Abstract:
In a TDD wireless communication system between transmitters and associated receivers, automatic gain control of a receiver is only applied during the corresponding time slot within the TDD signal time frame architecture. Successive received signal strengths are measured and gain levels are stored as estimates for an initial gain level in future time slots of the TDD signal. Estimating techniques, such as averaging or trending of received signal strength over successive time slots, and averaging or trending of gain level settings, provide improved estimation of future initial gain levels.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims priority to Provisional Patent Application No. 60/241,651 filed on Oct. 19, 2000. 
     
    
     
       BACKGROUND  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to wireless communication systems. More particularly, the invention relates to a method for selectively measuring a received portion of a transmission signal to be applied to an automatic gain control (AGC) system of a receiver.  
           [0004]    2. Description of the Prior Art  
           [0005]    The time division duplex (TDD) and time division multiplex (TDM) systems of spread spectrum wireless communication operate on the principle of repeating frames of data transmission that are divided into successive time slots. In TDD and TDM wireless systems, there is often a significant and sudden variation in received signal strength between one time slot and the next. This is caused by the fact that different transmitters, with possibly different transmit powers, and possibly vastly different path losses to the associated receiver, operate in consecutive time slots. Furthermore there is typically a so-called “guard” period inserted between time slots, during which no unit in the network is allowed to transmit. This causes another significant and sudden variation in signal strength as the allowed transmission period of one time slot ends, followed by the guard period in which no unit transmits, and then followed again by another transmission in the following time slot.  
           [0006]    These sudden and often dramatic variations in received signal strength wreak havoc with traditional automatic gain control (AGC) systems. Such systems are typically employed to adjust the receiver gain so that widely varying signal strengths received at the antenna are reduced to more modest variations in signal strength at the A/D converter, the detector or other devices within the receiver. Without such a reduction in the range of signal strengths, the operation of the A/D converter, the detector or other devices within the receiver can be severely impaired or rendered inoperable.  
           [0007]    Conventionally, AGC systems employ closed loop control systems which operate on the continuously received signal. The response speed of such AGC systems must often be limited so as to prevent instability and/or prevent the AGC from eliminating the rapid amplitude variations that are an inherent and essential part of many modulation schemes. Therefore, there are contradictory requirements which, on the one hand, call for a slow AGC response (so as to stabilize the system and not eliminate the essential amplitude variations), and on the other hand, call for a rapid AGC response in order to adjust to the rapidly varying received signal strength. It should also be noted that the information at the beginning of a received time slot, before the AGC has time to properly respond, may be lost or useless. In some systems it has been considered necessary to insert a period at the beginning of a time slot transmission in which no information is sent, even though the transmitter is active. Although this gives the receiver&#39;s AGC time to respond, this technique wastes precious bandwidth.  
         SUMMARY  
         [0008]    The present invention is a system or technique in which the automatic gain control of a received TDD or TDM signal for a given receiver is performed only during those specific time slots in which that particular receiver processes transmissions from its associated transmitter. One or more samples of gain control signals are stored until the next recurrence of the specific time slot in a subsequent frame. They are then treated as an estimate of the initial gain level required at the beginning of that subsequent time slot.  
           [0009]    The receiver AGC operates at the start of that subsequent time slot under the control of this estimate, derived from measured signal strength made during the same time slot in a preceding frame. In this way, the AGC operation during any given time slot becomes unaffected by excessive signal strength variations from one time slot to the next within a single time frame. The initial setting of the gain control level at the beginning of a specific time slot is especially improved as the AGC operation of the receiver resumes more smoothly over successive time frames.  
           [0010]    Further improvement can be obtained by deriving the estimate of initial gain level, not just from a single time slot in a prior frame, but rather from the average of several such prior time slots, or by determining trends in the prior gain control signals. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a simplified system diagram of a basic TDM system.  
         [0012]    [0012]FIG. 2 illustrates the typical signal format used in TDD or TDM architecture.  
         [0013]    FIGS.  3 A, and  3 B are simplified block diagrams an AGC system with a gain control signal synchronized to a designated time slot.  
         [0014]    [0014]FIG. 4 is a flow chart of the technique performed by the embodiments of FIG. 5.  
         [0015]    [0015]FIG. 5 is a simplified block diagram of an AGC system with means for gain control level storage.  
