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LTE Air Interface Dimensioning
Stockholm, 26.01.2009
Piotr GodziewskiNetwork Engineering
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Meeting Agenda
• Link budget target
• Transmitting end modeling
• Receiver end modeling
• Resource allocation modeling
• Required Signal to Interference Margin (SINR)
• System overhead
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Link budget target
• Estimating maximum allowable path loss for single radio link
PathLossmax_DL
PathLossmax_UL
R
Antenna gain, feeder/cable losses, noise figures, etc.
Transmitter/receiver end modeling
Clutter type, propagation model, channel model, etc.
Propagation environment
User data rate, system overhead, cell load, coverage reliability, BLER etc.
Requirements
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Transmitting end modeling
• EIRP – Effective Isotropic Radiated Power
MIMObodyTMAinsfeederantennaantennaTx GLLLGPEIRP +−−−+= _
TX power per antenna connector (DL 8/20/40/60 W license based control)(UL 23±2 dBm)
Antenna gain
Feeder loss (0 dB in UL with Tower Mounted Amplifier)(0 dB in DL with feederless solution,RRH or System Module close to the antenna)
TMA insertion loss (DL only) (~0.5 dB with Tower Mounted Amplifier)
Body loss (UE only) (0 dB for PC cards/laptops/etc.)
Total power increase due to single stream MIMO (TX diversity technique)(3 dB in DL for 2TX diversity, if not already considered in SINR)
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Receiving end modeling
• OFDM receiver sensitivity
SINRNFRBkHzHzdBmSRx ++⋅⋅⋅+−= )#1215log(10/174
Thermal Noise Density
Number of Physical Resource Blocks (DL: all available in the channel)(UL: allocated for transmission)
Noise figure (HW specific)
SINR (Signal to Interference Ratio)
DL: total TX power is shared between all subcarriersUL: total TX power is focused on allocated PRBsSensitivity must be modified in this way if we assume total TX power for the total channel in EIRP formula
OFDMA/SC-FDMA
3GPP allows for 15 kHz and 7.5 kHz
Subcarrier bandwidth
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Resource allocation modeling
• Basic rules (3GPP 36.213 Dec2008)– Separate MCS set for DL and UL– TBS mapping table– TBS mapping rules for dual-stream
▪ 1 ≤ #RB ≤ 55 take TBS for one-layer and multiply by 2
▪ 56 ≤ #RB ≤ 110 use specific mapping table
…
One-layer mapping
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Resource allocation modeling
• Transport Block– Number of user data bits transmitted to single
user during one TTI (1 ms)– Transport Block is defined for two
consecutive Physical Resource Blocks in time domain
EffectiveUserThroughput = 384 / (100% - 10%) = 427 kbps
MCS = 10-16QAM TBS index = 9
…search for TBS ≥ EffectiveUserThroughput
[#RB=3 and TBS=456] is the smallest possible configuration
456 bits / TTI = 456 bits / 1 ms = 456 kbps ≥ 427 kbps
…done!
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Resource allocation modeling
• Effective coding rate– Denotes coding rate applied on PDSCH/PUSCH with respect to the allocated
resource blocks, TBS and overheads
Number of allocated resource blocks
Number of resource elements per RB (84 for normal CP)(72 for extended CP)
Total number of resource elements within 1 ms (TTI)
Percentage of system overhead
Transport Block Size (depends on MCS/#RB combination)
orderMoverheadRERBTBSCR
⋅−⋅⋅⋅=
)1(2##
Modulation order (QPSK=2, 16QAM=4, 64QAM=6)
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Resource allocation modeling
• Example coding rates– DL MCS set– TBS for #RB=50
▪ 10 MHz bandwidth,all resource blocks allocated
– 23% system overhead▪ PRACH in every subframe▪ 3 OFDM symbols for signaling▪ Reference Signal at one antenna port
• Impact on– SINR interpolation
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Required Signal to Interference Ratio (SINR)
• Target SINR– Obtained from link level simulations– OFDM specific channel models– Considered antenna schemes:
▪ Downlink• 1Tx-2Rx• 2Tx-2Rx TX div.• 2Tx-2Rx Spatial Multiplexing
▪ Uplink• 1Tx-2Rx• 1Tx-4Rx
– Possible outcomes form▪ SINR value refers to the given
number of allocated resource blocks and MCS (modulation and coding scheme)
- Suitable for large channel bandwidths- Correlation between UE and eNB antennas in case of multi antenna systems- Doppler spread considered (Doppler frequency)- Large number of taps
EPA / ETU / EVA
light
UEcarrierDoppler v
vff ⋅=
EPA 5Hz:Doppler frequency=5Hz for 1800MHz and 3km/h
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Required Signal to Interference Ratio (SINR)
• Samples for DL 2Tx-2Rx
SINR within the one and the same MCS are worse for low number of resource blocks.Turbo coding efficiency decreases rapidly for small amount of input data (related to correlation increase between input and output bits of turbo coder)
Turbo coding degradation
Since there is practically no turbo coding degradation for #RB>25 it is possible to assume “flat” distribution of SINR for #RB>25
#RB>25
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Required Signal to Interference Ratio (SINR)
• Example– MCS-10 and #RB=3
SINR values for specific coding rates(known from simulation assumptions)
…6.57 dB does not refer to effective codingrate from link budget calculations
Additional interpolation needed(coding rate offset consideration)
General procedure for SINR estimation
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System overhead
• Downlink– Synchronization signals – occupies two last OFDM symbols in timeslot
0 and 10 in every radio frame (72 subcarriers per symbol)
Radio frame(20 timeslots)
Timeslot 0 and 10
72 subcarriers in OFDM symbol dedicated for synchronization signal
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System overhead
• Downlink– Reference signal – configuration depends on the chosen antenna
scheme
Res
ourc
e bl
ock
= 12
sub
carr
iers
1TX antenna
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System overhead
• Downlink– Reference signal – configuration depends on the chosen antenna
scheme
2TX antenna
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System overhead
• Downlink– Reference signal – configuration depends on the chosen antenna
scheme
Res
ourc
e bl
ock
= 12
sub
carr
iers
4TX antenna
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System overhead
• Downlink– PBCH (Physical Broadcast Channel) – 72 subcarriers in four OFDM
symbols per radio frame– PDCCH (Physical Downlink Control Channel) – up to four OFDM
symbols per subframe
4 symbols only in case of 1.4 MHz bandwidth (10 RBs)
Assume 3 symbols per subframe for standard macro/micro cell planning
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System overhead
• Uplink– Reference signal for PUSCH – 4th OFDM symbol in every resource
block dedicated for PUSCH transmission (3rd symbol in case of extended CP)
Timeslot
REs occupied by PUSCH reference signal
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System overhead
• Uplink– PRACH (Physical Random Access Channel) – six resource blocks– PUCCH (Physical Uplink Control Channel) – single PUCCH occupies
two resource blocks in two consecutive timeslots (one RB per onetimeslot)
Subframe
Resource block
...
Sys
tem
cha
nnel
Note:From the link budget point of view, reference signal for PUCCH does not need to be considered separately since it is included in the resources occupied by PUCCH
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System overhead
• Example figures– DL 2Tx-2Rx– UL 1Tx-2Rx– PRACH in every subframe– 3 OFDM symbols for PDCCH– CQI mode 2-0
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Thank you for attention