Abstract:
In the method and apparatus of controlling power of a transmitted communication signal, a communication signal is amplified and transmitted. At least one parameter on the transmitted signal is received, and a measure of interference with the transmitted signal is determined based on the received parameter. An average power level of the communication signal is increased by clipping the communication signal prior to amplification by an amount based on the determined measure.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the field of telecommunications, and more particularly, a method and apparatus for controlling the power of a transmitted signal. 
   2. Description of Related Art 
   Orthogonal Frequency Division Multiplexing (OFDM) is a special form of multi-carrier modulation having inherent robustness against multipath effect. For example, IEEE 802.11a specifies the Physical Layer Entry for an OFDM system that provides a wireless Local Area Network (LAN) with data payload communication capabilities from 6 to 54 Mbits/sec in the Unlicensed National Information Infrastructure (U-NII) frequency band. The system uses 52 sub-carriers which are independently modulated by using Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), 16-Quadrature Amplitude Modulation (16-QAM) or 64-Quadrature Amplitude Modulation (64-QAM) associated with different coding rate for different data speed. 
   A major challenge for an OFDM-based communication system is the inherent high crest factor (peak-to-average ratio) of multi-carrier systems. Considerable output power back-off from the power amplifier (PA) saturation region will be needed to avoid distortion and spectral regrowth. The back-off for the power amplifier, however, reduces its efficiency. Because the peak transmitted power is usually constrained by regulatory limits, a large back-off of the power amplifier design to deal with the high crest factor has the effect of significantly reducing the average transmit power. The low average transmit power introduces several problems such as reducing radio coverage and making the transmitted signal more susceptible to interference. 
   So far, several crest factor reduction techniques have been proposed such as Reed-Muller codes, Golay sequences, subsets of block coding that avoid transmitting codewords with a large crest factor, and selective sub-carrier mapping to reduce the crest factor. However, as the number of sub-carriers increases, the coding rate slows and the coding process becomes more complicated (e.g. extensive computation, search, look-up tables). Unlike cellular/PCS systems that can afford costly power amplifiers, the power amplifier used in a wireless LAN needs to be simple and cheap. Clipping the OFDM signal is another way to reduce the crest factor. Clipping can be described as limiting the peak amplitude of an OFDM signal to the power amplifier input so that the undesirable effect of the amplifier non-linearity problem can be controlled. However, inadequate clipping introduces excessive out-of-band distortion. 
   SUMMARY OF THE INVENTION 
   In the method and apparatus according to the present invention, interference with the transmitted signal is monitored. When long term interference is encountered, the average power of the transmitted signal is increased by a first amount. And, when short term interference is encountered, the average power of the transmitted signal is increased by a second amount greater than the first amount. Increasing the average signal power in this manner compensates for the determined interference. 
   The average signal power is increased without causing the power amplifier to enter the saturation region. Consequently distortion and spectral regrowth are avoided. To increase the average power of the transmitted signal, the signal for transmission is clipped to remove undesirably high peaks, and then the gain of the signal is increased. The clipping level and gain are adjusted based on the amount of determined interference. Accordingly, the clipping level is increased by, for example, the first amount when long term interference is determined, and increased, for example, by the second amount when short term interference is determined. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, wherein like reference numerals designate corresponding parts in the various drawings, and wherein: 
       FIG. 1  illustrates a block diagram of an apparatus employing the method of the present invention; 
       FIG. 2  illustrates waveforms output by elements in the block diagram of  FIG. 1 ; and 
       FIG. 3  illustrates a flowchart of an embodiment of the method according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a block diagram of a communication apparatus such as a wireless Local Area Network (LAN) card or base station employing the method of the present invention. While the method of the present invention will be described as implemented by a wireless LAN, the method is not limited to this implementation. For example, the method could be implemented by a cellular communication system. 
   As shown in  FIG. 1 , an antenna  8  receives and transmits signals to and from another transmission source  9  via a band-pass filter  10  and a duplexer  12 . The duplexer  12  supplies the received signals to a low-noise amplifier  14 . The amplifier  14  amplifies the signals and supplies them to a down converter  16 , which down converts the radio frequency signal from the amplifier  14  to an intermediate frequency. An analog-to-digital converter (ADC)  18  converts the analog output of the down converter  16  to digital. A baseband /medium access controller (hereinafter “controller”)  20  receives the output of the ADC  18 . The controller  20  transfers received signals to a host  19  (e.g., a computer). Based on the signals received from the ADC  18 , the controller  20  controls a limiter  26  and an automatic gain control (AGC)  28  according to the method of the present invention as described in detail below with respect to  FIGS. 2 and 3 . 
   Signals supplied from the host  19  to the controller  20  for transmission are supplied to a digital-to-analog converter (DAC)  22 . The digital output of the DAC  22  is received by an up converter  24 , which converts the received analog signals from an intermediate frequency to radio frequency. The limiter  26  clips the signals received from the up converter  26  based on control signals from the controller  20 , and the AGC  28  gain controls the output of the limiter  26  based on control signals from the controller  20 . A power amplifier  30  amplifies the output of the AGC  28 , and supplies the result to the duplexer  12 . The duplexer  12  passes the signal from the power amplifier  30  to the antenna  8  via the BPF  10 . 
