gsm applied cell planning gsm applied cell planning

GSM Applied Cell PlanningGSM Applied Cell Planning

David Stewart BA BSc(Hons) MIEEE

Section 1: Channel DimensioningSection 1: Channel Dimensioning

In this section we will look at:• Traffic channel dimensioning calculations

• SDCCH dimensioning

• CCCH configuration and dimensioning

Section 1 - Channel Dimensioning

Channel Dimensioning Channel Dimensioning -- ExampleExample

• In GSM channel dimensioning, the number of channels must be related to the number of carriers (frequencies) available:

• 8 channels (timeslots) per carrier

• Some channels will be required for signalling

• Example - in a particular cell:Mean call holding time = 90 seconds

Grade of Service = 1 %

Total number of available carriers = 4

3 timeslots allocated for signaling

How many subscribers can this cell support ?


Channel Dimensioning Channel Dimensioning -- SolutionSolution

• Mean call holding time of 90 s implies the average traffic per subscriber is 25 mE

• Number of channels available is given by:

(carriers x 8) - signalling channels

= 4 x 8 - 3 = 29 channels

• Using Erlang B tables for GoS = 0.01 and n = 29 channels, gives traffic that can be offered as 19.487 E = 19487 mE

• Number of subscribers that can be supported is: 19487 / 25 = 779


Trunking EfficiencyTrunking Efficiency

• Trunking efficiency or channel utilisation is given by:

carried traffic / number of channels

(where carried traffic = offered traffic modified by GoS)

Trunking Efficiency = A (1- GoS) / n

where A = offered traffic (erlangs)n = number of channels available

• In the Erlang B model:

• Using the previous example:A = 19.487 E, GoS = 0.01, n = 29

• Trunking Efficiency = 19.487 (1 - 0.01) / 29 = 0.665 = 66.5 %


• A Standalone Dedicated Control Channel (SDCCH) block is allocated to a user by the access grant channel (AGCH) in response to a random access (RACH) request for a channel

• SDCCH carries signalling between the MS and BTS while no traffic channel (TCH) is active

• The main activities on SDCCH and the mean holding times for these are shown here:

SDCCH DimensioningSDCCH Dimensioning

SDCCH Activity Mean Holding Time (s)

Call Set-up 2.5Location Updating (Automatic) 3.5Location Updating (Periodic) 3.5IMSI Attach 3.5IMSI Detach 3.0SMS Message 6.5Supplementary Services 2.5


SDCCH Grade of ServiceSDCCH Grade of Service

• One of the main functions of SDCCH is to carry call setup signalling

• Since access to a TCH is via SDCCH, the grade of service for SDCCH must be significantly better than for TCH - typically 2 to 4 times better - e.g. if TCH GoS = 2%, SDCCH GoS = 0.5% to 1%

SDCCHrequests

TCHrequests

Services using only SDCCHe.g. SMS

Blocking

Carried traffic on TCH

Blocking

Voice calls require SDCCH then TCH


SDCCH ExampleSDCCH Example

• Question:A cell is required is serve 500 subscribers

SDDCH grade of service is set at 0.5%

Typical SDCCH traffic in the busy hour is 5 mE

How many blocks are required for the SDCCH channel?

• Solution:Total SDCCH traffic = 500 x 5 = 2500 mE = 2.5 E

From Erlang B tables, using GoS = 0.005, this requires 8 blocks

• How are the required SDCCH blocks to be allocated?


SDCCH AllocationSDCCH Allocation• SDCCH blocks are allocated on the control channel multiframe structure in a

group of 4 (SDCCH/4) or 8 (SDCCH/8)

• Each SDCCH block comprises 4 timeslots and carries one SDCCH message

• SDCCH/4 is combined with other control channels on timeslot 0:

Note: SACCH 2 and 3 are carried on the next multiframe

SDCCH/4 allocation

Combined multiframe structure

• One SDCCH block may be replaced by CBCH if required

Downlink

S BCCHF CCCH S CCCHF CCCH SSDCCH

0F SF SF ISDCCH

1SDCCH

2SDCCH

3SACCH

0SACCH

1

Uplink

RR RACHSDCCH

0SDCCH

1SDCCH

3SACCH

0SACCH

1SDCCH

2RR


NonNon-- Combined Multiframe SDCCHCombined Multiframe SDCCH

SDCCH/8 may be allocated on a non-combined multiframe:

Other SACCH blocks are on the next multiframe

SDCCH/8 allocation

Non-combined multiframe structure

Downlink

ISACCH

0SACCH

1SDCCH

0SDCCH

1SDCCH

2SDCCH

3SDCCH

4SDCCH

5SDCCH

6SDCCH

7SACCH

2SACCH

3 I I

ISACCH

1SACCH

2SACCH

3 I ISACCH

0SDCCH

0SDCCH

1SDCCH

2SDCCH

3SDCCH

4SDCCH

5SDCCH

6SDCCH

7

Uplink


Practical SDCCH Dimensioning Practical SDCCH Dimensioning • Certain locations make greater use of SDCCH and will require particular allocation,

e.g.

• Cells at the border between location areas where location updating occurs frequently

• Airport:

Passengers disembark in large numbers and switch on their mobiles imposing a lot of pressure on SDCCH for location updating

Location updating may be prolonged for international roaming subscribers

Location area boundary cells


CCCH Configuration CCCH Configuration • On the downlink, CCCH consists of a number of blocks carrying paging (PCH) and

access grant (AGCH) messages

• A combined multiframe has only 3 CCCH blocks to allow for SDCCH and SACCH:

• A non-combined multiframe has 9 CCCH blocks on timeslot 0:

• A complete paging or access grant message takes four bursts (timeslots),i.e. one CCCH block


0F SF SF ISDCCH

1SDCCH

2SDCCH

3SACCH

0SACCH

1

S BCCHF CCCH S CCCHF CCCH S CCCHF CCCH S CCCHF CCCH S CCCHF CCCH I


CCCH PriorityCCCH Priority

• CCCH blocks are dynamically allocated to either PCH or AGCH according to the following priority:

Immediate Assignment Reject Message (AGCH)

Priority

High

Low

PCH

Immediate Assignment Message (AGCH)

• During periods of heavy paging, PCH could dominate, leaving no blocks for access grant messages

• To avoid this, some blocks can be reserved for AGCH


• In a non combined multiframe, up to 7 of the 9 blocks may be reserved for AGCH:

Reserving AGCH Blocks on CCCHReserving AGCH Blocks on CCCH

• In a combined multiframe, up to 2 of the 3 blocks may be reserved for AGCH:

• Additional CCCH capacity can be provided on other timeslots (2,4 or 6) of the BCCH carrier if required

• The number of AGCH blocks reserved is specified in the system information messages which the mobile reads on the BCCH

S BCCHF CCCH S CCCHF CCCH S CCCHF CCCH S CCCHF CCCH S CCCHF CCCH I


0F SF SF ISDCCH

1SDCCH

2SDCCH

3SACCH

0SACCH

1


Paging CapacityPaging Capacity

• Paging capacity is the number of mobiles that can be

paged per second

• This depends on:

• CCCH configuration

• AGCH blocks reservation

• Type of paging message used

• Paging message takes 4 bursts (1 CCCH block)

• This can page up to 4 mobiles depending on the

message type used


Calculating Paging CapacityCalculating Paging Capacity

X = number of mobiles paged per paging message (1 to 4)

Y = number of possible paging messages per multiframe

Duration of control channel multiframe = 0.235 seconds (235 ms)

• X depends on paging message type

• Y depends on CCCH configuration in the multiframe (e.g. 3 or 9) and the number of AGCH blocks reserved

235.0XY

Capacity Paging = mobiles / second


PCH DimensioningPCH Dimensioning

Paging channel requirement in blocks per multiframe is given by:

4.25 x 3600 x PMFM x PF x MT x Calls

Calls = Number of calls predicted for the location area during busy hour

MT = Fraction of calls which are mobile terminated

PF = Paging Factor = number of pages required per call

M = safety margin

PMF = Paging Message Factor = number of pages per message

Number of control channel multiframes per second = 4.25 (1 / 0.235)


This gives the following data:Calls = 50 000

MT = 0.3

PF = 2

PMF = 4

A typical safety margin for peak variations in number of calls is 1.2

PCH Dimensioning PCH Dimensioning -- ExampleExample

• PCH requirement =

• A particular location area contains 50 000 subscribers. It is predicted that 30% of these will receive a call during the busy hour. On average 2 pages are needed per call and only type 3 paging messages (TMSI) are used.

