LTE throughput calculation differs slightly between FDD (Frequency Division Duplex) and TDD (Time Division Duplex) due to the way uplink and downlink resources are allocated. While the basic data rate formula remains similar, the key difference lies in how spectrum and time resources are used.
Let’s break it down clearly.
Basic Throughput Formula (Common for Both)
The simplified LTE peak throughput formula is:
Throughput = RB × 12 × OFDM symbols × Bits per symbol × MIMO layers × 1000
Where:
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RB = Number of Resource Blocks (depends on bandwidth)
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Bits per symbol = Based on modulation (QPSK/16QAM/64QAM/256QAM)
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MIMO layers increase capacity linearly
This core formula applies to both FDD and TDD.
LTE FDD Throughput Calculation
In FDD:
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Uplink and Downlink use separate frequency bands
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DL and UL operate simultaneously
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Full 100% subframe availability for DL (excluding control overhead)
Key Point:
Throughput is calculated directly based on:
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Available bandwidth (RBs)
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Modulation
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MIMO configuration
There is no time-sharing reduction factor.
Example:
20 MHz, 64QAM, 2x2 MIMO → ~150 Mbps peak DL (LTE Cat 4)
FDD offers more stable and predictable throughput since DL resources are not time-shared.
LTE TDD Throughput Calculation
In TDD:
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Uplink and Downlink share the same frequency
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Transmission is separated in time domain
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UL/DL ratio depends on TDD configuration (Config 0–6)
Important Difference:
Only certain subframes are allocated for Downlink.
For example:
If DL:UL ratio is 6:4
→ Only 60% of subframes are DL
→ Effective DL throughput reduces accordingly.
Adjusted Formula for TDD:
Effective DL Throughput = Theoretical DL × (DL subframes / Total subframes)
So even with same bandwidth, modulation, and MIMO, TDD peak throughput is usually lower than FDD unless DL-heavy configuration is used.
Practical Comparison
| Parameter | LTE FDD | LTE TDD |
|---|---|---|
| Spectrum | Separate UL/DL | Shared UL/DL |
| DL Availability | Continuous | Depends on configuration |
| Throughput Stability | High | Depends on UL/DL ratio |
| Flexibility | Less flexible | More flexible (traffic-based) |
Real Network Considerations
In both modes, actual throughput reduces due to:
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Control channel overhead
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Reference signals
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HARQ retransmissions
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SINR & CQI variations
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UE category limits
However, TDD networks must additionally consider:
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Special subframes
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Guard periods
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UL/DL switching delays
Final Summary
The main difference is not in the base formula —
it is in resource availability.
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FDD → Frequency separation → Stable full DL utilization
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TDD → Time separation → Throughput depends on DL/UL configuration
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