee) user receiving the call Mobile Originating Call (MOC) Call originated by an MS Mobile Terminating Call (MTC) Call directed to an MS

19 Call establishment basics
Mobile Terminated Call Mobile Originated Call Fixed party Fixed party MS MSC MS MSC setup setup setup setup Call proceeding alerting Call confirmed alerting alerting Connect alerting Connect connect Connect Ack connect DATA DATA In ISDN ISUP: - setup = IAM (Initial Address Message); - Alerting = ACM (Address Complete Message); - Connect = ANS (Answer)

20 Call establishment steps
MS Mobile Terminated Call network MS Mobile Originated Call network Paging request Channel request Channel request Immediate Assignment Immediate Assignment Paging Response Service Request Authentication Request Authentication Request Authentication Response Authentication Response Ciphering mode command Ciphering mode command Ciphering Mode Complete Ciphering Mode Complete Setup Setup Call Confirmed Call proceeding Assignment Command Assignment Command Assignment Complete Assignment Complete Alerting Alerting Connect Connect Connect Acknowledge Connect Acknowledge


22 Routing calls to Roaming MS
ISDN (UK) MSC International Switching Center PLMN 2 (UK) International Switching Center ISDN (ita) Transit Exchange PLMN 1 (ITA) GMSC 1 MSC MSISDN MSRN +44.NDC Local Exchange HLR

23 Is the PRICE (!) to pay for simple routing and billing
“tromboning” Call to MSISDN MSC ISC (UK) PLMN 2 (UK) Call to MSISDN Is the PRICE (!) to pay for simple routing and billing ISC (ITA) PLMN 1 (ITA) GMSC 1 MSC MSISDN MSRN +44.NDC HLR

24 Number portability Subscriber may switch operator without changing his number First implemented in fixed network may 2002: extended to mobile networks Essential for fair competition among network operators UK 2002 survey: 42% of corporate subscribers were willing to change mobile operator; but 96% were, if number could be ported Resistence from leading operators Number portability helps newer operators to compete with traditional ones

25 Notation Donor switch The switch corresponding to a “ported” telephone number Recipient switch The switch to which the ported number is attached

26 Technical solutions a) call forwarding
Originating network Donor network switch switch switch Recipient network Originating switch sets-up trunk to donor switch Donor switch sets-up trunk to recipient switch Simplest solution, as call forwarding is a feature available in virtually all switches But extremely inefficient routing and trunking resource consumption!

27 Technical solutions b) query on release
Originating network Donor network SS7 ISUP IAM switch switch SS7 ISUP REL Number Portability DataBase switch Recipient network Donor switch “blocks” incoming call with a release message (REL) REL carries a QoR cause value, stating that called party number is ported Originating switch then queries Number Portability database

28 Technical solutions c) all-call query
Originating network Donor network switch switch Number Portability DataBase switch Recipient network Originating switch queries Number Portability database for every call!! - best solution if majority of numbers are ported (no interaction with donor) - but very high DB load, as EVERY number must be looked-up!

29 Mobile Number Portability
Same ideas as fixed number portability The donor switch is the GMSC of the donor network Donor GMSC Call forwarding (if more efficient fixed number portability not supported) While porting number, may also get MSRN! Recipient network HLR GMSC MSC Note: If path must cross GMSC: Use Intermediate Routing Number MSRN IRN MSRN (or IRN) Incoming call HLR GMSC Signaling relay function Donor network Clearly, still suffers of tromboning!