4.25 x 3600 x 41.2 x 2 x 0.3 x 50000

= 0.6

• 1 PCH block per multiframe will be adequate

PCH Requirement = Calls x MT x PF x M

PMF x 3600 x 4.25


AGCH DimensioningAGCH Dimensioning

• AGCH requirement is found by adding up the activities which need an AGCH message during the busy hour

• The following equation gives the number of AGCH blocks per multiframe:

4.25 x 2 x 3600M x SS) ID IA SMS LU (Calls +++++

AGCH required =

�The terms in brackets are the predicted numbers during the busy hour for:Calls, Location Updates (LU), SMS, IMSI attaches (IA), IMSI detaches (ID), Supplementary Services (SS)

�M = safety margin (e.g. 1.2)

�The PMF factor of 2 is because each AGCH block can carry 2 immediate assignment messages


AGCH Dimensioning AGCH Dimensioning -- ExampleExample• A cell has 1000 calls during the busy hour

• Other AGCH activities are modelled as multiples of the calls figure. A possible model is:

Activity Multiplier Total

LU 2 2000

SMS 0.1 100

SS 0.2 200

IMSI attach 0.2 200

IMSI detach 0.1 100

• This gives the total activity (including Calls) as 3600

AGCH required =4.25 x 2 x 3600

1.2 x 3600= 0.14 AGCH blocks per

multiframe


Accommodating a MultiAccommodating a Multi--Service systemService system

• The Erlang B formula relies on the variance of the demand equalling the mean (a Poisson distribution).

• If a particular service requires more than one “trunk” per connection, the demand is effectively linearly scaled and the variance no longer equals the mean.

• Methods to investigate:• Equivalent Erlangs

• Post Erlang-B

• Campbell’s Theorem


Equivalent Erlangs ExampleEquivalent Erlangs Example

• Consider 2 services sharing the same resource:• Service 1: uses 1 trunk per connection. 12 Erlangs of traffic.

• Service 2: uses 3 trunks per connection. 6 Erlangs of traffic.

• We could regard the above as equivalent to 30 Erlangs of service 1:• 30 Erlangs require 39 trunks for a 2% Blocking Probability

• Alternatively, we could regard the above as equivalent to 10 Erlangs of service 2:

• 10 Erlangs require 17 trunks, (equivalent to 51 “service 1 trunks”) for a 2% blocking probability

• Prediction varies depending on what approach you choose.


oror

12 Erlangs @ 1 TS

+

• Combine the two traffic sources together by converting one to the bandwidth of the other

• The trunking requirements will VARY with the bandwidth of equivalent Erlang that you choose!

• Not suitable for use due to this property

Difference in capacity

required for same GoS

6 Erlangs @ 3 TS

12 Erlangs

18 Erlangs18 Erlangs

@ 1 TS

30 Erlangs@ 1 TS

39Service 1Timeslots

4 Erlangs @ 3 TS

+

12 Erlangs @ 1 TS

4 Erlangs

6 Erlangs6 Erlangs @ 3 TS

10 Erlangs@ 3 TS

17Service 2

Connections

51EquivalentService 1 Timeslots

(@ n TS = n Timeslots per connection)

39Service 1

Connections

Equivalent ErlangsEquivalent Erlangs


• Consider 2 services sharing the same resource:• Service 1: uses 1 trunk per connection. 12 Erlangs of traffic.


• We could calculate the requirement separately• Service 1: 12 Erlangs require 19 trunks for a 2% Blocking Probability

• Service 2: 6 Erlangs require 12 trunks (equivalent to 36 “service 1 trunks”).

• Adding these together gives 55 trunks.

• This method is known to over-estimate the number of trunks required as can be demonstrated by considering services requiring an equal number of trunks.

Post ErlangPost Erlang--BBSection 1 - Channel Dimensioning

Post ErlangPost Erlang--BB

12 Erlangs @ 1 TS

• Calculate Traffic Capacity for each service and add the resulting Timeslot Requirement together

12 Erlangs

6 Erlangs6 Erlangs @ 3 TS

55EquivalentService 1 Timeslots

(@ n TS = n Timeslots per connection)

19Service 1

Connections


12Service 2

Connections



• Consider 2 services requiring equal resource:• Service 1: uses 1 trunk per connection. 12 Erlangs of traffic.

• Service 2: uses 1 trunk per connection. 6 Erlangs of traffic.

• We could calculate the requirement separately• Service 1: 12 Erlangs require 19 trunks for a 2% Blocking Probability

• Service 2: 6 Erlangs require 12 trunks.

• Adding these together gives 31 trunks.

• The accepted method of treating the above would be to regard it as a total of 18 Erlangs that would require 26 trunks.

• Post Erlang-B overestimates the requirement.

Post ErlangPost Erlang--BBSection 1 - Channel Dimensioning

Post ErlangPost Erlang--BB

12 Erlangs12 ErlangService A

• Combine the two traffic sources together after calculating required capacity

• Pessimistic about offered traffic supported to the same GoS

• Not suitable for use due to this property +

19 Timeslots

Illustration using 2 services with same trunk/connection ratio

6 ErlangService B

12 Timeslots

+

31Timeslots

18 Erlangs

Accepted correct method

12 ErlangService A

6 ErlangService B

26Timeslots

6 Erlangs


CampbellCampbell’’s Theorems Theorem• Campbell’s theorem creates a composite distribution where:

• c is known as the capacity factor

• The amplitude used in the capacity is the amplitude of the target service

• Once the offered traffic and Capacity are derived, GoS can be derived with Erlang-B -> similarly Required Capacity can be calculated if Offered Traffic and GoS target is known

( )c

aCCapacity ii −=

cfficOfferedTra

α=

�

�==

iiii

iiii

ba

bac

γ

γ

αν

2

α = meanυ = varianceγi = arrival rateai = amplitude of service bi = mean holding timeiib�=Traffic Offered Service


CampbellCampbell’’s Theorem Example(1)s Theorem Example(1)

• Consider the same 2 services sharing the same resource:• Service 1: uses 1 trunk per connection. 12 Erlangs of traffic.


• In this case the mean is:

• The variance is:

� � =×+×=×== 3063121Erlangs iiii aabγα

� � =×+×=×== 6636112Erlangs 2222iiii aabγν



• Capacity Factor c is:

• Offered Traffic for filtered distribution:

• Required Capacity for filtered distribution at 2% GoS is 21

2.23066 ===

αν

c

63.132.2

30 Traffic Offered ===c�



• Required Capacity is adjusted depending upon target service for GoS (in equivalent service 1 erlangs) :

• Target is Service 1: C1=(2.2 x 21) + 1 = 47

• Target is Service 2: C2=(2.2 x 21) + 3 = 49

• Different services will require a different capacity for the same GoS. In other words: for a given capacity, the different services will experience a slightly different GoS.


