Abstract:
A WLAN (Wireless Local Area Network) transmitter or another data communications apparatus is provided that includes a transmission section that is configured to generate signals to be transmitted, and a control section that is connected to the transmission section to control the transmission section dependent on at least two transmission parameters. The control section comprises a state transition controller that is configured to step through a plurality of predefined control states. The control section is configured to apply different transmission parameter modification mechanisms in different control states. The state transition controller is configured to determine the respective next control states based on transmission success and failure statistics.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention generally relates to data communication transmitters such as WLAN (Wireless Local Area Network) transmitters and transceivers, and corresponding methods, and in particular to the adjustment of transmission parameters. 
   2. Description of the Related Art 
   A wireless local area network is a flexible data communication system implemented as an extension to or as an alternative for, a wired LAN. Using radio frequency or infrared technology, WLAN systems transmit and receive data over the air, minimizing the need for wired connections. Thus, WLAN systems combine data connectivity with user mobility. 
   Most WLAN systems use spread spectrum technology, a wide-band radio frequency technique developed for use in reliable and secure communication systems. The spread spectrum technology is designed to trade-off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems. 
   The standard defining and governing wireless local area networks that operate in the 2.4 GHz spectrum, is the IEEE 802.11 standard. To allow higher data rate transmission, the standard was extended to the 802.11b standard that allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. This extension is backwards compatible as far as it relates to direct sequence spread technology, but it adopts a new modulation technique called CCK (Complementary Code Keying) which allows for realizing the speed increase. 
   In 802.11b compliant WLAN systems, all data rates specified in the IEEE 802.11 and 802.11b standards can be used to transmit digital data. The data rates of 1 and 2 Mbps were already possible in the 802.11 standard. The CCK related higher data rates that were introduced by the 802.11b standard, where the data rates of 5.5 and 11 Mbps. 
   Thus, an 802.11b compliant WLAN transmitter may select one of the above-mentioned four data rates for transmitting its signals. While choosing the highest data rate substantially increases the data throughput since choosing a higher data rate allows for transmitting more data in the same time, this mode cannot be used in all circumstances. For instance, if the present channel conditions are deteriorated by noise, signal reflections, interferences or other negative influences, the actual data throughput may be significantly less than what might be expected. This is because the error rate may increase so that signal retransmissions are required. 
   Another problem in WLAN systems is to choose the right transmission power. If a transmitter selects a low power level, the above signal deterioration by noise etc may still increase since the signal to noise ratio at the receiver side is decreased. However, choosing a high transmission power may not be the best choice since high power transmissions from one subscriber may then influence the signal quality of data transmissions of other subscribers. Moreover, using higher transmission powers increases the power consumption of the device what is particularly disadvantageous where the transmitter is a mobile station. 
   A technique for reducing the collision probability in WLAN systems and other data communication systems is the RTS/CTS (Request to Send/Clear to Send) mechanism. The exchange of RTS and CTS frames prior to the actual data frame is one way of distributing medium reservation information announcing the impending use of the medium. While this mechanism may significantly reduce the collision probability in high traffic systems it is not necessary in data communication systems where the channel quality is high. Rather, if the RTS/CTS exchange is performed in high quality systems, the data throughput is even reduced since the exchange of RTS and CTS frames increases the overall traffic volume where this mechanism is not necessary but nevertheless used. 
   Thus, there are a number of parameters in data communication systems which to choose may be a difficult task. As mentioned above, such parameters may be the data rate, the transmission power and the RTS/CTS threshold that indicates a frame length limit for controlling when to apply this mechanism. There may be many other parameters which need to be adjusted in data communication systems depending on the current channel situation. 
   In conventional data communication systems, adaptation techniques are applied that adjust one of these parameters to find an optimum and trace this optimum. However, such techniques usually work on only one of these parameters so that multiple mechanisms are required to optimizes all the different aspects mentioned above. This leads to a significant amount of control circuitry necessary to implement such mechanisms, and thus increase the circuit development and manufacturing costs. 
