Monthly Archives: July 2013

DVB-T implementation in GNUradio – part 3

Later edit: source code added here:

In this article I will describe my  DVB-T receiver implementation in GNUradio.

The blocks used for receiving are basically the transmitter blocks in the reverse order with a major difference that there is need for synchronization blocks in order to obtain a clean constellation.

OFDM symbol aquisition:

The task of this block is to synchronize the receiver so that a clean time domain signal is obtained before the FFT block. There are several subtasks that this block takes care of:

1. Use Cyclic Prefix (CP) to obtain the start of the OFDM symbol in time domain. Here I’ve chosen to use a MLSE algorithm that is presented in [REF1 – Van de Beek]. It minimizes a log likelihood function in order to obtain frequency correction, epsilon – a fractional of intercarrier frequency, and time correction, theta – number of samples to the detected start of OFDM symbol. Since the CP is added in front of the OFDM symbol and being a copy of the last part of the symbol there should be a correlation maximum that will signal the beginning of the symbol. This is called pre-FFT NDA (Non Data Aided) correction and basically will obtain sync on the beginning of the OFDM symbol and will assure the subcarriers will fit into the center of the FFT bin.

Note: The algorithms used in OFDM for 802.11 [Schmidl and Cox – REF2] are using a preamble that is specially created to create two identical halves in time domain. In DVB-T this option is not available and the pre-FFT correction is called Non Data Aided acquisition because no pilots are used to achieve the synchronization task. However, there will be pilots used for post-FFT synchronization and I will explain it later in this article.

2. Once the CP start is found and epsilon is also found, de-rotation of the signal is applied in order to obtain the correct time domain OFDM symbol.

Note: The MLSE algorithm is quite computational intensive due to the correlations necessary to be calculated. For that reason for the subsequent symbols only +/-4 samples is taken in consideration to assure the CP is found again. In case of a number of consecutive CP misses a full initial aquisition is triggered.

Note 2: The MLSE of Beek requires knowledge of SNR value. For now this is entered manually as a parameter to the processing block.

TODO: Add one methods of SNR estimation

3. The CP in front of the OFDM symbol is taken out

FFT for time to frequency domain conversion:

This is just plain FFT conversion for 2k, 8k or 4k depending on the parameters. Here the GNUradio block is used.

A plot of the constellation after the FFT is in order. One may see that the constellation is rotating and therefore requires more processing in order to be useful for de-mapping:

Plot is made when transmitting with USRP N210 and receiving with USRP N210, both equipped with WBX daughterboards. DVB-T parameters are 8Mhz bandwidth, FFT 2k, 16-QAM, FEC 1/2

Demodulation of Reference signals:
The DVB-T standard uses several sub-carriers to insert pilot signals that are used for synchronization, signal parameters transmission and equalization.

There are three pilot signals types used in DVB-T standard:

– scattered pilot signals

– continual pilot signal

– TPS (Transmission Parameter Signals)

1. Post-FFT DA (Data Aided) synchronization.

The continual pilots and (in my implementation) scattered pilots are used to obtain an integer frequency correction after the FFT is performed. The position of the scattered and continual pilots is known and therefore by using ML correlation of the signal with the expected values one can obtain an integer frequency correction. This is the number of bins the synchronizer needs to take in consideration when correcting the signal after the FFT.

There are two roots of frequency deviations that we need to take into account beside the nature of the transmission channel: Carrier and Sampling-clock deviations. These produces ICI (Inter Carrier Interferences) that are important due to the small intercarrier frequency and can be modelled as noise. See [Speth and all – REF3].

After pre/post-FFT carrier synchronization that is to be performed one time only, a continuous correction still need to be done for the residual correction that will appear during time using a PID-like algorithm described also in [Speth and all – REF3].

TODO: implement this correction.

2. Demodulation of TPS block

Transmission Parameter Signalling is used to assist the receiver in knowing the parameters of the stream that is received. The TPS is send using using predefined position pilots on witch  the data is modulated using DBPSK modulation. Being differential modulation, this is suitable for demodulation even though the constellation is rotating. The same bit of information is sent in an OFDM symbol so majority voting is used to decide the actual value of the data bit. There are 68 symbols in an frame and 4 frames in a superframe.

3. Equalization of signal based on pilot signals

A simple continual and scattered pilot based equalizer is used to perform correction of the data. The scattered pilots have a position inside the frame that is based on the following formula {k = Kmin + 3 × (l mod 4) + 12p p integer, p ≥ 0, k ∈ [Kmin; Kmax] } whre l is the symbol index. The idea behind scattered pilots is to allow for a better coverage of the whole channel when moving from one symbol to the other.

TODO: Implement DFE (Decision Feedback Equalizer) – See [Proakis – REF4]

The following plot shows the signal constellation for 16-QAM after the equalization:


[REF1]: Jaan-Jaap van de Beek, ML Estimation of Time and Frequency Offset in OFDM Systems

[REF2]: Timothy M.Schmidl and Donald C. Cox, Robust Frequency and Timing Synchronization for OFDM

[REF3]: Michael Speth, Stefan Fechtel, Gunnar Fock, Heinrich Meyr, Optimum Receiver Design for OFDM-Based Broadband Transmission – Part I and II

[REF4]: John G. Proakis, Masoud Salehi, Digital Communications