         [0016]    [0016]FIG. 6 is a simplified block diagram of an AGC system with means for RF input signal storage.  
         [0017]    [0017]FIG. 7A is an alternative embodiment of FIG. 5 using a microprocessor to incorporate various system functions.  
         [0018]    [0018]FIG. 7B is an alternative embodiment of FIG. 7A using a microprocessor to additionally include the variable amplifier of the system. 
     
    
     DETAILED DESCRIPTION  
       [0019]    Referring to FIG. 1, this shows a typical TDD/TDM system  10 . It consists of several transmitters denoted as T 1 , T 2 , T 3  and T 4 , and of several receivers, denoted as R 1 , R 2 , R 3  and R 4 . The number of transmitters and receivers, four (4) and four (4) respectively (and hence the corresponding number of time slots), is chosen for illustrative purposes and as such, other possible embodiments may comprise a greater or lesser number of time slots, transmitters and receivers. The transmitters T 1 -T 4  and receivers R 1 -R 4  communicate via a wireless medium  11 . These communications are so timed such that receiver R 1  processes signals from its associated transmitter T 1 , receiver R 2  processes signals from its associated transmitter T 2 , and so on.  
         [0020]    The timing architecture of these communications between associated transmitters and receivers is illustrated in FIG. 2, which shows a typical time frame N, during which communications between the four associated pairs of transmitters T 1 -T 4  and receivers R 1 -R 4  of FIG. 1 take place. To that end, frame N is subdivided into four consecutive time slots, designated TS 1 , TS 2 , TS 3  and TS 4 . During time slot TS 1 , receiver R 1  of FIG. 1 is intended to process signals from transmitter T 1  in FIG. 1. During time slot TS 2 , the same applies to receiver R 2  and its associated transmitter T 2 , and so on for time slots TS 3  and TS 4 . A time frame occurs again and again, each again subdivided into the four time slots shown for frame N. This is indicated in FIG. 2 by showing the last time slot TS 4  of the frame N−1 which precedes frame N and the first time slot TS 1  of the frame N+1, which follows frame N. The designation of time slots to particular transmitters and receivers is made herein for explanatory purposes. However, it should be understood by those of skill in the art that time slots will be assigned dynamically as needed in accordance with prior art techniques.  
         [0021]    Also shown in FIG. 2 is an expanded view of a time slot, using as an example time slot TS 2  of frame N. This shows a central portion  20 , within which data is transmitted and received, flanked by guard bands  21 , during which there is no data transmission or reception. The present invention will operate either with or without guard bands  21 .  
         [0022]    Referring now to FIG. 3A, an AGC system  30  is used, for example, in receiver R 1  of FIG. 1, responding to designated time slot TS 1 . The AGC system  30  comprises an input  31 , an output  36 , a synchronizer  38 A, and a closed feedback loop comprising a variable amplifier  35 , a measurement unit  32  and a reference and comparison unit  33 . The input  31  provides the RF input signal which has been detected by the receiver R 1 . The RF input signal comprises a plurality of repeating time frames, each including a plurality of time slots TS 1 -TS 4  as shown in FIG. 2. Although FIG. 2 shows four time slots TS 1 -TS 4 , those of skill in the art would clearly recognize that more or less time slots could be used as required by the particular application.  
         [0023]    The variable amplifier  35  receives the RF input signal from the input  31  and amplifies or attenuates the signal. The measurement unit  32  measures the output of the variable amplifier  35 . This measurement is forwarded to the reference and comparison unit  33  which compares the output of the measurement unit  32  with a predetermined reference. As a result of this comparison, the reference and comparison unit  33  outputs an error control signal  34  to the variable amplifier  35  to increase or decrease the amount of amplification or attenuation as desired, to keep the variable amplifier output  36  within a predefined operating range as required by the downstream electronic components (not shown). The synchronizer  38 A utilizes a switch to couple the input  31  to the AGC system  30  during time slot TS 1  and to decouple the input  31  during all other time slots TS 2 -TS 4 . The synchronized input ensures that the input  31  is timely coupled to the AGC system  30  during all occurrences of the applicable time slot, (in this example TS 1 ).  