   Moreover, even when implementing different optimization techniques that each relate to a different one of the above mentioned parameters, there are no synergy effects since the individual optimization techniques would operate completely independently from each other. Moreover, such cumulative optimization mechanisms tend to exhibit instabilities which may occur since controlling one mechanism will somehow influence the conditions that form the basis of controlling another mechanism. 
   SUMMARY OF THE INVENTION 
   An improved transmission adaptation technique in data communication systems is provided that may operate more efficiently, reliably and stable. 
   In one embodiment, a WLAN transmitter is provided that comprises a transmission section that is configured to generate signals to be transmitted, and a control section that is connected to the transmission section to control the transmission section dependent on at least two transmission parameters. The control section comprises a state transition controller that is configured to step through a plurality of predefined control states. The control section is configured to apply different transmission parameter modification mechanisms in different control states. The state transition controller is configured to determine the respective next control states based on transmission success and failure statistics. 
   In another embodiment, there may be provided a data communications apparatus that comprises a transmission section that is configured to generate signals to be transmitted, and a control section that is connected to the transmission section to control the transmission section dependent on at least two transmission parameters. The control section comprises a state transition controller that is configured to step through a plurality of predefined control states. The control section is configured to apply different transmission parameter modification mechanisms in different control states. The state transition controller is configured to determine the respective next control states based on transmission success and failure statistics. 
   In a further embodiment, an integrated circuit chip for use in a data communications transmitter comprises a transmission circuit that is configured to generate signals to be transmitted, and a control circuit that is connected to the transmission circuit to control the transmission circuit dependent on at least two transmission parameters. The control circuit comprises a state transition control circuit that is configured to step through a plurality of predefined control states. The control circuit is configured to apply different transmission parameter modification mechanisms in different control states. The state transition control circuit is configured to determine the respective next control states based on transmission success and failure statistics. 
   In still another embodiment, there is provided a method of operating a data communications device for transmitting data. The method comprises controlling the generation of signals to be transmitted, based on at least two transmission parameters. The control comprises stepping through a plurality of predefined control states, applying different transmission parameter modification mechanisms in different control states, and determining the respective next control states based on transmission success and failure statistics. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are incorporated into and form a part of the specification for the purpose of explaining the principles of the invention. The drawings are not to be construed as limiting the invention to only the illustrated and described examples of how the invention can be made and used. Further features and advantages will become apparent from the following and more particular description of the invention, as illustrated in the accompanying drawings, wherein: 
       FIG. 1  is a block diagram illustrating the components of a transmission parameter adaptation device for use in a data communication transmitter according to an embodiment; and 
       FIG. 2  is a state transition diagram illustrating the control mechanism performed by the state transition controller that is a component of the device shown in  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The illustrated embodiments of the present invention will now be described with reference to figure drawings. 
   Referring now to the drawings and particularly to  FIG. 1 , components of a WLAN transmitter or transceiver according to an embodiment are shown. As will be discussed in more detail below, the embodiments apply an adaptation algorithm to adjust the data rate, the transmission power, and/or the RTS/CTS threshold based on transmission statistics. While the present embodiments relate to WLAN systems, it is to be noted that other data communication systems may also make use of the described adaptation technique. In addition, embodiments exist where only two of the three mentioned parameters are adjusted by the adaptation technique or where some or all of the parameters may be temporarily disabled. 
   As can be seen from  FIG. 1 , an amplifier  100  is provided that receives input data and outputs an amplified signal. The amplifier  100  may further receive an RTS/CTS activation signal from RTS/CTS controller  110  to transmit these frames if the RTS/CTS mechanism is enabled. The amplification gain may be controlled by the transmission power adjustment unit  130  to control the output power. 
   The amplifier  100  may act as or be a part of a transmission section of the transmitter or transceiver. In one embodiment, the transmission section may be used for transmitting data whereas a reception section is used for receiving data. In another embodiment, the amplifier  100  (or its transmision section) may be a network interface card (NIC). 
   As apparent from the figure, the input and output data lines may be bidirectional for connecting to higher network layers and to the transfer channel. The amplifier  100  of the present embodiment is arranged to operate at any 802.11b compliant data rate, i.e., in the BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), CCK-5.5, and CCK-11 mode. It is to be noted that the available data rates may in other embodiments be restricted to only a subset of these rates. Moreover, data communication transmitters in non-WLAN systems may chose data rates that are not within the above set. 
