WO2004019545A1 - Method and apparatus for minima enlargement - Google Patents

Method and apparatus for minima enlargement Download PDF

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Publication number
WO2004019545A1
WO2004019545A1 PCT/US2003/025529 US0325529W WO2004019545A1 WO 2004019545 A1 WO2004019545 A1 WO 2004019545A1 US 0325529 W US0325529 W US 0325529W WO 2004019545 A1 WO2004019545 A1 WO 2004019545A1
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WIPO (PCT)
Prior art keywords
symbols
modulated signal
threshold
minima
adjustment
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PCT/US2003/025529
Other languages
French (fr)
Inventor
Darrell James Stogner
Richards S. Young
Karthik Narasimhan
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Motorola, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Publication date
Application filed by Motorola, Inc. filed Critical Motorola, Inc.
Priority to AU2003262689A priority Critical patent/AU2003262689B2/en
Priority to IL16148903A priority patent/IL161489A0/en
Priority to EP03793064A priority patent/EP1540873A4/en
Publication of WO2004019545A1 publication Critical patent/WO2004019545A1/en
Priority to IL161489A priority patent/IL161489A/en

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    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00Modulated-carrier systems
    • H04L27/26Systems using multi-frequency codes
    • H04L27/2601Multicarrier modulation systems
    • H04L27/2614Peak power aspects
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00Modulated-carrier systems
    • H04L27/26Systems using multi-frequency codes
    • H04L27/2601Multicarrier modulation systems
    • H04L27/2626Arrangements specific to the transmitter only
    • H04L27/2627Modulators
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L5/00Arrangements affording multiple use of the transmission path
    • H04L5/02Channels characterised by the type of signal
    • H04L5/06Channels characterised by the type of signal the signals being represented by different frequencies

