02 Rn20021en14gl1 (e)Gprs Network Audit

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RN 2002
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Presentation / Author
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Module objectives
After completing this learning element, the participant will be able to:
Theory:
Realize the difference between GPRS CSs and EGPRS MCSs
Explain the air interface principles such as modulation and link adaptation
Describe the EDAP functionality
Know capacity relevant parameters in the BSS
Explain how mobility is handled in GPRS networks
Explain Network controlled Cell Reselection and Network Assisted Cell Reselection
Depict the PCU functionality
Describe the Gb interface main structure
Know Gb enhancing features like Multipoint Gb or PCU pooling
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BSS - Content
GPRS Coding Schemes (CS)
GMSK and 8PSK
EDAP and PCU
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GPRS Coding Schemes- Link Adaptation
GPRS provides four coding schemes: Coding Scheme (CS) 1 to 4.
EDGE TRXs and PCU 2 are required for CS-3/4.
PCU1 gives support for CS-1 and CS-2 only.
Talk family only supports CS-1 and CS-2.
Link Adaptation (LA) allows the PCU to select the most suitable CS for one TBF.
In PCU1 the LA algorithm is based on detecting the occurred RLC block errors and calculating the block error rate (BLER). The operator can define by parameters, whether a TBF uses either a fixed coding scheme (CS-1 or 2) or Link Adaptation (LA) based on Block Error Rate (BLER). The Link Adaptation (LA) algorithm selects the optimum channel coding scheme (CS-1/CS-2) for a particular RLC connection to provide the highest throughput and lowest delay available.
A new LA algorithm is employed with PCU-2 based on Bit Error Probability values (see EGPRS)
In GPRS RLC acknowledged mode RLC data blocks which are not correctly received have to be retransmitted with the same Coding Scheme (CS).
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CS1 & CS2 – Implemented in all BTS types without HW change
CS3 & CS4 – not for Talk family BTS
GPRS
PCU2
Data
Error
Correction
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Precoded USF: 3 6 6
Code Rate 1/2 ~2/3 ~3/4
length: 456 588 676
Data rate (kbps): 9.05 13.4 15.6
USF
USF
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EGPRS has nine Modulation and Coding Schemes, MCS-1...9.
In general, a higher coding scheme has higher coding rate, and consequently higher peak throughput, but it also tolerates less noise or interference.
The figure shows throughput vs. C/I of EGPRS coding schemes in TU50 FH, without incremental redundancy (IR).
The basic unit of transmission is one radio block (= 4 bursts = 20 ms on average), which contains one or in case of MCS 7, 8 and 9 two RLC blocks.
0
10
20
30
40
50
60
0
5
10
15
20
25
30
MCS-1
MCS-2
MCS-3
MCS-4
MCS-5
MCS-6
MCS-7
MCS-8
MCS-9
C/I
kbps
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Ref: TS 43.064
Table 4: Coding parameters for the EGPRS coding schemes
NOTE: The italic captions indicate the 6 octets of padding when retransmitting an MCS-8 block with MCS-3 or MCS-6. For MCS-3, the 6 octets of padding are sent every second block (see 3GPP TS 04.60).
8,8
176
1
0.53
0.53
MCS-1
11,2
224
1
0.53
0.66
MCS-2
14,8
13,6
296
Modulation
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EGPRS MCS Families
In wireless networks the quality of the connection can change from very good to very poor in short time
EGPRS has nine different Modulation and Coding Schemes (MCS-1 – MCS-9) in order to optimize the performance in different radio conditions. All coding schemes have different amount of robustness and error correction.
When user bit rate is low, robustness is high (e.g. MCS-1)
When user bit rate is high, robustness is low (e.g. MCS-9)
Two modulations are used:
8-PSK (8-Phase Shift Keying) for MCS-5 – MCS-9
The MCS that offers the best performance in current radio environment should be selected
Automatic selection of most suitable MCS is called link adaptation (LA). The selection is based on link quality measurements
MCS-7
22
28
MCS-3
37
MCS-4
44
MCS-5
56
MCS-6
74
56
56
MCS-8
68
68
MCS-9
74
74
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22,5° offset
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Q0
I0
8-PSK (Phase Shift Keying) has been selected as the new modulation added in EGPRS
3 bits per symbol
Symbol rate and burst length identical to those of GMSK
Non-constant envelope high requirements for linearity of the power amplifier
Because of amplifier non-linearities, a 2-4 dB power decrease back-off (BO) is typically needed, NSN guarantees a BO of 2 DB for BTS
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Burst Structure
Burst structure is similar with current GMSK burst, but term 'bit' is replaced by 'symbol‘
One symbol has 3 bits when using 8-PSK
Training sequence has lower envelope variations
it is possible to use 8-PSK and GMSK on adjacent timeslots (not for MS!)