   The data rate actually used by amplifier  100  is chosen based on a control signal received from the data rate switch  120 . The data rate switch  120  of the present embodiment is the unit that issues the respective switch signals. 
   While the RTS/CTS controller  110 , the data rate switch  120 , and the transmission power adjustment unit  130  are depicted in  FIG. 1  as separate blocks, it is to be noted that these units may be implemented in other embodiments by means of one signal circuit only. Moreover, the units  110 ,  120 ,  130  may even be incorporated in the amplifier  100  or in other modules of the transmitter. 
   As can be seen from  FIG. 1 , there is further provided a state transition controller  140  that controls the RTS/CTS controller  110 , the data rate switch  120 , and the transmission power adjustment unit  130 . The state transition controller  140  of the present embodiment may be a finite state machine that is clock driven and frame based and that steps from one predefined state to another predefined state in a predetermined manner, following transition rules which may be hard coded or software based. 
   The state transition controller  140  of the present embodiment receives input values y and y R  to determine the next state which is to be to stepped to. Further, the state transition controller  140  receives control variables down_limit, power_up_limit, RTSfb_lim, and Pmax to control the transitions. These parameters may be pre-set and may be retrieved from read-only registers. 
   Further, the state transition controller  140  may receive adjustable parameters for deciding how to step through the states, i.e., parameters that are not fixed. For instance, the parameter up_limit may be received from a threshold adaptation unit  160 , and this parameter is updated depending on the current channel situation. 
   As may be further seen from  FIG. 1 , the state transition controller  140  may additionally receive control switches to enable or disable operational features of the adaptation technique. Such control switches may be the use_rts and power_adapt signals. 
   Moreover, the operation of the state transition controller  140  may depend on internal variables that may be set and updated by the state transition controller  140  when stepping through the states. Examples of such internal variables may be RTSfb and txad_stop stored in registers  180 ,  190  of the state transition controller  140 . 
   While the numerous variables, parameters, adjustable parameters, control switches and internal variables used by the state transition controller  140  of the present embodiment have been indicated so far without explaining in more detail the functionality behind these items, the following more detailed discussion will give insight into the peculiarities of the respective adaptation technique according to the embodiments. 
   Turning first to the variables y and y R , five low pass filters on transmission statistics are provided, four long term filters  170  and one short term filter  150 . All the low pass filters  150 ,  170  receive an input value x(k) of +1 if the transmission attempt of frame k was successful, and −1 if the transmission failed. The value x thus indicates transmission statistics on which the adaptation technique is based. 
   In the present embodiment, the value x may be input from the above-mentioned network interface card or reception section. 
   The short term filter  150  outputs for each input value x an output value y according to the following equation:
 
 y ( k )= c·x ( k )+ d·y ( k −1)
 
where c and d are constant values. In the present embodiment, the short term filter  150  is reset at each state transition except of individual transitions that have been preselected for not resetting the short term filter  150 . The preselected transitions will be specifically pointed to when discussing  FIG. 2 .
 
   The long term filters  170  also receive the transmission statistics value x but output long term filter values y R  where R denotes one of the different data rate modes, i.e. BPSK, QPSK, CCK-5.5, and CCK-11. The long term filter value y R  is not reset at each state transition but continuously develops in time. Similar to the above equation describing the short term filter, the long term filter values y R  are calculated based on the following equation:
 
 y   R ( k )= a·x ( k )+ b·y   R ( k −1)
 
where again, a and b are constant values.
 
   It is to be noted that the constant values a and b may be the same for each data rate mode R or may differ from mode to mode. Further, the constant values a and b may be chosen to be different from the constant values c and d. 
   It is to be noted that the short and long term filter values y, y R  of the present embodiment are chosen to grow with the transmission statistics value x. Thus, since the transmission statistics value x at a successful transmission is greater that the corresponding value at a transmission failure, higher short and long term filter values y, y R  indicate better channel conditions. 
   Before discussing in more detail the state diagram of  FIG. 2 , a synopsis is provided for explaining the various variables and parameters mentioned above. 
