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Interference Isolation in Wireless Systems

Wireless
Posted by James Donovan on 2 August 2017 Connect with James on LinkedIn Estimated reading time: 5 minutes

What causes RF interference and complications? We explore the causes, review considerations that must be made to avoid interference, and review example challenges that stand in the way effective signals?

 

RF Interference: What is it, and why does it occur?

An RF communications system that employs simultaneous, two-way flow of voice, data or other information is called a duplex system. Duplex communications systems combine multiple transmit and receive channels on a shared antenna, with information flowing both ways at the same time.

Imagine the simultaneous flow of traffic on a busy two-way street. You immediately see the importance of keeping the two different directions of traffic separated.

Just as vehicles on a busy, two-way street require clear lane markings to avoid collisions with oncoming vehicles, duplex RF channels also must be “isolated” from each other to avoid sources of interference. In RF terms, isolation is measured as the loss between two channel ports, either transmitter-to-transmitter or transmitter-to-receiver ports.

The higher the loss, or isolation, between the two ports, the cleaner the signal. To illustrate this concept, think about making a cell phone call from your car.

This simplest of duplex systems – one transmitter and receiver pair communicating with another transmitter and receiver pair – requires that both the phone and receiving station be able to receive and transmit at the same time, allowing a normal telephone conversation to take place (figure 1).

figure 1 - typical cell site architecture

(Figure 1) Duplex operation between two pairs of transmitters and receivers

To allow this communication to flow on a single antenna, a duplexer must be used with adequate isolation measures. Measured in dB, isolation is a critical consideration in the design of any duplex system.

Without proper isolation and elimination of all sources of interference, a transmitter will adversely affect the performance of its associated receiver, even though they may operate on different frequencies.

The specifications covering a particular receiver, for instance, may indicate that any RF signal outside the receiver’s passband (which can be as narrow as 15 kHz) will be attenuated, or weakened, by as much as 100 dB.

That means that the transmission’s power will be reduced to 1/10,000,000,000th of its original strength, making the communication unintelligible and useless in most cases.

You might think that such a selective receiver would prevent sources of interference from a transmitter operating on a frequency far outside the receiver’s passband. After all, if the interfering signal is 5 MHz away, how could it create complications when just being 5 kHz off the mark reduces the transmitter’s signal to virtually nothing?

The answer lies in the characteristics of modern receivers, and the way they can step high-frequency signals downward to achieve such precise frequency selectivity.

 

Addressing Interference Challenges

The First Challenge: Receiver Desensitization

Receiver desensitization is an inherent side effect of modern receiver design, which receive relatively high-frequency signals (often between 700 MHz and 3500 MHz). These signals pass through frequency-lowering stages in the receivers, which allow the receivers to feature such narrow, selective passbands (figure 2).

Once the signal has been lowered enough, only a small band remains and the circuitry can reject other bands within a margin measured in dB. A receiver’s specification sheet will include this measurement of overall selectivity.

Figure 2 - In a receiver, high-frequency signals are reduced in stages

(Figure 2) In a receiver, high-frequency signals are reduced in stages

The vulnerability is not at the end of this reducing process, but at its beginning. Remember that the initial signal was of higher frequency, and only after multiple stages of reduction was it lowered to the point where the receiver could use it.

The receiver’s earlier, broader stages cannot completely reject errant signals, even those several MHz away from the receiver’s operating frequency.

For optimum performance, critical voltage and current levels exist at certain points throughout the front-end stages of a receiver. If these sources of interference levels change significantly, the performance of the receiver suffers.

This happens when a nearby transmitter’s off-frequency signal enters the front-end stage. Such signals can be several MHz away from a receiving frequency, and radiate from sources several thousand feet away, and still cause significant interference.

The Second Challenge: Transmitter Noise

Transmitter noise is interference caused by carrier signals just outside of a transmitter’s assigned frequency. In an ideal world, a transmitter would channel 100% of its signal power into the narrow band of frequencies assigned to its transmission channel.

In the real world, however, this level of precision is simply not possible, and the result is called transmitter broadband noise radiation, or more commonly, transmitter noise.

While the vast majority of transmission power remains within the assigned channel, there remains a small fraction that “leaks” into channels above and below the intended carrier frequency.

Modern transmitters are equipped with filter circuits that eliminate a large portion of these errant signals, but even with these measures in place, enough transmitter noise escapes to degrade the performance of a receiver.

As the chart below illustrates, the effect of these sources of interference is most pronounced at frequencies closest to the transmitter’s carrier frequency (figure 3), but can also impact receivers operating several MHz away.

Figure 3 - Transmitter interference

(Figure 3) Transmitter interference is most pronounced near the assigned frequency (shown here as Tx frequency, located at zero on the horizontal axis)

We hear transmitter noise in a receiver as “on-channel” noise interference. Because it falls within the receiver’s operating frequency, it competes with the desired signal and cannot be filtered out.

To illustrate this kind of interference, imagine having a conversation with someone in a crowded room. If everyone else is talking, you’ll notice how hard it is to understand the other person, even if the overall noise level in the room is relatively low.

That’s because other voices – like unwanted transmitter noise – are similar to the voice you’re trying to hear. This is a key distinction between transmitter noise and receiver desensitization, which you’ll recall comes from signals far from the operating frequency of the receiver. Consider again the illustration of having a conversation.

Receiver desensitization is more like loud, disruptive sounds coming from a construction site next door. The interference is not similar to the voice you’re trying to hear, but it still distracts you from the other person’s voice.

Further Learning on Isolation of Interference

To find out more about the practicalities of RF Wireless Infrastructure, why not explore our RF Wireless Infrastructure Fundamentals [SP6500] course, with the objective to learn wireless transmission methods including modulation schemes, isolation systems, data rates and spectrum.

Or, read more in our eBook: Understand the Passive Infrastructure that Underpins Your Network.