Adaptive rf amplifier prelimiter

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DESCRIPTION

Prelimiter for adapter radio frequency amplifier.

Background of the invention

Field of the invention

This invention relates generally to spread spectrum code division multiple access (CDMA) communication systems. More particularly, the present invention relates to a system for adaptively limiting forward and reverse link transmit power within CDMA communication systems.

Description of the prior art

Wireless communication systems using spread spectrum modulation techniques represent the state of the art in digital communications and are increasing in popularity. In code division multiple access (CDMA) systems, data is transmitted using a wide bandwidth (spread spectrum) by modulating the data with a pseudo-random chip code sequence. The advantage obtained is that CDMA systems are more resistant to distortion of signals and frequencies that can interfere in the transmission channel than communication systems that use other multiple access techniques such as time division multiple access (TDMA). or frequency division multiple access (FDMA).

An indicator used to measure the performance of a communication system is the signal-to-noise ratio (SNR). At the receiver, the magnitude of the received wanted signal is compared to the magnitude of the received noise. Data from a transmitted signal received with a high SNR is easily recovered at the receiver. A low SNR leads to data loss.

A prior art CDMA communication system is shown in Figure 1. The communication system has a plurality of base stations (20_1? 20₂ ... 20_N) connected to each other through a Public Switched Telephone Network (PSTN). Each base station (20₁, 20₂ ... 20_N) communicates using spread spectrum CDMA with fixed and mobile subscriber units (22₁, 22₂ ... 22_N) within its cell area.

Figure 2 shows a simplified CDMA transmitter (24) and a receiver (26). A data signal having a given bandwidth is mixed with a spreading code generated by a pseudo-random chip code sequence generator that produces a digital spread spectrum signal for transmission. Upon receipt, the data is reproduced after correlation with the same pseudo-random chip code sequence used to transmit the data. By using different pseudo-random chip code sequences, many data signals or subchannels can share the same channel bandwidth. In particular, a base station (20₁) can communicate with a group of subscriber units (22₁, 22₂ ... 22_N) using the same bandwidth. Forward link communications are from the base station (20₁) to the subscriber unit (22₁, 22₂ ... 22_N), and reverse link communications are from the subscriber unit (22₁, 22₂ ... 22_N) to the base station (20₁).

For signal synchronization with a receiver (26), an unmodulated pilot signal is used. The pilot signal allows the respective receivers (26) to be synchronized with a given transmitter (24), which allows the depropagation of a traffic signal at the receiver (26). In a typical CDMA system, each base station (20₁, 20₂ ... 20_N) sends a single global pilot signal, received by all subscriber units (22₁, 22₂ ... 22_N) within communication range to synchronize forward link transmissions. Conversely, in some CDMA systems, for example in BCDMA^TM at the air interface, each subscriber unit (22₁, 22₂... 22_N) transmits a single assigned pilot signal to synchronize reverse link transmissions.

Figure 3 is an example of a prior art transmitter (24). Data signals (28₁, 28₂ ... 28_N), including traffic, pilot and maintenance signals, are propagated using the respective mixers (301, 30₂ ... 30_n) with single chip code sequences (30₁, 30₂ ... 30_N), respectively. Each mixer output is coupled to a combiner (34) that sums the individual mixed signals as a combined signal (44). The combined signal (44) is modulated to radio frequency (RF) by a mixer (36) mixing the combined signal (44) with an RF carrier, shown in Figure 3 as COS wt. The modulated signal is amplified to a predetermined transmit power level (TLP) by an amplifier (38) and radiated by an antenna (40).

Many CDMA systems use some form of adapter power control. In a CDMA system, many signals share the same bandwidth. When a subscriber unit (22₁, 22₂ ... 22_N) or base station (20₁, 20₂ ... 20_n) is receiving a specific signal, all other signals within the same bandwidth have noise characteristics relative to the specific signal. Increasing the power level of one signal degrades all other signals within the same bandwidth. However, if the TLP is reduced too much, undesirable SNRs are produced in the receptors (26). To maintain a desired SNR at the minimum transmit power level, an adapter power control is used.