   The parameters up_limit and down_limit denote upper and lower data rate switching thresholds to which the short term filter value y is compared to determine whether the data rate needs to be adjusted. More specifically, if the short term filter value y exceeds the parameter up_limit, this indicates that it might be favorable to increase the data rate. Similarly, if the short term filter value y is below the value of parameter down_limit, this might indicate that it would be helpful to decrease the data rate. 
   While the lower switching threshold down_limit is a constant parameter in the present embodiment, the parameter up_limit may be adjusted based on the long term properties of the data communications channels. More specifically, the present embodiments make use of an upper limit for switching the data rate according to the following equation:
 
up_limit= g+y   R ( k )
 
where g is a constant. For this purpose, the threshold adaptation unit  160  receives the long term filter value y R  from the respective long term filter  170  for calculating the switching threshold. It is to be noted that the switching value adaptation may cause a stabilization of the data rate selection during run time.
 
   The transition controller  140  further receives the constant parameter power_up_limit that may be used similar to the parameter up_limit but which is intended to control the transmission power adaptation rather than the data rate adaptation. In the present embodiment, the value of parameter power_up_limit is chosen to be greater than the value of parameter g. If the current short term filter value y exceeds the value of the parameter power_up_limit this may be an indication to decrease the transmission power. 
   Another parameter received by the state transition controller  140  is Pmax which is the maximum transmission power that the amplifier  100  can provide. 
   Moreover, the state transition controller  140  receives the parameter RTSfb_lim which is a fallback limit for controlling the RTS/CTS mechanism. The state transition controller  140  stores in register  180  an internal variable RTSfb which is incremented each time the transmitter enters a state or remains in such a state where the RTS/CTS mechanism is activated. If the variable RTSfb exceeds the fallback limit RTSfb_lim, then the RTS/CTS mechanism is deactivated to see if it is still necessary to avoid collisions. The fallback technique therefore advantageously prevents the transmitter from keeping the RTS/CTS mechanism active and wasting traffic volume if the channel conditions were good enough to operate the transmitter without having activated this mechanism. 
   As apparent from  FIG. 1 , the state transition controller  140  stores a further internal variable txad_stop in register  190 . This variable will be suitably set and unset by the state transition controller  140  to avoid instabilities in controlling the transmission power. This will be more apparent from the discussion of  FIG. 2  below. 
   Finally,  FIG. 1  shows that the transition controller  140  receives two control switch signals use_rts and power_adapt. These control switches are used to enable or disable the RTS/CTS threshold adaptation and the transmission power adaptation mechanisms, respectively. This allows the state transition controller  140  to simultaneously adapt each of the data rate, the RTS/CTS threshold, and the transmission power, or adapt only a subset of these parameters. 
   Turning now to  FIG. 2  which is a state diagram illustrating in more detail the process of performing the adaptation algorithm according to the embodiments, there are seven states  200 - 260  which the transmitter can enter. After switching on the transmitter, or performing a reset, the transmitter starts at state  200  with a default configuration. Dependent on the value y of the short term filter  150 , the state transition controller  140  will then step to one of the states  210 ,  240 ,  250  and  260 . The state transition controller  140  may then step from state to state with the exception of state  200  that cannot be reached anymore. Thus, the available states that can be entered when continuing the transmitter operation, are states  210 - 260 . These states will now be explained in more detail. 
   In state  210 , the RTS/CTS threshold is set to a default value while the data rate is switched to the next higher data rate, if possible. In the present embodiment, the default value for the RTS/CTS threshold is chosen to be 2346, i.e., the maximum frame length plus one. Since the RTS/CTS threshold is actually a frame length limit which needs to be reached or exceeded by the current frame to have RTS frames transmitted, no RTS frames will be sent if a threshold is set to the default value. This is because no frame can exist that has a frame length greater than the default value. That is, setting the RTS/CTS threshold to the default value means to (temporarily) deactivate the RTS/CTS mechanism. 
   The state  220  is a state where the transmission power is decreased, whereas the data rate is kept constant and the RTS/CTS threshold set to its default value. Likewise, the state  230  increases the transmission power while keeping the data rate constant and setting the RTS/CTS threshold to its default value. Increasing or decreasing the transmission power may be done in 3 dB steps. 
