Data transmission by concurrent relative phase coding in multiple spectral carriers

(54) Data transmission by concurrent relative phase coding in multiple spectral carriers (57) The bandwidth of an analogue link can provide a high throughput of digital data transmission if direct spectral synthesis/analysis is applied. The present invention provides a method and apparatus for data transmission, whereby a waveform composed of a plurality of sine pilots and carriers is generated and transmitted as a sequence of frames. Each frame carries a group of data bytes. Pilots and carriers are interleaved across the spectrum. The bound pilots are used to delimit the frames. Phase coding of the carriers is relative to the bound pilots and provides the high phase resolution required in the receiver, and robustness against phase distortion of the channel.

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Data transmission by concurrent relative phase coding in multiple spectral carriers.

This invention relates to a method and apparatus for data transmission by means of concurrent relative phase coding and decoding in multiple spectral carriers.

There are a number of well known methods and techniques for data transmission. These methods operate by coding the input data to analogue form, transmitting it over an analogue medium (eg. telephone line or radio waves), and converting it back to digital form at the receiving end. The apparatus used for implementing these methods is generally known as a modem.

Inefficient use of bandwidth in conventional, time domain modems results in low data throughput per bandwidth allocated for transmission.

The spectrum of the frequencies within the transmission bandwidth can be utilised much more efficiently by applying direct spectral synthesis/analysis and by the use of associated Digital Signal Processing.

The present invention provides a method and apparatus for data transmission. A waveform composed of a plurality of sine frequencies is generated and processed. Sine components are divided into groups of pilots and carriers, positioned in an interleaved order across the spectrum of the channel's bandwidth. The phases of the pilots are used to delimit the frames. The phases of the carriers are used to carry input data through the channel.

The sequence of input data is divided into groups of bytes. Each byte in the group is coded on its corresponding carrier, and the group of bytes is transmitted as one frame of a synthesized waveform, containing all of the carriers simultaneously. In the receiver, the group of bytes is recovered from the frame of carriers and output in its original order.

The specific embodiment of the invention will now be described by way of example, with reference to the accompanying drawing in which : Figure 1 shows a block diagram of the data transmission system.

Figure 2 shows a block diagram of the transmitter.

Figure 3 shows a block diagram of the receiver.

Figure 4 shows a bound pilot, consisting of two closely spaced sine waveforms.

2Figure 5 shows how the distance between the window and the start of the frame is recovered by means of the phase relationship in the bound pilot.

Figure 6 shows the interleaved positioning of the pilots and carriers across the spectrum.

Figure 7 shows the phase coding circle. Its values are used to translate an individual data byte to the phase shift of the corresponding carrier.

Figure 8 shows a perspective view of the spectral components at the start of the transmission frame. The frame is idle, since no data is encoded on the carriers.

Figure 9 shows a coded transmission frame, carrier lines illustrate phase shifts carriers. Deflection is relative to the plane determined by the bound pilots.

Deflections of the of the respective imaginary vertical

Figure 10 shows the analysed spectrum of the received window. It is the spectrum from Fig. 9 after having crossed the transmission channel.

Figure II shows the work of a decoder. The dotted line illustrates the interpolated start of the frame, and the shaded angles on that line represent the recovered original phase deflections of the carriers.

Figure 12 shows the decoding phase circle in the receiver.

Referring to the drawing, the apparatus comprises the transmitter (1) and receiver (2). In the transmitter there is an input buffer (3), carrier phase coder (4), pilot insertion module (5), sine waveform reference (6), waveform synthesizer (7), and the line driver (8).

The receiver comprises the signal conditioning module (9), window spectrum analyser (10), frequency component interpreter (11), pilot normalisation module (12), frame contour interpolation module (13), carrier phase decoder (14), and output buffer (15).

The phase of the sine waveform is building up constantly throughout the time of the waveform generation. Phase, like time, is a relative quality and incorporates the concept of a distance being measured with reference to a certain starting point.

If two frequencies are spaced closely (i.e. a fraction of an octave apart, Fig. 4), the phase difference between them is growing steadily and slowly. The speed of the phase gain of the higher frequency w2 (Fig. 5) over the lower frequency wl is equal to w2-wl, where w = 2PI/T (rad/sec), and T is a period of a sine waveform (sec). The time taken for this phase creep to make a full cycle (angle of 2PI) is equal to Tc = 2PI/(w2-wl).

closely spaced The generation phase equal to frame. The time the bound pilot of the frame is is possible to by measuring the s in a narrow

5).

In the present invention such a pair of frequencies is referred to as a bound pilot of either frequency in the pair starts with zero at the beginning of the transmission length of the frame and the frequencies of are designed in such a way that the duration less than the cycle Tc. In such a frame it determine the starting point unambiguously phase difference between component frequencie window placed anywhere within the frame (Fig.

This property is used to determine the start of the frame in the receiver, whereby only that portion of the frame contained in the window is utilised for spectral analysis. In particular, the time distance from the window to the start of a frame is equal to Pd/(w2-wl), where Pd is a phase difference (Δ0) measured in the window between two components of the bound pilot (Fig. 5).

The use of bound pilots is necessary because of the nature of spectral analysis in the receiver, whereby a sliding window which delimits data for analysis is at an unknown (random) distance from the beginning of the transmission frame.

The receiver has to determine the start of the frame in the received signal, so that the relevant measurements of the phase coding in the carriers can be made. To enable the unambiguous determination of the frame start a plurality of bound pilots are positioned across the spectrum. The purpose of this arrangement is to provide a local reference for carrier frequencies in the neighbourhood of the bound pilots, so as to achieve higher phase resolution and robustness against local phase delays incurred by the transmission channel.