30 Mobile Number Portability (with all call query approach)
Recipient network Query MSRN GMSC HLR MSC Return MSRN IRN Incoming call Number Portability DataBase Query IRN switch Return IRN

31 Mobile Number Portability improved – (with all call query approach)
Recipient network GMSC HLR MSC MSRN Signaling relay function Incoming call Number Portability DataBase Query MSRN switch Return MSRN

32 Wireless Cellular Networks (basics)
Part 4 – GSM radio interface

33 GSM Radio Spectrum Frequency [MHz] 2 x 25 Mhz band
Duplex spacing: 45 MHz 124 carriers x band 200 KHz channels Suggested use: only 122 Use top & bottom as additional guard 8 TDMA slots x carrier full rate calls – 13 Kbps If half-rate used, 16 calls at 6.5 kbps 960 DOWNLINK BS  MS 935 915 1 2 3 4 5 6 7 8 890.4 UPLINK MS  BS 890.2 “guard band” 890

34 Adjacent channels (due to GMSK)
60dB 35dB Specification: 9dB In practice, due to power control and shadowing, adjacent channels Cannot be used within the same cell…

35 Physical channel time slot time slot 7 time 577 ms
200 KHz bandwidth + GMSK modulation 1625/6 kbps gross channel rate ( kbps) 1 time slot = 625/4 bits bits 15/26 ms = ms time slot time slot 7 time 577 ms 1 frame = 60/13 ms = ms 26 frames = 120 ms (this is the key number)

36 Hybrid FDMA-TDMA physical channel = (time slot, frequency)
Total n. of channels: 992 200 KHz 200 KHz 200 KHz 200 KHz 200 KHz 200 KHz slot 200 KHz 200 KHz 200 KHz 577us 577us 577us 577us 577us 577us 577us 577us time

37 Slow Frequency hopping (optional procedure within individual cell)
Hopping sequence (example): …  f1  f2  f5  f6  f3  f7  f4  f1… Slow = on a per-frame basis - 1 hop per frame (4.615 ms) = 217 hops/second Physical motivation: - combat frequency-selective fading - combat Co-Channel Interference next slot may not interferere with adjacent cell slot (different hopping sequence) - improvements: acceptable quality with 9 dB SNR versus 11 dB

38 Duplexing MS uses SAME slot number on uplink and downlink
Uplink and downlink carriers always have a 45 MHz separation I.e. if uplink carrier is  downlink is 919.2 3 slot delay shift!! 1 2 3 4 5 6 7 DOWNLINK UPLINK 1 2 3 4 5 6 7 MS: no need to transmit and receive in the same time on two different frequencies!

39 Structure of a TDMA slot
Normal burst TB 3 DATA 57 S 1 Training sequence 26 S 1 Data 57 TB 3 GP 8.25 148 bit burst bit (15/26 ms = ms) Symmetric structure DATA: 2 x 57 data bits 114 data bits per burst “gross” bits (error-protected; channel coded) “gross” rate: 24 traffic burst every 26 frames (120 ms) 22.8 kbps gross rate 13 kbps net rate! S: 2 x 1 stealing bit Also called stealing flags, toggle bits Needed to grab slot for FACCH (other signalling possible)

40 Guard Period rationale
BTS d Assume the following synchro mechanism: BTS transmits at time 0 MS receives at time d/c MS transmits at time 3+d/c BTS receives at time 3+ 2d/c Offset depending on d! BTS downlink tx 1 2 3 4 MS downlink rx 1 2 3 4 MS uplink rx 1 BTS uplink rx 1 1 Expected RX time!

41 Guard period sizing … … … … Maximum cell radius:
dwlink slot 1 dwlink slot 4 dwlink slot 1 dwlink slot 4 MS time BTS time uplink slot 1 uplink slot 1 Maximum cell radius: Is there something wrong? (GSM says that cells go up to 35 km)

42 Frame synchronization
TA (transmitted in the SACCH) BTS Timing Advance (TA) Parameter periodically transmitted by BTS during MS activity 6 bits = 0-63 Meaning: anticipate transmission of TA bits TA=0: no advance I.e. transmit after bits after downlink slot TA=63: Transmit after bits time dwlink slot 4 dwlink slot 4 TA BTS time MS time uplink slot 1 uplink slot 1 uplink slot 1 TA avoids collision!