   In state  240 , the RTS/CTS threshold is again set to the default value, thus disabling the RTS/CTS mechanism. Further, the data rate is kept constant, and no transmission power adaptation takes place. 
   The state  250  substantially corresponds to state  210  but is the state where the next lower data rate is selected, if possible. Finally, the state  260  is the state where the RTS/CTS mechanism is activated by setting the RTS/CTS threshold to a suitable value. The data rate is kept constant. 
   As apparent from  FIG. 2 , each of the states  210 - 260  may be potentially stable states, at least for a given period of time. For instance, if the short term filter value y is equal to or greater than the value of parameter up_limit and the data rate is not yet at its maximum, the state  210  may be repeatedly entered to even more increase the data rate. Similarly, the state  230  may be stable as long as the short term filter value y is less than or equal to the value of parameter down_limit and the transmission power has not yet reached its maximum Pmax. Likewise, the state  240  may be continuously held if the short term filter value is within its limits or is beyond its limits but any data rate or transmission power adaptation is disabled. The state  250  will not be left as long as the short term filter value y is at least as low as the value of the parameter down_limit and the RTS/CTS mechanism is disabled. Finally, the state  260  may be stable as long as the short term filter value y is within its limits and also the internal variable RTSfb is below its failback limit, where the internal variable is repeatedly increased in state  260 . 
   While the state  220  is not shown in  FIG. 2  as being a stable state, embodiments exist where the state  220  may be repeatedly populated. 
   Beginning with the start configuration in state  200 , the short term filter value y is checked to be within or beyond the limits specified by parameters up_limit and down_limit. If the filter value is above up_limit, state  210  is entered. If it is within the limits, the state transition controller  140  proceeds to state  240 . If however the filter value is below the value of the parameter down_limit, the state  260  is entered if the RTS/CTS mechanism is activated, and state  250  is entered otherwise. When entering state  260 , the internal variable RTSfb stored in register  180  is reset to zero, as indicated by marking the transition with an encircled number 1. 
   If the short term filter value y exceeds the value of parameter up_limit, this is generally an indication of quite good channel conditions. As already mentioned above, the short term filter value y is linearly dependent on the transmission statistics value x which is +1 if a transmission attempt has succeeded, and −1 otherwise. Thus, if the short term filter value y exceeds the data rate adaptation upper limit up_limit, the state  210  can be entered to increase the data rate, if possible. If the short term filter value y falls below the value of parameter down_limit after having increased the data rate, the state transition controller  140  will step to state  260  or  250  dependent on whether the RTS/CTS mechanism is activated. If however the highest data rate is already reached and the power adaptation mode is activated, and if the short term filter value y does not only exceed the value of parameter up_limit but also the value of the parameter power_up_limit, the state transition controller  140  will step from state  210  to state  220  since the channel quality is good enough to decrease the transmission power. As shown in  FIG. 2 , the internal variable txad_stop is set to zero when entering state  220 . 
   When having entered state  220 , the short term filter value y is checked to be within or without the limits, similar to the procedure described above. If the short term filter value y is within the data rate adaptation limits, the state transition controller  140  steps to state  240 . At the transition that is indicated in  FIG. 2  by the encircled number 2, the internal variable freeze_y R  is set to the current value of the long term filter y R . 
   If starting from state  220 , the short term filter value y is equal to or lower than the value of parameter down_limit, the state transition controller  140  will enter state  230  to increase the transmission power. When entering state  230 , the state transition controller  140  will set the internal variable txad_stop to the value of one to control what will happen if the short term filter value y should exceed even the value of up_limit in state  230 . As can be seen from  FIG. 2 , if the short term filter value y exceeds that parameter, the process will step from state  230  to state  220  if the internal variable txad_stop is zero. Otherwise, the state  240  will be entered. Thus, the variable txad_stop is used to prevent the system from toggling between states  220  and  230 . 
