Implementation and Analysis of Multi-Carrier Synchronization Techniques

Multi-carrier synchronization for GnuRadio

Goal:

• Improve reliability of communications over frequency-selective channels

Approach:

• Split data into multiple narrow-band sub-streams
• Use coding to mitigate losses in these sub-streams

OFDM …

… from 10000 feet

Source: Google maps

FFTing Chunks of the input stream is essentially the same thing a waterfall diagram does

|x| and arg(x)

S&C detector with frequency offset

|x| and arg(x)

with frequency offset

Calculating the frequency offset from the S&C phase

Using the calculated frequency offset the symbols can be sampled correctly

GnuRadio

• … provides a set of DSP components
• … combines components to form a signal processing chain
• … lets developers write own components

GnuRadio levels of abstraction:

• Graphical block-diagram based interface
  → very intuitive
• Python programming interface
  → more flexible than the graphical interface
• C++ interface
  → can be a lot faster than python code

The Project

Problem:

• A Schmidl & Cox implemented using the graphical interface is too slow to detect WiFi frames on an usual Desktop computer

Solution:

• Integrate the functionality of the discrete blocks into a single, highly optimized block

The default configuration adds a tag to the sample stream whenever a S&C sequence is detected

Performs the raw S&C calculation and outputs a correlation value normalized to the mean input power

Source

Adds a tag to the sample stream when set thresholds are crossed

Source

Relocates incomming tags based on cross-correlation with stored preamble

Source

What i have learned

• OFDM
  → usecases / requirements / limitations
• Multicarrier synchronization
  → requirements / implementations / tradeoffs
• GnuRadio
  → internal structure / writing blocks in Python & C++ / vectorizing calculations

Next steps

• Performance evaluation
• Code cleanup for public use

The software sources are released under the terms of the
GNU GPLv3 license:

github.com/hnez/XFDMSync

This presentation is released under the terms of the
GNU FDLv1.3:

github.com/hnez/XFDMSync-Report