WO2002075933A2 - Communication system and method - Google Patents
Communication system and method Download PDFInfo
- Publication number
- WO2002075933A2 WO2002075933A2 PCT/GB2002/001260 GB0201260W WO02075933A2 WO 2002075933 A2 WO2002075933 A2 WO 2002075933A2 GB 0201260 W GB0201260 W GB 0201260W WO 02075933 A2 WO02075933 A2 WO 02075933A2
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- filter
- pole
- transmitter
- data sequence
- zero
- Prior art date
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L25/00—Baseband systems
- H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
- H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
- H04L25/03006—Arrangements for removing intersymbol interference
- H04L25/03343—Arrangements at the transmitter end
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L25/00—Baseband systems
- H04L25/38—Synchronous or start-stop systems, e.g. for Baudot code
- H04L25/40—Transmitting circuits; Receiving circuits
- H04L25/49—Transmitting circuits; Receiving circuits using code conversion at the transmitter; using predistortion; using insertion of idle bits for obtaining a desired frequency spectrum; using three or more amplitude levels ; Baseband coding techniques specific to data transmission systems
- H04L25/4902—Pulse width modulation; Pulse position modulation
Definitions
- the invention relates to a method of transmitting a predetermined data sequence to a receiver.
- the invention further relates to a communication system in which such a method is used and to a transmitting unit for transmitting such a predetermined data sequence to a receiver.
- a number of users communicate with a central station called an access point (AP) . All the users called subscriber units (SU) communicate with the AP over a shared common transmission medium.
- Wireless communication is inherently a broadcast process and consequently, only one SU can communicate with the AP at any one time if interference between SU transmissions is to be avoided.
- a mechanism has to be provided to share access to the AP via the common transmission channel in a fair and efficient manner.
- a random access scheme provides each SU with the maximum flexibility in gaining access to the channel whenever information is to be sent.
- There are a number of commonly used random access schemes which includes ALOHA and Slotted ALOHA.
- a consequence of using random access schemes is that it is inevitable that more than one.SU may- desire to transmit information at a given time. As a result, the SUs have to contend for channel access if collision is to be avoided between the transmissions arriving at the A .
- Each SU contends for channel access by transmitting a unique identifier called a contention word to the AP in a known contention frame.
- a contention word a unique identifier
- the AP can determine if more than one SU is contending for the channel at a given time. If only one SU requires access, the AP decodes the SU identifier and allocates an upcoming data frame to that SU.
- contention mechanism One consequence of this contention mechanism is that the AP cannot equalise the received contention word since equalisation at the AP would require knowledge about which particular SU is transmitting, which is something the contention mechanism is trying to establish. Consequently, any channel equalisation has to take place at the SU before the transmission of the contention word takes place.
- equalisation prior to signal transmission is commonly called pre-distortion.
- the pre-distorter may comprise a feed forward and feedback filter in reverse order to those in a standard decision feedback equaliser. Unlike the decision feedback equaliser, however, the stability of the pre-distorter is not guaranteed. This is because a decision feedback equaliser incorporates a decision device in the feedback loop which acts to limit the amplitude of the signal into the feedback filter. In a pre-distorter, no decision device is present and the signal into the feedback filter is unbounded and therefore the feedback filter is potentially an unstable element.
- One technique used to calculate the coefficients of the feed forward and feedback filters is root allocation. This technique allows a zero forcing pre-distorter to be realised for an arbitrary channel response.
- the procedure relies on separating the zeros of the channel impulse response into those that fall inside and those that fall outside of the unit circle.
- the root allocation technique has certain problems when zeros of the channel lie on, or in close proximity to, the unit circle. If the so called critical zeros lie close to but inside the unit circle, then they appear as underdamped roots in the feedback filter transfer function and- can lead to instability. If critical zeros lie just outside the unit circle, then these are absorbed into the feed forward filter transfer function and may result in a very long feed forward filter.
