Basic Concepts

Spurious Interference

Spurious interference refers to the additive interference generated by the interference source in the working frequency band of the interfered receiver. Spurious interference includes the out of band power leakage of the interference source, amplified background noise, and transmit inter-modulation product. Spurious interference can reduce the signal noise ratio (SNR) of the interfered receiver.

Spurious interference is generated by a transmitter and usually includes the thermal noise generated and amplified by the power amplifier (PA). Inter-modulation products can be generated during the multi-carrier operation and spurious signals can also be generated by frequency mixer.

Blocking Interference

Receivers usually work in linear areas. When a strong interference enters a receiver, it may also overdrive the receiver to work in non-linear state or even worse, in saturation mode as the out of band suppression ratio of the receiver may

Figure 5-1 Spurious interference

Figure 5-2 Blocking interference be limited. This type of interference is called blocking interference.

Generally, blocking interference is generated by a strong interference signal out of the receive band that makes the receiver work in saturation state and then reduces the gains. In addition, the interference signal may mix with the local oscillator signal and then generate the interference in the intermediate frequency (IF). Blocking interference can reduce the receiver gains and increase the noise.

Inter-Modulation Interference

When multiple strong signals with different frequencies enter a receiver at the same time, the inter-modulation product at the receiver is generated by the signals by the front end due to the non-linear circuit of the receiver. Unfortunately, the frequency of the inter-modulation product drops into the useful frequency band of the receiver and generates the inter-modulation interference.

When a strong signal is reflected back from the transmitting end of a transmitter back into the transmitter, this signal, together with the transmitted signal of the transmitter may also generate some inter-modulation products because of the non-linearity of the transmitter.

When multiple signals with different frequencies transverse across conductors at the same time, the inter-modulation product is generated because of the non-linearity of the metals.

Figure 5-3 Inter-modulation interference

Because of the non-linearity of the receiver and the limitation of out of band suppression, multiple harmonic waves are generated on the received signal. When two strong interference signals are received at the same time, the frequency

combination such as 2f1-f2 and 2f2-f1 of the two strong interference signals may drop into the band of the receiver and then generates interference.

The capability to resist inter-modulation is a feature of the receiver. If you want to eliminate the inter-modulation interference by installing a filter, you need to install a receive filter in the interfered system.

ACS

Adjacent channel selectivity (ACS) is a protection index to determine the capability of a receive filter. ACS refers to the capability to receive the power of the local in band channel when the interference signal from the adjacent frequency exists. ACS is the ratio of the receive filter attenuation on the assigned channel frequency to the receive filter attenuation on the adjacent channel(s). Hence, ACS is the ratio of the receive filter loss on the designated channel to the loss on the adjacent channel.

Figure 5-4 ACS

ACLR

Adjacent Channel Leakage power Ratio (ACLR) is measure of transmitter performance and it is defined as the ratio of the transmitted power to the power measured after a receiver filter in the adjacent RF channel. Alternatively, it can be referred to as the ratio of the average power on the designated frequency point to the average power on the adjacent channel. ACLR represents the suppression capability of the transmit filter to the adjacent channel.

As shown in the figure below, ACLR is the dB value of C (total transmit power of the designated frequency point) subtracting D (total in-band leakage power of the adjacent channel). The adjacent channel may be used by the same system or a different system. The bandwidth is determined based on the system in the adjacent channel.

ACLR was also formerly called Adjacent Channel Power Ratio and ACLR is specified in the 3GPP standard.

Figure 5-5 ACLR

Figure 5-6 Near-far effect of the interference in the adjacent channel ACIR

It is difficult to separate ACLR and ACS because they coexist; therefore, ACLR and ACS are usually considered together.

The result of the consideration is Adjacent Channel Interference Ratio (ACIR) and ACIR is the ratio of the total power transmitted from a source (base station or UE) to the total interference power affecting a victim receiver, resulting from both transmitter and receiver imperfections. The formula is as follows:

The interference in the adjacent channel affects both the system coverage and system capacity. When near-far effect exists, interference from the adjacent channel greatly affects the system coverage, and may even cause the dead zone.

As shown in the preceding figure, when the terminal in system A enters system B, the interference power of system B to system A is even greater than the useful power of system A. Therefore, the dead zone occurs, where the terminal cannot access the network.

In the uplink, the limiting design factor is the UE transmitter, which will dominate the uplink interference. The reason is that ACLRUE << ACSBS, which implies that uplink ACIR ≈ ACLRUE. Thus, in an uplink simulation, it is essentially the UE ACLR performance that is simulated.

In the downlink, the limiting design factor is the UE receiver, which will dominate the downlink interference. The reason is that ACSUE << ACLRBS, which implies that downlink ACIR ≈ ACSUE. A downlink simulation will thus essentially be a simulation of UE ACS performance.

OFFSET/BW/GB

Figure below is the relationship of the Offset (frequency offset value), BW (channel bandwidth) and Guard Band.

Figure 5-7 Frequency offset relationship