GB2376858A

GB2376858A - Pulse-based communication system

Info

Publication number: GB2376858A
Application number: GB0115446A
Authority: GB
Inventors: Nicholas Charles Lee
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 2001-06-23
Filing date: 2001-06-23
Publication date: 2002-12-24
Anticipated expiration: 2021-06-23
Also published as: GB0115446D0; GB2376858B; GB0414839D0; GB2401016B; GB2401016A

Abstract

An ultra wideband impulse radio system conveys information by slight delays and advances in the timing of successive very short impulses from their expected times in a low duty cycle. The expected times are varied to according to time hopping codes to further spread the power spectrum over the frequency range reducing interference to other systems and providing multiple code channels. During transmission pulses are spread in time due to multi-path interference effects. To recover the delay and advance information received pulses are correlated 6 with a pulse shape template 5 expected from a pulse that has travelled through the current multi-path environment. The template pulse is determined by averaging previously received pulses. The template pulse is captured and stored using a time-hopping version of a sampling method used by sampling oscilloscopes, and regenerated at an expected time of a pulse to be received using a cyclic-FIR circuit.

Description

Pulse-based communication system This invention relates to impulse based radio systems and has particular but not exclusive application to Ultra Wide band radio systems.

Impulse-based radio systems can operate by conveying information by virtue of variability of successive pulses in a signal. It includes Ultra Wideband (UWB) which is a based on the transmission of discrete very narrow pulses, typically of sub-nanosecond duration. Typically the duty cycle of transmission of such pulses is in the order of 1%. If transmitted on a regular basis the signal would generate a line spectrum. The lines of the spectrum could interfere with the operation of any other radio systems whose RF bandwidth encompassed the frequency of one of theses lines. In order to mitigate this effect, a time hopping code is commonly introduced. This provides a pseudo random variation in the times at which the pulses are transmitted within the nominal available period. This results in many more spectral lines at closer spacing and with lower power. The line spacing is then related to the length of the time hopping code. The use of time hopping codes allows several users to co-exist in the same bandwidth; the receiver will expect a pulse at a particular time and only listen for a pulse in a narrow window of time. If a different time hopping code is devoted to each link then interference between two links will take place only when the time of the received pulse from the interfering transmitter coincides with the time of the received pulse from the wanted transmitter. If synchronised orthogonal codes are used then this will never happen. In a practical, real world,

environment this is impossible to achieve and codes with good cross correlation properties over a wide range of time offsets are used.

The pulses may be modulated in several different ways. Typically pulse position modulation is used wherein the time of transmission of the pulses is varied from the expected time of arrival. The pulse is transmitted either with a slight delay or advance and so the receiver will determine this and will register a"0"or"I", depending on a late or early pulse; hence digital information can be conveyed. The reception problem therefore becomes one of determining the precise timing of the received pulse, or more specifically determining whether a received pulse arrived earlier or later than the nominal timing as specified by the time hopping code.

In practical UWB communications links, the pulse radiated from the transmitter not only has a direct path to the receiver; it also reaches the receiver via reflections off objects in the environment. This is known as multi-path interference. With UWB systems, this multi-path interference affects the received pulse by spreading the pulse shape (typically a 0. 5ns Gaussian monocycle), into a series of pulses whose average amplitude decreases over a relatively long time span (e. g.

100nus). This means that the energy of the transmitted pulse is spread over time and therefore the informational content of the signal is also spread out over time. A typical pulse in a real system can be represented by that shown in Figure 1, as shown it extends over a finite period and comprises a number of ripples. In known systems the received waveform is correlated with a single Gaussian monocycle to enable the

detection as to whether the pulse is slightly late or early. In practice therefore this correlation is over a short portion of the pulse and the correlation essentially looks for the first zero-crossing point i. e. the first cycle as shown by arrow A. The problem with this method is that the extra ripples contain energy and therefore information and such correlation loses about 12 dB in signal to noise ratio.

