Ferkans — Interactive Telecom Tutor

Joint Communication and Sensing

OFDM is not just for communication: the same waveform can be used for radar sensing. This dual-function capability (JCAS) is a key feature of 6G research. The OFDM radar processor extracts range and Doppler from the communication signal.

Definition:
OFDM Radar Range-Doppler Processing

The OFDM radar receiver forms a range-Doppler map from $M$ OFDM symbols, each with $N$ subcarriers:

Per-symbol division: $Z[k, m] = Y[k, m] / X[k, m]$ removes data
IFFT across subcarriers (range): resolves delay bins
FFT across symbols (Doppler): resolves velocity bins

Range resolution: $\Delta R = c / (2B)$ where $B = N \Delta f$ . Velocity resolution: $\Delta v = c / (2 f_c M T_s)$ .

def ofdm_range_doppler(Y, X, N_fft):
    Z = Y / X  # element-wise division
    range_profile = np.fft.ifft(Z, axis=0)  # IFFT over subcarriers
    range_doppler = np.fft.fft(range_profile, axis=1)  # FFT over symbols
    return np.abs(range_doppler)

Example: OFDM Radar Target Detection

Simulate an OFDM radar detecting a target at 150 m range and 30 m/s velocity. Form the range-Doppler map.

Solution

Implementation

# See code supplement ch22/python/ofdm_radar.py

OFDM Radar Range-Doppler Map

Simulate OFDM radar processing and visualize the range-Doppler map.

Parameters

OFDM Radar Processing Flow — OFDM radar processing pipeline: data division, range IFFT, Doppler FFT, forming the 2D range-Doppler map.

Quick Check

What determines the range resolution of an OFDM radar?

Number of subcarriers

Total signal bandwidth

Carrier frequency

Number of OFDM symbols

Correction:

Total signal bandwidth

Correct: $\Delta R = c/(2B)$ where $B = N\Delta f$ is the total bandwidth.

Key Takeaway

The same OFDM signal used for communication can serve as a radar waveform. Range processing uses IFFT across subcarriers, and Doppler processing uses FFT across symbols. This dual-function capability (JCAS) is a cornerstone of 6G systems.

Why This Matters: OFDM Sensing in 6G

6G envisions integrated sensing and communication (ISAC) where the same base station simultaneously serves users and detects targets. OFDM radar provides range-Doppler maps at communication frame rates, enabling applications like autonomous driving, gesture recognition, and environmental monitoring.

See full treatment in Chapter 25

JCAS / ISAC

Joint Communication and Sensing / Integrated Sensing and Communication: using the same waveform and hardware for both data transmission and radar sensing.

Range-Doppler Map

A 2D representation of radar returns with range on one axis and velocity (Doppler) on the other; formed by 2D FFT processing.

OFDM as a Sensing Waveform