Definitions

  • the present invention relates generally to a method and apparatus for minima enlargement.
  • Wireless telecommunication systems sometimes are divided into a series of cell areas covering a service area.
  • Each cell area has a transmitting base station using an operating frequency set comprising a plurality of radio channels to communicate with mobile subscribers.
  • Each channel represents an information signal at a particular frequency carrier or band.
  • the channels can all be combined by a broadband signal combiner into a multi-subchannel signal at lower power levels and then amplified by a single linear amplifier (or its equivalent, a plurality of linear amplifiers in parallel, each amplifying a reduced power version of the same multi-carrier subchannel signal) to raise the multi-subchannel signal to an appropriate transmit power level.
  • a single linear amplifier or its equivalent, a plurality of linear amplifiers in parallel, each amplifying a reduced power version of the same multi-carrier subchannel signal
  • Another application of multi-subchannel technology is to split the single, high symbol rate modulation into a plurality of lower rate sub-channels that each has a low enough modulation bandwidth to avoid the need for an adaptive equalizer.
  • Highly linear multi-subchannel modulations have large dynamic ranges where the minimum voltage can approach -50 dB or lower from the signal mean. This will essentially take the linear amplifier used to increase the power of the signal prior to transmission down to idle bias current and cause a spike in the load impedance with unpredictable results.
  • supply modulation techniques used to increase the linear amplifier efficiency if the supply voltage approaches zero, the linear amplifier could have problems with phase, noise and stability.
  • floor clamp circuits have been employed to prevent the voltage from dropping below a specified threshold.
  • FIG. 1 illustrates a block diagram of a transmit modem in accordance with the present invention
  • FIG. 2 illustrates a pictorial representation of minima enlargement in accordance with the present invention.
  • the present invention discloses an improved method and apparatus for minima enlargement in a modulator.
  • the present invention allows control of the compression in accordance with the type of information being sent and will tailor symbols on multiple subchannels, including empty subchannels, in a controlled, individualized manner to improve the minima to average power ratio.
  • FIG. 1 illustrates a block diagram of the transmit modem with the iterative minima enlarger.
  • the block diagram comprises an unmodulated symbols input 100, a frequency division multiplexing ("FDM") modulator 102, a minima enlarger 104, and a summation component 106.
  • the unmodulated symbols, X(k,m) 100 is the complex baseband symbol matrix with k selecting the frequency axis, and m selecting the time axis.
  • the FDM modulator 102 modulates symbols according to the following equation:
  • x(n) the 7 ⁇ th sample of the modulated output
  • g the pulse shaping filter
  • N s the pulse shaping filter length (in units of symbol periods)
  • I the filter interpolation rate
  • M the number of subchannels.
  • the minima enlarger 104 detects signal minima and computes an adjustment matrix that when combined with the unmodulated symbols, X(k,m), 100 enlarges the signal above the minima threshold, M p .
  • the signal minima is defined as any output sample whose magnitude is less than a specified minima threshold, M p .
  • the minima threshold, M p can either be predetermined or dynamically adjusted based on system needs. When the signal drops below the minima, the problems discussed in the background manifest themselves.
  • the adjustment matrix of the present invention brings the signal above the threshold and avoids the aforementioned problems.
  • the summation component combines the unmodulated symbols, X(k,m),
  • the minima enlarger 104 must find the ⁇ (k,m) such that:
  • FIG. 2 pictorially illustrates minima enlargement.
  • the magnitude of x(p) 200 is close to the origin, and hence the output of the linear amplifier is no longer reliable.
  • ⁇ p 202, the phase of X ⁇ (P) 204 is the same as the phase of x(p)200. Making all contributions towards X ⁇ (P) 204 coherent in this direction minimizes the magnitude.
  • the pulse-shaping filter is acting as a weighting function to the symbol adjustment matrix. Moving symbols that align with large filter coefficients have a greater impact on the magnitude than others.
  • the pulse-shaping filter coefficients themselves can be used to weight the changes.
  • ⁇ (k,m) it is noted that the various symbol types have different importance. For example, in most applications pilot and synchronization symbols should be altered much less than data symbols.
  • W(k,m) as a matrix the same dimension as X(k,m) that contains weights for each entry in X(k,m). The complete equation for ⁇ (k,m) is given by:
  • w r (n,m) is defined as the one-dimensional LFFT of W(k,m) along the frequency axis:
  • the present invention provides a method and apparatus for receiving a plurality of symbols, and modulating the plurality of symbols to create a first modulated signal.
  • the first modulated signal is then compared to a threshold. If the first modulated signal is above the threshold, the first modulated signal is transmitted. If the first modulated signal, however, is below the threshold, an adjustment to the received plurality of symbols is computed and added to the received plurality of symbols to create an adjusted plurality of symbols.
  • the adjusted plurality of symbols is then modulated to create a second modulated signal.
  • the second modulated signal is transmitted if it exceeds the threshold; otherwise, the steps of computing, adding and the second step of modulating are repeated until the modulated signal exceeds the threshold.
  • the present invention provides a method and apparatus for receiving a plurality of symbols, mixing the plurality of symbols using an inverse fast Fourier transform to create a plurality of mixed symbols, and pulse shaping the plurality of mixed symbols to create a first modulated signal.
  • the first modulated signal is then compared to a threshold. If the first modulated signal is above the threshold, the first modulated signal is transmitted. If the first modulated signal is below the threshold, an adjustment to the plurality of mixed symbols is computed and added to the plurality of mixed symbols to create an adjusted plurality of mixed symbols.
  • Pulse shaping is performed on the adjusted plurality of mixed symbols to create a second modulated signal.
  • the second modulated signal is transmitted if the second modulated signal exceeds the threshold; otherwise, the steps of computing, adding and the second step of pulse shaping are repeated until the modulated signal exceeds the threshold.

Abstract

A modulator (102) modulates a plurality of symbols to create a modulated signal. A transmitter is coupled to the modulator (102). The transmitter transmits the modulated signal if the modulated signal is above a threshold. A minima enlarger (104) is coupled to the modulator (102). The minima enlarger (104) computes an adjustment to the plurality of symbols if the modulated signal is below the threshold. A summer (106) is coupled to the modulator (102) and the minima enlarger (104). The summer (106) adds the adjustment to the plurality of symbols if the modulated signal is below the threshold.