To recover this informational content the inventor has determined that it is advantageous to measure not only the direct path element of the received pulse, but also all of the multi-path reflected elements i. e. over the entire waveform or a substantial portion thereof. In other words it would be advantageous to correlate the received pulse with a template pulse that had a shape that corresponded to a pulse that had travelled through the current multi-path reflecting environment.

The invention comprises, in an impulse based communication system, a method of extracting information from a signal comprising pulses, by correlating received input pulses to a representative pulse in synchronisation and over a substantial portion of the pulse duration Moreover the inventor has devised an apparatus which maks it possible to implement this.

Preferably the representative pulse used in said correlation has a resolution of less than 5 nanoseconds, preferably less 2 nanosecond and preferably less than 1 nanosecond or better. The term"representative pulse"may be alternatively referred to as a"model pulse"or"pulse

template" ; the important point is it is a stored pulse shape which is correlated with an incoming pulse.

A further problem is that the characteristics of the pulse change over time. Thus if the receiver moves, the shape of the pulse tend to change as well. For example, the channel characteristics would change if the transmitter or receiver were moved, or if any nearby RF reflecting object in the environment were moved. The inventor has determined that the pulse template should advantageously be adaptive and change to match the transmission link's channel characteristics. Preferably therefore previously received input pulses are used to obtain the pulse template. For example the pulse template may updated by being formed from the averaged shape of the last 10 or so received pulses. Thus the use of an adaptive pulse generator.

To generate the template pulse, there are two processes. The first process is to sample the pulses at the receiver to determine their shape.

The second process is to reconstruct that pulse shape at a time determined by the time hopping code sequence and to mix that constructed pulse with a received pulse for correlation purposes.

The invention will now be described in more detail by means of example only and with reference to the following figures of which.

Figure 2 shows a simplified block diagram of one embodiment of the invention.

Figure 3 shows a diagrammatic representation of how incoming pulses can be sampled to provide the template pulse.

Figure 4 shows an example of one implementation of the pulse template generator component of figure 2.

Figure 5 shows an example of a further preferred implementation of the pulse generator component of figure 2.

Example 1 Basic configuration of overall system Figure 2 shows the basic components of one example of a system according to an embodiment of the invention. The system comprises an antenna 1 which is input via switch 2 to a scanning receiver 3. The scanning receiver operates as described above and effectively scans successive pulses so as to provide a pulse template. A necessary input to the scanning receiver is from a time hopping sequence generator 4 which tells the scanning receiver when to expect the next pulse. The scanning receiver generates the pulse shape (template) and updates it on a regular basis. The template or pulse representation is then fed into a pulse template output generator 5 which in the example is a Finite Impulse Response Filter (FIR) which stores the template as a series of coefficients or alternatively termed"tap values". The job of the FIR is to output at the correct time the template pulse so as to be correlated with an incoming pulse. Hence the FIR has an input from the time hopping sequence generator. Operation of the FIR will be described in

more detail below. The system also includes a correlator 6 which is shown in general by a dotted line and includes a mixer 7 and an integrator 8. This correlator functions to correlate the received pulse and pulse template; the latter of which is timed to be output for correlation in order to ascertain whether the incoming pulse is slightly delayed or advanced. In other words in normal operation with the switch in position"b", the pulse from the antenna arrives at the mixer either slightly before or slightly after the time that the pulse from the FIR filter gets to the mixer. This time difference will depend upon whether a 0 or a I is being transmitted. The time hopping code sequence generator acts to zero the integrator ready for each pulse. This integrator integrates the output of the mixer over time. This is then thresholded to give a decision output of a 1 or a 0.