43 Timing Advance analysis
Downlink propagation delay: d/c Uplink propagation delay: Uplink delay with TA: d/c-TA Perfect resynchronization occurs when TA = 2d/c Maximum cell size for perfect resync: 8.25 bits Guard time additionally available for imperfect sync (+/- error)

44 And when the user is not connected? But wants to connect…
Solution: USE A DIFFERENT BURST FORMAT Access Burst: much longer Guard Period available drawback: much less space for useful information Access burst TB 8 Training sequence 41 Data 36 TB 3 GP 68.25 88 bit burst bit (0.577 ms) No collision with subsequent slot for distances up to 37.8 km

45 Logical vs Physical channels
Logical channels (traffic channels, signalling (=control) channels) Physical channels (FDMA/TDMA) Physical channels Time given frequencies Issues: modulation, slot synchronization, multiple access techniques, duplexing, frequency hopping, etc Logical channels Built on top of phy channels Issue: which information is exchanged between MS and BSS

46 Logical – physical mapping
Physical Channel: data rate r, time slot i frequency Logical Channel Mapping: Different channels may share a same physical channel frequency Frame 8 Frame 9 Frame 10 Frame 11 Frame 12 Logical channel A: data rate r/3, time slot i, frame 3k Logical channel B: data rate 2r/3, time slot i, frame 3k+1, 3k+2

47 GSM logical channels Traffic channel (TCH) TCH/F TCH full rate MSBSS TCH/H TCH half Rate Broadcast channel BCCH Broadcast control BSSMS (same information to all MS in a cell) FCCH Frequency Correction SCH Synchronization Common Control channel (CCCH) RACH Random Access MSBSS (point to multipoint channels) AGCH Access Grant (used for access management) PCH Paging Dedicated Control channel (DCCH) SDCCH Stand-alone Dedicated control (point-to-point signalling channels) SACCH Slow associated control (dedicated to a specific MS) FACCH Fast associated control Additional logical channels available for special purposes (SMS, group calls, …)

48 Traffic channels (TCH/F)
Periodic pattern of 26 frames (120 ms = 15/26 ms/TS * 8 TS/frame* 26 frame) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 24 TCH frames over 26 Same TS in every frame 2 1 3 4 5 6 7 2 1 3 4 5 6 7 2 1 3 4 5 6 7 2 1 3 4 5 6 7 Theoretical rate: 1/8 channel rate: r=33.85 kbps 2 signalling frames: r  kbps Burst overhead (114 bits over ): r 22.8 kbps GP 8.25 TB 3 DATA 57 S 1 Training Seq. (26) Data 148 bit burst bit (0.577 ms)

49 Slow Associated Control Channel
Always associated to instaurated call on TCH (TCH + SACCH = TACH) On the same Time Slot Periodic (order of ½ second) time-scale information for radio link control TCH/F(0…7) TCH/F(0…7) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 SACCH(0…7) IDLE frame SACCH-0 SACCH-1 SACCH-2 SACCH-3 SACCH-4 SACCH-5 SACCH-6 SACCH-7 1 SACCH burst (per TCH) every 26 frames (120 ms)

50 Broadcast Channel (usual) organization
51 frame structure vs 26 ms period (vs 120 ms) Sub-blocks with 10 frames Starting with Frequency Correction Channel (FCCH) Immediately followed by Synchronization Channel (SCH) Other frames (numbered from #0 to #50): #50 idle #2,3,4,5 BCCH Remaining: Paging (PCH) / Access Grant (AGCH) [=PAGCH] 51 frame structure - downlink F S B B B B P P P P F S P P P P P P P P F S P P P P P P P P F S P P P P P P P P F S P P P P P P P P 10 frame sub-block