Traffic Analysis Methods ComparedTraffic Analysis Methods Compared

• Equivalent Erlangs• Optimistic if you use the smallest amplitude of trunk (39)

• Pessimistic if you use the largest amplitude of trunk (51)

• Post Erlang-B• Pessimistic (55)

• Trunking efficiency improvement with magnitude ignored

• Campbell’s theorem• Middle band (47 - 49)

• Different capacities required for different services - realistic

• Preferred solution for dimensioning, but not ideal...


Capacity Dimensioning with CampbellCapacity Dimensioning with Campbell’’s s TheoremTheorem

• Consider the following service definition and traffic forecast.

• Based on a theoretical availability of 15 voice trunks per cell and using voice as the ‘benchmark’ service, determine the number of cells required to serve the above traffic levels and the traffic offered per cell for each service

Service Amplitude ForecastVoice 1 250E

HSCSD 2 63E



• Assuming we have n cells, we can determine the loading per cell.

nnc

c

nnn

nnn

282335.1376mean

trafficoffered

335.1376502

meanvariance

)factor(capacity

502263250variance

376263250mean

2

=×

==

===

=×+=

=×+=



• Unfortunately, we cannot now look up “282/n” in the Erlang B tables.

� � represents the total number of cells in the network

• To calculate �, first the captured traffic per cell is calculated based on a cell trunking capacity of 15 (given at the start of the problem)

nnc282

335.1376mean

trafficoffered =×

==



• Considering the equation

• Ci is predefined as 15. ai depends on the service we use as our “benchmark”. Choosing the voice service as the “benchmark” service make ai equal to 1.

• 10.5 (or, rather, 10) trunks will service 5.08 Erlangs.

caC ii −= Capacity

( )5.10

335.1115 =−=iC



• As 10 trunks will service 5.08 Erlangs:

• Therefore, the cell requirement is established at 56 cells.

• Each of the cells will service:• 4.46 Erlangs of voice (250E / 56 cells)

• 1.13 Erlangs of HSCSD (63E / 56 cells).

5.55

08.5282

=

=

nn


Assessing Cell Loading using CampbellAssessing Cell Loading using Campbell’’s s TheoremTheorem

• After placing sites on the coverage map and spreading the traffic, the next stage is to assess the cell loading.

• If mixed services are used, it is necessary to use Campbell’s Theorem to assess the required number of timeslots to satisfy the likely demand.

• Consider the case where a particular cell captures 7 Erlangs of voice and 2 Erlangs of HSCSD traffic that requires 2 timeslots per connection.


Assessing Cell Loading using CampbellAssessing Cell Loading using Campbell’’s s TheoremTheorem

• Using Campbell’s Theorem:

• Hence 20 timeslots required.

( ) 20136.114 :benchmark as voiceTakingrequired. trunks14 B, Erlang From

09.836.1

11 trafficoffered

36.11115

15227variance

11227mean2

=+×

==

==

=×+=

=×+=

c


Dimensioning Micro and PicoDimensioning Micro and Pico--CellsCells• Erlang B assumes that demand doesn’t vary as connections are allocated.

• E.g. 800 subscribers producing 20 Erlangs would require 28 timeslots.

• Statistics of demand would reduce slightly as connections were allocated.

• Reduction from 800 to 772 would produce only a tiny reduction in demand.

A1

Demand when no connections

allocated produced by 800 subscribers

A2

Demand when 28 connections



The Engset DistributionThe Engset Distribution• Ignoring the reduction in demand is not justifiable if a cell covers a small number

of high-demand subscribers.

• E.g. a cell covering just 6 subscribers who offer 3 Erlangs of traffic would produce a required provision of 7 connections if Erlang B formula is used.

• This is clearly wrong.

• Engset distribution considers the reduction in demand as connections are allocated.

A1

Demand when no connections


A2

Demand when 5 connections

allocated produced by 1 subscriber.

• Engset formula suggests 5 connections should be provided.


SummarySummary

• Traffic channel dimensioning calculations: Erlang B tables, channel calculations, trunking efficiency

• SDCCH dimensioning: Grade of Service, channel calculations, SDCCH allocation- combined and non-combined multiframes, practical considerations

• CCCH configuration and dimensioning: CCCH configurations, PCH/AGCH priority, AGCH reservation, paging capacity, PCH dimensioning, AGCH dimensioning

• Multi-service traffic dimensioning using:

Erlang C, post-Erlang B, Campbell’s Theorem

• Micro & Pico-Cell Dimensioning using Engset


Section 2: Practical Frequency PlanningSection 2: Practical Frequency Planning

In this section we will look at:

• Practical frequency planning

• Multiple Reuse Patterns

• Frequency Hopping

Section 2 – Practical Frequency Planning

Practical Frequency PlanningPractical Frequency Planning

• Practical factors which must be considered include:

• Base stations do not radiate same power• Different cells may use different antennas• Variations in propagation due to clutter and

terrain

• Use planning tool (e.g. ILSA) to assign carriers

• Adjust frequency plan manually to optimise C/I

Worst interference

Average interference Total cost

Number of iterations


Adjustments for CapacityAdjustments for Capacity• Simple re-use patterns assign same

number of carriers to each cell

• Practical traffic may not be evenly distributed

• Moving carriers to other cells to handle traffic will introduce new interference problems

• This can be avoided by reducing base station power - e.g. introduce an overlay cell

Moving any carrier from B3 to C2 will decrease C/A (with C3) and C/I (with B3 in neighbouring cluster)

A1

A2A3

B1

B2B3

C1

C2C3

A1

A2A3

B1

B2B3

C1

C2C3

Under loaded cell

Heavily loaded cell


272625242322212019

181716151413121110

987654321

C3B3A3C2B2A2C1B1A1

Multiple ReMultiple Re--use Patternsuse Patterns

• MRP :• A technique to vary the reuse pattern for different channels and different

levels of quality of service (QoS)

• Combines conservative control channel reuse with aggressive traffic channel reuse to achieve a tighter average reuse

• Frequency Hopping, Power Control and DTX are necessary

• Frequencies can be reserved for microcells and picocells

• Best used with lots of spectrum• Performance results with 15 MHz (75 GSM carriers) are better than for 5 MHz

(25 GSM carriers) because there are more frequencies to hop across

• In ASSET, carrier layers are used to represent these subsets


Planning with Planning with MRPsMRPs

• The subset with the greatest number of carriers is used exclusively to plan the BCCHchannels

• The subset with the second greatest number of of carriers is used exclusively to plan the first TCH (TCH1) channel on cells

• The third greatest subset is used exclusively to plan the second TCH (TCH2) channel on cells

• The next subset is used exclusively to plan the third TCH (TCH3) channel on cells and so on

BCCH

TCH1

TCH2

TCH3


MRP: Planning the BCCH LayerMRP: Planning the BCCH Layer

• 12 available carriers (GSM Carriers 1-12) available for the BCCH Layer

• Maximum allocatable carriers per cell for the BCCH is 1

• The 12 BCCH carriers are then spread throughout the network using a 4/12 Reuse pattern

A1 B1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D31 2 3 4 5 6 7 8 9 10 11 12


A1

A2A3

C1

C2C3

D3

D2D1

B1

B2B3

A1

A2A3

C1

C2C3

D3

D2D1

B1

B2B3

A1

A2A3

C1

C2C3

D3

D2D1

B1

B2B3

MRP Planning the TCH1 LayerMRP Planning the TCH1 Layer

• 9 carriers (GSM Carriers 13-21) available for the TCH1 Layer

• Maximum allocatable carriers per cell for the TCH1 is 1

• The 9 TCH1 carriers are then spread throughout the network using a 3/9 Reuse pattern

