Adjacent-Channel Rejection

Adjacent-channel rejection describes how well a receiver rejects energy in a nearby channel while preserving the wanted channel. It is a practical coexistence metric because real filters have finite skirts, real adjacent transmitters have finite spectral containment, and real receiver front ends can respond to strong nearby energy even when the nominal channel assignment looks clean.

The term is receiver-side. Adjacent-channel leakage ratio describes how much a transmitter leaks into a neighboring channel. Adjacent-channel rejection asks whether the receiver can tolerate that neighboring signal after filtering, gain control, demodulation and implementation limits.

Basic Metric

A simple adjacent-channel rejection statement compares the maximum tolerated adjacent-channel interferer with the wanted signal level for a stated degradation criterion:

where:

• P_{adj,max} is the adjacent-channel signal level that causes the specified limit;
• P_C is the wanted carrier or signal level;
• both powers use the same receiver reference plane.

The criterion matters. The limit might be a bit-error rate increase, packet-error threshold, EVM limit, SINR floor, throughput drop, false alarm rate or loss of lock.

Filter-Skirt View

Receiver filtering provides attenuation at the adjacent-channel offset:

where G(f_C) is gain at the wanted channel and G(f_C+\Delta f) is gain at the adjacent interferer offset. A larger A_{adj} generally improves selectivity, but may increase insertion loss, group-delay distortion, ringing, cost or tuning sensitivity.

An effective adjacent interferer after filtering can be screened as:

using dB arithmetic at the stated reference plane. The adjacent-channel carrier-to-interference ratio is then:

This is a useful first screen before more complete modem or field testing.

Worked Example

A telemetry receiver sees a wanted signal:

An adjacent-channel transmitter is measured at the receiver input:

The channel filter provides adjacent-offset attenuation:

The effective adjacent interference is:

The adjacent-channel ratio is:

If the waveform requires:

then the selectivity margin is:

The receiver is not failing because the wanted signal is absent. It is failing because adjacent-channel energy is not rejected enough.

Improved Selectivity Screen

If a sharper filter, wider channel spacing or better channel plan raises adjacent rejection to:

then:

and:

The new margin is:

This looks acceptable only if the filter insertion loss, group delay, temperature drift and manufacturing tolerance do not consume other receiver margins.

Difference From Linearity

Adjacent-channel rejection is not the same as one-dB compression point or third-order intercept point. P1dB checks whether strong total input power compresses the receiver. IP3 screens whether nonlinear mixing creates in-band products. Adjacent-channel rejection checks whether a nearby-channel signal leaks through the receiver selectivity path enough to corrupt the wanted demodulation.

All three can matter together. A very strong adjacent signal may first challenge compression, then create intermodulation, and also leak through the channel filter. The diagnostic task is to identify which mechanism controls the service failure.

Engineering Use

Adjacent-channel rejection is used for channel planning, receiver selection, site surveys, private wireless deployments, telemetry coexistence, cellular planning, radar receivers, industrial wireless and lab qualification. It connects spectrum occupancy to receiver performance. A clean link budget is not enough if a nearby transmitter occupies the next channel during real operation.

Mitigations include channel change, greater frequency separation, better receiver filtering, narrower occupied bandwidth, improved transmitter spectral mask, antenna isolation, lower adjacent transmitter power, scheduling, directional antennas or a waveform with stronger interference tolerance.

Validation Evidence

A defensible adjacent-channel rejection review includes wanted level, adjacent level, channel spacing, measurement bandwidth, waveform, modulation and coding, filter configuration, receiver gain state, AGC behavior, EVM, SINR, packet error, detector settings, occupancy, duty cycle, temperature, antenna state and uncertainty.

Adjacent-channel problems are often intermittent. The field record should therefore include time-stamped spectrum captures or receiver logs, not only one average trace from a quiet period.

Common Mistakes

Common mistakes include treating adjacent-channel rejection as a transmitter-only spectral-mask issue, measuring the adjacent interferer at a different reference plane than the wanted signal, ignoring filter insertion loss, assuming occupancy percentage is enough without amplitude, confusing selectivity with sensitivity, and fixing the problem by increasing wanted transmit power without checking coexistence consequences.

The practical rule is to state the channel spacing and degradation criterion, measure the adjacent signal at the receiver boundary, convert it through the receiver selectivity path, then compare the resulting margin with service evidence.