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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

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)
  • Rcoding – 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.


Practical OFDM Data Rate Calculator

R = ν ⋅ Qm ⋅ R ⋅
Nsc
Ts + Tg
 ⋅ (1 - η)
ν: MIMO Rank Qm: Modulation Order R: Coding Rate Nsc: Subcarriers Ts: Useful Symbol Time η: Overhead Factor

Further Reading

  1. MIMO Rank
  2. OFDM Subcarriers and Symbols
  3. Data Rtae in OFDM using MATLAB

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