A1 B1 C1 A2 B2 C2 A3 B3 C313 14 15 16 17 18 19 20 21


A1

A2A3

B1

B2B3

C1

C2C3

A1

A2A3

B1

B2B3

C1

C2C3

A1

A2A3

B1

B2B3

C1

C2C3

MRP Planning the TCH2 LayerMRP Planning the TCH2 Layer

• 3 carriers (GSM Carriers 22-24) available for the TCH 2 Layer

• Maximum allocatable carriers per cell for the TCH2 is 1

• The 3 TCH2 carriers are then spread throughout the network using a 1/3 Reuse pattern

A1 A2 A322 23 24


A1

A2A3 A1

A2A3A1

A2A3

Example of MRPExample of MRP• MRP example with 5 MHz of Spectrum

• 24 GSM RF Carriers (excluding one for guard band)

• 12/7/5 reuse

• Average cluster size = (12 + 7 + 5) / 3 = 8

81 2 3 4 5 6 7 12 151413119 10 242322212019181716

12 BCCH Frequencies

7 TCH Group 1

5 TCH Group 2


Interference in Interference in MRPsMRPs

• MRP uses progressively fewer carriers on each layer:• Progressively tighter frequency reuse

• Progressively worse interference on each carrier layer

• Ways of reducing interference:• not every cell will employ all carriers - increases the reuse distance

• MRP is often used in conjunction with:• frequency hopping

• discontinuous transmission

• downlink power control

• These techniques reduce the impact of interference on calls and allow close reuse distances to work more reliably


Frequency HoppingFrequency Hopping• When using frequency hopping, the actual carrier frequency used by a

TRX changes on each frame (8 timeslots)• The frequency follows either a sequential or pseudo-random pattern:

4 2 6 5 1 3 4 21 3 4 2 6 5 1 3

Frames cycle through carriers 1 to 6 :

Hopping sequence

• One frame is 4.6 ms long

• Rate of hopping = 1/ (4.6 x 10-3) = 217 hops / second

• This is also known as Slow Frequency Hopping (SFH) to distinguish it from Fast Frequency Hopping used in CDMA systems


Frequency Hopping at the BTSFrequency Hopping at the BTS• If the BTS has implemented SFH:

• TRXs used only for traffic channels will hop through set sequences

• TRX used for the BCCH carrier will not hop - mobiles must be able to access this for periodic signal level measurements

• 64 hopping sequences are available in GSM:• 1 sequence is cyclic - 1,2,3 …, 1,2 …

• 63 others are pseudo random patterns

• Hop Sequence Number (HSN) defines the sequence in use• HSN = 0 indicates the cyclic sequence

• The set of carrier frequencies assigned to the sequence (Mobile Allocation) may be the same for each TRX provided the sequence starts at a different point (Mobile Allocation Index Offset, MAIO)


Frequency Hopping at the MobileFrequency Hopping at the Mobile

• Base stations need not implement frequency hopping

• Mobile must be capable of SFH in case it enters a cell in which it is implemented

• In addition to hopping in step with the BTS, the mobile must also make measurements on adjacent cells

• This is why the rate of hopping is limited to SFH in GSM

• The mobile needs to know:• Frequencies used for hopping (Mobile Allocation) - coded as a subset of the

Cell Allocation frequencies

• Hop Sequence Number (HSN)

• Start frequency (Mobile Allocation Index Offset, MAIO)


Frequency Hopping and HandoverFrequency Hopping and Handover

• Scenario:• Mobile is frequency hopping in a cell• It is being handed over to a new cell in which it can continue hopping

• Requirement:• Handover command message must contain information to start the

hopping process in the new cell• Channel Description in the message must include:

• Hopping / Non-hopping flag• MAIO• HSN

MS BSS 1

Handover Command


SummarySummary

• Practical frequency planning: assumptions , planning tools, capacity adjustments

• Multiple Reuse Patterns: principle of MRP, planning with MRP, interference effects

• Frequency Hopping: effects of Frequency Hopping at the BTS, Mobile and during Handover.


Section 3: Integrating New SitesSection 3: Integrating New Sites


• An Initial Situation

• Configuration of a New Site• Carriers, Sectorisation, Diversity, antenna downtilt

• Neighbour Issues

Section 3 – Integrating New Sites

Integrating New SitesIntegrating New Sites

• Consider the situation where an existing suburban area with a near-uniform traffic distribution has been planned. Then a new office building creates a hotspot that puts a strain on the network.


Integrating New SitesIntegrating New Sites

• Suppose that, in the first instance, 24 carriers are allocated to the area with a 4x12 re-use pattern.

• Each site is configured on a 2+2+2 basis.


Questions to be AnsweredQuestions to be Answered

• Configuration of New Site:

• Frequency Allocation

• Sectorisation

• Diversity

• Neighbour Issues


TRxTRx RequirementsRequirements

• The number of TRx’s required depends on capacity requirements.

6 TRXs: 27 Erlangs6 TRXs: 35 Erlangs

3 TRXs: 9 Erlangs3 TRXs: 15 Erlangs

Sectorised SiteOmni-directional Site


The MicroThe Micro--Cell LayerCell Layer

• It is possible that separate carriers are available for micro-cells.

• If not, then a new frequency plan must be created.


A New Frequency PlanA New Frequency Plan

• Generally speaking, frequency planning is no longer a manual task.

• After the new site has been configured, a new plan can be created by using an automatic software planning tool.

• This will involve extensive changes to the frequencies applied at sites throughout the network.


SectorisationSectorisation and Diversityand Diversity

• Sectorisation and Diversity may be required depending on the range and capacity requirements of the new cell.

• Diversity allows the uplink and downlink to be balanced at higher transmit powers thus allowing greater range (not usually necessary with hotspots).

• The decision regarding Sectorisation is not straightforward as different factors exert influence.


DiversityDiversity

• Diversity provides a gain in the uplink only.

• This will allow the uplink and downlink to be balanced when there is a greater difference between the transmit powers (that is, when the downlink transmit power is very high).

• This allows a greater coverage range.

• Sites covering small hotspots do not usually employ diversity.

SectorisationSectorisation

• Does the micro-cell have new frequencies allocated to it?

• If not, it is usually best to fit in with existing configuration.


Further Considerations Further Considerations –– downdown--tilting antennastilting antennas

• Even if the new site is not formally assigned to the micro-cell layer, coverage of hotspots have a lot in common with micro-cells.

• The small geographical area over which coverage is required means that the antennas should be steeply down-tilted to avoid interference with nearby cells using co- or adjacent channels.

• Remember that only sectored antennas can be mechanically tilted.


ReRe--directing directing NeighbouringNeighbouring AntennasAntennas

• The coverage provided by the new site will mean that the existing sites coverage patterns will be changed.

• Thus the pre-existing antennas will have to be re-directed.


NeighbourNeighbour IssuesIssues

• Once the frequency plan has been finalised, it is necessary to consider and define new neighbour relations.

• This will affect both the new and existing sites.

• Along with neighbour relations, it is necessary to define hysterisis margins for the new neighbours

Integrating New Sites Integrating New Sites -- SummarySummary

• The major problem is frequency allocation.

• Ideally, frequencies separate from those allocated to the existing macro-cell layer will be available.

• If not, a new frequency plan must be produced.

• It is usually easier to produce the new frequency plan if the new site is sectorised.