51 BCCH contents 184 bits Coded in 456 bits and interleafed in 4 bursts
same coding and interleaving as SACCH BCCH capacity 184 bits / (51*8*15/26 ms) ~ 782 bps Information provided Details of the control channel configuration Parameters to be used in the cell Random access backoff values Maximum power an MS may access (MS_TXPWR_MAX_CCCH) Minimum received power at MS (RXLEV_ACCESS_MIN) Is cell allowed? (CELL_BAR_ACCESS) Etc. List of carriers used in the cell Needed if frequency hopping is applied List of BCCH carriers and BSIC of neighboring cells

52 BCCH carrier placement
On Downlink Corresponding uplink dedicated to Random Access Channel 51 frame structure - uplink R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R On one frequency per cell (beacon) MUST BE on Time Slot #0 Other Time slots may be used by TCH Provided that: All empty slots are filled with DUMMY bursts Downlink power control must be disabled

53 Fast Associated Control Channel
FACCH: urgent signalling Used when several signalling information needs to be transmitted Call setup Handover FACCH block = 184 456 after coding Interleaved as voice block Spreaded on 8 bursts Replaces a voice block (20 ms) on the TCH Via stealing bits Voice block(s) discarded Maximum FACCH bit rate 184*6/120 [bits/ms] = 9.2 kbps (vs 383 bps of SACCH!)

54 FACCH insertion in TCH Via Stealing bits - upper bit = odd bits stolen
- lower bit = even bits stolen - both bits = all burst stolen Figure: shows example of 2 FACCH blocks stealing a TCH (note begin and end behavior due to interleaving) time

55 Why paging Channel assignment: Paging
only upon explicit request from MS Paging needed to “wake-up” MS from IDLE state when incoming call arrives to MS MS accesses on RACH to ask for a channel Generally SDCCH (but immediate TCH assignment is possible) 1) paging BSS/MSC MS 2) Random access 3) Channel assignment Paging channel: PCH Access Grant Channel: AGCH Random Access Channel: RACH CCCH Common Control CHannel PAGCH

56 Paging Paging message generated by MSC Which receives incoming call
Transferred to subset of BSC Paging limited to user’s location area Paging message contains: List of cells where paging should be performed Identity of paged user (IMSI or TMSI) Paging message coded in 4 consecutive bursts over the air interface Same coding/interleaving structure of SACCH (184456 bits) Paging for more MSs may be joined in one unique paging message

57 An example procedure involving signalling
Setup for an incoming call (call arriving from fixed network part - MS responds to a call) Steps: paging for MS MS responds on RACH MS granted an SDCCH authentication & ciphering on SDCCH MS granted a TS (TCH/FACCH) connection completed on FACCH Data transmitted on TCH

58 Radio Resource allocation three standardized solutions
Non-Off Air Call Set-Up (Non-OACSU) Normally used (previous description) Off Air Call Set-Up (OACSU) TCH assigned only when the called party actually responds! Best utilization of radio resource (avoids allocation if callee not available) Call drop if no TCH is available at this point Very Early Assignment (VEA) Immediate assignment of TCH Fastest signalling process Waste of resources VEA RACH TCH (FACCH) TCH (DATA) Non-OACSU RACH SDCCH TCH (FACCH) TCH (DATA) OACSU RACH SDCCH TCH (DATA) Connection established Callee responds

59 handover Procedure in which an MS releases a connection with a BTS, and establishes a connection with a new BTS, while ensuring that the ongoing call is maintained The MS remains in dedicated state (unlike cell reselection, where MS is in idle state) Handoff: synonymous of handover Needs two mechanisms Handover preparation: detection of cell-border crossing Based on radio link quality measurements Handover execution: setup of a new channel in a cell, and tear-down of a previous channel Improved handover mechanisms: Seamless handover: when active call performance is not impaired Not possible in GSM: for about ms, communication is interrupted Soft Handover: when two channels are simultaneously set-up (old and new) Not possible in GSM; possible in UMTS

60 Hard, Seamless, Soft handover
MSC BSS 1 BSS 2 MS f1 f2 before during after Hard handover (GSM) Seamless (DECT) Soft (UMTS)