In case of max output power only, back-off applied to 8-PSK (there is no DL power control)
Bit Number (BN)
guard period
subclause 5.2.8
where the "tail bits" are defined as modulating bits with states as follows (bits are grouped in symbols separated by ;):
(BN0, BN1 .. BN8) = (0,1,0;1,1,1;1,1,0) and
(BN435, BN436 .. BN443) = (0,1,0;1,1,1;1,1,0)
where the "training sequence bits" are defined as modulating bits with states as given in the following table according to the training sequence code, TSC. For broadcast and common control channels, the TSC must be equal to the BCC, as defined in GSM 03.03 and as described in this technical specification in subclause 3.3.2.
Training
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8-PSK Modulation – Back-off Value
Since the amplitude is changing in 8-PSK the transmitter non-linearities can be seen in the transmitted signal.
These non-linearities will cause e.g. errors in reception and bandwidth spreading.
In practice it is not possible to transmit 8-PSK signal with the same power as in GMSK due to the signal must remain in the linear part of the power amplifier.
The back-off value is taken into account in link budget separately for UL / DL and bands: 900/850, 1800/1900)
Too high initial MCS (8PSK) can lead to unsuccessful TBF establishment, if the MS is on cell border with low signal level (so the back-off is taken into account) and / or low C/I
Back Off for MS (4dB) is higher than for BTS (2dB)
Peak to Average of about 3,2 dB
Pin
Pout
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User data
Header part, robust coding for secure transmission
Adding redundancy
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EGPRS Channel Coding (MCS-9)
EGPRS channel coding is done separately for data and header, as shown in the figure for MCS-9 downlink.
Coding of data part:
Data part includes user data, two information from RLC header, BCS (block check sequence) and tail bits.
Coded using 1/3 convolutional code.
Punctured with a selectable puncturing scheme (P1, P2 or P3).
Two separate RLC blocks for MCS 7, 8 and 9.
Header part:
Includes RLC/MAC header information including information on the coding of the data part (like used puncturing scheme).
Convolutional coding + puncturing.
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34 octets
34 octets
34 octets
34 octets
The MCSs are divided into different families A, B and C
Each family has a different basic unit of payload: 37 (and 34), 28 and 22 octets respectively.
Different code rates within a family are achieved by transmitting a different number of payload units within one Radio Block.
For families A and B, 1 or 2 or 4 payload units are transmitted, for family C, only 1 or 2 payload units are transmitted
When 4 payload units are transmitted (MCS 7, MSC-8 and MCS-9), these are splited into two separate RLC blocks (with separate sequence BSN numbers and BCS, (Block Check Sequences)
The blocks are interleaved over two bursts only, for MCS-8 and MCS-9.
For MCS-7 the blocks are interleaved over four bursts
34+3 octets
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EGPRS Link Adaptation
Link Adaptation (LA)
The task of the LA algorithm is to select the optimal MCS for each radio condition to maximize RLC/MAC data rate, so the LA algorithm is used to adapt to situations where signal strength and or C/I level is low and changing slowly with time. Ideal LA would follow the envelope of the throughput of different MCSs.
- The PCU selects the data block and additionally selects the MCS depending on radio link quality and amount of available dynamic Abis channels
- LA is done independently for each UL and DL TBF on RLC/MAC block level, but the LA algorithm is same for uplink and downlink
- The MCS selection is not the same in case of initial transmission and retransmission
- LA algorithm works differently for RLC acknowledged mode and unacknowledged mode
- RLC control blocks are transmitted with MCS-1 coding
C/I
kbps
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50
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5
10
15
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25
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MCS-1
MCS-2
MCS-3
MCS-4
MCS-5
MCS-6
MCS-7
MCS-8
MCS-9
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Incremental Redundancy (IR)
IR is a combination of two techniques sometimes called ARQ type II:
Automatic Repeat reQuest (ARQ)
Forward Error Correction (FEC)
In the ARQ method the receiver detects the errors in a received RLC block and requests and receives a re-transmission of the same RLC block from the transmitter.
