JPL's Wireless Communication Reference Website

Chapter: Analog and Digital Transmission


Multi-Carrier Modulation

Multi-Carrier Modulation (MCM) is the principle of transmitting data by dividing the stream into several bit streams, each of which has a much lower bit rate, and by using these substreams to modulate several carriers.

The first systems using MCM were military HF radio links in the late 1950s and early 1960s. A special form of MCM, Orthogonal Frequency Division Multiplexing (OFDM), with densely spaced subcarriers with overlapping spectra of the modulating signal, was patented in the U.S. in 1970. OFDM abandoned the use of steep bandpass filters that completely separated the spectrum of individual subcarriers, as it was common practice in older Frequency Division Multiplexing systems (e.g. in analogue SSB telephone trunks) and still occurs in (asynchronous) Frequency Division Multiple Access (FDMA) private radio systems. Instead, OFDM time-domain waveforms are chosen such that mutual orthogonality is ensured even though spectra may overlap. It appeared that such waveform can be generated using a Fast Fourier Transform at the transmitter and receiver. For a relatively long time, the practicality of the concept appeared limited. Implementation aspects such as the complexity of a (Fast) Fourier Transform, appeared prohibitive, not to speak about other problems, such as the stability of oscillators in transmitter and receiver, the linearity required in RF power amplifiers, and the power back-off associated with this.

After many years of further intensive research in the 1980's, today we appear to be on the verge of a breakthrough of MCM techniques. Many of the implementational problems appear solvable and MCM has become part of several standards.

Progress in Multi-Carrier Modulation

MCM benefited from considerable research interest for the military applications, but certainly to a much lesser extent than direct-sequence (DS) spread spectrum. The current overwhelming attention to spread spectrum and Code Division Multiple Access (CDMA) can at least partly be explained by many years of active exploration of this field in military labs. CDMA led to new insight in communication theory, that proved extremely valuable for finding reliable and efficient transmission methods suitable for adverse, i.e., both dispersive and time-varying, mobile communication channels with severe limitations by interference. DS-CDMA and MCM have in common that investigation into both schemes heavily relies on the insight in communications provided by Shannon, in particular, in his "geometric" theory, considering waveforms to be a point in a Euclidean space, allowing definitions of orthogonality.

It was known from earlier experiments with wireless data transmission that the selection of the modulation technique is highly critical. In the early days of mobile communications, many attempts to connect a telephone modem to a cellular phone failed miserably, mainly because of the poor anticipation to the mobile channel anomalies. Although entrepreneurs rapidly recognized the demand for wireless data communications, experiments and product tests rapidly revealed that the mobile fading channel needed specific solutions, for modulation scheme, bit rates and the packet length. Among the many proposals, Multi- Carrier Modulation appeared one of the most elegant solutions for wireless digital transmission at high symbol rate. The signal waveform used for Multi- Carrier transmission has intriguing properties. The rapid increase in digital signal processing power in (software programmable) radio receivers has opened the way for large scale use of this idea. Nowadays this method is considered to be particularly suitable for high-performance digital radio links.

Special Forms of MCM

Coded MCM

Multi-Carrier Modulation on its own is not the solution to the problems of communication over unreliable multipath channels. The channel time dispersion will excessively attenuate some subcarriers such that the throughput on these sub-channels would be unacceptable small. Only if the joint signal of many subcarriers is processed appropriately, the diversity advantages of MCM can be exploited.

MC-CDMA

Multi-Carrier Code Division Multiplexing or Multiple Access (MC-CDMA) is another interesting approach is to spread the signal over different subcarriers. It uses DS-CDMA merely for multiplexing, but chooses the signal waveforms using the OFDM principle. Signals to different users are added linearly onto a multiplex of Multi-Carrier CDMA signals.

MCM Applications

MCM has the elegant waveform properties that make it useful for a wide variety of applications. In particular, we mention digital transmission over telephone lines, applications in broadcasting and in wireless LANs.

Digital Transmission over the Telephone System

MCM has been successfully tested for digital transmission at high rates over the twisted-pair telephone subscriber loop. It is proposed for the Asymmetric Digital Subscriber Loop (ADSL).

Digital Audio Broadcasting

 

The concept of Orthogonal Frequency Division Multiplexing (OFDM) for Digital Audio Broadcasting (DAB) goes back to the end of the 1980s. Nowadays, experimental systems are in operation, and introduction to the mass-audience is soon to follow. It may substantially improve mobile reception of radio broadcasts. Chipsets for DAB are now being developed in the European JESSE project, a necessary step towards mass production of receivers at low price.

Marketeers previously noticed that the introduction of a new technology is often more successful if it replaces an old existing service than if it only offers previously unknown services. But in our personal opinion, improved mobile radio reception by itself may not guarantee the real breakthrough of digital broadcasting. A lesson learned from High-Definition Television (HDTV) was that quality of reception is not the main motivation for customers to replace their equipment. However novel multi-media oriented services can easily be introduced once DAB is operational, which may appear to be a substantial market.

Presumably, one of the reasons to choose OFDM as the DAB standard was the possibility to deploy single frequency networks. As MCM is robust against fading caused by natural multipath, it can also work if signals are received from two different transmitter sites: the mutual interference is simply experienced as artificial multipath reception. This possibility guarantees very efficient use of scarce radio spectrum, particularly if nationwide coverage is aimed at.

Digital Television

Developments in MPEG-2 video encoding showed that good quality television signals can be distributed over a channel of 3 to 8 Mbit/s. Multi-Carrier signals with FFT sizes on the order of 2k to 8k points have been proposed as the Digital Video Broadcasting (DVB) standard to ensure reliable mobile reception of digital Terrestrial Television broadcasting (DTTB).

As far as we can see, business plans for the introduction of DTTB appear not yet to be fully worked out. Mobile reception of television broadcasts may not justify the enormous costs of replacing the broadcast infrastructure: within DVB, marketing interests appear to focus on provision of television services over cable, the twisted pair telephone subscriber loop, or directly via satellite.

Wireless Local Area Networks

MCM was not adopted as a standard for the European HIgh PErformance Radio LAN (HIPERLAN), presumably because of the need for highly linear RF amplifiers which are difficult to build with the limited power available for PCMCIA add- ons. However, HIPERLAN II and the Wireless 1394 standards adopted OFDM transmission.

The deregulation of the use of Industrial Scientific and Medical (ISM) bands for communication allows many manufacturers to develop their own wireless equipment, independent of the time-consuming standardization processes, that are mostly dominated by commercial interests of a few major players. It is particularly in this field that many small companies in the U.S. are researching novel modulation methods and improved access methods, including OFDM and MC-CDMA.



JPL's Wireless Communication Reference Website Jean-Paul M.G. Linnartz, 1993, 1995.