The switch has 2 positions"a"and"b". When the switch is in position "b", the scanning receiver samples pulses from the antenna and the resultant waveform shape is then stored and passed onto the FIR as a set of tap values, as described above. For calibration purposes, the switch can be put in position"a". The pulses produced by the FIR are then fed back into the scanning receiver and the actual output of the FIR

compared with the previously stored waveform shape sampled from the antenna. By comparing these two waveform shapes, the FIR tap values can be modified until the two waveforms match. This feedback system can compensate for non-linearity in the FIR filter electronics and in the delay times between each stage of the FIR.

Components of the system will now be explained in more detail below.

Generation of Pulse Template To capture the shape of a received pulse, a scanning receiver is used. The scanning receiver uses a time hopping code sequence generator to

supply the variable T which is the time between one pulse and the next successive pulse. The pulse is sampled at a time increments of 1 : +8, where 8 is a small increment, i. e. a small fraction of the pulse duration.

1 is of course variable where a time hopping code is used. Fast but repetitive waveform shapes are captured by sampling the pulse amplitude at successive pulses but at steadily increasing offsets. This slowly builds up an image of the pulse shape, and is similar to techniques common to many designs of oscilloscope. Figure 3 which represents 5 successive pulses. Samples of values are taken at five points a, b, c, d, and e and these are used to build the representative pulse. This overcomes the problem of sampling the same signal pulse at 5 time instants which is practically impossible due to the short duration of the pulse.

Pulse Constructor Having obtained the average shape of the received pulses, the next task performed by the system of figure 2 is to reconstruct that pulse at a time indicated by a time hopping code sequence generator. It is extremely difficult to reproduce representative pulse quickly especially at a precisely defined time for the purpose of correlating with an incoming pulse. Various methods of achieving this with varying effectiveness are now discussed below:

a) Digital to Analogue Converter Where the values representing the pulse shape (template) are stored in memory it is not practical to output them on a digital to analogue converter (DAC) as the pulse is so short in duration that a DAC would not be quick enough. b) FIR This may be implemented using also e. g. a finite impulse response filter FIR. The taps of the FIR can be initially taken directly from the output of the scanning receiver as they are already in the required form. Figure 4 shows a representation of a FIR filter. This comprises a number of mixers 7 input to which are a series of coefficients (bO, b I, b2,.....) representing the pulse template i. e. values of amplitude with time. The mixers also have an input from one of a series of time delays 10. These delays are extremely short and can be implemented by flip flops. They are cascaded to form a bank. In operation the input Xn is a pulse which ripples along the time delay bank. In this way the output value y is successively bo, bi, b, b3, b4, bs factored by the input along the x line.

The equations defining this filter are as follows.

It can be seen from the above equations that where Xn consists of a unit impulse at n=0 and is zero thereafter and the FIR taps bo to bq are the samples from the scanning receiver, then the output Yn will be a reconstruction of the required template pulse waveform. The delays are implemented as discrete digital elements and the propagated impulse is mixed (i. e. multiplied) by the analogue tap values.

The disadvantage of the above such conventional FIR filter implementation for this application is the large number of taps required.

Each filter tap requires its own digital to analogue converter and mixer, adding to the cost and power usage to the design.

For a typical UWB implementation that uses a 0. 5ns impulse and ringing of up to 128ns, this would require a FIR filter with 256 mixers and digital to analogue converters. c) Novel pulse template generator.

To remedy the above problems, a novel design has been developed by the inventor. This enables practical implementation of the invention whereby generation of the analogue representation of the pulse signal is possible which overcomes the problems of using a conventional FIR circuit.

Figure 5 shows an example of a system according to an example of the novel filter design to reconstruct the pulse signal, where the pulse signal comprises 288 discrete values varying with time. It has some similarities to the conventional FIR of figure 2 but there are only 16

taps comprising a series of 16 time delays and mixers. To each mixer is an input from a DAC, 11, of which there are 16, each DAC having an input from a memory, 12.

