LTE network testbed with USRP and general purpose PC EECRT2 Project Kalle Ruttik Contributions by: G.M. Crespo,J. Kerttula, C. Guo, Y. Beyene, N. Malm Department of Communications and Networking Aalto, School of Electrical Engineering EECRT2 27.11.2013
EECRT2 Cognitive Radio test-bed Purpose Project creates a “living lab” cognitive radio test-bed – Living lab: Transmission of the real application data over the air interface – Test-bed is a realistic radio network – Test-bed is designed for investigating RRM algorithms • Test-bed used in – TEKES EECRT project – One of METIS official test-beds
Test-bed general properties • Composed of 24 USRP nodes – USRP N200+SBX: 0.4 – 4.4 GHz – License to transmit in 620 – 650 MHz • Implementation of TDD-LTE type PHY in software – BB processing in C++ – RRM in Python • Currently BB and RRM not combined • Implementation of BS and UE units – Two-directional TDD communication • Currently under the test – Can address users – Can allocate resource blocks
SDR implementation
Software platform operates as a radio system simulator RRM Data Python MAC C++ BB Buffer scheduler Buffer is a wrapper IP around UHD driver switch - It hides synchronization RRM Data related issues from the rest of the code - Handles TDD direction MAC BB Buffer switching scheduler
Operates as HW in a loop simulator RRM Data Python MAC C++ BB Buffer USRP scheduler Buffer is written as a wrapper around UHD driver RRM Data - It hides synchronization related issues from the MAC rest of the code BB Buffer USRP - Handles TDD direction scheduler switching
Software RAN in VM • Implementation of TDD LTE physical layer in software – Can run Radio Access processing in virtual machine (VM) – Separation of BB processing and sending data to RF – Can be used in servers with remote radio heads BB BB BB RF AD/DA RF VM OS VM OS VM OS AD/DA RF Host OS AD/DA
Software architecture
PHY Implementation • Bit exact DL PHY – PHY – PSS, SSS – PCFICH – PHICH – PDCCH • UL PHY – PUCCH – PUSCH
SDR with multiple USRP units
Using SDR with multiple USRPs • Using multiple 1 Gbit Ethernet ports for connecting multiple USRP to a “server” – The USRP units do not have common clock • Problem – Frame synchronization? • The USRP N200 units do not have global clock • The packets are transmitted at the arbitrary times – Clock drift • Over the time one transmitter will have have one sample more than other
Synchronizing multiple transmitters • Use software for synchronizing the transmitters – 2 Tx and 1 Rx connected to same computer – Rx receives both TX signals and computes correction factor – The clocks of transmitters are continuously adjusted by adjusting the samples • Initial calibration (synchronization) – Setting the frames to start at the same time – Use different PSS sequences • Tracking – Keeping the transmitters synchronized
N200 vs USRP-2932 • 2.5 ppm TCXO frequency • 2.5 ppb OCXO reference • 0.01 ppb w/GPSDO option • 0.01 ppm w/GPSDO option • Sample rate and RF frequency both derived from the same oscillator • Can not simply shift RF frequency • Sampling difference gives • Phase error • Different amount of samples over time
Tx2 Tx1 Test Rx
Tx Timing Mismatch Calibration Once the Rx work station estimates the Tx timing on mismatch from the off correlators‘ outputs, it on indicates the Tx which off antenna must delay its transmission by N number of samples Tx process Add Tx1 source delay data Add Tx2 source delay data Correlator 1 Timing delay estimation Correlator 2 Tracking process UDP/IP UDP/IP
Frame synchronization Frame starting after synchro Initial frame location • Addressable
Comparison of freq. drifts between 2 Tx N200 USRP-2932 N200 2tx timing drift USRP-2932 2tx timing drift 5 1 0 0.5 -5 0 0 2 4 6 8 10 12 0 5 10 15 20 25 30 35 4 1 2 0 0 -1 0 2 4 6 8 10 12 0 5 10 15 20 25 30 35 5 1 0 0.5 -5 0 0 2 4 6 8 10 12 0 5 10 15 20 25 30 35 2 1 1 0.5 0 0 0 2 4 6 8 10 12 0 5 10 15 20 25 30 35 time (seconds) time (seconds)
Histogram of the frequency drifts 10 s. and one LTE symbol N200 USRP-2932 N200 USRP NI2932 0.4 0.8 0.3 0.6 pdf pdf 0.2 0.4 0.1 0.2 0 0 0 1 2 3 4 5 6 0 1 2 3 4 5 6 Phase (radians) Phase (radians) -5 -6 x 10 x 10 Mean = 3.4681 Std = 1.12 Mean = 1.6111 Std = 0.50163 0.4 0.8 0.3 0.6 pdf pdf 0.2 0.4 0.1 0.2 0 0 1 2 3 4 5 6 1 2 Samples in 10 s Samples in 10 s
Performance • Drift between the transmitters clocks – Drift figure – Histogram of drift • Error in one LTE OFDM symbol 66e-6 s – Histogram of symbol error
Radio link performance measurements
Measurements • Ongoing measurement campaign for indentifying impact of using different DL/UL sub-frame configurations in different transmitters • Here: SINR and BER measurements in one radio link – Using different methods for measuring SINR DL: Downlink SP: Special subframe UL: Uplink
Measurement campaign • Interference from outside Tx to inside Rx – Two TDD radio links • one outside one inside – Measure if transmitters • Sychronised • Nonsynchronised – Performance is measured as SINR and BER in radio links • Performance is measured per sub-frame • Currently the measurement campaign is going on
SINR measurements • EVM based measurement – The channel is feed with coded data – The data is received decoded and encoded – EVM is computed from difference of received data and decoded and re-encoded data • SINR estimation from the spectrum – Difference between the pilots based signal power estimate and signal plus noise estimate from the resource elements with data • RSSI • RSRQ
Estimation of signal strength at different USRP units
Measured BER on USRP-2932 at 630 mHz
Conclusions • We have TDD LTE type BS that can operate in as server – The system allows real time operations – The USRP do not have common clock and they are synchronized over the air • The software system scales for testing TDD based radio network – We can measure and control interference in the designed network
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