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Typically, a transmitter (24) will send a signal to a particular receiver (26). On receipt, the SNR is determined. The determined SNR is compared to a desired SNR. Based on the comparison, a signal is sent on the reverse link to the transmitter (24), either increasing or decreasing the transmit power. This is known as direct channel power control. Conversely, the power control from the subscriber unit 22 to the base station 20 is known as reverse channel power control.

Amplifiers (64₁, 64₂ ... 64_n) are used for adapter power control in figure 3. The amplifiers (64i, 64₂ ... 64_n) are coupled to the inputs of the combiner (34) to individually control each level of signal power.

WO 98/44668 describes a direct sequence code division multiple access (DS-CDMA) cellular telecommunication system, where a cellular base station communicates with multiple mobile units while maintaining exclusive communication links with the mobile units. Each unique link is assigned a code that is orthogonal to all other codes. In this case, the Walsh code is used, a code that has the property that all its element codes are orthogonal to each other. This property of Walsh codes allows a base station in the DS-CDMA system to transmit a combination of carrier signals, where each carrier signal is encoded with an assigned Walsh code. Each receiver unit extracts the desired information based on the assigned Walsh code and ignores other orthogonal Walsh codes.

In a particular embodiment, a waveform with peak reduction is estimated and summed with a composite signal to reduce a mean-to-peak power ratio of the composite signal. The peak reduction waveform estimate is modified to have orthogonal Walsh code components for the assigned Walsh codes. An iterative process of estimating the next peak reduction waveform is performed to produce a peak reduction waveform that, if summed with the composite signal, results in a composite signal that has a mean-to-peak power ratio. to a desired level and thus does not have the remodulation effects of the assigned Walsh codes.

WO 99/18686 describes a coding and modulation system including a combined modulator / demodulator and a propagation / depropagation mechanism. The propagation / depropagation mechanism includes an information vector generator, a pivot table coupled to the output of the information vector generator, a multiplication block coupled to the output of the pivot table, a Walsh function generator coupled to the information block multiplication, a sum block coupled to the output of the multiplication block, and a dynamic limiter to limit the output of the sum block, placed in a base station of a mobile communication system transmitting and receiving a plurality of communication channels by a direct extension code division multiple access (DS-CDMA) system and ensures the suppression of peak transmission power while transmitting. The system comprises a plurality of propagation units for the propagation of transmission data of a plurality of communication channels by means of different propagation codes to emit propagation signals, a summing synthesizer to sum the propagation signals coming out of the propagation units to output a multiplexed propagation signal, and a limiter to perform amplitude limiting of the multiplexed propagation signal output from the summing synthesizer. The system further comprises a progressive cut filter to carry out spectrum modeling of the multiplexed propagation signal of limited amplitude so that an occupied bandwidth of the multiplexed propagation signal of limited amplitude is included within a predetermined value. , a digital-to-analog converter for converting a digital baseband signal from the amplitude-limited multiplexed propagation signal to an analog baseband signal, a modulator for converting the analog baseband signal into a radio frequency signal, a transmitting power amplifier for amplifying the radio frequency signal coming out of the modulator, and a transmitting antenna for transmitting the amplified radio frequency signal coming out of the transmit power amplifier.

Figures 4a, 4b, 4c and 4d show a simplified illustration of three spread spectrum signals (42i, 42₂, 42₃) and a resulting combined signal 44. Although each signal (42₁, 42₂, 42₃) is expanded with a different pseudo-random chip code sequence, each signal (42₁, 42₂, 42₃) is synchronous with the chip speed. When the individual chips within the sequences are summed, the combined signal can have the transient ends 46, 48, where the chip energies combine or reduce the transient ends 47 as they subtract.

Very transient peaks are undesirable. For each 3 dB increase in the peak, twice the base amplification power in watts is required. Not only do transient peaks load the amplifier, but the power coming out of the amplifier must have a greater capacity than the maximum transient that can be anticipated. This is particularly undesirable in portable battery powered devices. Additionally, designing higher power levels as a consequence of high transients requires more complex amplifier circuitry or trade-offs between amplifier gain, battery life, and communication time outcome.