61 Handover classification
Classification by motivation Classification by typology Rescue handover (mandatory handover) Driven by radio channel quality degradation Confinement handover (network-directed handover) Target: minimize radio interference Assign new channel when old channel results critical for total interference Traffic handover (network-directed handover) Driven by traffic congestion conditions Also called load-balancing Internal handover Intra-BTS New radio channel in the same cell Not termed as “handover” but as“subsequent assignment” Inter-BTS (Intra-BSC) Under control of same BSC External handover Inter-BSC (Intra-MSC) Change reference BSC; may imply a location area update Inter-MSC Most complex: need to change MSC

62 managed the current call Switching point for all inter-MSC handover
Types of handover Anchor MSC: the MSC that first managed the current call Relay MSC: the MSC that currently manages the call Switching point for all inter-MSC handover Switching point for inter-BSC handover A-MSC R-MSC A Switching point for internal handover BSC BSC BSC A-bis BTS BTS BTS BTS radio interface

63 Handover taxonomy BCHO: Base station Controlled Handover
Handover detection: BS Handover Execution: BS MCHO: Mobile Controlled Handover Handover detection: MS Handover Execution: MS MAHO: Mobile Assisted Handover GSM: somehow a BCHO with a flavor of MAHO Handover decision always taken by BSC Based on measures taken at both BTS and MS New channel selection decision taken at BSC or R-MSC or A-MSC (depending on handover type) based on traffic consideration

64 Handover preparation Measurements performed at BTS
RX signal level From (dBm) To RXLEV_0 - -110 RXLEV_1 -109 RXLEV_2 -108 RXLEV_3 -107 RXLEV_62 -49 -48 RXLEV_63 Measurements performed at BTS Up-link signal level received from MS lower than threshold RXLEV_UL < L_RXLEV_UL_H Up-link signal quality (BER) received from MS RXQUAL_UL < L_RXQUAL_UL_H Distance between MS and BTS adaptive timing advance parameter > MAX_MS_RANGE Interference level in unallocated time slots. Measurements performed at MS. Down-link signal level received from serving cell RXLEV_DL < L_RXLEV_DL_H Down-link signal quality (BER) received from serving cell RXQUAL_DL < L_RXQUAL_DL_H Down-link signal level received from n-th neighbor cell RXLEV_NCELL(n) > RXLEV_MIN(n) Bit error Ratio From (%) To RXQUAL_0 - 0.2 RXQUAL_1 0.4 RXQUAL_2 0.8 RXQUAL_3 1.6 RXQUAL_4 3.2 RXQUAL_5 6.4 RXQUAL_6 12.8 RXQUAL_7

65 Handover preparation – additional metrics
Transmission power Maximum MS transmission power Maximum serving BTS transmission power Maximum neighboring BTSs transmission power congestion status of serving BTS of neighboring BTSs provided they can support the MS. Handover Margin To avoid ping-pong handover effect 5-10 dB in normal operation; up to 30dB in urban operation (to fight shadowing) RXLEV (cell A) RXLEV (cell B) Handover RXLEV (cell A) RXLEV (cell B) Handover HANDOVER ALGORITHM: operator-dependent! GSM standard SUGGESTS a simple reference algorithm, but implementation left to operator hysteresis

66 handover procedure skeleton
1) Handover request goes up to switching point 1 MSC BTS BSC 5) Old channel/path torn down 5 2) Switching point prepares new path on fixed net 2 3) Switching point sends HO command to MS 3 4) MS accesses new channel 4

67 Inter-MSC handover X X X
More complex, as an ISDN circuit must be set between MSCs We’ll not enter into details (just the basic ideas) Two cases First MSC change (basic handover) Second MSC change (subsequent handover) X MSC-A MSC-R1 MSC-A MSC-R1 MSC-R2 X X Note the role of the Anchor MSC!

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