Abstract:
A low IF mixer and method for down-converting a signal at a desired frequency are disclosed with improved selectivity performance. The energy of sidebands on each side of the desired frequency is evaluated; and a local oscillator frequency is selected based on the evaluation of the energy. Generally, the local oscillator frequency associated with the sideband having a lower energy is selected. The desired frequency may have a frequency of RF and the sidebands have a frequency of the desired frequency plus or minus an offset frequency (RF+IF or RF−IF). The signal at the desired frequency may be multiplied by the selected local oscillator frequency to down-convert the signal.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to wireless communication systems, such as wireless local area networks (WLANs), and more particularly, to a low IF mixer for down-conversion of a received signal.  
       BACKGROUND OF THE INVENTION  
       [0002]     Low Intermediate Frequency (LIF) receivers have been proposed for use in wireless networks to provide improved sensitivity and noise characteristics. Generally, communication signals are transmitted at a desired frequency, f o , obtained by multiplying the original base band signal by a carrier frequency. In a Low-IF receiver, the desired frequency, RF, is first down-converted to an intermediate frequency, IF, so that the unwanted DC content can be filtered out, before further down conversion is performed to the information carrying base band signal, in a known manner. The down-conversion process at the receiver causes adjacent channel interference (ACI) at a frequency of LO−IF, as shown in  FIG. 1 .  
         [0003]     These partial images of the desired frequency signal on each side of the desired frequency cannot be filtered out. Thus, following the down conversion process, the adjacent channel interference will cause distortion in adjacent channels (i.e., increased out-of-channel energy). Thus, Low-IF receivers are said to exhibit poor performance for handling adjacent signals. Nonetheless, Low-IF receivers demonstrate superior sensitivity since they do not suffer from in-channel distortion, such as DC offset and IQ imbalance distortion. A need therefore exists for a low IF receiver that exhibits improved adjacent channel rejection.  
       SUMMARY OF THE INVENTION  
       [0004]     Generally, a low IF mixer and method for down-converting a signal at a desired frequency are disclosed with improved selectivity performance. The energy of sidebands on each side of the desired frequency is evaluated; and a local oscillator frequency is selected based on the evaluation of the energy. Generally, the local oscillator frequency associated with the sideband having a lower energy is selected. The desired frequency may have a frequency of RF and the sidebands have a frequency of the desired frequency plus or minus an offset frequency (RF+IF or RF−IF). The signal at the desired frequency may be multiplied by the selected local oscillator frequency to down-convert the signal.  
         [0005]     A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  illustrates the frequency spectrum prior to down-conversion at the receiver;  
         [0007]      FIG. 2  illustrates a wireless network environment in which the present invention can operate;  
         [0008]      FIG. 3  is a schematic block diagram of an exemplary transmitter/receiver station incorporating features of the present invention;  
         [0009]      FIG. 4  is a schematic block diagram of a low IF mixer incorporating features of the present invention;  
         [0010]      FIGS. 5 and 6  illustrate the RF power as a function of frequency;  
         [0011]      FIG. 7  is a flow chart describing an exemplary sideband selection process for selecting the upper or lower sideband in the low IF mixer of  FIG. 4 . 
     
    
     DETAILED DESCRIPTION  
       [0012]      FIG. 2  illustrates a wireless network environment  200  in which the present invention can operate. The wireless network environment  200  may be, for example, a wireless LAN or a portion thereof. As shown in  FIG. 2 , a number of stations  300 - 1  through  300 -N, collectively referred to as stations  300  and discussed below in conjunction with  FIG. 3 , communicate over one or more wireless channels in the wireless digital communication system  200 . An access point  220  is typically connected to a wired distribution network  205  with other access points (not shown). The access point  220  typically provides control and management functions, in a known manner. In addition, the access point  220  acts as a central node through which all traffic is relayed so that the stations  300  can rely on the fact that transmissions will originate from the access point  220 . The wireless network environment  200  may be implemented, for example, in accordance with the IEEE 802.11 standard or the various extensions to the 802.11 standard, such as 802.11a, b and g, or the HIPERLAN/2 standard.  
         [0013]     The exemplary IEEE 802.11 protocol specifies that all communications are relayed via the access point  220 , so each transmission that is of interest (other access points  220  may be active on the same radio channel) is from the access point  220  the stations  300  is associated with. An example of such a communications protocol is the Basic Service Set (BSS) mode of the IEEE 802.11 protocol, in which stations  300  are associated with an access point  220  that relays all communication. An alternate scenario is the Independent Basic Service Set (IBSS) where two stations can directly communicate.  