In operation, as a pulse ripples down the set of time delays, as input into

a counters so that at only one of the points 10, Ii, 12, 13, 14. is there is a voltage present, such that only one at a time the value b from each memory (via the DAC) is output onto the integrator and thus the output therefrom, yin. on the first pass therefore, the first address contents of each memory are thus output to line Yn in consecutive order. This process is repeated such that the contents of the second address of each memory are thus output to line Yn in consecutive order, etc. At the end of the time delay bank the pulse inputs to a 4 bit counter, 13. This counts the number of times a pulse has ripple down the time delay bank.

This value C is also fed into the each of the memories. This procedure will now be described in more detail below.

The reset procedure for this circuit would be to preload the counter with 0 and to clock each of the 16 memory chips so that their first address contents are presented at their outputs, then to increment the counter to 1. Each of the 16 memory chips has 16 address locations that are filled with the filter tap values. MemO contains taps 0,16, 32... 240, Meml contains taps 1,17, 33... 241 and so on up to Meml5 which contains taps 15,31, 47... 255.

The impulse now arrives at X and proceeds to travel through the discrete (non-clocked) delays. The impulse at 10 is multiplied by bo the

output of D/AO and fed to Y via the summer. The D/AO output voltage bo is set by the binary value from the first memory's first address contents. When the impulse travels through the delay to 11 it then becomes multiplied by bo and fed to Y via the summer.

The impulse at Ii also triggers MemO to output the memory contents whose address is C to D/AO. At this point C is 1. This process continues until the impulse reaches 115. After this delay, the impulse is fed into the counter. It is also fed back to the beginning of the FIR filter. By the time that the impulse has travelled through all 16 delays and back to the beginning, the first memory and D/A have had time to set their output voltage. The counter ensures that the impulse travels around the loop 16 times only. After completing a full sequence of 16 loops, the memory contents and loop counter are automatically set to the correct values for the next impulse to arrive at X. This system arrangement conveys 256 tap values to the output Y in response to a single input impulse from the time hopping code sequence generator whilst only using 16 delays, mixers, D/As and memories. Due to the looping structure, the response times required of each individual D/A and memory are divided by 16 relative to that required by a system that feed all the taps to a single stage of D/A and memory. The looping structure also uses 16 times less hardware than a conventional FIR filter implementation. These characteristics act to reduce the cost and power consumption of the circuitry.

It can be seen from this application specific example that different division ratios could be chosen to trade off number of hardware

elements versus speed of operation of each of those taps. If slower hardware elements are desired, then more elements must be used and the size of the loop counter correspondingly reduced. Conversely, if less hardware is required, then fewer but faster elements may be used and the loop counter increased so that the impulse travels through each of the elements more times.

The principle advantages of the invention are a large proportion of the received radio energy is used to determine the informational content of the system. This confers increased signal processing gain and therefore system efficiency, unlike existing systems. Compared to a fixed filter system, these techniques permit adaptive matching to the changing multi-path reflection environment. This process maintains system efficiency even if the communications link is mobile, unlike other solutions. The core technology of the cyclic-FIR filter circuit permits the precise time synchronised generation of fast, arbitrary time varying waveform shapes without requiring the ultra high-speed electronics that would be required for a conventional FIR or single A/D implementation. This technology is not restricted to the given ultra wideband receiver application.

Claims

Claims 1. In an impulse based communication system, a method of extracting information from a signal comprising pulses, by correlating received input pulses to a representative pulse in synchronisation and over a substantial portion of the pulse duration.
2. A method as claimed in claim I wherein said representative pulse used in said correlation has a resolution of less than 5 nanoseconds or better.
3. A method as claimed in claim I wherein said representative pulse used in said correlation has a resolution of less than 2 nanoseconds or better.
4. A method as claimed in claim 1 wherein said representative pulse used in said correlation has a resolution of less than 1 nanoseconds or better.
5. A method as claimed in any of the above claims wherein said correlation is used to ascertain any time delay or advance of the pulse from the expected arrival time.
6. A method claimed in any preceding claim wherein said system is a radio system and said signal is a radio signal.

