ELEC5508 Chap.4 Cellular Reuse and Interference
Cellular Reuse and Interference
This chapter treats frequency reuse as an explicit trade-off. Giving every cell more spectrum increases degrees of freedom, but closer co-channel transmitters reduce SINR. OFDMA can orthogonalise users within one cell while leaving out-of-cell interference. Power control, coordinated scheduling and spatial processing attack different terms in the SINR expression and carry different costs.
The correct comparison keeps bandwidth and SINR visible together. A universal-reuse design may improve cell-average throughput yet harm a boundary user whose desired and interfering powers are similar. Conversely, a conservative pattern can protect SINR while wasting spectrum near the cell centre. This user-specific geometry motivates fractional reuse and coordinated schemes rather than one universal answer.
It also explains why the received-signal expression should be written before a rate formula.
What this chapter covers
- 01
Cellular reuse patterns and co-channel distance
- 02
Universal reuse and fractional reuse
- 03
SINR numerator and denominator
- 04
OFDMA in-cell orthogonality
- 05
Interference averaging and avoidance
- 06
Power, coordination and beamforming controls
Worked example · free
Compare two reuse choices
- +1Universal SINR is 2.4/(1.0+0.2) = 2.0.
- +1Universal rate is 10 log2(3) = 15.85 Mbit/s.
- +1Split SINR is 2.4/(0.15+0.2) = 6.857.
- +1Split rate is 5 log2(7.857) = 14.87 Mbit/s.
- +1Universal reuse wins slightly in this geometry because bandwidth offsets interference.
Key terms
- Frequency reuse
- Repeated allocation of the same spectrum to sufficiently separated cells.
- Co-channel interference
- Unwanted power from another transmitter using the same time-frequency resource.
- SINR
- Desired received power divided by noise plus aggregate interference power.
- OFDMA
- Multiple access that assigns orthogonal OFDM resources to users within a cell.
- Power control
- Adjustment of transmit power to balance desired coverage against interference caused elsewhere.
Cellular Reuse and Interference FAQ
Why can universal reuse improve capacity?
It gives each cell the full available band, increasing degrees of freedom. The benefit survives only if interference management keeps SINR from falling too far. Cell-edge users may lose even when the cell-average throughput rises.
Does OFDMA remove cellular interference?
It can prevent scheduled users in the same cell from sharing one orthogonal resource, but neighbouring cells can still use that resource simultaneously. Their received powers remain in the SINR denominator unless coordination, reuse planning or beamforming reduces them.
How should I compare reuse patterns?
Compute both bandwidth and SINR effects with the same rate model. Then identify which user location or network objective is being evaluated. A pattern that maximises average throughput may not protect edge reliability or fairness.
What is a reliable workflow for an interference comparison?
Write desired power in the numerator and every simultaneous co-channel contribution plus noise in the denominator. Convert logarithmic powers before summing them. Compute SINR for each reuse or coordination option, then apply the same bandwidth-aware rate expression to all options. Keep bandwidth and SINR visible as separate columns so a throughput change can be attributed to one or both.
Repeat the comparison for a centre and edge condition when location matters, because one average can hide weak users. Finish by identifying what a proposed lever changes: transmit power, resource overlap, beam direction, cooperation or scheduling. Also state the information and coordination delay that the lever requires.
Exam move
Write the received signal and SINR before inserting values. Track desired power, each interferer, noise and bandwidth separately so the reason for a rate change remains visible. Start by sketching a small hexagonal network and label co-channel cells. Explain how moving a repeated label farther away changes interference and spectrum per cell in opposite directions.
Build a reusable calculation template with desired signal in the numerator and every simultaneous co-channel term plus noise in the denominator. Convert decibel-milliwatts to linear power before summing, and convert the final ratio back to decibels only if requested. Compare two reuse patterns with B log2(1+SINR) using the same power model.
Then repeat the comparison for a centre user and an edge user so the distribution is not hidden by one average. Create a separate lever table for power control, time-frequency coordination, fractional reuse, beamforming and multi-site cooperation. For each lever, state which term it changes, what information it needs, how fast it can react and which resource it consumes.
In tutorial-style problems, write the complete received signal before simplifying orthogonal resources. In lab-style explanations, report desired power, interference, noise, SINR and bandwidth separately. If throughput improves, say whether the cause is more degrees of freedom, less interference or both. This structure prevents OFDMA in-cell orthogonality from being confused with the removal of neighbouring-cell interference.
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