High value transients force amplifier 38 into the non-linear region of its dynamic range resulting in more out-of-band emissions and reduced amplifier efficiency.

Accordingly, there is a need for an RF adapter transmitter system that addresses the problems associated with the prior art.

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Summary of the invention

The present invention provides a base station for transmitting signals using a code division multiple access (CDMA) technique in accordance with independent claim 1. Preferred embodiments of the invention are reflected in the dependent claims.

The claimed invention can be better understood by taking into account the arrangements described below. In general, the described arrangements describe preferred embodiments of the invention. The careful reader will note, however, that some aspects of the described arrangements extend beyond the scope of the claims. As far as the described provisions extend beyond the scope of the claims, the described provisions should be considered as supplementary information that do not constitute definitions of the invention per se. The same applies to the subsequent "Brief Description of the Drawings" as well as the "Detailed Description of Exemplary Arrangements".

In particular, according to the present invention, the base station for transmitting signals using a CDMA technique comprises means for combining a plurality of spread spectrum data signals, where the output of the combining means is a combined signal having a power level fluctuating and means for modulating the combined signal to produce an RF signal for transmission. Furthermore, the base station comprises means for measuring an average of an absolute value, an approximate variance, an approximation of the standard deviation, an average of a square of the output of said combining means for a given period of time and means for limiting adaptively an output of the combining means at a calculated power level based on, at least in part, said measured average of an absolute value, the approximate variance, the approximation of the standard deviation, or the average of the square of the output of said combining means for a given period of time.

Consequently, the invention reduces transient peaks in signals transmitted in CDMA communication systems. A plurality of spread spectrum data signals are combined into a combined signal having a fluctuating power level corresponding to the combination of the data signals. The combined signal is modulated to produce an RF signal for transmission. The average power of the combined signal is measured over a selected period of time. The combined signal power level is adaptively limited to a calculated power level based on at least part of the measured power.

Brief description of the drawings

Figure 1 is an illustration of a prior art CDMA system.

Figure 2 is an illustration of a prior art CDMA transmitter and receiver.

Figure 3 is a system block diagram of a prior art transmitter.

Figure 4a is an illustration of a first pseudo-random chip code sequence.

Figure 4b is an illustration of a second pseudo-random chip code sequence.

Figure 4c is an illustration of a third pseudo-random chip code sequence.

Figure 4d is an illustration of the combined chip code sequences of Figures 4a-4c.

Figure 5 is a system block diagram of an embodiment of the invention with the power measurement device coupled to the amplifier.

Figure 6 is a system block diagram of an alternative embodiment of the invention with the power measurement device coupled to the modulator.

Figure 7 is an illustration of the probability distribution function of the power levels of a combined signal.

Figure 8 is a graph of loss in received signal-to-noise ratio versus limiting level.

Figure 9 is a graph of loss in received signal to noise ratio versus limiting level in a CDMA communication system using adapter power control.

Figure 10 is a system block diagram of an alternate embodiment of the invention with the processor controlling the gain of the amplifier.

Description of exemplary provisions

Preferred arrangements will be described with reference to the figures of the drawings where the same numbers represent the same elements.

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Figures 5 and 6 represent transmission systems. A group of data signals (28i, 28₂ ... 28_N) that include traffic, pilot and maintenance signals are mixed with different chip code sequences (28₁, 28₂... 28_N) and are all added in a combiner (34) as a combined signal (44). Combiner 34 is coupled to an adjustable signal limiter 50 (limiter) where the power levels of the signals are limited to + β and -β dB. Power levels between + β and-β are not affected. The limited signal (45) is RF modulated by a mixer (36). The modulated signal is amplified by an amplifier (3) 8 to a predetermined power level and radiated by the antenna (40).