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7. A method as claimed in any preceding claim wherein previously received input pulses are used to obtain the representative pulse.
8. A method as claimed in any preceding claim wherein said correlation is performed at time interval dependent on particular timing sequence
9. A method as claimed in any preceding claim wherein the representative pulse is determined by taking samples of amplitude from a number of previously received pulses.
10. A method as claimed in any of the above wherein said representative pulse is stored as a set of discrete values.
11. A method as claimed in claim 10 wherein the representative pulse is stored as claimed in claims 23 or 24 below.
12. A method as claimed in claim 10 wherein said representative pulse is output according to any of the methods of claims 25 to 27 below.
13. An impulse based communication system having means to extract information from a signal comprising pulses comprising means to correlate received input pulses to a representative pulse in synchronisation over a substantial portion of the pulse duration.

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14. A system as claimed in claim 13 wherein said representative pulse has a resolution of 2 nanoseconds or better.
15. A receiver for implementing any of the above methods as claimed in claims 1 to 12 comprising an antenna to receive said signal containing pulses, an scanning receiver connected to said antenna adapted to take values of successive input pulses to provide a representative pulse, means to output said representative pulse to a correlator to be correlated with an incoming pulse in sychronisation.
16. A receiver as claimed in claim 15 wherein said synchronisation is provided by a time sequence hopping generator.
17. A receiver as claimed in claim 15 or 16 wherein the scanning receiver has an input from a time sequence hopping generator.
18. A receiver as claimed in claims 15 to 17 wherein said means to output said representative pulse comprises an apparatus as claimed in claims 21 or 22 below.
19. A receiver as claimed in claims 15 to 18 having means to feed the representative pulse output to the correlator to also the scanning receiver in order to reduce non-linearities.
20. A receiver as claimed in claim 19 wherein said reduction of non-linearities is by means of pre-distortion.

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21. An apparatus for rapidly outputing a waveform template, said waveform being stored as a set of n discrete values comprising: a) a number of memory units, each memory unit storing a number q of said discrete values and where n =pq ; b) a number of DAC's each connected to its corresponding memory unit; c) a number p of time delays arranged in series; d) a number p of mixers, each mixer having an input from one of said DAC's and an input from an individual connection adjacent a time delay and e) an integrator having an input from each mixer and whose output is representative of the waveform template to be output; f) means to send a pulse to ripple q times along the time delay bank such that in operation, the analogue value representing successive discrete values are output one at a time from each memory via said DAC's to the integrator, and after each ripple or loop this operation is repeated until all discrete values have been output and wherein no adjacent (successive) discrete values are stored on the same memory unit.
22. An apparatus as claimed in claim 21 including a counter which determines the amount of said repeats, this counter having an input into said memories.
23. A method of storing a signal shape template comprising storing a number n of discrete and consecutive values mi = in,, m2, m3..... min}, on a number p of memory units such that each

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consecutive (adjacent) discrete value is stored units in adjacent (consecutive) memories.
24. A method of storing a signal shape template comprising storing a number n of discrete and consecutive values nmi, mz, m3..... mon}, on a (ring of) number p of memory units such that each memory stores a number q of said discrete values, said values being

mz, where i are separated by (mz+p, mz+2p, mz+3p,....... mz+qp) where z represents the memory unit number.
25. A method of outputing a series of discrete values representing a waveform in consecutive order, comprising outputting a single value from each of a set of memories in turn, via a DAC, from the first to the last memory, and then repeating this until all of said discrete values have been output consecutively.
26. A method as claimed in claim 25 wherein the switching between the memories is performed by sending a pulse to ripple through a series of time delays, allowing the output one of said discrete value from one memory unit at a time.
27. A method as claimed in claim 26 wherein the output of said discrete unit from each memory unit is performed by the output from each memory being connected via its said corresponding DAC to a mixer, each mixer being connected to different points in said time delay array.