LTE PSS Synchronization from Recording

Detects the LTE primary synchronization signal in a real 1.92 MS/s RTL-SDR downlink recording. The gr-lte chain estimates OFDM symbol timing from the cyclic prefix, extracts the central six resource blocks, identifies N_id_2, and prints the detected physical-layer identity to the console.

Author GNU Radio World
Category gr-lte
Blocks 16
Connections 15
File gr-lte/lte_pss_sync_recording.grc

Press Load to open this flowgraph in an editor embedded above, then ▶ to run it. Nothing is downloaded until you do.

Open in the full editor ▶ Download the .grc

Blocks used

Block	Module	Used
Variable	`GNU Radio`	3
CP-based Rough Symbol Sync Vec	`gr-lte`	1
Extract subcarriers	`gr-lte`	1
FFT	`GNU Radio`	1
GR World Recording	`GNU Radio`	1
GUI Layout	`GNU Radio`	1
IChar To Complex	`GNU Radio`	1
Message Debug	`GNU Radio`	1
Note	`GNU Radio`	1
Null Sink	`GNU Radio`	1
PSS Calculator	`gr-lte`	1
PSS Frame Tagger	`gr-lte`	1
PSS Symbol Selector	`gr-lte`	1
Throttle	`GNU Radio`	1

Notes in this flowgraph

The console prints the detected LTE physical-layer identity N_id_2. The hosted SigMF source comes from Jiao Xianjun's LTE-Cell-Scanner regression corpus (GPL-3.0), captured at 1815.3 MHz with an RTL-SDR.