• Down-tilting and re-pointing antennas will help reduce inter-cell interference.

• The neighbour list must be updated to acknowledge the existence of the new cells.

• The process will need several iterations to arrive at the optimum configuration.


Section 4: GSM 1800 and DualSection 4: GSM 1800 and Dual--band Networksband Networks


• 1800 MHz Frequency Spectrum

• Network Evolution and GSM 1800

Driving Forces

• Propagation Issues

• Dual Band Antennas

• Handset Issues

• BSS Issues

Section 4 – GSM 1800 and Dual-band Networks

GSM 1800 and DualGSM 1800 and Dual--band Networksband Networks

• Initially known as DCS 1800

• Spectrum “available” since 1991

• Re-named GSM 1800

• New spectrum more than doubles the amount

available.


DCS DCS -- 1800 Spectrum1800 Spectrum

Uplink Downlink

1710 1785 1805 1880 MHz

Duplex spacing = 95 MHz

Guard Band100 kHz wide

Channel Numbers (n) (ARFCN)200 kHz spacing

Range of ARFCN:512 – 885 (374 carriers)

1 nGuard Band100 kHz wide

Fu(n)

2 3 4


1800 MHz Utilization1800 MHz Utilization

A possible distribution of frequencies among operators is:

DECT: Digital Enhanced Cordless Telecommunications

GSM 900

overflow

1710 1721.5 17851781.5

MHz

1800 MHz Operator “A”

DE

CT

1805 1816.5 18801876.5

MHz

Uplink

Downlink

One 2 One

1800 MHz Operator “B”

1751.5

1846.5


EE--GSM Spectrum (Extended GSM)GSM Spectrum (Extended GSM)

Uplink Downlink

880 915 925 960 MHz

Duplex spacing = 45 MHz


Channel Numbers (n) (ARFCN)200 kHz spacing

Range of ARFCN:1 – 124975 - 1023 1 n


Fu(n)

2 3 4


Evolution of GSM CapacityEvolution of GSM Capacity

GSM 900:25 MHz

E-GSM 900:35 MHz

GSM 1800:Extra 75 MHz

• Original allocation of 25 MHz allowed for 124 200 kHz carriers.

• Extending to 35 MHz bandwidth increased limit to 173 carriers.

• GSM 1800 allows for 374 carriers pushing total to 547.

• Note 1800 MHz band has most of the capacity.


Evolution of a GSM NetworkEvolution of a GSM Network

Coverage

Increase TRXs

Split Cells and Add Sites

• After the initial drive to provide coverage, the

main objective is to meet customer demand.

• Implementation of Dual Band is one of the final

steps in achieving maximum capacity.

Deploy Micro-cells

Implement Dual Band Network


The Driving Force The Driving Force -- RevenueRevenue

1 TS: $0.10 per minute

1 TRX: $0.70 per minute

Assuming 10% occupancy:$100 per day

• A timeslot will serve one telephone call.

• Throughout a network, a single extra TRX on

each cell will lead to substantial revenue

possibilities.

$36500 per year

10,000 cell network: $365 million per year per TRX


GSM 1800 GSM 1800 –– Propagation IssuesPropagation Issues

• Okumura-Hata model for Macrocell prediction:

• A and B have different values for the different frequency bands:

• At 900 MHz, A~69.55 and B~26.16: At 1800 MHz, A~46.3 and B~33.9

• Thus, for a 30 metre BTS, the standard models would be:

( ) dhhfBA BTSBTS 10101010 loglog55.69.44log82.13logLoss −+−+=

MHz) (1800 log35136(dB) Loss

MHz) (900 log35126(dB) Loss

10

10

d

d

+=+=


Dual Slope Loss CharacteristicDual Slope Loss Characteristic• Okumura-Hata is valid at large distances from the base station.

• The “Path Loss : Distance” graph usually displays a definite “kink”.

1 km

K1 far

K1 nearK2 near

K2 far

Break Point

Loss

• Typically, “K2 far” will be approximately 35.

• “K2 near” will be approximately 20.

Distance (log scale)


Dual Slope Loss CharacteristicDual Slope Loss Characteristic

• The existence of this Break Point has an impact at all frequencies.

• The position of the Break Point depends on the height of the Base Station and the frequency.

• Approximate Formula:

( ) ( )metres

50mMHz

4DistancePoint Break BTSMSBTS hfhh ≈≈λ


Dual Slope Loss CharacteristicDual Slope Loss Characteristic( ) ( )

metres 50

mMHz4DistancePoint Break BTSMSBTS hfhh ≈≈

λ

1040 m540 m30 m

720 m360 m20 m

360 m180 m10 m

Break-point at 1800 MHz

Break-point at 900 MHz

Height of BTS

• Area within break point is much larger at 1800 MHz than at 900 MHz.

• This has implications for coverage.



• Determining “K1 near”

1 km

K1 far

K1 nearK2 near

K2 far

Break Point

Distance

Loss

• The value of “K1 near” will depend upon the values of the other “K values” and the Break Point distance.



K1 near = K1 far + (log10 [Break Point])(K2 far – K2 near)


• Beyond the break point, the difference in path loss will be approximately 10 dB.

• Within the break point, the difference in loss will be larger (~13 dB).

1800 MHz

Break Points


Loss

900 MHz


AntennasAntennas

• Antennas are generally scaled in proportion to the wavelength of operation.

• An 1800 MHz antenna can be expected to be smaller than a 900 MHz antenna.

• It is easier to implement antennas with higher gains (~18 dBi) in 1800 MHz systems than it is in 900 MHz systems.


Coverage ComparisonsCoverage Comparisons

• The increased path loss might be partially compensated for by a higher gain antenna at 1800 MHz

• For indoor coverage a path loss of 136 dB could be tolerated at 1800 MHz whereas the limit could be 132 dB at 900 MHz.

• Generally, coverage is greater at 900 MHz, by a factor of approximately �2.

• This gives rise to the general rule that twice the number of sites would be required to give continuous coverage at 1800 MHz compared to 900 MHz.


Dual Band AntennasDual Band Antennas

• It is possible to employ a single antenna that will serve both the 900 MHz and 1800 MHz bands.

• A special-purpose diplexer will be required.

• Even with a single antenna, polarisation diversity will usually be used.

Tx/Rx 900/1800

Dual Band – Dual PolarisationAntenna

LNA

LNA LNA

LNA

Rx Diversity 900/1800

Tx/Rx 1800

1800 RxDiversity

Tx/Rx 900

900 RxDiversity


Dual Band NetworksDual Band Networks

• A network utilising both bands yields gains in terms of flexibility and capacity.

• A strategy has to be adopted:•900 MHz layer to provide continuous coverage? In which case:

– BCC would be on 900 MHz frequency only.

•1800 MHz frequency for micro/pico cells.


Dual Band Networks: Handset IssuesDual Band Networks: Handset Issues

• To successfully implement a dual band network the mobiles must:

• Be able to transmit and receive on both frequencies

• Tune rapidly between the bands

• These requirements can lead to:• Larger handset size

• Shorter battery life


Dual Band Networks: BSS IssuesDual Band Networks: BSS Issues

• 900 MHz and 1800 MHz equipment widely available.

• Easily fit into standard cabinets

• New antenna systems required

• Handover strategy:• If signalling for handover between bands must go via MSC this will slow handover and lead to excessive MSC traffic.

• Ideally, handover between bands should be handled by the BSC.


