In this post, we will study an unwanted component of cellular communication: channel interference. We will study why it is undesirable, its various types, and how to reduce its effects in the network.
In general, interference refers to the process of disruptive modification of a communication signal as it travels along a communication channel between the source and receiver. Interference may occur due to a variety of reasons, ranging from the addition of an external noise signal to the message signal to the interfering of adjacent channel frequencies with the message signal.
Why is interference an issue?
As we saw in the introduction, interference is considered undesirable, and engineers try to avoid it as much as possible. Some of the reasons that make it undesirable are:
- It can reduce the signal strength between the MS (Mobile Station, aka your handset) and the BTS (Base Transceiver Station, aka tower). You can read more about the components of a cellular system here.
- The chances of dropped calls increases due to the weakened signal reception.
- Increase in noise in the message signal, which can hinder the quality of the call.
- May render a system useless even with a moderate level of undesired extra load.
Types of Interference
We generally classify the concept of interference into the following types depending on how the noise signal causes a disruptive modification in the message signal.
- Cochannel Interference
- Adjacent Channel Interference
- Self Interference
- Multiple Access Interference
- Inter-symbol Interference
- Cochannel Interference (CCI) occurs in those channels that reuse the same range of frequencies.
- It causes crosstalk, a phenomenon where signals in one channel affect the ones in the other. Have you ever heard some weird bits of conversation leaking into your phone call? That’s crosstalk.
- Cochannel interference is also known as inter-cell interference.
- As we saw about frequency reuse in one of our earlier posts, no two adjacent cells can operate on the same range of frequencies. As CCI occurs only when the message signal and the unwanted signal are of the same range of frequencies, this essentially means that the transmitter transmitting the unwanted signal is located somewhere far away and is not in the adjacent cell.
- Adverse weather conditions, poor frequency use planning, which causes an overly crowded radio spectrum, and bleeding from adjacent bands are some of the reasons that lead to cochannel Interference.
How can we reduce cochannel interference in cellular communication?
- Better frequency reuse designs help in minimizing the effect of CCI.
- Another method of dealing with CCI is to employ cooperating transmitters, that transform the interference channel into a broadcast channel. Multiple such cooperating transmitters can share data traffic amongst themselves. You can read more about the mitigation of interference using transmitter cooperation here.
Adjacent Channel Interference
- Adjacent channel interference (ACI) occurs due to multiple channels that communicate in the same geographical location using neighboring ranges of frequencies.
- Extraneous power from an adjacent channel is the major cause of ACI.
- The adjacent channel interference experienced by a receiver is gauged by the Adjacent Channel Leakage Ratio (ACLR).
- The process of the power interfering with the signal in the adjacent channel is known as adjacent channel leakage.
- We use a parameter, known as Adjacent Channel Selectivity (ACS), to measure the ability of a radio receiver to receive the message signal in the presence of the noise signal from an adjacent channel.
How can we reduce adjacent channel interference in cellular communication?
- ACI is not as serious as CCI and can be easily brought to a minimum (definitely not eliminated) by managing the split of the frequency spectrum such that there exists a distinct “gap” between two adjacent channels.
- This “gap” is known as the guard band as it prevents the two ranges of frequencies on either side from interfering with each other.
- Self-interference (SI) is due to interference induced among signals that share a transmitter.
- The amount of interference induced depends on the modulation type (such as OFDM) in use.
- Non-linearities in the transmitter and receiver, such as amplifier non-linearity (where the amplifier present does not show a constant gain for a range of frequencies), may also be a source of self-interference.
- Interference between the uplink and downlink transmissions in a duplex system may also be classiﬁed as self-interference, as it occurs among signals on the same two-way connection.
How can we reduce self-interference in cellular communication?
- Self Interference Cancellation (SIC) techniques help in minimizing SI.
- Some of these techniques include employing duplex filters that manage uplink and downlink channels, and analog and digital cancellation at the transmitter.
Multiple Access Interference
- Multiple Access Interference (MAI) results from multiple users accessing the same range of frequencies on the same channel.
- In CDMA systems, adjacent cells reuse the ranges of frequencies available, and hence this type of interference is predominant in the CDMA systems.
- MAI can prove to be a significant problem if the power level of the message signal is significantly lower (mainly due to distance) than the power level of the interfering user.
- Multiple Access Interference comprises of two types of interference:
- Intracell interference – caused by users in the same cell.
- Intercell interference – caused by users in another cell due to the reuse of the same CDMA channel in the neighboring cells.
- The following image gives a pictorial representation of the concept:
How can we reduce multiple access interference in cellular communication?
- As MAI is predominantly present in CDMA systems, CDMA employs spread-spectrum technology to counter this.
- Spread-spectrum technology effectively spreads out a signal generated with a specific bandwidth, increasing the net bandwidth.
- Successive Interference Cancellation and Differential Detection are two methods that work in reducing MAI in optical CDMA. (Source)
- Another method to combat MAI is to implement a certain coding scheme where each transmitter in the same frequency channel gets a unique code.
- Inter-Symbol Interference (ISI) results from the spreading of the pulse of bits beyond its allotted time interval, which causes it to interfere with the neighboring pulses.
- In other words, inter-symbol interference is a form of distortion of a signal in which one symbol, which essentially refers to one or several transmitting bits, interferes with subsequent symbols.
- This is a threat to look out for as the previous symbols act just like external noise, making the communication less reliable.
How can we reduce inter-symbol interference in cellular communication?
Some methods to counter ISI are:
- Design systems with a shorter impulse response to prevent the energy of one symbol from overlapping with another.
- Apply a device called the equalizer at the receiver’s end, which effectively undoes the changes that occurred on the message signal during transmission.
- Apply a sequence detector at the receiver, which attempts to figure out the original sequence of the symbols.
In this post, we had a detailed overview of the concept of interference, its effects in the communication system, its types, as well as methods to counter them. The comments section awaits your queries in case you have any.