Description

METHOD AND APPARATUS FOR MINIMA ENLARGEMENT
Field of the Invention
The present invention relates generally to a method and apparatus for minima enlargement.
Background of the Invention
Wireless telecommunication systems sometimes are divided into a series of cell areas covering a service area. Each cell area has a transmitting base station using an operating frequency set comprising a plurality of radio channels to communicate with mobile subscribers. Each channel represents an information signal at a particular frequency carrier or band.
In many instances it is advantageous to combine these channels for transmission purposes. The channels can all be combined by a broadband signal combiner into a multi-subchannel signal at lower power levels and then amplified by a single linear amplifier (or its equivalent, a plurality of linear amplifiers in parallel, each amplifying a reduced power version of the same multi-carrier subchannel signal) to raise the multi-subchannel signal to an appropriate transmit power level. As data rate requirements rise, the symbol rate necessary in forthcoming protocols will cause the modulation bandwidth to exceed the coherence bandwidth of the channel. This requires an expensive equalizer at the receiver to compensate for intersymbol interference created by the time dispersion in a multipath channel. Another application of multi-subchannel technology is to split the single, high symbol rate modulation into a plurality of lower rate sub-channels that each has a low enough modulation bandwidth to avoid the need for an adaptive equalizer. Highly linear multi-subchannel modulations have large dynamic ranges where the minimum voltage can approach -50 dB or lower from the signal mean. This will essentially take the linear amplifier used to increase the power of the signal prior to transmission down to idle bias current and cause a spike in the load impedance with unpredictable results. In addition, with recent supply modulation techniques used to increase the linear amplifier efficiency, if the supply voltage approaches zero, the linear amplifier could have problems with phase, noise and stability. In the past, floor clamp circuits have been employed to prevent the voltage from dropping below a specified threshold. This results in frequency domain splatter and a reduction in linearity of the signal. Furthermore, different types of signals can tolerate different amounts of distortion and thus different amounts of compression. Prior-art techniques have not attempted to tailor symbols on multiple subchannels, including empty subchannels, in a controlled, individualized manner to improve the minima to average power ratio.
Clearly then, a need exists for an improved method and apparatus for minima enlargement.
Brief Description of the Figures
A preferred embodiment of the invention is now described, by way of example only, with reference to the accompanying figures in which:
FIG. 1 illustrates a block diagram of a transmit modem in accordance with the present invention; and FIG. 2 illustrates a pictorial representation of minima enlargement in accordance with the present invention.
Detailed Description of the Preferred Embodiment
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to each other. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate corresponding elements.
The present invention discloses an improved method and apparatus for minima enlargement in a modulator. The present invention allows control of the compression in accordance with the type of information being sent and will tailor symbols on multiple subchannels, including empty subchannels, in a controlled, individualized manner to improve the minima to average power ratio.
Turning to the figures, FIG. 1 illustrates a block diagram of the transmit modem with the iterative minima enlarger. As illustrated, the block diagram comprises an unmodulated symbols input 100, a frequency division multiplexing ("FDM") modulator 102, a minima enlarger 104, and a summation component 106. The unmodulated symbols, X(k,m) 100, is the complex baseband symbol matrix with k selecting the frequency axis, and m selecting the time axis. The FDM modulator 102 modulates symbols according to the following equation:
Figure imgf000004_0001
where, x(n) = the 7ϊth sample of the modulated output; g = the pulse shaping filter; Ns = the pulse shaping filter length (in units of symbol periods); I = the filter interpolation rate;
D = the filter decimation rate; and
M = the number of subchannels.
The inner sum of equation (1) performs the mixing of the subchannels, and the outer sum of equation (1) performs the pulse shaping and rate change as described in greater detail in U.S. patent 6,134,268, titled "Apparatus for Performing a Non- Integer Sampling Rate Change in a Multichannel Polyphase Filter," which is herein incorporated by reference. It is known in the art that equation (1) can be efficiently implemented using an inverse fast Fourier transform ("IFFT") to perform the mixing as shown in equations (2) and (3) below: x(n) = (2)
Figure imgf000005_0001
where xr(n,m) is given by:
Figure imgf000005_0002
It is important to note that the present invention is not limited to any particular method for performing FDM modulation, but rather, it should be obvious to those skilled in the art that the present invention can utilize any of the many available methods of performing FDM modulation and still remain within the spirit and scope of the present invention.
The minima enlarger 104 detects signal minima and computes an adjustment matrix that when combined with the unmodulated symbols, X(k,m), 100 enlarges the signal above the minima threshold, Mp. The signal minima is defined as any output sample whose magnitude is less than a specified minima threshold, Mp. The minima threshold, Mp, can either be predetermined or dynamically adjusted based on system needs. When the signal drops below the minima, the problems discussed in the background manifest themselves. The adjustment matrix of the present invention, however, brings the signal above the threshold and avoids the aforementioned problems. The summation component combines the unmodulated symbols, X(k,m),
