Evolution from "V.dot" Modem Technology

In many ways, ADSL modems are an extension of current analog modem technologies. Shared features include:

Each of these will now be discussed with relations to V.dot modems and ADSL modems. The use of the Discrete Multi-Tone Modulation method in ADSL will be discussed on the next page.

Quadrature Amplitude Modulation (QAM)

With QAM, two independent messages are each separately modulated onto orthogonal versions of the same carrier frequency (e.g. sine and cosine)

A QAM signal can be represented by:

Where x (t) and y (t) are called the in-phase and quadrature phase components respectively. QAM is a form of multilevel signaling, and a 4-symbol QAM is simply called 4-QAM. In essence, QAM modulates both the amplitude and phase of the carrier signal. We represent QAM signals diagrammatically using Constellation Maps. For 4-QAM, its constellation map would look like:

 

Figure 5. 4-QAM Constellation Map

 

The bit values for this are:

Bit Values

Amplitude

Phase

00

1

45o

01

1

135o

11

1

225o

10

1

315o

Table 3. Bit values for 4-QAM

 

Clearer, by increasing the constellation size, we get a corresponding increase in bits/symbol resulting in a better utilisation of bandwidth. However, due to channel noise and/or interference, increasing the constellation size decreases the accuracy associated with determining the correct point on the map. This is because as more and more symbols appear on the constellation map (e.g. 64-QAM), the amplitude and phase difference between adjacent symbols become smaller and ambiguities can occur during deciphering of symbols in presence of noise. The following diagram illustrates this:

Figure 6. Ambiguity in 64-QAM. Source: [9]

Multi-Dimensional Trellis Coding

From the above scenario, it follows that increasing the distance between symbols decreases the ambiguities of detecting the correct symbols. This is exactly what Trellis Coding attempts to do. This method introduces redundant bits into the bit stream, and expands the constellation map without increasing the bandwidth. This advancement was first implemented in V.32 modems and has been used in all other V.dot modems eversince. An improvement on this is 2-D Trellis coding, which in effect doubles the constellation size by introducing redundancy in 2D space. However, for large constellations, as the constellation size increases, ambiguities in deciphering are re-introduced. It is therefore desirable to add redundancy to the bit stream without increasing the constellation size. Taking this one step further, if the redundant bits were distributed in multi-dimensional space, then the number of bits/symbol mapped to a 2D space would be less. Hence it is possible to introduce redundancy without doubling the constellation size. 4D Trellis coding is considered the best compromise between complexity and performance. For more information on Multi-Dimensional Trellis Coding, please refer to textbooks on coding and modulation.

Echo Cancellation

Echo cancellation modems have overlapping upstream and downstream channels. Echo cancellation is necessary to minimise the reflections hence allowing the modem to achieve full duplex. See explanation on previous page.

Handshaking protocol

Handshaking allows two modems to negotiate features and mode parameters. (E.g. for V.34 modems, the handshake protocol is V.8). Features include: which modulation, error correction, and data compression to use and at what speed. The handshake protocol for ADSL is G.hs defined by the ITU. This protocol will allow two ADSL modems to negotiate the most suitable parameters and features to use during transmission.

Training

During initialisation, one modem transmits a known bit stream to another modem. Using the received bit stream and comparing it with the original, the modem is able to determine the line condition and compensate for any non-linearity. Non-linearities are caused by the equipment and by the twisted pair. Higher frequencies get attenuated more than lower frequencies in the wires. Since ADSL modems are "always connected", it will be necessary to perform training periodically. Training consumes time, especially for modems with high transmit rates. Therefore, fast training procedures are critical for ADSL modems since they reach speeds of up to 8Mbps. Additional requirements will be necessary to support ADSL modems without a POTS splitter.