In an attempt to utilise effectively the entire spectrum of the transmission channel, there is provided a plurality of carriers spaced across the spectrum and interleaved with bound pilots. Fig. 6 shows an example of ten carriers interleaved with three bound pilots.

starts at the of a carrier the individual

There are a finite number of values for this initial The phase circle of the carrier is divided into a of phase coding points (Fig. 7). The size of the carrier frequency . The initial phase coding the data on

The generation of the beginning of the frame waveform is used for carrier phase. number sector dividing adjacent coding points depends on the particular design, resolution of the apparatus and properties of the transmission channel.

Fig. 9 shows the coding of several carriers in relation to the bound pilots. Coded phases are represented by the shaded angles.

In this example there are 257 coding points present on the phase circle. 256 points enable the 8-bit data bytes to be coded on a single carrier. One additional coding point is reserved for coding the idle carrier condition, i.e. for carriers that are not coded when fewer data bytes than the full capacity of the frame were presented for transmission.

Since each frame has a plurality of carriers, it has the capacity to carry simultaneously a number of data bytes, one byte per carrier. Data to be transmitted is collected in the input buffer (3), and each data byte is converted to the corresponding value of the phase offset, via the phase circle. Phase values, obtained from the phase circle, are allocated to respective carriers in module 4.

Initial phase offsets for all carriers are ready at the beginning of the frame. The waveform synthesizer (7) creates a composite waveform containing all bound pilots and all carriers. Module 6 provides sine reference values for the synthesizer. The synthesizer generates the waveform throughout the duration of the frame. At the beginning of the next frame, the initial phases are readjusted according to the new set of input data, and the next frame is generated. A succession of frames is generated continuously.

The synthesized waveform is passed on from the synthesizer (7) to the line driver (8). The line driver converts the waveform to a form appropriate to the transmission medium, and passes on the waveform to the transmission channel. This may be an analogue medium, pulse code modulation link, optical fibre, radio waves etc.

As the composite signal is transmitted through the channel it is subject to phase distortion, and this is dependent on frequency. Phase distortion causes an uneven phase shift of the spectral components within the frame. This phase shift affects both carriers and bound pilots. However, as a result of their interleaved arrangement, pilots are shifted together with the adjacent carriers and act as a 'floating reference'. In relation to the local pilots, the phase of the adjacent carriers is affected much less than their absolute values. This feature provides the high resolution of the phase required in the receiver.

In the receiver, the signal is received from the transmission· channel by module 9. This module also provides antialias filtering, converts the analogue signal to the sampled digital form, and provides a suitable conditioning of the signal, eg. equalisation and echo cancellation.

In module 10 the sampled waveform is divided into a succession of windows, and each window is subject to spectral analysis. The length of the window is designed in such a way that it is a fraction of the length of the transmission frame. In this example, the window length is 3 ms (frame - 10ms), and the 1024 point Fast Fourier transform is performed by an array of eight TMS320-C25 Digital Signal Processors, operating at a speed of 10 MIPS (Million Instructions Per Second) each. Spectral analysis provides the results in polar form, i.e. magnitude and phase of the individual frequencies.

The phases of the individual components are meaningless at this stage, since they contain a distortion due to transmission channel delays and window positioning. It is the task of subsequent modules to process phases of the components and recover the carried data.

The phases of all components, i.e. pilots and carriers are passed on to the component interpreter (11). This module determines the role of each spectral component obtained from the window analysis and passes it on to either of the two' subsequent modules. Pairs of frequencies determined as bound pilots are passed on to the frame contour reference module (12), and carriers are passed on to the decoding module (14).

Straight boundaries of the individual frames across the spectrum do not exist in the received window, due to uneven phase distortion (Fig. 10). In addition, the position of the start of the frame with reference to the analysis window is unknown. Several steps are therefore required to recover data (Fig. 11). .Initially, through the local bound pilot the distance is calculated as the time between the start of the frame and the present position of the window. The calculation is made from the two known frequencies of the bound pilot and their present phases, as shown in Fig. 5.

This procedure is repeated in module 12 for each bound pilot. Distances are different (eg. tl and t2, Fig. 11), since the phase distortion is a function of frequency. Once the calculations have been completed, the resultant distances create the set of starting points that delineate the starting contour of the received frame (see dotted line, Fig. 11).

From this set of points the frame contour is interpolated in more detail in module 13, in particular the starting positions for all carrier frequencies. In the present example the interpolation between two adjacent bound pilots is linear, but other functions can be used if a more accurate result is required.

For each carrier, its phase is unwrapped in module 14 from the current phase in the window back over the time distance to the interpolated starting point. The resultant unwrapped starting phase corresponds to the original value that was coded on this carrier in the transmitter (eg. shaded angles on the dotted line, Fig. 11).

The recovered starting phase of the carrier, is mapped on the phase circle (Fig. 12), and the angular sector determines the value of the data byte received on this carrier.

Data bytes recovered from the individual carriers are passed . on to the output module 15. They are buffered in the slots pertaining to the number of the carrier from which they were recovered. Some carrier bytes are recovered in the early windows, others in subsequent windows, due to the uneven delay of the portions of the frame. However, after the analysis of the last window in the frame all data bytes are recovered from the carriers.

Once buffering is complete, module 15 outputs data bytes in the original format and order.

The maximum throughput of data in the present example is 100 kilobits/sec in the voice grade 3kHz telephone channel, and can be calculated as follows: one frame contains 125 carriers, each carrying an 8-bit input data byte. Therefore one frame carries 1000 bits. The duration of the frame is 10 ms, and there are 100 contiguous frames per second, giving the total throughput of 100,000 bits per second.