OVERVIEW OF ADVANCED GPRS

BY: NAVJIT SINGH

JULY 2008

Several GPRS Users can share one Channel (PDCH)

CONTENTS:• 1. GPRS OVERVIEW

• ARCHITECTURE & INTERFACES

• 2. GPRS ADVANCED TERMINOLOGY

• CHANNEL ADMINSTRATION

• CELL SELECTION/RESELECTION

• PAGING

• LINK ADAPTATION

• MS POWER CONTROL

• 3. GPRS ACCESS PERFORMANCE

• 4.OVERVIEW OF GPRS NETWORK OPTIMIZATION

NETWORK ELEMENT AND PRIMARY FUNCTIONS :

• PDCHs are allocated in PSETs.

• A complete PSET consists of 8 PDCHs

• Allocation of two types are possible:

• Dedicated: the number of PDCHs set by the operator. Allocated from

the CSD to the PSD.

• On-demand: taken from the CSD only when needed. They

• are returned to the CSD when there is no need or when

• needed by the CSD.

• PDCHs that are not reserved for GPRS traffic, are put in a PSD

• idle list. After a limited time in the PSD idle list, the PDCHs

• are deallocated from the PSD and returned to the CSD.

• A request for PDCH pre-emption is sent to the PSD, in the case

• there are any on-demand PDCHs in the cell.

• On-demand PDCHs that become idle in both directions,• are linked into the PSD idle list .• The timer PILTIMER is started When the timer expires for a PDCH, the

PDCH is deallocated in the PSD and returned to the CSD• The value range of the PILTIMER is 1 s. to 3600 s.

• Configuration of dedicated PDCHs• • Up to eight dedicated PDCHs can be configured per cell• • The dedicated PDCHs are only available for GPRS traffic • • The dedicated PDCHs in a cell can either be placed on• non hopping TCH ,on the BCCH frequency as a first choice, • or as no preference according to the parameter PDCHALLOC.

• Handling of Packet Data Traffic (Abis interface):

• When recourses are needed for a PS connection, PDCHs

• are reserved in accordance with:

1. MS Type (GPRS capable or not)

2. Channel Load

3. Available channel resources

4. 16K Abis data rate is not sufficient for GPRS CS-1 to CS-4

5. The feature SARA handles the 64K Abis allocation.

• Service Oriented Allocation of Abis Resources

• (SARA) :

• • 64k Abis needed for:

• Adapt Abis interface to CS3/4 and EGPRS

• • Increase Abis bit rate per Air-interface TS to 64kbit/s

• • 16k Abis used for:

• – CS-1 and CS-2 .

Mobility Management (MM) states:• Idle State: The MS is turned on but not GPRS-attached. The

• MS is "invisible" to GPRS, i.e. outside the GPRS coverage area.

• Standby state: The MS is attached to the GPRS Mobility

• Management (MM). It sends Routing Area (RA) updates to the

• SGSN and performs GPRS cell selection and re-selection.

• Ready state: A packet transfer is ongoing or has recently

• ended. A READY timer defines how long time the MS shall

• remain in ready state after transfer. The time is decided by

• SGSN and can take values from zero to infinity, i.e. the MS shall

• never go back to Standby state. The MS performs cell update

• when changing cell in ready state.

Why GPRS Cell Selection ?• To be able to access the system from anywhere

in the network .• To be able to register its current location to the

Network.• To support the GPRS MS with information about• the structure of the network.• To enable the MS to choose the most suitable cell• to camp on, based on signal strength.

• Cell selection is based on :• Initial cell selection according to C1-criterion .

GPRS CELL SELECTION/RESELECTION :

• Cell reselection:

• GPRS cell reselection is performed if (at least) one

• criterion below is fulfilled :

• 1. C1<0 in serving cell

• 2. N-cell considered ”better” than s-cell

• 3. S-cell becomes barred

• 4. DL signaling failure

• It depends on PBCCH is in serving cell or not :

1. if PBCCH in s-cell: according to C1/C31/C32 criterion .

2. if no PBCCH in s-cell: according to C1/C2 criterion .