- the invention provides a method of transmitting a predetermined data sequence to a receiver over a transmission channel comprising the steps of; a) determining the impulse response of the channel, b) providing a pre-distortion arrangement at the transmitter, having a response that approximates to the inverse of the channel response, the pre- distortion arrangement comprising a filter having a critical pole, c) causing a zero of the predetermined data sequence to coincide with the critical pole of the filter; and d) cancelling the zero of the data sequence and critical pole from the filter.
- a critical zero can be removed from the feed forward filter- transfer function and enable a shorter feed forward filter to be realised.
- the transmitter can be arranged so that the filter in the pre-distortion arrangement has a minimum length.
- Step c) of the method may be performed by modifying the phase of the zeros of the predetermined data sequence .
- This technique is referred to in this application as root rotation.
- the unique contention word ascribed to each SU consists of 24 chips. These 24 chips convey a unique customer identifier to the access point and are transmitted as a succession of six pulse position modulation (4-PPM) symbols of length four. Consequently, only one chip in every slot of four chips is high and there exists 4 6 which equals 4096 distinct identification words. These identification words are amplitude encoded only and consequently, no information is carried in their phase. As a consequence, the phase of the contention word maybe varied without altering the information at the receiver. This enables the zero of the contention word to be aligned with a critical pole of the filter.
- 4-PPM pulse position modulation
- the invention further provides a transmitter for transmitting a predetermined data sequence to a receiver over a transmission channel, the transmitter comprising a pre-distortion arrangement having a response that approximates to the inverse of the channel response, the pre-distortion arrangement comprising a filter having a critical pole removed by causing a zero of the predetermined data sequence to coincide with the critical pole of the filter and cancelling the pole-zero pair, and means for transmitting the predetermined data sequence modified by the pre-distortion arrangement.
- the predetermined data sequence comprises ⁇ chips of pulse position modulated (PPM) symbols of length b and the PPM symbols are encoded in overlapped pulse position modulated (OPPM) form as (jb,c)-OPPM symbols, where c is an integer greater than one
- the phase of the pulse shape of the OPPM symbol may be modified so that a zero of the pulse shape coincides with a critical pole of the filter and the filter characteristic is modified by cancelling the pole-zero pair.
- Figure 1 shows a communication system according to the invention
- Figure 2 illustrates the contention process between two subscriber units seeking access to the access point
- Figure 3 shows a pre-coder for use in the SUs of Figure 1
- Figure 4 illustrates a general pre-distorted communication system
- Figure 5 is a modification of the system of Figure 4 in which one method according to the invention is illustrated
- Figure 6 shows the zeros of a predetermined data sequence and a critical pole of the pre-distortion filter
- Figure 7 shows the format of PPM and (4,2) -OPPM symbols
- Figure 8 shows a predetermined data sequence encoded as (4, 2) -OPPM symbols
- Figure 9 shows the predetermined data sequence before root rotation, after root rotation, and after passing through the pre-distortion filter, and shows the filter length before and after critical pole removal.
- FIG. 1 shows in block schematic form a wireless point to multipoint transmission system.
- the transmission is frequency division duplex (FDD) , that is the AP transmits on one frequency and the SUs transmit on a different frequency, and adaptively modulated either as QPSK or QAM depending on the distance, transmitter power, and quality of the radio link.
- the traffic is managed by time division multiple access.
- a number of users communicate with a central station 1.
- the central station 1 is referred to as an access point (AP) .
- the users communicate with the AP by means of subscriber units (SU) .
- the subscriber units communicate with the AP via transmission paths 3-1, 3-2, ... 3-N.
- Wireless communication is inherently a broadcast process and consequently only one SU can communicate with the AP at any one time if interference between SU transmissions is to be avoided, since the SUs all transmit at the same frequency. Therefore, a mechanism is provided to share • access to the AP via the common transmission channel in a • fair and efficient manner.
- the information transmitted from the SU to the AP occurs in bursts, since an SU will only transmit when a subscriber wishes to communicate, and the most efficient means of sharing access to the common channel is by random access techniques.
- a random access scheme provides each SU with maximum flexibility in gaining access to the channel whenever information is to be sent.