         [0024]    [0024]FIG. 3B shows an alternate embodiment wherein the synchronizer  38 B comprises a sample and hold unit in which sampling of the input  31  is synchronized to the frequency of time slot TS 1 . A control signal from the synchronizer  38 B selectively overrides the signal generated by the reference and comparison unit  33 . During the applicable time slot (i.e. TS 1 ), the synchronizer  38 B allows the gain control function provided by the measurement unit  32 , the reference and comparison unit  33  and the amplifier  35  to operate normally. During time slots other than TS 1 , the signal from synchronizer  38 B overrides the signal from the reference and comparison unit  33 , in order to hold the gain of the variable amplifier  35  at the level that existed at the end of time slot TS 1 .  
         [0025]    By having synchronizer  38 A or  38 B in the AGC system  30 , sampling of the input  31  is synchronized to the occurrence of the desired time slot TS 1 . This allows the variable amplifier  35  to operate at a level much closer to the required level, particularly at the beginning of the next occurrence of time slot TS 1 , than would otherwise be possible had the AGC system  30  been allowed to vary across time slots TS 1 -TS 4  from one frame to the next. The result is an improved AGC system  30  with respect to setting the initial gain level during time slot TS 1 .  
         [0026]    Referring to FIG. 5, an alternate embodiment of an AGC system  50  is shown. This embodiment of the AGC system  50  includes components similar to the prior embodiments, but further includes a control storage unit  51  and an estimate enhancement unit  53 . The control storage unit  51  stores the control signal  34  output from the reference and comparison unit  33  for the designated time slot, such as TS 1 , over several time frames. The stored control signal  34  may comprise a single sample (such as at the end of time slot TS 1 ), or may comprise an average of several samples of the control signal  34  over the entire duration of time slot TS 1 . This provides a more accurate estimate for time slot TS 1  compared with a single sample of signal strength within time slot TS 1 . The synchronizer  38 A, measurement unit  32 , reference and comparison unit  33  and variable amplifier  35  all serve the same functions as the corresponding components shown in FIG. 3A.  
         [0027]    In a first embodiment of AGC system  50 , the estimate enhancement circuit  53  performs calculations of control signals stored in the control storage unit  51 , including averaging a sequence of stored control signals derived during the time slot TS 1  over several prior time frames. For example, a value of 0.2 for the control signal  34  would be the result of averaging performed on control signal  34  values of 0.1, 0.2, 0.2, 0.3, stored for time slot TS 1  over four time frames. In another embodiment, the estimate enhancement circuit  53  may also perform a calculation to determine a rising or falling trend of a sequence of stored control signals  34 . For example, a value of 0.5 for the control signal  34  would be the result of a trending calculation performed on control signal  34  values 0.1, 0.2, 0.3, 0.4 of stored values for time slot TS 1  over four time frames. Thus, the output of the estimate enhancement circuit  53  provides an improved estimate of the appropriate gain required for the next occurrence of time slot TS 1 . There are many statistical trending algorithms available in the prior art, and any of these algorithms may be utilized by the present invention. A detailed discussion of such algorithms is outside the scope of the present invention. It should be noted that although the estimate enhancement circuit  53 , the control storage unit  51 , the reference and comparison unit  33  and the measurement unit  32  are illustrated as separate components, they may be combined together as a single component as desired, such as a microprocessor (not shown). The variable amplifier  35  may also be incorporated into such a microprocessor to provide a single, unitary “smart AGC.” 
         [0028]    The process  400  used by each respective receiver R 1 -R 4  to demodulate a signal intended for that particular receiver in accordance with the present invention is shown in the flow diagram of FIG. 4. For this process  400 , it is assumed that each receiver R 1 -R 4  has been synchronized to the repeating time frames, and each receiver R 1 -R 4  has also been preassigned to a particular time slot. The prior description notwithstanding, it also should be understood that although a single time slot for each receiver R 1 -R 4  is used as an example for simplicity of explanation, multiple time slots (for example two time slots such as TS 1  and TS 2 , or TS 1  and TS 3 , or even more than two time slots), may be assigned to a particular receiver, (for example R 1 ), for higher data rate communications. The foregoing discussion also assumes that the received RF signal has been downconverted and despread. However, it should be recognized that the individual signals in each time slot may be spread using different spreading codes and, therefore, only the code associated with the desired time slot(s) is despread.  
