Multicarrier Modulation, MCM Tutorial
- multicarrier modulation, MCM is being used increasing as a modulation format for high data rate transmissions. OFDM is the most popular format, but there are other technologies and techniques that are now becoming viable.
Multicarrier modulation, MCM is a technique for transmitting data by sending the data over multiple carriers.
Multicarrier modulation techniques are particularly beneficial because when the data rates increase, so wider bandwidths are needed. When this happens, different frequencies within the bandwidth are subject to different path lengths and different fading conditions. This can distort the transmission making it difficult to copy. MCM provides a way of increasing the bandwidth whilst still being able to tolerate the varying fading conditions present.
A further advantage of multicarrier systems is that they are less susceptible to interference than single carrier system as interference may only affect a small number of the carriers.
Multicarrier modulation basics
Multicarrier modulation operates by dividing the data stream to be transmitted into a number of lower data rate data streams. Each of the lower data rate streams is then used to modulate an individual carrier.
When the overall transmission is received, the receiver has to then re-assembles the overall data stream from those received on the individual carriers.
It is possible to use a variety of different techniques for multicarrier transmissions. Each form of MCM has its own advantages and can be sued in different applications.
Development of MCM
The history of multicarrier modulation can said to have been started by military users. The first MCM were military HF radio links in the late 1950s and early 1960s. Here several channels were sued to overcome the effects of fading.
Originally the concept of MCM required the use of several channels that were separated from each other by the use of steep sided filters of they were close spaced. In this way, interference from the different channels could be eliminated.
However, multicarrier modulation systems first became widely used with the introduction of broadcasting systems such as DAM digital radio and DVB, Digital Video Broadcasting which used OFDM, orthogonal frequency division multiplexing. OFDM used processing power within the receiver and orthogonality between the carriers to ensure no interference was present.
Later OFDM was used for systems such as wireless / cellular telecommunications and networking standards including WiMAX, Wi-Fi 802.11, and also LTE, the Long Term Evolution for cellular systems.
Also other cellular systems have used multicarrier techniques to achieve high data rates by using two or more carriers from a standard cellular system. Dual Carrier HSPA is one example.
With new networking and cellular systems on the horizon, other multicarrier techniques have been investigated and their use seems likely in the near future.
Multicarrier modulation systems
There are many forms of multicarrier modulation techniques that are in use of being investigated for future use. Some of the more widely known schemes are summarised below.
- Orthogonal frequency division multiplexing, OFDM: OFDM is possibly the most widely used form of multicarrier modulation. It uses multiple closely spaced carriers and as a result of their orthogonality, mutual interference between them is avoided. Read more about OFDM
- Generalised Frequency Division Multiplexing, GFDM: GFDM is a multicarrier modulation scheme that uses closed spaced non-orthogonal carriers and provides flexible pulse shaping. It is therefore attractive for various applications such as machine to machine communications.
- Filter Bank Multi Carrier, FBMC: FBMC is a form of multicarrier modulation scheme that uses a specialised pulse shaping filter known as an isotropic orthogonal transform algorithm, IOTA within the digital signal processing for the system. This scheme provides good time and frequency localisation properties and this ensures that inter-symbol interference and inter-carrier interference are avoided without the use of cyclic prefix required for OFDM based systems. Read more about FBMC
By Ian Poole
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