100 with an adjustment computed by the minima enlarger component 104.
Let us now focus the discussion on the minima enlarger component 104. When the minima enlarger component 104 detects a minima at output sample p with phase of φp, as described by the following equation,
Figure imgf000005_0003
it computes an additive symbol adjustment to X(k,m) called Δ(k,m) such that passing X'(k,m) = X(k,m) + Δ(k,m) through the FDM modulator would result in x'(p) ≥MP, where x'(p) is given by the following equation:
Figure imgf000006_0001
Noting that the system is linear, the contribution of Δ(k,m) to x'(p) can be treated separately:
Figure imgf000006_0002
The minima enlarger 104 must find the Δ(k,m) such that:
Figure imgf000006_0003
In practice, this may be accomplished by adjusting the minima such that
Figure imgf000006_0004
=
Mp-a, with a > 1. That is:
Figure imgf000006_0005
FIG. 2 pictorially illustrates minima enlargement. First, it should be noted that the magnitude of x(p) 200 is close to the origin, and hence the output of the linear amplifier is no longer reliable. Second, it should also be noted that φp 202, the phase of XΔ(P) 204, is the same as the phase of x(p)200. Making all contributions towards XΔ(P) 204 coherent in this direction minimizes the magnitude.
Working from equation (6):
Figure imgf000007_0001
The optimal solution to this is to require each component of the double summation to have the necessary phase:
pD 2π
ZΔ k, m φp k(p)M for O ≤ k ≤ M -1 and O ≤ m ≤ N, - ! ill) p M
Now consider the magnitude XΔ(P) 204. The total symbol error introduced is given by:
Figure imgf000007_0002
In order to minimize this quantity, examine the magnitude of XΔ(P) from equation (1):
Figure imgf000007_0003
The pulse-shaping filter is acting as a weighting function to the symbol adjustment matrix. Moving symbols that align with large filter coefficients have a greater impact on the magnitude than others. The pulse-shaping filter coefficients themselves can be used to weight the changes. As a final component to Δ(k,m), it is noted that the various symbol types have different importance. For example, in most applications pilot and synchronization symbols should be altered much less than data symbols. Define W(k,m) as a matrix the same dimension as X(k,m) that contains weights for each entry in X(k,m). The complete equation for Δ(k,m) is given by:
Figure imgf000008_0001
The constant C is in place to ensure that the minima is sufficiently enlarged according to equation (8), after which, the linear amplifier will be operating in a region where the output is reliable:
Figure imgf000008_0002
Pulling C to the front, noting that \g \ = g and that W(k,m) is positive and real:
Figure imgf000008_0003
Computing Δ(k,m) in this manner requires the re-computation of the IFFT for Xr(n,m) in equations (2) and (3). It is more efficient to solve for x'r(n,m), the IFFT of Δ (k,m). Since the system is linear, the component from the symbol adjustment matrix can be computed and summed with xr(n,m):
xr (n, m) = xr (n, )+ δr ψ, m) (22) where,
Figure imgf000009_0001
Substituting for Δ(k,m) and using the pointer updates for k and m:
Figure imgf000009_0002
■ C - e1*'
Figure imgf000009_0003
nD δr(n,m)= C -eJΦ> - g[mI + (nD)M (n - p)M .. - m (27)
Where wr(n,m) is defined as the one-dimensional LFFT of W(k,m) along the frequency axis:
Figure imgf000009_0004
Thus, in a first embodiment, the present invention provides a method and apparatus for receiving a plurality of symbols, and modulating the plurality of symbols to create a first modulated signal. The first modulated signal is then compared to a threshold. If the first modulated signal is above the threshold, the first modulated signal is transmitted. If the first modulated signal, however, is below the threshold, an adjustment to the received plurality of symbols is computed and added to the received plurality of symbols to create an adjusted plurality of symbols. The adjusted plurality of symbols is then modulated to create a second modulated signal. The second modulated signal is transmitted if it exceeds the threshold; otherwise, the steps of computing, adding and the second step of modulating are repeated until the modulated signal exceeds the threshold. In an alternative embodiment, the present invention provides a method and apparatus for receiving a plurality of symbols, mixing the plurality of symbols using an inverse fast Fourier transform to create a plurality of mixed symbols, and pulse shaping the plurality of mixed symbols to create a first modulated signal. The first modulated signal is then compared to a threshold. If the first modulated signal is above the threshold, the first modulated signal is transmitted. If the first modulated signal is below the threshold, an adjustment to the plurality of mixed symbols is computed and added to the plurality of mixed symbols to create an adjusted plurality of mixed symbols. Pulse shaping is performed on the adjusted plurality of mixed symbols to create a second modulated signal. As in the first embodiment, the second modulated signal is transmitted if the second modulated signal exceeds the threshold; otherwise, the steps of computing, adding and the second step of pulse shaping are repeated until the modulated signal exceeds the threshold.
While the invention has been described in conjunction with specific embodiments thereof, additional advantages and modifications will readily occur to those skilled in the art. The invention, in its broader aspects, is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. Various alterations, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Thus, it should be understood that the invention is not limited by the foregoing description, but embraces all such alterations, modifications and variations in accordance with the spirit and scope of the appended claims.