         [0014]     Thus, typically, a station  300  has associated with a nearby access point  220  and has been assigned a channel for the link. Another nearby AP (not shown) will typically be operating on an adjacent channel. The station  300  could actually be in a location where the unwanted adjacent channel signal from the neighboring access point  220  or another client is actually stronger than the wanted signal from the access point  220  in the link, referred to as the Near-Far Problem.  
         [0015]     The present invention recognizes that in most Wireless LAN environments, however, the long-term average of upper/lower adjacent channel interference shows unequal energy levels, with one of the adjacent channels being a more dominant source of interference. This is illustrated in  FIG. 6 , where the exemplary lower adjacent channel interference has a higher energy level than the upper adjacent channel interference.  
         [0016]     According to one aspect of the invention, a receiver  300  employs a low IF mixer  400 , as discussed further below in conjunction with  FIG. 4 , that exhibits improved adjacent channel rejection. As discussed hereinafter, the receiver scans the energy in the upper and lower adjacent channels and selects the Local Oscillator (LO) frequency for down-conversion based on the adjacent channel that has the lower interference.  
         [0017]      FIG. 3  is a schematic block diagram of an exemplary transmitter/receiver station  300  (or alternatively, an access point  220 ) incorporating features of the present invention. The stations  300  may each be embodied, for example, as personal computer devices, or any device having a wireless communication capability, such as a cellular telephone, personal digital assistant or pager, as modified herein to provide the features and functions of the present invention. As shown in  FIG. 3 , a signal is received by an antenna  340  that provides the signal to RF circuitry  330 . As shown in  FIG. 3 , the RF circuitry  330  includes a low IF mixer  400 , as discussed further below in conjunction with  FIG. 4 , that operates during the down conversion to baseband, before the signal is applied to a baseband processor  320 . Thereafter, the baseband signal is supplied to a Medium Access Controller (MAC)  305 .  
         [0018]      FIG. 4  is a schematic block diagram of a low IF mixer  400  incorporating features of the present invention. The low IF mixer  400  provides a selectable Upper/Lower Sideband of operation, whereby the LO frequency of the upper or lower sidebands is selected for down-conversion based on the upper or lower adjacent channel having the lower interference.  
         [0019]     As shown in  FIG. 4 , the low IF mixer  400  includes an amplifier  410  that amplifies the received signal. A local oscillator  415  generates an LO signal of either RF+IF or RF−IF. As discussed below in conjunction with  FIG. 7 , the local oscillator  415  generates the LO signal based on a scan of the energy in the upper and lower sidebands. The upper or lower sideband having the lowest interference is selected and the corresponding LO frequency is generated. The amplified received signal and the generated LO signal are applied to a multiplier  420 . The output of the multiplier  420  is filtered by a complex low pass filter  430 , based on the selected sideband  425 .  
         [0020]     Thus, the low IF mixer  400  has a selectable upper/lower sideband of operation. The upper or lower sideband is selected based on the upper and lower sidebands having the lowest measured ACI. In this manner, the selectivity performance of the LIF receiver is improved over a receiver with a sideband of operation.  
         [0021]      FIG. 5  illustrates the RF power as a function of frequency, where the ACI of the upper sideband  530  is higher than the ACI of the lower sideband  510  as determined by the energy scan of the sidebands (first scenario). As discussed below in conjunction with  FIG. 7 , the low IF mixer  400  then chooses a local oscillator frequency of RF−IF such that the receiver image is then placed in the lower adjacent channel where the energy level is lower, effectively giving the receiver improved selectivity performance. In other words, the LO frequency is selected just below the desired channel band.  
         [0022]      FIG. 6  illustrates the RF power as a function of frequency, where the ACI of the lower sideband  610  is higher than the ACI of the upper sideband  630 , as determined by the energy scan of the sidebands (second scenario). As discussed below in conjunction with  FIG. 7 , the low IF mixer  400  then chooses a local oscillator frequency of RF+IF such that the receiver image is then placed in the upper adjacent channel where the energy level is lower, effectively giving the receiver improved selectivity performance. In other words, the LO frequency is selected just above the desired channel band.  
         [0023]      FIG. 7  is a flow chart describing an exemplary procedure for selecting the upper or lower sideband in the low IF mixer  400  of  FIG. 4 . As shown in  FIG. 7 , the sideband selection process  700  initially scans the energy in the lower and upper adjacent channels during step  710 . Thereafter, the LO frequency is selected for down-conversion during step  720  based on the adjacent channel having the lowest energy.  
         [0024]     It is noted that while the sideband selection process  700  is illustrated as a flow chart, the selection algorithm can also be implemented in hardware, as would be apparent to a person of ordinary skill in the art.  
         [0025]     It is to be understood that the embodiments and variations shown and described herein are merely illustrative of the principles of this invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.