GSM in the field BTS To BSC / MSC To BSC / MSC BTS Handover Handover BTS To BSC / MSCTo BSC / MSC BTS
Some practical experiences.. No network access (= no „Telenor‟ in the display) – Lack of coverage – Network overload situation (users barred from access) – Network failure. 3 beeps/3-tone beeps at call setup – Blocking/error situations somewhere in the network – User error (e.g. non-existent number) Poor speech quality – Marginal coverage – Interference between base stations Dropped call – Poor quality of radio link – Blocking in target cell (moving mobile, handover situation) – Network failure (transmission) The mobile is the weakest link in the chain: Mass production units – Internal antenna – Increased software and processing complexity (WAP/GPRS/HSCSD/games&features)
GSM in the field BTS To BSC / MSC To BSC / MSC BTS No traffic channels available Handover Handover to non-optimal cell BTS Poor speech quality To BSC / MSC Dropped callTo BSC / MSC BTS
How is Network Qualitymonitored by the operator? Coverage – Measurements by technical staff. Test telephone + laptop. – Measurements by remotely controlled test systems (installed in bus, taxi etc). – Customer complaints and performance stats as input. Capacity – Network Maintenance Center / Operations center – The network counts # of events (pos./neg.) Post processing in Performance Management tool. – Tools for prognosis on capacity needs vs. required quality of the network (Grade of Service)
How is Network Qualitymonitored by the operator? Speech quality – The network counts Dropped calls and to some extent report on speech quality – Measurements by remotely controlled test systems / tech.staff. Services quality – Services are built up by combining equipment and software – External tools needed to ensure quality of delivery • Correct content / Response time • 3rd party content delivery • 3rd party Service Level Agreements (SLA)
Capacity and quality– a trade off at a given funding
Capacity in the GSM radio network Base station with antennas connected in a mast – BTS contains a number of TRX based on the need for capacity. Tx DOWNLINK Rx Rx UPLINK Tx TRX TRX TRXBTS cabinet
Radio network capacity Building blocks for capacity at the base stations: TRX – 8 channels that are used for signalling (SMS, suppl.services etc) and speech/data. Call duration -> Erlang – 1 Erlang = 1 hour of speech for 1 user – Call duration 90 sec: 90s / 3600s = 0,025 Erl = 25 mErl – I.e. 1 Erlang: 40 calls of 90 seconds each. (1 Erl = 40 x 25 mErl) Theoretical capacity: Erlang‟s model and chosen Grade of Service. – GoS (%) Number of call attempts that may meet blocking in a busy hour. • Typical values used: 2-5% – Example, GoS=2% – 1 TRX 7 speech channels 2.9 Erlang 41% – 2 TRX 15 speech channels 9.0 Erlang 60% – 3 TRX 22 speech channels 14.9 Erlang 68% – 4 TRX 30 speech channels 21.9 Erlang 73%
Radio network capacity Base station configuration example: 3 TRXes, Omni directional cell – 1 cell with 3 TRX, GoS=2%: • Capacity = 14.9 Erl • 90 sec call duration (25mErl) • 14.9 / 0.025 = 596 calls in a busy hour (theoretical max!) TRX TRX TRX BTS If we accept higher blocking figures we can carry more traffic with less customer satisfaction. 5% GoS used in this example => 17.1 Erl capacity => 684 calls (+15%)
Radio network capacity Base station configuration example: 3 TRXes, Sectorised 1/1/1. – 3 cells with 1 TRX each: • Capacity = 3 x 2,9 Erl = 8,7 Erl • 90 sec call duration • 8,7 / 0,025 = 348 calls in a busy hour TRX TRX TRX BTS If we accept higher blocking figures we can carry more traffic with less customer satisfaction. 5% GoS used in this example => 11.1 Erl capacity => 444 calls (+28%)
Frequency planningRe-use of GSM channels isnecessary. In this example afrequency is re-used in every12th cell => reuse factor 12
Resource: Frequency spectrum Spectrum size strongly affect base station roll out. Re-use of GSM channels is necessary. 1 TRX use 1 GSM channel Example, Malaysia: – 24.8 MHz => 124 GSM channels – # of TRX per cell: >6. Good capacity per base station (xx Erlang). Example, Russia: – 4 MHz => 20 GSM channels – Maximum ~1,5 TRX per cell = a denser base station network will be needed for serving increasing traffic (xx Erlang) Good radio/frequency planning and use of “smart” features enables a tight re-use of GSM channels – Power control • Mobile and BTS adjust their output power as low as possible, adjustment 2 times per sec. – Discontinuous Transmission (DTX) • Mobile and BTS only transmit when user actually speak. (User benefit: less power consumption) – Frequency hopping • Mobile and BTS jump from GSM channel to GSM channel (480ms), less affected by interference.
Spectrum vs. investmentsCapacity case Malaysia case.. – 6TRX/cell= 34,6 Erl/cell – 900TRX, 150 cells and 50 BTS needed Area= 50km2 Population= 1 M Russia case.. Subscribers= 346k – 1,5 TRX/cell= 5,8 Erl/cell 15 mErl/sub – 1343TRX, 895 cells and 299 BTS needed Traffic= 5190 Erl We need 6 times as many sites for capacity reason!For illustration purpose only – the conclusions and calculations are simplified
Network Quality,some KPIs Minutes per dropped call – Summary of all traffic minutes divided by number of dropped calls during a period of time (e.g. Week/month) – Typical values: 50-150 minutes. Blocking/Congestion – Call attempts that meet blocking, all resources are occupied. – The network is dimensioned to meet a certain traffic level in the busy hour. Typical dimensioning values (GoS): 2 - 5 % Typical values measured in the networks: 0 - 50+ %
Network Quality, some KPIs Network Utilisation Cell utilization 160 140 120 100Utilization (%) 80 60 40 20 0 % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % 10 13 16 19 22 26 29 32 35 38 41 44 47 50 53 57 60 63 66 69 72 75 78 81 84 88 91 94 97 1 4 7 0 10 Cell utilization distribution About 22% of the cells in the network are utilised > theoretical max = customers experience more blocking than acceptable in these cells!
Network Quality,challenges How to measure customer perceived quality? – Unmanned active measurement units. New and advanced services – Fast development and deployment of new services. – Increased complexity, both for network and mobile. 3.party content providers – Timeout... Not available... – Service Level Agreements. How do you know what is delivered? – New and advanced measurement systems for test&monitoring. Roaming – How well are we able to keep foreign roamers in our network – and how are our roamers abroad doing? – Advanced measurement systems (signalling) for test&monitoring.
EnvironmentBase station location: Aim to minimise the effect of masts and antennas on the local environment.