Achievable Data Rate in OFDM

In general, the data rate in OFDM is calculated as:
Data Rate = Number of Subcarriers × Bits per Symbol × OFDM Symbol Rate

In OFDM (Orthogonal Frequency Division Multiplexing), the achievable data rate depends on several parameters: bandwidth, modulation scheme, number of subcarriers, and the signal-to-noise ratio (SNR).

Key Parameters Affecting Achievable Data Rate

• Total Bandwidth (W): The total available bandwidth for transmission. More bandwidth allows more subcarriers, increasing the data rate.
• Number of Subcarriers (N): The total bandwidth is divided into smaller subcarriers; more subcarriers means more parallel data streams.
• Modulation Scheme: Determines bits per symbol:
  • QPSK – 2 bits/symbol
  • 16-QAM – 4 bits/symbol
  • 64-QAM – 6 bits/symbol
  • 256-QAM – 8 bits/symbol
• Coding Rate (R): The ratio of useful data bits to total transmitted bits (includes error correction).
• Symbol Duration & Guard Interval: The guard interval prevents inter-symbol interference but slightly reduces throughput.
• Signal-to-Noise Ratio (SNR): Higher SNR allows higher-order modulation for increased data rates.

Formula for Achievable Data Rate

The achievable data rate R can be approximated by:

R = N × (bits per symbol) × (symbol rate) × Rcoding × (efficiency factor)
  

Where:

• N – number of subcarriers
• bits per symbol – depends on modulation scheme
• symbol rate – inverse of symbol duration
• Rcoding – coding rate (e.g., 1/2, 2/3, 3/4)
• efficiency factor – accounts for guard intervals and practical losses

Example Calculation

Assume the following OFDM system parameters:

• Total Bandwidth W = 20 MHz
• Subcarrier Spacing Δf = 15 kHz
• Modulation Scheme: 64-QAM (6 bits/symbol)
• Coding Rate R = 2/3
• Guard Interval = 25% of symbol duration

Number of subcarriers:

N = W / Δf = 20,000,000 / 15,000 ≈ 1333 subcarriers
  

Symbol duration: T = 1 / 15,000 = 66.67 μs

Hence, achievable data rate:

R = 1333 × 6 × (1 / 66.67×10⁻⁶) × (2/3)
  

This gives a rough estimate of the achievable throughput before accounting for guard intervals and other inefficiencies.

OFDM Data Rate Calculator

Total Bandwidth (Hz): Subcarrier Spacing (Hz): Modulation Scheme: QPSK (2 bits/symbol) 16-QAM (4 bits/symbol) 64-QAM (6 bits/symbol) 256-QAM (8 bits/symbol) Coding Rate (e.g., 2/3 = 0.67): Guard Interval (fraction of symbol duration):

Formula

R = N × (bits per symbol) × (symbol rate) × Rcoding
  

where:

• N – Number of subcarriers
• Bits per symbol – Based on modulation (e.g., 2 for QPSK, 6 for 64-QAM)
• Symbol rate – Inverse of effective symbol duration (includes guard interval)
• R_coding – Coding rate (e.g., 0.67 for 2/3)

In modern systems like LTE, Wi-Fi 6, and 5G NR, OFDM enables throughput ranging from several Mbps to multi-Gbps depending on configuration.

Further Reading

1. Data Rtae in OFDM using MATLAB