The FEC method adds redundant information to the re-transmitted information at the transmitter and the receiver uses the information to correct errors caused by disturbances in the radio channel
IR needs no information about link quality in order to protect the transmitted data but can increase the throughput due to automatic adaptation to varying channel conditions and reduced sensitivity to link quality measurements
For each MCS there are 2 or 3 Puncturing Schemes (PS) defined
Data block
Transmitter
Receiver
P2
P3
P1
P1
P1
P1
P2
P2
P3
No data recovered
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Link Adapatation is ‘slow’ process and is enabled to adapt to
path loss
Incremental Redundancy works only for RLC acknowledged mode, the retransmission process is based on IR in UL and DL.
Support of IR for MS is mandatory
Support of IR for BTS it is optional and implemented in all NSN BTS types supporting EGPRS.
LA must take into account
if IR combining is performed at the receiver
the effect of finite IR memory (in case of full memory buffers it may be necessary to switch to lower MCSs in order to allow the buffers especially in MS to get empty.
EGPRS Link Adaptation & Incremental Redundancy
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Modulation and Coding Schemes - MCS Selection
The PCU selects the used MCS. This selection can be divided in four classes:
Initial MCS to be used when entering packet transfer mode (set by parameters)
Modulation selection
MCS selection for initial transmissions of each RLC block (Link Adaptation, LA)
MCS to be used for re-transmissions (must be the same family, only for ACK mode )
Link Adaptation algorithm depends if RLC protocol works in acknowledged or unacknowledged mode. In acknowledged mode, LA algorithm is designed to optimize channel throughput in different radio conditions. In unacknowledged mode, the algorithm tries to keep the TBF below a specified Block Error Rate (BLER) limit.
The Link Adaptation (LA) algorithm for EGPRS is the same for PCU 1 and 2. It can be disabled by parameter. After the start of the TBF LA starts to work based on Bit Error Probability (BEP) measurements performed at the MS (downlink TBF) and the BTS (uplink TBF).
In DL case the MCS selection is based on EGPRS Channel Quality Report received in EGPRS PACKET DOWNLINK ACK/NACK message sent from the MS to network using PACCH to indicate the status of the downlink RLC data blocks received.
The MCS selection is based on using the BEP (Bit Error Probability) measurement data which contains the current averaged BEP value as well as the variation of this value (CV-Coefficient of Variation).
In UL case the MCS selection is based on the respective BEP measurement values which are measured by the BTS and given to PCU in the UL PCU frames.
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Value ranges are the same for 8-PSK and GMSK!
big variations
BEP as well as CV_BEP values are measured and reported
by MS (DL) and BTS (UL) to PCU
More details 3GPP 46.008
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GPRS Coding Schemes (CS)
GMSK and 8PSK
EDAP and PCU
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Territory border
The Territory method is used to divide the resources (timeslots) between CS and PS users. Timeslots within a cell are used for CS or PS users. The Territory border can change dynamically based on CS/PS load and parameters increasing or reducing the number of resources for (E)GPRS users.
The system is able to adapt to different load levels and traffic proportions, offering an optimized performance under a variety of load conditions. The Territory is configured by the system at the end of one TRX.
It is the choice of the operator if territory is configured to the BCCH TRX or on TCH TRXs
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TRX 1
TRX 2
The (E)GPRS territory can contain dedicated, default and additional capacity of course only on TRXs where GPRS is enabled. GPRS support is prerequisite for EGPRS support.
Dedicated capacity: Number of timeslots are allocated to (E)GPRS on a permanent basis i.e. are always configured for (E)GPRS and cannot be used by the circuit switched traffic. This ensures that the (E)GPRS capacity is always available in a cell. It is defined by parameter (Default GPRS Capacity, CDEF). It can be equal to zero.
Default capacity: It is the area that always is included in the instantaneous (E)GPRS territory, provided that the current CS traffic permits this. The Dedicated capacity is part of the Default capacity. It is defined by parameter (Default GPRS Capacity, DMAX).
