JPL's Wireless Communication Reference Website

Chapter: Analog and Digital Transmission
Section: Multi-Carrier Modulation, CDMA

Multi-Carrier CDMA

Multi Carrier Code Division Multiple Access (MC-CDMA) is a relatively new concept. Its development aimed at improved performance over multipath links. MC-CDMA is a modulation method that uses multi-carrier transmission (more precisely OFDM) of DS-CDMA-type signals.

This scheme was first proposed at PIMRC '93 in Yokohama by Linnartz, Yee (U. of California at Berkeley) and Fettweis (Teknekron, Berkeley, currently at U. of Dresden, Germany). Independently, Fazal and Papke proposed a similar system. Linnartz and Yee showed that MC-CDMA signals can also be detected with fairly simple receiver structures, using an FFT and a variable gain diversity combiner, in which the gain of each branch is controlled only by the channel attenuation at that subcarrier. At PIMRC '94 in The Hague, optimum gain control functions were presented. Results showed that a fully loaded MC-CDMA system, i.e., one in which the number of users equals the spread factor, can operate in a highly time dispersive channel with satisfactory bit error rate. These results appeared in contrast to the behaviour of a fully loaded DS-CDMA link that typically does not work satisfactorily with large time dispersion.

Since 1993, MC-CDMA rapidly has become a topic of research. At the keynote address of the ISSSTA conference 1996, Prof. Hamid Aghvami predicted that the hottest topic in spread-spectrum, viz. multi-carrier cdma, would attract 80% of the research by 1997. Around 2000, we see that MC-CDMA has attracted tremendous attention, with entire conference sessions devoted to this. Mc-CDMA is praised as a modulation solution that merges the insights due to Shannon (particularly those relating to CDMA) with insights due to Fourier (particularly those explaining why OFDM has advantages in a dispersive channel).

What is orthogonal MC-CDMA?

There are many equivalent ways to describe MC-CDMA:

MC-CDMA is a form of CDMA or spread spectrum, but we apply the spreading in the frequency domain (rather than in the time domain as in Direct Sequence CDMA).
MC-CDMA is a form of Direct Sequence CDMA, but after spreading, a Fourier Transform (FFT) is performed.
MC-CDMA is a form of Orthogonal Frequency Division Multiplexing (OFDM), but we first apply an orthogonal matrix operation to the user bits. Therefor, MC-CDMA is sometimes also called "CDMA-OFDM".
MC-CDMA is a form of Direct Sequence CDMA, but our code sequence is the Fourier Transform of a Walsh Hadamard sequence.
MC-CDMA is a form of frequency diversity. Each bit is transmitted simultaneously (in parallel) on many different subcarriers. Each subcarrier has a (constant) phase offset. The set of frequency offsets form a code to distinguish different users.

The MC-CDMA method described here is NOT the same as DS-CDMA using multiple carriers. In the latter system the spread factor per subcarrier can be smaller than with conventional DS-CDMA. Such a scheme is sometimes called MC-DS-CDMA. This does not use the special OFDM-like waveforms to ensure dense spacing of overlapping, yet orthogonal subcarriers. MC-DS-CDMA has advantages over DS-CDMA as it is easier to synchronize to this type of signals.

Possible Transmitter Implementation

Figure: possible implementation of an Multi-Carrier spread-spectrum transmitter.

Each bit is transmitted over N different subcarriers. Each subcarrier has its own phase offset, determined by the spreading code.

MC-Code Division Multiple Access systems allow simultaneous transmission of several such user signals on the same set of subcarriers. In the downlink multiplexer, this can be implemented using an Inverse FFT and a Code Matrix.

Figure: FFT implementation of an MC-CDMA base station multiplexer and transmitter.

MC-CDMA as a special case of DS-CDMA

Figure: possible implementation of a Multi-Carrier spread-spectrum transmitter. Each bit is transmitted over N different subcarriers. Each subcarrier has its own phase offset, determined by the spreading code.

The above transmitter can also be implemented as a Direct-Sequence CDMA transmitter, i.e., one in which the user signal is multiplied by a fast code sequence. However, the new code sequence is the Discrete Fourier Transform of a binary, say, Walsh Hadamard code sequence, so it has complex values.

Figure: Alternative implementation of a Multi-Carrier spread-spectrum transmitter, using the Direct sequence principle.

Receiver design

Because of delay spread and frequency dispersion due to multipath fading, subcarriers are received with different amplitudes. An importance aspect of the receiver design is how to treat the individual subcarriers, depending on their amplitude r_i. Options are

Linear combining, by weighting the ith subcarrier by a factor d_i according to
- Maximum Ratio Combining: d_i = r_i. This optimally combats noise, but does not exploit interference nulling. (See also MRC diversity)
- Equal Gain: d_i = 1. The simplest solution. (See also EGC diversity)
- Equalization: d_i = 1/r_i. This perfectly restores orthogonality and nulls interference, but excessively boosts noise.
- Wiener filtering: d_i = r_i/(r_i² + c). This gives the best post-combiner signal-to-noise-plus-interference ratio.
Maximum likelihood detection

Doppler

See also our page on MC-CDMA with Doppler, i.e., with rapid time variations of the channel.

What are the advantages of MC-CDMA?

Compared to Direct Sequence (DS) CDMA.
DS-CDMA is a method to share spectrum among multiple simultaneous users. Moreover, it can exploit frequency diversity, using a RAKE receiver. However, in a dispersive multipath channel, DS-CDMA with a spread factor N can accommodate N simultaneous users only if highly complex interference cancellation techniques are used. In practice this is difficult to implement. MC-CDMA can handle N simultaneous users with good BER, using standard receiver techniques.
Compared to OFDM.
To avoid excessive bit errors on subcarriers that are in a deep fade, OFDM typically applies coding. Hence, the number of subcarriers needed is larger than the number of bits or symbols transmitted simultaneously. MC-CDMA replaces this encoder by an NxN matrix operation. Our initial results reveal an improved BER.

Publications for scientific reference:

The first paper on Multi-Carrier CDMA appeared in 1993:

[93-C4] N. Yee, J.P.M.G. Linnartz and G. Fettweis, "Multi-Carrier CDMA in indoor wireless Radio Networks", IEEE Personal Indoor and Mobile Radio Communications (PIMRC) Int. Conference, Sept. 1993, Yokohama, Japan, pp. 109-113. PDF 2.8MB | PDF LInnartz PIMRC 93 MC CDMA

N. Yee and J.P.M.G. Linnartz, "Multi-Carrier in an indoor wireless radio channel", Memorandum UCB/ERL M94/6, U.C. Berkeley, 1994. http://www.eecs.berkeley.edu/Pubs/TechRpts/1994/ERL-94-6.pdf (UCB bib data) (The original U.C. Berkeley Technical report)

[94-C10] PS PDF PDF2 N. Yee and J.P.M.G. Linnartz, "Wiener filtering for Multi-Carrier CDMA", IEEE / ICCC conference on Personal Indoor Mobile Radio Communications (PIMRC) and Wireless Computer Networks (WCN), The Hague, September 19-23, 1994, Vol. 4, pp. 1344-1347.

PDF J.P.M.G. Linnartz, "Performance Analysis of Synchronous MC-CDMA in mobile Rayleigh channels with both Delay and Doppler spreads", IEEE VT, Vol. 50, No. 6, Nov. 2001, pp 1375-1387.