Figure 7 illustrates a typical probability distribution function of the combined signal power level. A combo chip (46, 47, 48) as shown in figure 4d will have an associated power level. The probability of a given combo chip having a particular power level is shown in Figure 7. The two extreme power levels are (+ K and -K). As shown in Figure 7, the probability of a given combo chip having a power level of (+ K or -K) is extremely low. However, the probability of a combo chip having a power level in the middle of the two extremes is high. Since a spread spectrum signal is spread across a wide communication bandwidth and there is a low probability that a combo chip will have a power level at the ends of the distribution, the pooled signal (44) can be limited. below these extremes with negligible loss.

The transmission system adjusts the limiting levels, β, to eliminate transient signals with only a small reduction in the transmission signal-to-noise ratio (SNR). Figure 8 is a graph illustrating the relationship between SNR and level limiting for a system that does not use an adapter power control. The solid line, the dashed line, and the dotted line represent communication channels with different operational SNRs. As shown in Figure 8, for a β set at a limiting level of two standard deviations the loss in SNR is negligible and at a limiting level of one standard deviation the loss is only about 0.2 dB.

For a system using adapter power control, Figure 9 is a graph of SNR versus limiting level. The results are similar to those obtained in a system that does not use adapter power control. As shown in Figure 9, with a limitation level of two standard deviations, the loss in SNR is again negligible. Therefore, the limiting circuit is applicable to systems that use adapter power control and systems that do not use adapter power control.

Referring again to Figure 5, to determine β, a power measurement device (52) and a processor (54) are used. The power measurement device (52) is coupled to the output of the RF amplifier (38) as shown in Figure 5 or to the mixer (36) as shown in Figure 6. Preferably, the power measurement device (52) determines the average of the square of the magnitude of the transmitted signal in a predetermined period of time. The output of the preferred power measurement device (52) approximates the variance of the mixed signal (49) or the signal (51) being transmitted. Alternatively, the power measurement device (52) determines an approximation of the standard deviation by taking the average of the absolute value of the signal (49, 51) or the power measurement device (52) measures the magnitude of the signal. (49, 51) with the processor determining either the variance or the standard deviation.

The output of the power measurement device (52) is coupled to a processor (54). If the power measurement device (52) is coupled to the output of the amplifier (38), the processor (54) progressively reduces the output of the power measurement device (52) by the gain of the amplifier (38). Processor 54 determines the appropriate limiting level for β. Depending on the desired SNR and bandwidth, the value for β will be a multiple of the standard deviation. If the power measurement device (52) approximates the variance, the processor (54) will take the square root of the device output as the standard deviation. In a preferred arrangement, β will be twice the standard deviation.

In certain situations, the processor (54) overrides the determined value of β. For example, if the transmitter (25) was used in a base station (20₁, 20₂... 20_N), a large increase in the number of users can cause β to be temporarily set too low. This will cause an undesirable SNR to be received. When supplied to the processor (54) via line (60), the number of users usually in communication with the base station (20₁, 20₂ ... 20_N) to change β or temporarily disable the limiter (50) to allow all signals to pass through unaltered when appropriate.

Additionally, since the probability distribution function assumes a large sample size, a small number of users can result in an unwanted received SNR. Therefore, if users were to communicate with the base station (20₁, 20₂ ... 20_N), the limiter (50) could be disabled. Also, when there are only a small number of active users the range of the dynamic amplifier is not reached. Consequently, there is no need to limit the combined signal. In other situations it may be necessary to override the limiter (50). For example, in some CDMA systems short codes are used during the initial power boost. Since these codes are not long enough to approximate a random signal, by chance a code can assume a large number of high transients within the signal. Limiting these transmissions can dramatically reduce the received SNR and unnecessarily delay the initial power increase during the procedure. In these situations, a signal will be sent to processor 54 through line 62 to override limiter 50.

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In an alternative arrangement shown in Figure 10, the processor (54) is also used to control the gain of the amplifier (38) through the line (58). The amplifier gain characteristic is stored in the processor. The gain of the amplifier is adjusted to prevent the amplifier from entering the non-linear operating region. Consequently, out-of-band emissions and interference to services in contiguous frequency bands are reduced.