     FIG. 2A  illustrates the output of the up converter  24 . Because of the high peak-to-average ratio of this signal, an increase in the gain of the signal will cause the power amplifier  30  to enter the saturation region, and distortion and spectral regrowth will result.  FIG. 2B  shows the limiter  26  clipping the output of the up converter at a clipping level set by the controller  20 . Having clipped the peak of the signal, the gain of the signal can be increased by the AGC  28  as shown in  FIG. 2C  such that the average power of the signal is increased without causing distortion and spectral regrowth. 
   The method by which the controller  20  controls the limiter  26  and the AGC  28  will now be described in detail with respect to  FIG. 3 .  FIG. 3  illustrates a flow chart of the embodiment of the present invention employed by the controller  20 . As shown, in step S 10 , the controller  20  receives signals via the antenna  8 , the BPF  10 , the duplexer  12 , the amplifier  14 , the down converter  16  and the ADC  18  from the other transmission source  9  such as a remote station (not shown). The signals either include a measurement of the signal-to-noise ratio (SNR) made by the other transmission source  9  or provide a signal strength measurement of the signal transmitted by the apparatus of  FIG. 1  as measured by the transmission source. Using the signal strength measurement, the controller  20  calculates the SNR in the well-known manner. 
   Next in step S 12 , the controller  20  compares the received or calculated SNR to a long threshold. If in step S 12  the controller  20  determines that the received or calculated SNR (hereinafter “the SNR”) is not less than the long threshold, then in step S 14 , the controller  20  sets the clipping level of the limiter  26  and the gain of the AGC  28  to predetermined levels. Also, in step S 14  the long and short counters, discussed in detail below, are reset. However, if the controller  20  determines the SNR is less than the long threshold, then the controller  20  determines that the possibility of long term interference exists (hence the name long threshold) and in step S 16  the controller  20  increments a long counter. 
   Subsequent to step S 16 , the controller  20  determines if the SNR is less than a short threshold in step S 18 . If the controller  20  determines that the SNR is not less than the short threshold, then in step S 20 , the controller  20  determines if the long counter exceeds a long count threshold. If not, then in step S 22  the controller  20  sends the SNR calculated in step S 10  to the remote station and processing returns to step S 10 . 
   In step S 20 , if the long counter does exceed the long count threshold, then the controller  20  determines that long term interference (e.g., a more permanent change in the environment affecting the SNR) exists. In step S 24 , the controller  20  determines if the current clipping level plus a first predetermined amount (e.g., 0.1 to 0.5 dB) is less than a maximum clipping level. If so, then in step S 26 , the controller  20  increments the clipping level of the limiter  26  by the first predetermined amount, increases the gain of the AGC  28 , and resets the long and short counters. In a preferred embodiment, the gain of the AGC  28  is increased by the same first predetermined amount, but it will be appreciated from this disclosure that the present invention is not limited to increasing the gain in this manner. After step S 26 , processing proceeds to step S 22 . 
   In step S 24 , if the current clipping level plus the first predetermined amount is not less than the clipping maximum, then in step S 28 , the clipping level of the limiter  26  is set at the clipping maximum, and the gain of the AGC  28  is increased by the same amount required to increase the current clipping level to the clipping maximum; however, the present invention is not limited to affecting gain of the AGC  28  in this one-for-one manner. Also, in step S 28 , the long and short counters are reset. Processing then proceeds to step S 22 . 
   Returning to step S 18 , if the SNR is less than the short threshold, then in step S 30  the controller  20  determines that the possibility of short term interference exists and increments a short counter. In subsequent step S 32 , the controller  20  determines if the short counter exceeds a short count threshold. If the short counter does not exceed the short count threshold, then processing proceeds to step S 20 . However, if the short count exceeds the short count threshold, then the controller  20  determines that short term interference (e.g., a transmission by a different transmission source) exists. In step S 34 , the controller  20  determines if the current clipping level plus a second predetermined amount (e.g., 1 to 3 dB), greater than the first predetermined amount, is less than the maximum clipping level in step S 34 . If so, then in step S 36 , the controller  20  increments the clipping level of the limiter  26  by the second predetermined amount, increases the gain of the AGC  28 , and resets the short and long counters. In a preferred embodiment, the gain of the AGC  28  is increased by the same second predetermined amount, but it will be appreciated from this disclosure that the present invention is not limited to adjusting the gain in this manner. After step S 36 , processing proceeds to step S 22 . 
   In step S 34 , if the current clipping level plus the second predetermined threshold is not less than the clipping maximum, then processing proceeds to step S 28 . 
   As will be appreciated from the above description, when long term interference is encountered, the clipping level is slowly increased, while for short term interference, a quick increase in the clipping level occurs. In this way, the controller  20  is responsive to and compensates for the type of interference encountered. This methodology also prevents increasing the clipping level by too great a margin such that an unnecessarily large increase in the average signal power does not occur; thus, preventing undue interference caused by the transmitted signal. 
   The invention being thus described, it will be obvious that the same may be varied in many ways. For example, instead of or in addition to resetting the long and short counters, the long and short counters could be decremented at, for example, step S 22  or other times at the discretion of the system designer. As another alternative, the long and short counters could be kept over a moving window of time or data samples. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications are intended to be included within the scope of the following claims.