Claims

ClaimsWe claim:
1. A method comprising the steps of: receiving a plurality of symbols; modulating the plurality of symbols to create a first modulated signal; comparing the first modulated signal to a threshold; if the first modulated signal is above the threshold, transmitting the first modulated signal; and if the first modulated signal is below the threshold, computing an adjustment to the received plurality of symbols, adding the adjustment to the received plurality of symbols to create an adjusted plurality of symbols, modulating the adjusted plurality of symbols to create a second modulated signal, and transmitting the second modulated signal if the second modulated signal exceeds the threshold; otherwise, repeating the steps of computing, adding and the second step of modulating.
2. The method of claim 1 wherein the adjustment is computed with the following equation:
Figure imgf000012_0001
3. The method of claim 1 wherein the threshold is predetermined.
4. The method of claim 1 wherein the threshold is dynamically adjusted.
5. The method of claim 1 wherein the plurality of symbols are divided in time and frequency.
6. The method of claim 5 wherein the step of modulating comprises at least one of mixing, pulse shaping, and rate changing the plurality of symbols.
PCT/US2003/025529 2002-08-22 2003-08-14 Method and apparatus for minima enlargement WO2004019545A1 (en)

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AU2003262689A AU2003262689B2 (en) 2002-08-22 2003-08-14 Method and apparatus for minima enlargement
IL16148903A IL161489A0 (en) 2002-08-22 2003-08-14 Method and apparatus for minima enlargement
EP03793064A EP1540873A4 (en) 2002-08-22 2003-08-14 Method and apparatus for minima enlargement
IL161489A IL161489A (en) 2002-08-22 2004-04-19 Method and apparatus for minima enlargement

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US10/225,982 US6754284B2 (en) 2002-08-22 2002-08-22 Method and apparatus for minima enlargement

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US6754284B2 (en) 2004-06-22
EP1540873A4 (en) 2010-06-16
IL161489A (en) 2009-07-20
EP1540873A1 (en) 2005-06-15
TWI239185B (en) 2005-09-01
AU2003262689B2 (en) 2005-09-01
AU2003262689A1 (en) 2004-03-11
TW200420069A (en) 2004-10-01
IL161489A0 (en) 2004-09-27
US20040042540A1 (en) 2004-03-04

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