CatapuLTE ==> CatG

CatapuLTE application is a tool for analyzing LTE frames. The analysis is based on three aspects and checks out that the frames provided are in accordance with the current LTE standard. Hence, it provides information about frames and spots the standard-breaking elements. Three aspects enabling frame analyzing are listed and described below:

Spectral analysis

In a wireless system in which users share a common bandwidth, it is imperative to respect the resources allocation in order not to interfere with other users or technologies. That is why the application particularly focuses on the signal spectral occupation.

Constellation analysis

The application detects the constellation used. It enables signal demodulation and checks out that the correct constellation is used.

Information contained in frames analysis

The application detects the synchronizing channels P-SCH and S-SCH and localizes their position. It also decodes the PBCH channel and displays the parameters.

To provide this information, input parameters are requested:

• samples
• whether the “main path” is active or not
• whether the “aux path” is active or not
• MIMO or SIMO mode

The general schematic of the technical modules is presented below,

Synchronization

The first step for LTE frame decoding consists in time and frequency synchronizations.

The goal of time synchronization is dubbled. First, it enables the first sample to be detected. Hence, the cell ID is determined by computing the cross correlation between reference synchronization signals and the signal.

The Doppler influence is estimated by the frequency synchronization.

Signal processing

Now, the signal is synchronised in time and frequency. The propagation channel has to be equalized. Its impulse response is estimated by comparing some parts of the signal with pilot symbols (reference signals). Then, identification and correction of the digital symbol constellation is computed after OFDM demodulation.

Demodulation

The symbol contained in PBCH can now be selected before digital demodulation. Hence, a descrambling is performed to reorder the bit sequence.

Decoding

The final step is the information decoding. Firsts, the de-rate matching performs an meaning on the redundancy bits (the same information appears four times in one PBCH). Then the bit sequence is decoded using the Vitervi algorithm before checking the CRC. CRC includes a parity sequence to which a offset is added depending on the antennas configuration in transmission.

PBCH contains Hybrid ARQ (3 bits), the bandwidth (in term of Ressource Block) and the 8 most significant bits of the System Frame Number, which are the same for 4 consecutive frames. That permits the 2 least significant bits to be determined.