A LTE Receiver Framework Implementation in GNU Radio · A LTE Receiver Framework Implementation in...

Communications Engineering Lab (CEL)Univ.-Prof. Dr.rer.nat. Friedrich K. Jondral

A LTE Receiver Framework Implementation inGNU Radio

Johannes Demel, Sebastian Koslowski, Friedrich K. Jondral

KIT – University of the State of Baden-Wuerttemberg and

National Laboratory of the Helmholtz Association

www.kit.edu

http://www.kit.edu

Contents

LTE air interface overviewBasic system parameters

Project roadmapImplementation

Implementation overviewSynchronizationOFDM operationPhysical channel decodingTest with recorded data

Performance results

Conclusion

2 A LTE Receiver Framework Implementation in GNU RadioJohannes Demel, Sebastian Koslowski, Friedrich K. Jondral

Communications Engineering LabUniv.-Prof. Dr.rer.nat. Friedrich K. Jondral CELCEL

LTE overview

Air interface basics (downlink)

OFDM signal (15 kHz subcarrierbandwidth)different modes possible

variable bandwidth (up to 20 MHz)MIMO capabilites (up to 4x4)

6 physical channels3 transport channels3 control information channels

resourceblock (RB)

time

frequency

pilot

reserved



Roadmap

Taskssynchronization

time, frequency, frame timing

OFDM operationradio channel estimation,equalization

demodulationSTBC, FECphysical channel demultiplexing

extract system parameterscell IDMIMO configurationsystem bandwidth

Implementation goals

modular block-based structure

separate handling of data andcontrol information

use stream- and event-basedprocessing



Implementation overview

Receiver framework components at work: Example flowgraph with MIB decoding

synchronization

OFDM operation



Synchronization

cyclic prefix (CP)-based synchronization

recover coarse symbol timing n̂0

calculate sliding window correlation with fixed lag of NFFT

n̂0 = arg maxn|γ(n)| , γ(n) =

n+NCP−1∑m=n

r(m) r∗(m − NFFT)

stream tagstags indicate symbol start



Synchronization

primary synchronization symbol (PSS) detection

recover fine symbol timing and half-frame timing

extract cell ID number NID2 from PSSstream tags

indicate half-frame startpropagate cell ID number NID2



Synchronization

frequency offset detection and correction

recover fractional frequency offsethalf-frame timing needed

different CP-lengths within each slot



Synchronization

secondary synchronization symbol (SSS) detectionrecover frame timingextract cell ID group NID1

receive NID2 tagcalculate cell ID NID = 3 ∗ NID1 + NID2

message portpublish NID for dynamic block configuration

stream tagsindicate frame start



OFDM operation

inverse OFDM operation

remove cyclic prefix

compute FFTextract subcarriers of interest

complexity reduction

channel estimation

get pilot positions

calculate channel coefficients

linearly interpolation

output data stream andchannel estimates for antennaport 0 and 1





synchronization

OFDM operation



Decode PBCH

MIMO configuration still unknown at this pointtrail & error: different configurations interleaved in output

inverse Alamouti Operation

deinterleave layers

demodulation: PBCH always uses QPSKdescrambling

scrambling sequence depends on NID



Decode BCH

BCH is transmitted on PBCH

Deinterleaving (block-based)Viterbi decoder

hierarchical blockparameterized GNU Radio Viterbi decoder

Calculate CRCCRC checksum depends on MIMO configurationCRC match indicates number of TX antennas





synchronization

OFDM operation



Test with recorded data

IQ baseband samples asinput

recorded using aUSRP N210

flowgraph outputfixed parameters

MIMO: 2x1RB: 50 equals10 MHzPHICH parameters

system frame number

decoding rate 97.8%

tests indicate real timecapabilities



Effect of sample rate on performance

varying sample rateFFT-length depends onsample rate

relative performance changeslower sample rate

less multiplicationssmaller correlationsequencessmaller FFT-length

Sync

hron

izat

ion

OFDM

Deco

de P

BCH

Deco

de B

CH

MIB

106

107

108

109

1010

Inst

ruct

ion

Fetc

h

Instruction Fetch per blockFFT-length

2048512



Effect of computed resource blocks onperformance

varying number of RBsFFT-length always 2048number of RBs limited byFFT-length

channel estimation is morecomplex

great increase of powerconsumption in OFDM part

Sync

hron

izat

ion

OFDM

Deco

de P

BCH

Deco

de B

CH

MIB

105

106

107

108

109

1010

Inst

ruct

ion

Fetc

h

Instruction Fetch per blocknumber of RBs

1006



Conclusion

LTE overviewintroduction to our GNU Radio LTE receiver

synchronization, OFDM operation, PBCH extractionexample output

performance analysisdifferent parameters

possibilitiesDetect LTE cells with parameters

What’s nextextend flowgraph with additional channels and uplink

source code available at github.com/kit-cel/gr-lte



github.com/kit-cel/gr-lte

The End

Thanks for your attention!gr-lte source code available at github.com/kit-cel/gr-lte



github.com/kit-cel/gr-lte

A LTE Receiver Framework Implementation in GNU Radio · A LTE Receiver Framework Implementation in...

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