Section 5: Frequency HoppingSection 5: Frequency Hopping


• Principles of Frequency Hopping

• Implementing Frequency Hopping

• Implications for Planning

Section 5 – Frequency Hopping

Principles of Frequency HoppingPrinciples of Frequency Hopping

• Providing a useful communication

channel in a GSM network relies on

delivering an acceptably low BER.

• This, in turn, requires a sufficiently

high Signal to Noise plus Interference

ratio.

��

��

=0

erfc21

NE

P bb



Probability of Error against Eb/No

0

0.1

0.2

0.3

0.4

0.5

0.6

0 5 10 15 20

Eb/No (dB)

P(e

rror

)

Probability of Error againstEb/No

BEREb/No

(dB)

2.98E-1416

3.11E-1115

6.99E-0914

4.72E-0713

1.24E-0512

0.00015711

0.00111410

0.0050629

0.0162538

0.0399037

0.0796386

0.1355245

0.2040724

0.2798073

0.3571172

0.4314991

0.50



��

��

=0

erfc21

NE

P bb

Detailed View

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0 5 10 15 20

Eb/No (dB)

P(e

rror

)

Detailed View

•Area of Interest

The target value of C/I of about 11 dB includes a margin such that the value of Eb/No remains above 5 dB.


Interference in GSM networksInterference in GSM networks

• Co-channel interference will occur

between cells that share the same

TRX frequency.

• If Group planning is implemented,

multiple carriers will be allocated to

each cell.

• These carriers are always on

regardless of demand.

• Downlink interference is therefore

at a constant level.


• As the re-use pattern gets tighter, so the interference level rises.

• Introducing frequency hopping can improve the user experience in such situations.

• The following two phenomena contribute to the improvement.

• “Interference Averaging”

• Demand-led allocation of TRXs – if only one TRX is required, only one need be allocated.

A1

A2A3

B1

B2B3

C1

C2C3

A1

A2A3

B1

B2B3

C1

C2C3

A1

A2A3

B1

B2B3

C1

C2C3

Interference in GSM networksInterference in GSM networksSection 5 – Frequency Hopping

Multipath FadingMultipath Fading

• A mobile radio channel

often utilises more than

one path between the

mobile and the base

station.

• This results in the

channel experiencing

Rayleigh fading.

Relative Signal Strength (dB)

0

+10

-10

Distance (wavelengths)

0

5 10 15



• Users will experience a

variable signal quality

and the target value for

planning purposes has a

margin included to

accommodate this.


0

+10

-10


0

5 10 15



• If a channel hops across

several frequencies, the

chances of a sustained

“null” are very small (bit

interleaving helps with

this).

• The margin required is

lower hence the average

C/I can be lower for a

given performance.


0

+10

-10


0

5 10 15

F1

F2

F3



• Gain due to frequency

hopping relies on low

“coherence” between the

frequencies.

• The more multipath (and,

in particular, the greater

the delay spread) the

less coherence.


0

+10

-10


0

5 10 15

F1

F2

F3


Delay SpreadDelay Spread

• If an impulse is transmitted,

several impulses will be

received if a multipath

environment exists.

Relative Power (P�)

00

Delay (�) �s

1

0.5 1.0 1.5

Impulse Response of Mobile Channel ( )

τ

τ

τ

τ

π

ττττ

Sf

P

dP

S

c

tot

21

0

2

=∆

−

=−

∞

�Delay Spread,

Coherence Bandwidth,


Coherence BandwidthCoherence Bandwidth

• Frequency hopping provides the greatest benefit if the hopping

distance is comparable to, or greater than, the coherence

bandwidth.

• Hopping not of great benefit in micro-cell or indoor applications.

( )

τ

τ

τ

τ

π

ττττ

Sf

P

dP

S

c

tot

21

0

2

=∆

−

=−

∞

�Delay Spread,

Coherence Bandwidth,


Example ConfigurationExample Configuration

• Suppose a total of 36 carriers

are allocated to a layer on a

network that will implement

frequency hopping.

• Each cell will have four TRXs.

• In the non-hopping

configuration, each cell would

have four fixed frequencies

allocated to it.

A1

A2A3

B1

B2B3

C1

C2C3

A1

A2A3

B1

B2B3

C1

C2C3

A1

A2A3

B1

B2B3

C1

C2C3



• The introduction of hopping

should improve the user

experience by providing

“frequency diversity” and

“interference averaging” so

that no user finds themselves

“stuck” on a frequency that

experiences a hostile

propagation environment.

A1

A2A3

B1

B2B3

C1

C2C3

A1

A2A3

B1

B2B3

C1

C2C3

A1

A2A3

B1

B2B3

C1

C2C3



• If one of the frequencies contains the BCCH channel (TS0 on the

nominated carrier), then that TS cannot hop.

• TSs 1 to 7 can hop on all four frequencies but TS0 for the BCCH

frequency (F0) is fixed and hopping can only occur amongst the other

three frequencies.

F0

F0

F1

F2F3F2F3

F1

F0

F2

F3

F1

TS0 TS1 TS2 TS4 TS6TS3 TS5 TS7



• Hopping occurs once per frame: every 4.615 ms.

• MS has to adjust its transmit frequency and receive frequency

• BTS is always transmitting and receiving on all frequencies but has to

re-configure channel map each frame.

F0

F0

F1

F2F3F2F3

F1

F0

F2

F3

F1

TS0 TS1 TS2 TS4 TS6TS3 TS5 TS7


Alternative ConfigurationAlternative Configuration

• An alternative to hopping

within the four “original”

frequencies is to allow each

TRX to hop across all 36

frequencies allocated to the

cell layer (only 4 frequencies

being transmitted on any one

cell simulataneously).

• BCCH not on the hopping

layer.

A1

A2A3

B1

B2B3

C1

C2C3

A1

A2A3

B1

B2B3

C1

C2C3

A1

A2A3

B1

B2B3

C1

C2C3


Hopping SequencesHopping Sequences

• Interference is reduced to a

minimum by allocating

appropriate hopping

sequences to each TRX.

A1

A2A3

B1

B2B3

C1

C2C3

A1

A2A3

B1

B2B3

C1

C2C3

A1

A2A3

B1

B2B3

C1

C2C3

1 2 3 4

2 4 3 1

3 2 4 1

4 2 3 1

1 3 4 2

Different sequences for four frequencies

“Cyclic”

“Pseudo Random”

“Pseudo Random”

“Pseudo Random”

“Pseudo Random”

• Different sites are “free-

running” with respect to each

other – occasionally a clash

will occur.


Site Hopping SequencesSite Hopping Sequences

• TRXs on the same site will

have the same sequence but

will be given different

offsets. These will be

synchronised.

A1

A2A3

B1

B2B3

C1

C2C3

A1

A2A3

B1

B2B3

C1

C2C3

A1

A2A3

B1

B2B3

C1

C2C3

1 3 4 2

2 4 1 3

3 1 2 4

4 2 3 1

Different offsets within the same sequence.

• Each site shown will have 12

TRXs each transmitting one

of 36 possible frequencies.


Types of Hopping: Synthesiser HoppingTypes of Hopping: Synthesiser Hopping

• A single channel using synthesiser hopping has only one TRX output and would not require a combiner

• If several channels are to be combined and fed to one antenna, the combiner must have a wide bandwidth to deal with the range of frequencies from the synthesiser

• Hybrid combiners must be used in this case – these are wideband but lossy.

Tuning controller

BasebandData Signal 1 TRX 1


HybridCombiner

Load

Antenna


Types of Hopping: Synthesiser HoppingTypes of Hopping: Synthesiser Hopping

• Multiple stages of combination introduce further losses.