- contention for channel access between SUs invariably occurs. That is, more than one SU may wish to transmit information at a given time and hence, these SUs must contend for access to the channel if a collision is to be avoided between the transmissions arriving at the AP.
- each SU contends for channel access by transmitting a unique identifier called a contention word to the AP in a known contention frame.
- a contention word a unique identifier
- the AP can determine whether more than one SU is contending for the channel . If only one SU requires access, the AP decodes the SU identifier and allocates an upcoming data frame to that SU.
- the registration word is amplitude encoded only and no information is carried in the phase. Other SUs maybe simultaneously contending for access to the channel.
- the PPM format of the contention word allows the access point to detect the presence of two or more contending users.
- contention mechanism One consequence of this contention mechanism is that the AP cannot equalise the received contention word since equalisation at the AP would require knowledge about which particular SU is transmitting and this is something the contention mechanism is trying to establish. Consequently, any channel equalisation has to take place at the SU before transmission of the contention word takes place.
- equalisation prior to signal transmission is commonly called pre-distortion but may also be referred to as pre- coding.
- FIG. 2 illustrates a contention frame where two subscribers are both contending for access.
- Subscriber 1 has the identifier 043233 while subscriber 2 has the identifier 202231.
- the AP can detect that there are two chips high in at least some of the four chip slots and can use this information to determine that more than one subscriber is seeking access at a particular time. In this case, the AP will not know which two subscribers are contending merely that there are two contending subscribers at the same time and consequently, neither will be allocated a transmission channel.
- two SUs are contending for a transmission channel there must be a difference between the two unique identifiers otherwise they would not be unique. Consequently, at least one of the six time slots will have more than one high chip .
- FIG. 3 shows the general arrangement for pre-distorting or pre-coding the contention word at the SU.
- the pre- codes comprises an input 30 which is connected to a first input of a summing arrangement 31 whose output is connected to the input of a feed forward filter 32 and to the input of a feedback filter 33.
- the output of the feedback filter is connected to a second input of the summing arrangement 31 while the output of the feed forward filter 32 is connected to an output 34 of the pre- coder.
- the pre-coder comprises a feed forward (FF) 32 and feedback (FB) 33 filter in reverse order to those in a standard decision feedback equaliser (DFE) .
- DFE standard decision feedback equaliser
- a DFE incorporates a decision device in the feed back loop which acts to limit the amplitude of the signal into the feedback filter.
- no decision device is present and the signal into the feedback filter is unbounded and therefore, the feedback filter is potentially an unstable element.
- One technique used in calculating the coefficients of the feed forward and feedback filters is root allocation.
- the root allocation technique allows a zero forcing pre-coder to be realised for an arbitrary channel response. The procedure relies on separating the zeros of the channel impulse response into those that fall inside and those that fall outside the unit circle. For an estimated channel impulse response which has a z-transform H(z) root finding is used to factorise H(z) into two components as follows;
- H(z) H 1 (z ⁇ 1 (z)
- K_ ⁇ z) consists of all the routes of H(z) lying inside the unit circle (stable roots) and H 2 (z) consists of all the roots of H(z) lying outside the unit circle (unstable roots) .
- a new polynomial H 3 (z) is formed by reversing the order of the coefficients of H 2 (z) .
- H 3 (z) ⁇ h n z ⁇ n + h n _ lZ - n+1 + + z ⁇ x + h 0 ]
- the polynomial H 3 (z) has routes at the reciprocal locations to the routes of H 2 (z).
- the feedback filter 33 of the pre-coder is set equal to
- the feed forward filter 32 of the pre-coder is set equal to
- H x (z) H 3 (z) has all its roots inside the unit circle the feedback filter is now stable.
- the disadvantage with this root allocation method is that it is not always possible to cleanly separate- the zeros of H(z) into those outside the unit circle and those inside the unit circle. Many channels have some zeros lying on or very ' close to the unit circle. These zeros near the unit circle are sometimes known as critical zeros .