   If the state transition controller  140  has controlled the transmitter to enter state  240 , it is checked whether the long term filter value y R  differs from the previously stored freeze value by a predefined amount α. If so, the controller  140  will step from state  240  to  230 . The same step will be performed if the short term filter value y is equal to or below the parameter down_limit and the current transmission power has not yet reached its maximum Pmax. 
   If in state  240 , the short term filter value y is equal to or above the value of parameter power_up_limit, and if the data rate is at its maximum value (i.e. that of the CCK-11 mode), and if further the power adaptation mechanism is enabled, the state transition controller  140  will step to state  220  if the value of the variable txad_stop is zero. 
   If the short term filter value y in state  240  is equal to or below the value of parameter down_limit and the transmission power is at its maximum Pmax, the process will continue with state  250  or  260 , depending on whether the RTS/CTS mechanism is enabled. When entering state  250 , the short term filter value y is checked to be within or without the data rate adaptation limits, and state  240  may be entered if it is within the limits. If however the short term filter value y is equal to or above the upper limit, the state transition controller  140  will step to state  210 . If it is equal to or below the lower limit, the state  250  will be kept active if the RTS/CTS mechanism is turned off, and state  260  will be entered otherwise. 
   As mentioned above, state  260  is the RTS/CTS mode state and will be kept active at least until the fallback limit is reached. If however in state  260 , the short term filter value y is equal to or above the data rate adaptation upper limit, the process will step to state  210 . If the short term filter value y is equal to or below the lower limit, the process will continue with state  250 . That is, if the short term filter value y leaves the data rate limits, the RTS/CTS mechanism will be turned off and the data rate will be adapted. 
   Finally, it is to be mentioned that the power adaptation states  220 ,  230  and the data rate adaptation states  210 ,  250  are states that are not populated for a long time. If starting from these states, the short term filter value y comes within its limits so that the state transition controller  140  will be controlled to enter state  240 . 
   Given the above description of the embodiments, a transmission technique is provided where one, two or all of the following parameters are adjusted according to transmission statistics: the data rate, the RTS/CTS threshold, and the transmission power. This is done by using error filters  150 ,  170 , and an additional adaptation stability is achieved by varying the upper switching threshold up_limit. It is to be mentioned that in other embodiments, further parameters may be introduced, and all of the presently discussed and the additional parameters may be made dependent on the current statistics. 
   By using the above described technique, the lowest possible transmission power can be selected at any one time. Further, the MAC (Medium Access Control) transmission parameters RTS/CTS threshold and data rate can be adapted to currently existing WLAN conditions. It is to be noted that the technique may nevertheless apply to other data communications system than WLAN systems. 
   Moreover, by using the above described adaptation technique, a maximum network throughput can be achieved at the lowest possible transmission power. Further, RTS/CTS protection is ensured even at appearing hidden nodes. Further, the RTS/CTS fallback is ensured at disappearing hidden nodes. Moreover, it is ensured that the transmission power reduction will not take place until the carrier sensing threshold is reached, to avoid hidden nodes. 
   The above described technique according to the embodiments advantageously achieves synergetic effects by providing a unique mechanism that combines the adaptation of multiple transmission parameters. For instance, while limits are given for controlling given adaptations, the embodiments are not restricted to adapt the respective transmission parameter even if one of its limits is exceeded. Rather, another parameter may be adapted instead to achieve a more stable and reliable control. For instance, if starting from state  240  and having a short term filter value y that exceeds the upper data rate limit up_limit, the state transition controller  140  may decide in some cases to increase the transmission power rather than the data rate, and will then step to state  230  rather than state  210 . Thus, the technique of the embodiments is an in-depth mixture of multiple adaptation techniques leading to better adaptation results than just using multiple individual separate mechanisms. 
   While the invention has been described with respect to the physical embodiments constructed in accordance therewith, it will be apparent to those skilled in the art that various modifications, variations and improvements of the present invention may be made in the light of the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention. For instance, whenever a transmitter was mentioned in the above description, it is to be understood that this term may relate to any transmitting device including transceivers. 
   In addition, those areas in which it is believed that those of ordinary skill in the art are familiar, have not been described herein in order to not unnecessarily obscure the invention described herein. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrative embodiments, but only by the scope of the appended claims.