Tuning controller



HybridCombiner

Load

Tuning controller



HybridCombiner

Load

HybridCombiner

Load

Antenna


Types of Hopping: Types of Hopping: BasebandBaseband HoppingHopping

• The baseband signal is fed to one of several TRXs in turn by a switch

• The TRX outputs must be combined to be fed to the antenna

• The combiner must be able to handle a wide bandwidth of signals

• This can be achieved using either:• hybrid combiners - several stages causing large loss

• cavity filters - one associated with each TRX - maximum loss ~ 5 dB

TRX

TRXBasebandData Signal

TRX

Antenna

Switch controller

Cavity filter method is preferred as it gives lower loss


Choosing the Best Type of HoppingChoosing the Best Type of Hopping

• Baseband hopping: One TRX required for every frequency used. Would be very expensive if hopping is across the entire layer. Advantage is that narrow-band, low loss, cavity filters can be used to combine the outputs. Could be the better method for the situation where hopping is only within the cells “normal” frequency allocation.

• Synthesiser hopping: One TRX required only for each simultaneous frequency transmission. Economises on TRXs. Requires wideband, hybrid combiners which are lossy.


Implications for PlanningImplications for Planning

• Hopping within the frequency allocation.• Allows lower average C/I to be tolerated.

• Can plan for average C/I to be only 9 dB and assume that hopping will “make it work”.

• Hopping across “all frequencies”.• Don’t plan – hop!

• Strategies will be adopted largely on the basis of experience

• Performance improved by automated optimisation procedures.

• Can help when TRXs need to be added to a cell.


Section 6: GPRS PlanningSection 6: GPRS Planning


• Structure of a Packet Service Session

• Potential Throughput

• Channel Coding Schemes

• Data Rate Arrays

• Planning for GPRS

• EDGE

• BTS Location Planning

Section 6 – GPRS Planning

GPRS PlanningGPRS Planning

• General Packet Radio Services

• Always On

• Maximum Throughput

• Theoretical Maximum Throughput 171 kbits/s


Structure of a Packet Service SessionStructure of a Packet Service Session

Reading Time

Packet Service Session Duration

Packet Size Interarrival Time

Packet Call

• A Packet Service Session consists of a number of calls. (A period “surfing” the internet would form a service session.)

• Each call consists of a number of packets.

• Time intervals between packets and calls are important parameters


Potential ThroughputPotential Throughput

• GSM Frame contains 57 traffic bits every 4.615 ms.

• This corresponds to 24.7 kbits/s

• Voice uses half this rate

• Possibility exists to pass data rates closer to 24.7 kbit/s over a single timeslot

57 bits 57 bits

4.615 ms


Potential ThroughputPotential Throughput

• GSM Frame contains 114 traffic bits

every 4.615 ms.

•This corresponds to 24.7 kbits/s

•Voice uses half this rate

•Possibility exists to pass data rates

closer to 24.7 kbit/s over a single

timeslot

57 bits 57 bits

4.615 ms

• In practice 21.4 kbits/s per timeslot

is the highest that is offered.

• If all 8 timeslots are used, potential

is for 171.2 kbits/s to be offered.

• Actual offered maximum rate is

likely to be less than this.


Channel Coding SchemesChannel Coding Schemes• The maximum value of 21.4 kbits/ s requires a very good C/I (>25 dB).

• Different Channel Coding Schemes are used to get the maximum throughput

for each C/I value.


Data Rate ArraysData Rate Arrays• C/I is predicted as part of the normal GSM Planning Procedure

• This can be converted to give a picture showing the maximum throughput

possible for each area.

Typically, the higher data rates are only achievable in areas close to cells.


GPRS Planning GPRS Planning –– Base Station Subsystem (BSS)Base Station Subsystem (BSS)

• Implementing GPRS will have implications for the configuration of the BTSs the

BSCs and the Core Network.


GPRS Planning GPRS Planning –– The BTSThe BTS

• For CCS-1 and CCS-2 only software

changes are necessary.

• For Higher schemes (throughputs

greater than 16 kbps per timeslot) new

hardware is required.


GPRS Planning GPRS Planning –– The BSCThe BSC

• BSC’s will require a Packet Control Unit (PCU).

• The PCU will have capacity limitations for:

• Number of TRXs

• Number of BTSs

• Total Transmission Capacity

• PDP Contexts

• Further the capacity of interfaces must be assessed.

• Redundancy issues must be addressed.


Network SubNetwork Sub--System (NSS)System (NSS)

• The addition of two new nodes• The Serving GPRS Support Node (SGSN)

• The Gateway GPRS Support Node (GGSN)

• Both inter-work with the HLR, MSC/VLR, and BSS


GPRS Planning GPRS Planning –– The GGSNThe GGSN

• Provides a gateway between the GPRS

network and the Packet Data Network

(PDN) or other GPRS networks.

• Provides authentication and location

management functions

• Connects to HLR via Gc interface

• Counts the number of packets transmitted

for accurate subscriber billing

• Connected to SGSN via IP Backbone


GPRS Planning GPRS Planning –– The SGSNThe SGSN

• Controls the connection between the

network and MS

• Provides session management and GPRS

mobility management functions such as

handover and paging

• Attaches to HLR via the Gr interface and to

the MSC/VLR via the Gs interface

• Also counts the number of packets routed

• Each Packet Processing Unit is capable of

handling a certain amount of traffic.


Air Interface Planning for GPRSAir Interface Planning for GPRSThroughput influenced by:

• Channel Coding Scheme

• C/I

• Characteristics of Channel

(including mobile speed affects).

Additionally, Frequency Hopping

facilitates the use of higher level

CCSs.

Coverage for CCS-2 generally similar

to that for GSM voice.


Capacity Planning for GPRSCapacity Planning for GPRSMeasuring Subscriber Traffic:

• Traffic measured in Megabits during the busy hour (not a constant rate in kbps).

• E.g. 1 Mbit during the busy hour equates to an average of approximately 280 bits

per second.

• 4000 such subscribers would generate an average demand of approximately

1100 kbits/s.

• Number of timeslots required depends on C/I and use of CCSs.

• At 11 kbit/s per timeslot, 100 timeslots would be required.

• Availability of timeslots would influence delays.


Capacity Planning for GPRSCapacity Planning for GPRS

Sharing Capacity with CS Traffic:

• CS traffic will take priority.

• Dimensioning for a small (2%) Blocking

Probability will mean that there is

usually extra capacity available.


Capacity Planning for GPRSCapacity Planning for GPRS

LoadTarget Load

Non Controllable Real Time Load

• E.g. 3 TRXs with 22 traffic timeslots will serve

14.9 E of traffic.

• Average of 7.1 timeslots available for packet

traffic.

• This would serve 7.1 Erlangs if traffic was

always being offered when capacity available.

• Typically, these timeslots would have a

combined throughput of 80 kbit/s capable of

serving 280 subscribers with an average of

280 bps.

• Working to an estimate of 80 – 90%

occupancy suggests 240 subscribers is a

more realistic value.

• Remember that GPRS is

a new service.

• Subscriber behaviour not

yet known.


EDGE EDGE • Bandwidth fundamentally limits the Symbol rate.

• Symbol rate is linked to bit rate by the modulation scheme.

BPSK: 1 bit per symbol

8PSK: 3 bits per symbol

0 1

• GSM uses a binary scheme.

• Absolute limit of 50 kbit/s per timeslot

in a 200 kHz bandwith

• Practical limit approx 25 kbit/s

• EDGE uses a 8-state scheme.

• Absolute limit of 150 kbit/s per timeslot

in a 200 kHz bandwidth

• Practical limit approx 63 kbit/s


EDGE and C/IEDGE and C/I• EDGE achieves a higher data rate than conventional GSM by utilising an 8PSK

modulation scheme. GSM uses a form of BPSK.