Additional (E)GPRS capacity: It means the extra time slots beyond the default capacity which are assigned due to many (E)GPRS users. It can be restricted by parameter (MAX GPRS Capacity, CMAX, CMAX has to be bigger than CDEF).
Default Capacity
Additional Capacity
Dedicated Capacity
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Territory setting parameters - example
The values of the territory parameters are given in percentage per BTS. Each TRX can be disabled for EGPRS and GPRS traffic (GTRX = N).
Table below provides example how same parameter setting can result in different territory sizes with different GRTX/TRX configurations:
Any setting 1…20% of CDEF with 1 TRX configuration (GTRX=1) will result 1 TSL Default territory.
# of TRXs (GTRX=Y)
1
2
2
3
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TRX 1
TRX 2
For Load considerations one has to consider that the system tries to keep the Timeslots at the (E)GPRS Territory border (within the CS Territory) idle. How many timeslots depends on
Parameters and amount of TRXs. (CSD and CSU parameter)
Whether an Territory upgrade or downgrade took place
Of course there will be no free timeslots inside the CS territory when the CS territory has taken all available Timeslots!
Tip!: If mixed configurations of Dual Rate and Full Rates Timeslots are required it is recommended to keep them away from the territory border!!!
= Free TSL for CSW, part of CS territory
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Territory Method – Parameters 3
Example how to determine the Idle Timeslots: With default settings and 2 TRX 1 Timeslot after CS downgrade and 2 Timeslots after CS upgrade are kept free (as well by intra cell handovers). So in the dimensioning, it can be assumed that on average there are 1.5 free timeslots kept free (the same amount of upgrades as downgrades takes place).
Default
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TRX 1
TRX 2
= (E)GPRS Territory
= CSW Territory
Downlink Dual Carrier (DLDC) requires the territory to span at least 2TRXs. There is no special requirement for deploying DLPD (PCU2 is required) but
BTS level resources
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Dynamic Territory upgrades and downgrades apply only for standard cells.
In the case of extended (up to 70 KM cell radius) or super-extended (up to 105 KM cell radius) no dynamic up- and down-grades of (E)GPRS territories apply.
EGTCH (Extended (E)GPRS Traffic Channel) timeslots are not available for CS and so similar to dedicated capacity
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Cell selection and Reselection
In the GMM state READY the MS has to indicate cell changes towards the SGSN.
Unlike in GSM dedicated mode, there is no handover for (E)GPRS networks. Existing idle mode criteria are used to select the best suitable cell. In case of cell change the MS has to initiate a cell update to SGSN (the TBF in the old cell simply drops)
When in GSM the LA changes during a call the LA update is initiated by the MS when the call ends. In (E)GPRS there is no handover and the MS has to do first the RA/LA update, before the data transfer can be continued on LA/RA border.
In all cases (as well without RA border) there is a longer interruption time called ‘cell outage’ in the data transfer. Most non-real time applications manage to recover.
For an MS in DTM mode the cell update happens after the handover.
SGSN-1
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Cell Selection / Re-selection
The network may request measurement reports from the MS and control its cell re-selection
Depending on the NC (Network Control) mode set by the network, the MS shall behave as follows:
NC0: Normal MS control; the MS shall perform autonomous cell re-selection
NC1: MS control with measurement reports; the MS shall send measurement reports to the network and shall perform autonomous cell re-selection
NC1 is not supported
NC2: Network control; the MS shall send measurement reports to the network with Network Controlled Cell Reselection (NCCR). The feature is not applicable for DTM capable devices in dedicated or DTM mode.
BSC-1
PCU
which cell to change
the MS when to change
and to which cell to change
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-Path loss criterion (C1) or if defined:
-Cell reselection criteria (C2)
These criteria are used for the cell reselection during GMM ready in the same way as in GMM standby state.
BSC-1
RA-1
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Cell Selection / Re-selection – NC2
NC2: Network control is deployed with Network Controlled Cell Reselection (NCCR). The MS has to send measurement reports and the PCU will make the decision to reselect a certain cell based on C31/C32 criteria.
C31: Signal Strength threshold criterion
C32: Cell ranking
NC2 is only apply in GMM (GPRS Mobility Management) Ready state. In GMM Standby state, the MS will always use NC0 mode!
TBF
TBF
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02 Rn20021en14gl1 (e)Gprs Network Audit