How is blocking measured?

The blocking level is the power of an off-channel CW signal that causes 1 dB desensitization (1 dB increase in the noise floor or 1 dB reduction in SINAD) of the desired signal at the receiver's sensitivity level. Specified at various frequency offsets: in-band (e.g., +/- 3 MHz) and out-of-band (e.g., > 10 MHz). 3GPP defines specific blocking requirements for each UE category.

What causes blocking?

A strong interferer drives the LNA or mixer into gain compression. In compression, the gain for all signals (including the desired one) drops. The interferer does not need to be in the desired channel; it only needs to pass through the preselector filter with enough power to compress the active stages. Reciprocal mixing of the interferer with LO phase noise also contributes.

How to improve blocking performance?

Higher IP1dB LNA (increases compression threshold). Sharper preselector filters (attenuate off-channel signals before the LNA). AGC that backs off gain when strong signals are detected. SAW/BAW filters with steep roll-off. In 5G NR, the UE must handle -15 dBm in-band blockers per 3GPP TS 38.101.

Wireless System Design

Blocking

Blocking is the desensitization of an RF receiver when a strong off-channel signal drives the front-end (LNA, mixer) into gain compression, reducing the effective gain for the desired signal. The blocker does not need to be in the desired channel; it only needs sufficient power after the preselector to compress active stages. Blocking performance is specified as the interferer power level that causes 1 dB desensitization at a given frequency offset.

Category: Wireless System Design

5G NR Req: −15 dBm in-band

Understanding Blocking

Consider a desired signal at −100 dBm with an LNA providing 20 dB gain and P1dB of −10 dBm input. A blocker at −15 dBm passes through the preselector with 20 dB rejection, arriving at the LNA at −35 dBm. After 20 dB gain, the blocker is at −15 dBm output, well below P1dB. But if preselector rejection is only 5 dB, the blocker arrives at −20 dBm, and with 20 dB gain the output reaches 0 dBm, compressing the LNA and reducing gain for the desired signal.

Reciprocal mixing also contributes: the LO phase noise at the blocker's offset frequency mixes with the blocker, creating noise in the desired channel bandwidth.

Blocking Desensitization

1 dB desensitization level:
P_blocker at which desired signal SNR degrades 1 dB

LNA input compression:
P_in,blocker = P_blocker − L_preselector
Compression if P_in,blocker > P1dB_input − 10 dB

3GPP Blocking Requirements (NR FR1)

Scenario	Offset	Blocker Level	Desired Signal
In-band	≥ Channel BW	−15 dBm	Ref sensitivity + 6 dB
Out-of-band (close)	15 MHz	−15 dBm	Ref sensitivity + 6 dB
Out-of-band (far)	> 60 MHz	−15 dBm	Ref sensitivity + 6 dB
Narrow-band	CW at image	−44 dBm	Ref sensitivity + 6 dB

Common Questions

Frequently Asked Questions

How measured?

Off-channel CW power causing 1 dB desensitization at sensitivity. Specified at multiple offsets. 3GPP defines per UE category.

What causes it?

Strong signal compresses LNA/mixer gain. Reciprocal mixing with LO phase noise also contributes. Preselector rejection is key.

How to improve?

Higher IP1dB LNA, sharper preselector (SAW/BAW), AGC with fast back-off, and improved LO phase noise.

Related Terms

← Blockage Efficiency Blocking Switch →

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