- Critical zeros cause problems with root allocation because they result in an unstable feedback filter if they are placed in the feedback filter transfer function or else require a very long feed forward filter if placed in the feed forward filter transfer function. This problem has made the root allocation method unsuitable for channels with critical zeros .
- MMS ⁇ minimum mean square error
- the zeros of the feedback filter are not always located within the unit circle.
- the magnitude of the input to the feedback filter will increase exponentially with time irrespective of the length of the input sequence.
- One indicator of an unstable feedback filter implementation is the presence of any feedback filter coefficients with a magnitude larger, than unity.
- An unstable implementation can, however, still result even when all the feedback filter coefficients have a magnitude smaller than unity and the only definite means of identifying feedback filter instability is to determine the exact location of the zeros of the feedback filter polynomial, that is root finding.
- the pre-distortion filter now has a transfer function of;
- a polynomial expansion for 1 can be calculated readily by long division.
- D 2 (z) This is because the zeros of D 2 (z) can lie outside the unit circle and under those circumstances long division results in a polynomial with unbounded coefficients.
- a stable implementation can, however, be obtained by reversing the coefficients of D 2 (z) before performing the long division, and then reversing the coefficients of the resulting quotient afterwards. This procedure is explained, for example, in the textbook entitled Equalizer for Digital Modems by A. P. Clark, published by Pentech Press Ltd ISBN 0-7273-0504-2 at pages 169-173.
- the final polynomial expression for F(z) is obtained by multiplying N(z) by the polynomial expression for 1 and __1 ⁇ _ ( Z ) D 2 (z) obtained through long division.
- any critical zeros act to increase the length of F(z) (in fact F(z) will have infinite length if any zero is located on the unit circle) . It has been shown that pre-distortion based on either root finding or the MMSE algorithm suffer from two problems which arise because of the presence of critical roots. First, the length of the pre-distortion filter approaches infinity as any of the roots of the channel impulse response approach unity and, secondly, the amplitudes of the signal out of the pre-distortion filter may become large.
- the present invention provides a method of eliminating critical zeros in the filter characteristic. This is achieved by taking advantage of the key properties of the registration word, that is it has finite length and is not phase sensitive. These properties- are utilised to force pole zero cancellation between the pre-distortion filter and the registration word thereby removing a number of critical zeros.
- Figure 4 illustrates a general pre-distorted transmission system.
- the pre-distortion filter F(z) if calculated according to the root allocation algorithm will have a response which approximates to the inverse of the channel response H(z) .
- the filter F(z) is assumed to have a length n and to have (n-1) poles which correspond to the positions of the zeros of H(z). That is,
- the input registration word (z) has a length m with a z- transform
- W(z) may alternatively be expressed in terms of its zeros
- the pole removed from F(z) happens to be a critical pole then it may be possible to reduce the length of the resulting pre-distortion filter such that the length of a pre- distortion filter given by F(z) (1- ⁇ z "1 ) may have a significantly reduced length.
- the following description discloses two techniques for the removal of critical poles from the pre-distortion filter by pole zero cancellation.
- the first technique which may be called root rotation removes one critical zero by modifying the phases of all the zeros of the registration word.
- Figure 6b illustrates the superposition of the zero 62 on the pole 61.
- w ⁇ are the roots of W(z) and ⁇ is the angle of rotation.
- Figure 6 shows the results of pole-rotation for an example Rician channel of:
- This channel has a Rician K factor of 2.0 and a critical zero located just inside the unit circle (the magnitude of the critical root is 0.989) .
- Figure 9b shows the magnitude of the registration word before (solid line) and after (dashed line) root rotation and removal has taken place. Note that the registration word after root rotation, although still amplitude limited to unity, bears little resemblance to the original word.
- Figure 9c results.
- the received signal is shown in Figure 9a. As expected, the root rotation and removal process has not affected the amplitude information of the received signal.
- Figure- 9d shows the impulse response of the pre-distortion filter (dashed line) when root rotation and pole-zero cancellation has taken place.
