Co-channel and adjacent-channel interference – how interference affects maximum achievable throughput and link distance
Interference and channel loading by too many radios are primary concerns within the license-exempt bands. The licensed band is not immune to interference either. The ability to operate a link using a required modulation depends on Carrier-to-Interference (C/I) performance within the bandwidth being used.
Carrier-to-interference susceptibility gets progressively worse with increasing modulation order e.g. QPSK (9dB (European Communications Office (ECO), January 2010)), 16QAM (19dB), 64QAM (26dB). So, while an interference power level at 9dB below the received carrier level will start causing link errors for a link using QPSK, a link using 64QAM must have its carrier 26dB above the interference level.
Radios that use Adaptive modulation will change modulation method if they detect errors when attempting to use high-order-modulation. This can be problematic if IP Radios continually adjust their modulation schemes to cater for a changing ‘license-exempt’ band environment.
PLEASE NOTE: if license-free radios are on the SAME channel and work according to standards such as EN 300 328 or EN 300 893, it is a regulatory (mandated) requirement for these radios to ‘listen’ to the channel before transmitting – the result is that the radios automatically SHARE the band. So, the presence of other radios on the same channel does not result in ‘interference’ per se, but rather, there is an ‘enforced sharing’ of the radio channel resource as radios must ‘back-off’ for a random time and attempt transmission at another time.
So, what is the solution when there is the possibility of interference? Looking from the receiver’s perspective, the key objective is to maximize the ‘Carrier signal’ relative to noise and interference power levels (governed by regulatory EIRP constraints and channel-access sharing mechanisms, at least for co-channel operation). Since interference can be self-generated (multipath) or due to other equipment transmitting asynchronously on adjacent channels, approaches include:
- link design planning to analyze the possible effects of multipath loss
- management of frequency usage on masts
- co-locating FDD equipment that transmit using the same frequency channels
- using larger (higher gain) antennas with narrower -3dB beamwidths to minimize unwanted receive signal power.
- using ‘high performance’ antennas with shields (‘collars’) to minimize signal reception via antenna sidelobes.
- balancing the receive power levels at sites using ATPC (Automatic Transmit Power Control).
When frequency channels (from TDD radios possibly operating asynchronously) are operated too close to each other, power from another radio transmitter that is leaked into an adjacent channel can affect (‘deafen’) the adjacent channel’s receiver. Whereas the receive sensitivity may for example be -87dBm, an adjacent channel’s transmit power could have -70dBm levels (in the power spectrum spillover ‘skirts’) leak into the adjacent channel’s receive chain.
For TDD systems, the solution is use synchronization which ensures that ALL transmitters coordinate their transmission times (this is impractical when competing operators use BRAN equipment from different vendors).
Furthermore, advanced radio solutions go even further by synchronizing the arrival times of packets at a receiver from distant transmitters – this ensures that the receiver is not overwhelmed when the number of remote devices increases. Low-cost BRAN solutions in license-exempt bands do not implement these advanced features.
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