Modulation Techniques

In ADSL, there are two competing modulation schemes.

1. Carrierless Amplitude Phase (CAP)

2. Discrete Multi-Tone (DMT)

Both CAP and DMT use the same fundamental technique of Quadrature Amplitude Modulation (QAM). QAM is a bandwidth conservation modulation method commonly used in modems as discussed earlier.

1. CAP

CAP was developed and patented by AT&T. This technique can be regarded as a modified version of QAM. Since the carrier contains no information, it is suppressed before transmission and is reassembled at the receiving end, hence the term "carrierless". Suppressing the carrier has two distinct advantages:

• Less expensive digital transceiver implementation is required as the computation intensive multiplication operations needed for carrier modulation and demodulation becomes unnecessary.
  

• The absence of a carrier increases flexibility as changing the spectrum’s properties, such as the center frequency and the shape, can easily be done.

At start-up, CAP tests the quality of the twisted pair and implements the most efficient version of QAM (e.g. 16-QAM) for the channel. It is a relatively simple single carrier system and has been a popular choice for some earlier implementations of ADSL modems. It is however being replaced by the DMT method, which is superior in performance but more complex.

2. DMT

The Discrete Multi-Tone has been selected by the American National Standards Institute (ANSI), European Telecom Standards Institute (ETSI), and the International Telecommunications Union (ITU) as the "standard" for ADSL transmissions. This method is a multi-carrier (carrier spacing of 4kHz) modulation technique that separates the available bandwidth into 256 subchannels using Discrete Fourier Transform methods. Because high frequency signals on twisted pairs suffer more in the presence of noise, the transmission of data can be allocated so that the lower frequency channels are utilized more. Like CAP, the system checks the line quality at start-up to determine the capacity of each of the 256 subchannels. To minimise the overlap in the subchannels, which leads to interference, other techniques such as the Discrete Wavelet Multi-Tone (DWMT) are used to increase the isolation between adjacent channels.

DMT based ADSL modems can be thought of as 256 "mini modems" running simultaneously. Each sub-channel is modulated using QAM and can carry between 0-15 bits/symbol/Hz. The actual number of bits/symbol/Hz used is allocated depending on the line condition. Therefore, some subchannels can be operating at the maximum 15 bits/symbol/Hz, while some may be unused due to poor line conditions. The following diagram illustrates this.

 

Figure 7. DMT Source: [9]

 

Based on the above assumptions, the theoretical maximum upstream and downstream rates can be calculated as [9]:

Upstream: 25 channels x 15 bits/symbol/Hz/channel x 4kHz = 1.5 Mbps

Downstream: 249 channels x 15 bits/symbol/Hz/channel x 4 kHz = 14.9 Mbps

Advantages over CAP

During initialisation, DMT measures the signal to noise ratio S/N separately for each channel, and assigns the rate of the sub-channel accordingly. Typically, lower frequency sub-channels are utilised more than higher frequency sub-channels because signals are attenuated more at higher frequencies. This procedure allows for a better performance than CAP under practical line conditions. Another feature of DMT is Rate Adaptation, where it monitors the line conditions and dynamically adapt the transmit rate of the modem.