Flowgraph source

gr-lte/lte_pss_sync_recording.grc

options:
    parameters:
        author: GNU Radio World
        comment: ''
        copyright: ''
        description: Detects the LTE primary synchronization signal in a real 1.92 MS/s RTL-SDR downlink recording. The gr-lte chain estimates OFDM symbol timing from the cyclic prefix, extracts the central six resource blocks, identifies N_id_2, and prints the detected physical-layer identity to the console.
        generate_options: qt_gui
        id: LTE_PSS_Synchronization_from_Recording
        title: LTE PSS Synchronization from Recording
    states:
        coordinate: [10, 10]
        rotation: 0
        state: enabled
blocks:
-   name: fft_len
    id: variable
    parameters:
        comment: "1.92 MS/s / 15 kHz\nLTE subcarrier spacing"
        value: '128'
    states:
        coordinate: [1200, 20]
        rotation: 0
        state: enabled
-   name: resource_blocks
    id: variable
    parameters:
        comment: "Central six resource\nblocks contain the PSS"
        value: '6'
    states:
        coordinate: [980, 20]
        rotation: 0
        state: enabled
-   name: sample_rate
    id: variable
    parameters:
        comment: RTL-SDR recording sample rate
        value: '1920000'
    states:
        coordinate: [720, 20]
        rotation: 0
        state: enabled
-   name: blocks_throttle2_0
    id: blocks_throttle2
    parameters:
        affinity: ''
        alias: ''
        comment: ''
        ignoretag: 'True'
        limit: auto
        maximum: '0.1'
        maxoutbuf: '0'
        minoutbuf: '0'
        samples_per_second: sample_rate
        type: complex
        vlen: '1'
    states:
        coordinate: [700, 180]
        rotation: 0
        state: enabled
-   name: central_subcarriers
    id: lte_extract_subcarriers_vcvc
    parameters:
        N_rb_dl: resource_blocks
        affinity: ''
        alias: ''
        comment: Extract the central 72 subcarriers around DC
        fftl: fft_len
        maxoutbuf: '0'
        minoutbuf: '0'
    states:
        coordinate: [880, 450]
        rotation: 0
        state: enabled
-   name: detected_identity
    id: blocks_message_debug
    parameters:
        affinity: ''
        alias: ''
        comment: Print each detected LTE N_id_2 value to the console
        en_uvec: 'True'
        log_level: info
    states:
        coordinate: [1590, 460]
        rotation: 0
        state: enabled
-   name: fft
    id: fft_vxx
    parameters:
        affinity: ''
        alias: ''
        comment: Transform candidate OFDM symbols without shifting DC
        fft_size: fft_len
        forward: 'True'
        maxoutbuf: '0'
        minoutbuf: '0'
        nthreads: '1'
        shift: 'False'
        type: complex
        window: '[1.0] * fft_len'
    states:
        coordinate: [510, 430]
        rotation: 0
        state: enabled
-   name: iq
    id: blocks_interleaved_char_to_complex
    parameters:
        affinity: ''
        alias: ''
        comment: Convert signed interleaved RTL-SDR I/Q bytes to normalized complex samples
        maxoutbuf: '0'
        minoutbuf: '0'
        scale_factor: '127'
        vector_input: 'False'
    states:
        coordinate: [370, 200]
        rotation: 0
        state: enabled
-   name: note_0
    id: note
    parameters:
        affinity: ''
        alias: ''
        comment: ''
        note: The console prints the detected LTE physical-layer identity N_id_2. The hosted SigMF source comes from Jiao Xianjun's LTE-Cell-Scanner regression corpus (GPL-3.0), captured at 1815.3 MHz with an RTL-SDR.
    states:
        coordinate: [300, 10]
        rotation: 0
        state: enabled
-   name: pss
    id: lte_pss_calculator_vcm
    parameters:
        affinity: ''
        alias: ''
        comment: Correlate against all three LTE PSS sequences
        fftl: fft_len
        maxoutbuf: '0'
        minoutbuf: '0'
    states:
        coordinate: [1220, 430]
        rotation: 0
        state: enabled
-   name: pss_selector
    id: lte_pss_symbol_selector_cvc
    parameters:
        affinity: ''
        alias: ''
        comment: Select candidate synchronization symbols
        fftl: fft_len
        maxoutbuf: '0'
        minoutbuf: '0'
    states:
        coordinate: [110, 430]
        rotation: 0
        state: enabled
-   name: recording
    id: wasm_gr_world_recording
    parameters:
        affinity: ''
        alias: ''
        comment: "80 ms LTE downlink capture at\n1815.3 MHz from LTE-Cell-Scanner"
        length: '0'
        maxoutbuf: '0'
        minoutbuf: '0'
        offset: '0'
        progress: 'True'
        recording: lte/lte_downlink_1815_3mhz_rtlsdr
        repeat: 'False'
        type: byte
    states:
        coordinate: [20, 190]
        rotation: 0
        state: enabled
-   name: rough_sync
    id: lte_rough_symbol_sync_cc
    parameters:
        affinity: ''
        alias: ''
        comment: Locate OFDM symbol boundaries from cyclic-prefix correlation
        fftl: fft_len
        maxoutbuf: '0'
        minoutbuf: '0'
        vlen: '1'
    states:
        coordinate: [1040, 190]
        rotation: 0
        state: enabled
-   name: sink
    id: blocks_null_sink
    parameters:
        affinity: ''
        alias: ''
        comment: Consume the synchronized and tagged sample stream
        type: complex
        vlen: '1'
    states:
        coordinate: [1920, 240]
        rotation: 0
        state: enabled
-   name: synchronized_stream
    id: lte_pss_tagger_cc
    parameters:
        affinity: ''
        alias: ''
        comment: Apply half-frame and N_id_2 tags to the sample stream
        fftl: fft_len
        maxoutbuf: '0'
        minoutbuf: '0'
    states:
        coordinate: [1600, 190]
        rotation: 0
        state: enabled
-   name: wasm_gui_layout_0
    id: wasm_gui_layout
    parameters:
        affinity: ''
        alias: ''
        columns: '12'
        comment: ''
        layout: '{}'
        row_height: '60'
    states:
        coordinate: [1370, 20]
        rotation: 0
        state: enabled
connections:
- [blocks_throttle2_0, '0', rough_sync, '0']
- [central_subcarriers, '0', pss, '0']
- [fft, '0', central_subcarriers, '0']
- [iq, '0', blocks_throttle2_0, '0']
-   src_blk_id: pss
    src_port_id: N_id_2
    snk_blk_id: detected_identity
    snk_port_id: print
-   src_blk_id: pss
    src_port_id: N_id_2
    snk_blk_id: synchronized_stream
    snk_port_id: N_id_2
-   src_blk_id: pss
    src_port_id: half_frame
    snk_blk_id: pss_selector
    snk_port_id: half_frame
-   src_blk_id: pss
    src_port_id: half_frame
    snk_blk_id: synchronized_stream
    snk_port_id: half_frame
-   src_blk_id: pss
    src_port_id: lock
    snk_blk_id: pss_selector
    snk_port_id: lock
-   src_blk_id: pss
    src_port_id: lock
    snk_blk_id: synchronized_stream
    snk_port_id: lock
- [pss_selector, '0', fft, '0']
- [recording, '0', iq, '0']
- [rough_sync, '0', pss_selector, '0']
- [rough_sync, '0', synchronized_stream, '0']
- [synchronized_stream, '0', sink, '0']
metadata:
    file_format: 2
    grc_version: 3.11.0.0