- impulse response of the pre-distortion filter has reduced from 148 samples to a mere 9 samples when causing the impulse response to fall to 5% of the peak values.
- Figure 8 shows an example of a (4,2) -OPPM based registration word corresponding to the identification code 132300. This longer OPPM symbol length increases the total length of the registration word by 6 (L+N-l) chips, The final length in chips for various OPPM formats is given in the table below.
- W reg (z) contains the positional information of the registration word
- both a and b will have magnitudes close to unity, it is possible for the second term to have a magnitude as large as two. To transmit such a pulse shape will require a corresponding back off in transmitter power to avoid any signal clipping at the transmitter.
- the invention may be performed using other PPM arrangements, for example the use of higher-order PPM schemes, such as 8-PPM or 16-PPM has advantages.
- a higher-order PPM scheme results in a contention word with a greater number of root positions, so allowing critical filter poles to be cancelled more accurately.
- the increased number of root positions does increase the computational complexity of the root-finding process.
Abstract
Description
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Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
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AU2002241117A AU2002241117A1 (en) | 2001-03-16 | 2002-03-15 | Communication system and method |
Applications Claiming Priority (2)
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GB0106604.2 | 2001-03-16 | ||
GB0106604A GB2373420B (en) | 2001-03-16 | 2001-03-16 | Communications system and method |
Publications (2)
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WO2002075933A2 true WO2002075933A2 (en) | 2002-09-26 |
WO2002075933A3 WO2002075933A3 (en) | 2002-11-21 |
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PCT/GB2002/001260 WO2002075933A2 (en) | 2001-03-16 | 2002-03-15 | Communication system and method |
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AU (1) | AU2002241117A1 (en) |
GB (1) | GB2373420B (en) |
WO (1) | WO2002075933A2 (en) |
Families Citing this family (7)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
WO2003013087A2 (en) | 2001-07-26 | 2003-02-13 | Cambridge Broadband Limited | Predistorting of contention signals |
GB2384666B (en) * | 2002-01-25 | 2005-02-23 | Cambridge Broadband Ltd | Method and apparatus for predistorting data |
EP1550280A1 (en) * | 2002-10-08 | 2005-07-06 | M/A-Com, Inc. | Apparatus, methods and articles of manufacture for pre-emphasis filtering of a modulated signal |
US7340007B2 (en) | 2003-09-16 | 2008-03-04 | M/A-Com, Inc. | Apparatus, methods and articles of manufacture for pre-emphasis filtering of a modulated signal |
US7519129B2 (en) * | 2002-12-12 | 2009-04-14 | Cambridge Broadband Networks Limited | Precoding of contention words in a fixed wireless access system |
JP4295050B2 (en) * | 2003-09-09 | 2009-07-15 | 株式会社エヌ・ティ・ティ・ドコモ | Communication system, transmitting station and receiving station |
US9231805B2 (en) | 2012-07-09 | 2016-01-05 | Telefonaktiebolaget L M Ericsson (Publ) | Device for carrier phase recovery |
Citations (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
WO2000049779A1 (en) * | 1999-02-19 | 2000-08-24 | Adaptive Broadband Ltd. | Stabilized precoder for data transmission |
-
2001
- 2001-03-16 GB GB0106604A patent/GB2373420B/en not_active Expired - Lifetime
-
2002
- 2002-03-15 WO PCT/GB2002/001260 patent/WO2002075933A2/en not_active Application Discontinuation
- 2002-03-15 AU AU2002241117A patent/AU2002241117A1/en not_active Abandoned
Patent Citations (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
WO2000049779A1 (en) * | 1999-02-19 | 2000-08-24 | Adaptive Broadband Ltd. | Stabilized precoder for data transmission |
Also Published As
Publication number | Publication date |
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WO2002075933A3 (en) | 2002-11-21 |
AU2002241117A1 (en) | 2002-10-03 |
GB2373420A (en) | 2002-09-18 |
GB0106604D0 (en) | 2001-05-09 |
GB2373420B (en) | 2004-04-14 |
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