Although the invention has been described in part with detailed reference to certain specific provisions, such details should be considered as instructive rather than restrictive. Those skilled in the art will appreciate that many variations in structure and mode of operation can be made without departing from the scope of the invention as described in these instructions. In a preferred aspect of the invention there is provided: a transmitter for use in a CDMA communication system station, the transmitter comprising: means for combining a plurality of spread spectrum data signals into a combined signal having a power level fluctuating corresponding to the combination of the data signals; means for modulating the combined signal to produce an RF signal for transmission; means for measuring the average power of the combined signal over a selected period of time; and means for adaptively limiting the power of the combined signal to a calculated power level based, at least in part, on said measured power.

The transmitter, as defined above, where said measuring means measures an average power of the RF signal in the selected period of time.

The transmitter, as defined above, further comprising an amplifier for amplifying the RF signal prior to transmission; wherein said measuring means measures an average power of the RF amplified signal in the selected period of time.

The transmitter, as defined above, where said measurement means determines a variance of the combined signal power where said adaptive limiting means limits the combined signal power to the power level calculated in part based on the approximation of the variance. The transmitter, as defined above, where said measuring means determines an average of one square of the combined signal power; wherein said adaptive limiting means limits the combined signal power to the power level calculated in part based on the mean squared. The transmitter, as defined above, where said measuring means determines an average of an absolute value of the combined signal power; wherein said adaptive limiting means limits the combined signal power to the power level calculated in part based on the mean of the absolute value. The transmitter, as defined above, wherein said measurement means will determine a magnitude of the combined signal power and said measurement means has processing means for determining a variance of the combined signal power based on the determined magnitude; wherein said adaptive limiting means limits the power of the combined signal to the power level calculated in part on the basis of the determined variance. The transmitter, as defined above, wherein said measuring means has processing means for determining a standard deviation of the combined signal power and said adaptive limiting means limits the combined signal power to the power level calculated in part in based on the determined standard deviation. The transmitter, as defined above, where the calculated power level is two of the determined standard deviations. The transmitter, as defined above, where the calculated power level is the determined standard deviation. The transmitter, as defined above, wherein said processing means disables said adapter limiting means in response to a number of active users. The transmitter, as defined above, where said processing means disables said adaptive limiting means during the transmission of short codes. The transmitter, as defined above, further comprising an amplifier for amplifying the RF signal prior to transmission and processing means for determining the calculated power level; wherein a gain of said amplifier is adjusted by said processing means in response to the calculated power level and stored gain characteristics of said amplifier. In a preferred aspect there is also provided a method for transmission in a CDMA communication system, the method comprising: combining a plurality of spread spectrum data signals into a combined signal having a fluctuating power level corresponding to the combination of the data signals; modulating the combined signal to produce an RF signal; measuring the average power of the combined signal over a selected period of time; adaptively limiting the combined signal power to a power level calculated at least in part based on said measured power; and transmit the RF signal. The method, as defined above, where the measurement is of an average power of the RF signal over a selected period of time. The method, as defined above, further comprising amplifying the RF signal prior to transmission; where the measurement is of an average power of the amplified RF signal in the selected period of time. The method, as defined above, where the calculated power is based on a variance of the combined signal. The method, as defined above, where the calculated power is based on the standard deviation of the combined signal. The method, as defined above, where the calculated power is one of the standard deviations. The method, as defined above, where the calculated power is two of the standard deviations. The method, as defined above, where the adaptively limiting phase is not performed in response to a number of active users. The method, as defined above, where the adaptively limiting phase is not performed in response to a short code transmission. The method, as defined above, further comprising: amplifying by an amplifier the RF signal by a gain factor prior to transmission; and adjusting the gain factor in response to the calculated power level and the stored gain characteristics of the amplifier.

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Documents cited in description

This list of documents cited by the applicant has been compiled exclusively for the information of the reader and is not part of the European patent document. It has been made with the greatest diligence; However, the EPO does not assume responsibility for eventual errors or omissions.

Patent documents cited in the description • WO 9844668 A [0011] • WO9918686 A [0013]