ECE4024 Chap.7 OFDMA and SC-FDMA
OFDMA and SC-FDMA
Multiple access turns an OFDM-style time-frequency grid into scheduled user resources. This chapter distinguishes OFDMA allocation from modulation, explains the DFT spreading that gives SC-FDMA its single-carrier-like envelope, and builds payload rate after pilots, coding, cyclic-prefix time and scheduling are accounted for exactly once. It closes with the uplink and downlink reasons behind different waveform choices.
What this chapter covers
- 01
Time-frequency resource grids
- 02
OFDMA user and resource-block allocation
- 03
Scheduling with channel quality
- 04
Adaptive modulation and coding
- 05
SC-FDMA and DFT spreading
- 06
Peak-to-average power ratio
- 07
Pilot, control, coding and prefix overhead
- 08
Uplink power efficiency and downlink flexibility
Worked example · free
Calculate scheduled user payload
- +1Each 64-QAM symbol carries log2(64) = 6 coded bits.
- +1One fully loaded interval contains 96 x 6 = 576 coded bits.
- +2Apply code rate two-thirds: 576 x 2/3 = 384 information bits.
- +1Apply the usable fraction: 384 x 0.85 = 326.4 information bits per average interval.
- +1Divide by 100 microseconds: 326.4/0.0001 = 3.264 Mbit/s.
- +1State that the total duration already includes cyclic-prefix time and the 96 count already excludes non-data subcarriers.
Key terms
- OFDMA
- A multiple-access method that assigns subsets of orthogonal time-frequency resources to different users.
- SC-FDMA
- A DFT-spread multicarrier method with a more single-carrier-like envelope, used when transmitter power efficiency matters.
- DFT spreading
- A transform applied to a user symbol block before subcarrier mapping, correlating the occupied tones and reducing envelope variation.
- PAPR
- Peak instantaneous waveform power divided by average power, a measure that drives power-amplifier back-off requirements.
- Resource block
- A scheduled collection of time-frequency elements used to carry payload, pilots or control information.
- Link adaptation
- The adjustment of modulation or coding to channel quality so rate and reliability follow current conditions.
OFDMA and SC-FDMA FAQ
What does OFDMA allocate to a user?
It allocates selected time-frequency resources such as subcarriers or resource blocks. Modulation then determines how many coded bits each allocated element carries, so allocation and constellation order are separate decisions.
Why is SC-FDMA attractive on an uplink?
DFT spreading produces a more single-carrier-like waveform with lower peak-to-average power ratio than plain OFDMA in typical settings. A handset can then operate its power amplifier with less back-off and better efficiency.
Why does timing alignment matter between users?
Orthogonality depends on observing the correct time interval and frequency grid. Separate uplink users have different propagation delays and oscillators, so a base station must manage timing and carrier alignment to limit mutual interference.
How should pilot overhead enter a rate calculation?
Remove pilots from the data-subcarrier count or apply a pilot efficiency, but do not do both for the same resources. Read whether the stated count already means data-only before multiplying any factor.
What is adaptive modulation and coding?
It selects constellation order and coding rate according to channel evidence. Strong conditions can support denser constellations or higher code rates, while weaker conditions need more robust settings to meet an error target.
Exam move
Sketch a time-frequency grid and explain separately who receives each cell and what modulation fills it. Work payload rates as a ledger: data subcarriers, symbols, bits per symbol, coding, allocation and total time. Cross out any overhead that is already excluded. Calculate PAPR from power, then convert to decibels with ten times the logarithm.
For compare-and-contrast answers, name the shared cyclic-prefix and FFT structure, DFT spreading, envelope behaviour, amplifier consequence and synchronization requirement. Avoid claiming that either waveform is universally superior. Revise multiple access by annotating a resource grid rather than listing acronyms.
Mark time, subcarrier index, scheduling interval, guard resources, pilots and two users with different allocations. For each coloured region, write the modulation order and code rate separately from the allocation size. This prevents a scheduler decision from being confused with waveform generation.
In a throughput calculation, begin with assigned data resource elements, remove explicitly stated pilot or control overhead, multiply by coded modulation bits and code rate, then account for layers and elapsed time. If the problem has already supplied the number of usable data elements, do not subtract overhead again. For SC-FDMA, draw symbol mapping, the DFT-spreading stage, subcarrier mapping, IFFT and prefix insertion.
Compare it with OFDMA at the same points in the chain and explain how the extra transform changes the time-domain envelope and frequency-domain interpretation. Practise localised and interleaved mapping sketches without assuming both have identical scheduling behaviour. For PAPR, identify peak and average powers over the same interval, form a linear ratio and only then use ten log base ten.
Connect the result to power-amplifier back-off, efficiency and handset constraints, while acknowledging that practical PAPR depends on data and implementation. Close with synchronization: independent uplink users must arrive within timing and frequency tolerances or their orthogonality can be damaged.
A complete comparison therefore covers allocation flexibility, envelope behaviour, receiver processing, amplifier cost and coordination requirements.
Working through OFDMA and SC-FDMA in ECE4024? Sia is AskSia’s AI Electrical Engineering tutor — ask any ECE4024 OFDMA and SC-FDMA question and get a clear, step-by-step explanation grounded in how ECE4024 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
