Overcoming interferences in a duplex RF system

Maintaining a simultaneous two-way communication - talking and listening during a call - is a complex process.

Southeast Asia’s mobile phones market grew by 24% in volume from July 2011 to June 2012, with smartphone accounting for 61% of the market. Millions of consumers today are talking and listening on their mobile devices, downloading data and surfing the web. Consumers have come to expect clear signals and good caller connectivity when making a simple phone call, yet few have thought about the innovation that made it possible. However, maintaining a simultaneous two-way communication - talking and listening during a call - is, in fact, a complex process.

When a person is communicating to someone else over a mobile phone, information is constantly flowing both ways via the receiver and transmitter. The simultaneous, two-way flow of voice and data is made possible by duplex communications systems, which combine multiple transmit and receive channels on a shared antenna. Such systems are widely deployed especially in telephony and computer networking.

The transmitting and receiving frequencies are usually very close in a duplex communications system. To minimise interference between the channels, it is important for telecom operators to isolate the different channels properly.

Challenges with duplex communications systems

An inherent side effect of the modern receiver design is receiver desensitisation. This happens when a strong off-channel signal appears at the input to a receiver. As these signals pass through the multiple stages of frequency reduction, the lower-frequency signal becomes unduly weakened, thus impeding the receiver’s ability to receive it.

The second challenge is transmitter noise, which is usually caused by carrier signals just outside of a transmitter’s assigned frequency. Ideally, a transmitter should channel 100% of its signal power into the narrow band of frequencies assigned to its transmission channel. In the real world, this level of precision is simply not possible and a small fraction leaks into channels above and below the intended carrier frequency.

Isolating the interference

The key to overcoming receiver desensitisation and transmitter noise is isolation. This can be done by first examining the interference from both sources and then identifying the optimal isolation level that will assure reliable operation of paired communications devices.

Once the isolation level has been determined, operators can start looking at one of two methods to achieve the desired outcome.

The first method is to use two antennas that are physically separated by a given distance. This can be done by separating the transmitter and receiver either horizontally or vertically at an appropriate distance to achieve the required level of isolation.

Alternatively, telecom operators can consider using an appropriate duplexer in a single-antenna system. A duplexer replaces one of the two antennas and two lengths of coaxial cable by allowing both transmitter and receiver to operate at the same time, on the same antenna.

This option reduces the needed infrastructure by half and brings about significant cost benefits to telecom operators. For example, using a duplexer, operators only need to lease one tower space instead of two, cutting down expensive leasing costs by half. Additionally, telecom operators can be assured that the duplexer reliably isolates transmitter and receiver regardless of external circumstance or terrain. A duplexer also eliminates the need for telecom operators to worry about the coverage area needed to place the transmitter and receiver antennas.

Modern two-way communications networks must contend with the interfering effects of both receiver desensitisation and transmitter noise. Faced with the challenges of space, cost and antenna availability, operators should not ignore the benefits offered by duplexers. The isolating properties offered by a duplexer allow both transmitter and receiver to operate efficiently while reducing transmitter noise and receiver desensitisation. With a duplexer installed in a single-antenna system, operators achieve a compact, efficient and reliable communications network that delivers a high quality of service for consumers.