Cell Reselection Algorithm :

Path Loss Criterion:• C1=( RLA – GPRS_ RXLEV_ ACCESS MIN) - max(GPRS _MS TXPWR_ MAX

_CCH – P,0).

• RLA is the Received Level Average power. At least 5 received signal

• strength measurement samples are required per BCCH to receive a valid

RLA .

• GPRS_RXLEV_ACCESS_MIN is the minimum received level

• at the MS required for accessing the system.

• GPRS_MS_TXPWR_MAX_CCH(s+n) is the maximum

• transmitter power level an MS may use when accessing the system.

• P is the maximum output power of the MS according to its

• Class.

Signal strength threshold criterion C31:

• C31(s) = RLA(s) - HCS_THR(s)• C31(n) = RLA(n) - HCS_THR(n) - TO(n) * L(n)

• HCS_THR is the signal strength threshold for applying HCS re-selection.

• TO is the Temporary Offset. • The Temporary Offset• TO(n) = GPRS_TEMPORARY_OFFSET(n) *H(GPRS_PENALTY_TIME(n) -

T(n))

• L(n) = 0 if PRIORITY_CLASS(n) = PRIORITY_CLASS(s)• L(n) = 1 if PRIORITY_CLASS(n) < > PRIORITY_CLASS(s)

•

• Cell Ranking Criterion C32:• C32(s) = C1(s)• C32(n) = C1(n) + GPRS_RESELECT_OFFSET(n) – TO(n) * (1- L(n))

• GPRS_RESELECT_OFFSET applies an offset and• hysteresis value to each cell.

• GPRS Cell Re-Selection• Finding the ”best” cell:• S = The set of all cells with C31>=0• P = The subset of S consisting of all cells in S with the• highest value of GPRS_PRIORITY_CLASS

• Finding the ”best” cell:• i) S is empty:• The cell with highest C32 among all cells, is ”best• ii) S is non-empty:• The cell with highest C32 in P, is ”best”• Parameter values:

PAGING :

• Page a mobile to establish a downlink packet transfer

• MS classes of operation:

• Class A mode of operation allows an MS to have a circuit switched

• connection at the same time as it is involved in a packet transfer.

• Class B mode of operation allows a MS to be attached to both circuit switched and packet switched connections, but it can not use both services at the same time.

• Class C mode of operation allows an MS only to be attached to one service at a time. An MS that only supports GPRS and not circuit switched traffic will always work in class C mode of operation.

Physical Channel Multiframe and Layers Mapping:

Why GPRS Link Adaptation?

• Four Coding Schemes available on DL (CS-1 to CS-4)• GPRS Link Adaptation dynamically selects the most optimal coding scheme

depending on the interference situation• The algorithm is applied per downlink TBF• CS-1 and CS-2 is supported on UL and will be static

Mapping relationship of GPRS channel coding scheme and C/I:

Why High C/I For High Data Rate CS :1. The reason is that half of the bit rate (CS-1) and one third of the bit

rate (CS-2) in the radio link control (RLC) blocks is applied to the forward error correction (FEC). Though this reduces the requirement of C/I, it reduces the transmission rate of the data.

• 2. Though the transmission rate of the data under the CS-3 (15.6Kbit/s) and CS-4 (21.4Kbit/s) is relatively high (including the RLC block header), it is enhanced through reducing and canceling the error correction bits, so the CS-3 and CS-4 coding schemes require the C/I to be a greater value.

Radio link quality measurements:

• MS will send Channel quality report to the BTS on request:• • The initial coding scheme is set by CHCSDL• • The signal variance (SIGN_VAR) and a filtered RXQUAL value are used

as input to the algorithm• • The coding scheme will be changed to maximize the throughput.

Parameter Used in LA :

• If Channel quality report is missing...• If Channel quality reports are missing, the coding scheme• will change slowly to a more robust one:

• GPRS Network Optimization Principles:

Fully use the existing equipment resources and spectrum resources; enhance the investment-to-profit ratio and meet the service development requirements.

Enhance the GPRS service quality as much as possible while ensuring the quality of the GSM circuit services.

Adjust the parameters and resources configured for the radio access network according to market position and GPRS network operation.