• EDGE will require a better SNR than GSM to achieve higher (> 40 kbps)

throughputs.

• 30 kbps possible for C/I > 15dB – a clear improvement over GSM

BPSK: 1 bit per symbol 8PSK: 3 bits per symbol


Implementing EDGEImplementing EDGE

• Hardware changes to the network and the mobile required.

• EDGE will help overall capacity by delivering very high data rates in areas of

high C/I. This influences location planning. If the BTS can be place near an

area of high demand, the throughput will be increased.

•EDGE area•GPRS area


BTS Location OptimisationBTS Location Optimisation• In traditional GSM networks the capacity of a cell is generally unaffected by the

position of users within its coverage area.

• With GPRS, it is possible to enhance the capacity of the cell by ensuring that areas

of high demand experience a high C/I.

• This is a further consideration for network planning.

High Throughput Area

Low Throughput Area


Why Optimise?Why Optimise?

• Improving C/I at a point from, for example, 14 dB to 17 dB will have a

noticeable affect on achievable bitrate.

• This will reduce the timeslot demand from GRPS, increasing capacity of

voice.

• Also, it will improve the coverage, capacity and efficiency of a voice network.

• This is particularly noticeable in multi-layer networks in which aggressive

frequency re-use strategies have been adopted.


Examples of ImprovementExamples of Improvement

• Group planning was applied to a

cluster of 28 sites using a 4/12 re-use

pattern. C/I was predicted.

• For particular coding schemes, these

values of C/I can be converted into a

GPRS data rate.

• It is possible to improve the values of

C/I observed by using an Automatic

Frequency Planning tool.



• AIRCOM’s proprietary frequency

planning tool, ILSA, was used to

improve the plan.

• The same number of carriers was

allocated but ILSA was able to

optimise the plan so that the

percentage area affected by

interference was minimised.

• Most of the area was receiving a C/I of

16 dB or better.



• A more detailed examination revealed

that significant areas were

experiencing values of C/I of better

than 24 dB.



• Zooming into the view

reveals that C/I is azimuth-

dependent at a given

range.

• Variations of 5 dB over a

60 degree movement are

typical.



• Examining the Channel

Coding Schemes shows

that C/I values of greater

than 12 dB can only be

exploited if Schemes 3 and

4 are implemented.

• These higher level

schemes require hardware

modifications.


GPRS and Multiple ReGPRS and Multiple Re--use Patternsuse Patterns

• Most benefit from improving

C/I comes at low levels of

initial C/I.

• For example. C/I improvement

from 3 dB to 6 dB increases

throughput by 50%.

Improvement from 12 dB to 15

dB increases throughput by

14%.


GPRS and Multiple ReGPRS and Multiple Re--use Patternsuse Patterns

• Message is that:

• GPRS will operate in

environments too hostile

for voice.

• Improvements in those

environments will be

beneficial from the

viewpoint of throughput.


A ReA Re--use Factor of 3use Factor of 3

• The same 28 site cluster was

manually planned with a re-use

factor of 3.

• C/I values varied from 4 to 26

dB.

• Throughputs expected to vary

from 5 kbps to 20 kbps.

• Typical value: 10 dB (11 kbps).

• 70% of area “good for voice”

(greater than 9 dB C/I).


A ReA Re--use Factor of 3use Factor of 3

• Because of the strict limitation

on the number of carriers, no

improvement was possible

through the use of an

Automatic Frequency Planning

Tool.


Introducing an Extra CarrierIntroducing an Extra Carrier

• Introducing a fourth carrier

should increase the average

C/I experienced.

• This will increase overall

throughput possible.

• We can evaluate whether it is

worth the extra carrier.

• Should also increase

percentage “good for voice”.



• Result is that the percentage

“good for voice” increases to

80%.

• Many areas have much-

improved C/I.

• Some areas experience a

worse C/I.



• Data Rate capacity was not

even (geographically) when 3

carriers were used but each cell

had about the same capacity.

• Now, by targetting a minimum

overall interference, the

capacity of cells will vary widely.

• This is of interest when

providing for hotspots.



• Providing the extra carrier did

not provide a uniform

improvement.

• It may be best to provide a

basic coverage using the 3

carriers, holding the fourth

carrier in reserve for hotspots.

• The “hotspot issue” is of great

interest in GPRS planning and

has implications for MRP

strategies for GSM voice.


Location OptimisationLocation Optimisation

• Placing sites and orienting

antennas so that hotspots

receive the highest possible C/I

has advantages from the

viewpoint of minimising the

number of timeslots necessary

to provision for demand.

• AIRCOM’s planning tool,

ASSET, was used to simulate

three different situations.



• Firstly: 150 kbps demand from

an area of Low C/I.

• Frequency Re-use factor of 3

employed.

• 5 kbps per timeslot achievable.

• 37 Timeslots required assuming

80% occupancy.

•Low C/I




an area of Moderate C/I.


employed.

• 11 kbps per timeslot

achievable.


80% occupancy.

• Note: this can be achieved by

changing antenna azimuth.

•Moderate C/I




an area of High C/I.


employed.

• 20 kbps per timeslot

achievable.


80% occupancy.

• Note: requires location

planning.

•High C/I


Other Possibilities: adding a repeaterOther Possibilities: adding a repeater

• Repeaters can improve the local

C/I.

• An example is shown “before”

and “after” the introduction of a

repeater.

•Area of interest •Note: colour changes in 4 dB increments


Other Possibilities: adding a repeaterOther Possibilities: adding a repeater

• C/I improvement is very

localised :- within a few hundred

metres of the repeater site.

• Improvement lifts C/I to

approximately that which could

be achieved by adjusting

azimuths.

• Not as effective as moving the

site; but probably much less

costly.

•Area of interest •Note: colour changes in 4 dB increments


The impact on GSM voiceThe impact on GSM voice

• Techniques focussed on

improving thoughput in GPRS.

• Most beneficial when tight re-use

factor (e.g. 3 is employed).

• This will have an impact on voice

services, particularly when

different re-use factors are

employed on different layers.

Voice area covered by layer with re-use factor of 12



The impact on GSM voiceThe impact on GSM voice• The greater the area that can be

served by both layers, the higher

the trunking efficiency.

• E.g. if the two areas shown are

equal and each have 7 timeslots,

each can serve 3 Erlangs: total 6

Erlangs.

• If the whole area has access to

14 timeslots, then 8 Erlangs can

be served.

• Great advantage in serving

hotspots with all available layers.




EDGE and C/IEDGE and C/I

57 kbps21 kbps30 dB

42 kbps19 kbps20 dB

33 kbps14 kbps15 dB

19 kbps11 kbps10 dB

10 kbps7 kbps5 dB

EDGEGMSKC/I• Comparison of bitrates possible

for Binary and 8PSK modulation

schemes.

• Note that, due to space and

transmission constraints, a

typical EDGE transmitter may

operate at a few dB lower Tx

power than a standard

transmitter.

• Nevertheless, conclusion is that

EDGE will offer superior bitrates.


SummarySummary

• GPRS packet data utilises different coding schemes that will permit data transfer

at a rate that is dependent on the C/I experienced at the receiver.

• GPRS can operate at C/I levels too low to permit voice communication.

• Improvements in GPRS capacity can be achieved through “smart” planning rather

than heavy investment.

• In particular, planning to accommodate demand from hotspots can be profitable.

• EDGE provides potentially much higher throughput than Binary modulation

schemes.


gsm applied cell planning gsm applied cell planning

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