         [0029]    Using the AGC system  50  of FIG. 5 as an example, in step  401 , the initial AGC level of an initial time frame is set based on the closed loop feedback as in a typical AGC circuit without the benefit of a stored estimate. The transmitted signal  31  sent from T 1  contained in time slot TS 1  of frame N−1 is then measured in step  402 . Next, in step  404 , the signal is compared to a predetermined reference level, and an appropriate amplification or attenuation is calculated in step  406 . An error control signal  34  for the variable amplifier  35  is then generated in step  408 , based on the result of the calculation step  406 , and is also stored in step  410  as an estimate for the next time frame. If desired, an improved estimate of the required control signal for the next time frame is determined in step  411 , whereby an average or trend is calculated for a plurality of stored control signals stored over several earlier time frames. However, this is an optional feature and the “unenhanced” control signal stored at step  410  may be utilized for further processing. The AGC system  50  is then “deactivated”, “suspended”, or “switched out” in step  412 , using synchronizer  38 A until the next occurrence of TS 1  in the subsequent frame is available for measurement, at which point the AGC system  50  is reactivated as shown in step  414 . Finally, in step  416 , the stored control signal of step  410  (or alternatively, as calculated in step  411 ) is used to set the variable amplifier  35  at an estimated level of amplification.  
         [0030]    As shown in FIG. 4, the process repeats starting at step  402 , continuing through step  416  over the course of subsequent time frames, thereby providing a sequence of stored control signals. If an average or trend is calculated, each subsequent reoccurrence of a time slot will update the control signal measurement, thereby providing a “rolling sequence” over a number of consecutive recurring time slots. The average or trend calculation of step  411  is performed on the “rolling sequence” of stored values. Utilizing this procedure, the AGC system  50  will expect the signal level of the next occurrence of TS 1  to be within a certain range of the signal level in the prior occurrence of TS 1 . This permits much more stable and accurate operation of the AGC system  50 .  
         [0031]    [0031]FIG. 6 shows another alternative embodiment of the present invention. Rather than using feedback on the output  36  of the variable amplifier  35  to determine the control signal for variable amplifier  35 , the AGC system  60  analyzes the RF input signal  31  before being processed by the variable amplifier  35 . The AGC system  60 , also a closed loop type, comprises an input  31 , a variable amplifier  35 , preamplified a measurement unit  62 , a reference and comparison unit  64 , storage unit  61 , a synchronizer  38 A, an estimate enhancement unit  63  and an output  36 .  
         [0032]    The synchronizer  38 A ensures that the AGC system  60  acts on the RF input signal  31  only during the designated time slot for the subject receiver. The preamplified measurement unit  62  measures the receive signal strength of the RF input signal  31 . The measured signal strength is then stored by input storage unit  61  for providing an estimate for subsequent receive signal strengths. Over the course of several time frames, a sequence of stored RF input signal strengths are retrieved by the estimate enhancement unit  63 , which refines the estimate of the received signal strengths of several earlier occurrences of the designated time slot by analyzing the recorded sequence for increasing or decreasing trends, or by calculating an average of the sequence. This enhanced estimate is forwarded to a converter  65  which converts the refined RF input signal from the estimate enhancement unit  63  into a gain control signal  66  using a predefined target value for the output signal  36 . The reference and comparison unit  64  utilizes the estimated gain control signal  66  only at the beginning of each desired time slot to produce the initial error control signal  34  at the beginning of the time slot.  
         [0033]    Subsequent to the beginning of the time slot, the measurement unit  32 , reference and comparison unit  64  and the variable amplifier  35  operate as a typical AGC circuit to control the gain of the variable amplifier  35  and increase or decrease the amount of amplification or attenuation as required. The output  66  is ignored during this time.  
         [0034]    Although the preamplified measurement unit  62 , the input storage  61 , the estimate enhancement unit  63 , the converter  66 , the reference and comparison unit  64 , the measurement unit  32  and the synchronizer  38 A have been described herein as separate and discrete components, it should be noted that they perform functions that may be incorporated as part of a microprocessor  71  having an associated memory (not shown), as illustrated by the embodiment shown in FIG. 7A. The variable amplifier  35  may also be incorporated into the programmed microprocessor  71  to provide a single, unitary “smart AGC”, as illustrated in FIG. 7B.  
         [0035]    While the present invention has been described in terms of the preferred embodiments, other variations which are within the scope of the invention as outlined in the claims below will be apparent to those skilled in the art.