Abstract:
A decision feedback loop for a receiver for channel estimation and de-rotation of complex input symbols derived from a sampled information signal received via a channel which contains data organized into successive time slots, is configured to retrain and reinitialize the loop during each slot in order to mitigate slot to slot propagation of the estimation error.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to telecommunications systems, for example wireless digital cellular systems, employing complex or quadrature modulated information signals containing data organized into successive time slots, each slot containing a series of pilot bits and a series of data bits, and more specifically to an apparatus and method for channel estimation and de-rotation of such an information signal received via a channel. 
     2. Description of the Related Art 
     In planned third generation digital wireless cellular systems known as Uniform Mobile Telephone System (UMTS), Wideband Code Division Multiple Access (W-CDMA), and Third Generation (e.g. 3G Partnership Project) spread spectrum information signals are used which contain data grouped into slots, each slot consisting of a predetermined series of N pilot  pilot bits in a first portion of the slot and a series of N data  data bits in the second portion of the slot. It is known to use various types of filtering schemes, ranging from simple to complex, to achieve channel estimation and de-rotation of a despread received spread spectrum information signal by synchronizing with the pilot bits. The nature of the estimation error achieved with prior art filtering schemes varies, but generally the error propagates from slot, sometimes increasing over time. 
     While decision feedback loops are known for other purposes, the prior art has not considered the possibility of using a decision feedback loop for channel estimation and de-rotation of a spread spectrum signal received via a channel. 
     OBJECT AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a decision feedback loop apparatus and method for a receiver for channel estimation and de-rotation of a received signal. It is a further object that the decision feedback loop is implemented in a manner to mitigate propagation of the estimation error from slot to slot. 
     This and other objects of the present invention are satisfied by a decision feedback loop which uses the known sequence of pilot bits to initialize and train the feedback loop during each slot. This continued sloe by slot re-initialization and re-training prevents the estimation error in one slot from propagating to the next. 
     In accordance with the invention, a decision feedback loop apparatus for a receiver for channel estimation and de-rotation of complex input symbols derived from a sampled information signal received via a channel, which information signal contains data organized into successive time slots, each slot containing a predetermined series of pilot bits during a first portion of the slot and a series of data bits during a second portion of the slot, comprises a first multiplier for multiplying the complex input symbols with estimated conjugate channel coefficients, which are derived from a feedback signal, to form complex soft symbols to be used for channel decoding, a hard decision device for forming complex hard symbols from the complex soft symbols, a pilot generator for generating a series of complex pilot symbols corresponding to the predetermined series of pilot bits in the information signal, and a second multiplier for multiplying a first signal at a first input, which is derived from the complex input signal, with a second signal at a second input to form a feedback signal at an output. The invention is characterized in that the second signal is derived from the complex pilot symbols during the first portion of the slot and from the complex hard symbols during the second portion of the slot. 
     Similarly, in accordance with the invention, a decision feedback loop method for a receiver for channel estimation and de-rotation of complex input symbols derived from a sampled information signal received via a channel, which information signal contains data organized into successive time slots, each slot containing a predetermined series of pilot bits during a first portion of the slot and a series of data bits during a second portion of the slot, comprises multiplying the complex input symbols with estimated conjugate channel coefficients, which are derived from a feedback signal, to form complex soft symbols to be used for channel decoding, forming complex hard symbols from the complex soft symbols, generating a series of complex pilot symbols corresponding to the predetermined series of pilot bits in the information signal, and multiplying a first signal, which is derived from the complex input signal, with a second signal to form a feedback signal. The inventive method is characterized by the act of deriving the second signal from the complex pilot symbols during the first portion of the slot and from the complex hard symbols during the second portion of the slot. 
     Another aspect of the invention is that the estimated channel coefficients are derived by applying a filter to the feedback signal. 
     Other objects, features and advantages of the present invention will become apparent upon perusal of the following detailed description when taken in conjunction with the appended drawing, wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a simplified decision feedback loop for channel estimation and de-rotation of complex input symbols derived from a sampled information signal in accordance with the present invention; 
     FIG. 2 shows the time slot structure of the information signal; 
     FIG. 3 shows a more detailed decision feedback loop which corresponds to an alternative embodiment to that shown in FIG. 1; 
     FIG. 4 shows a wireless telecommunications system including a mobile station having a channel estimation and de-rotation decision feedback loop in accordance with the invention; and 
     FIG. 5 shows a wireless handset or mobile station for incorporating the decision feedback loop for receiving purposes. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIG. 4 of the drawing for the purpose of orientation, there is shown a mobile station MS transceiver in communication with a base station BS of a wireless cellular telecommunication system, e.g. of the UMTS, W-CDMA, 3GPP types employing spread spectrum signals transmitted and received via the antennae A′, A. At the functional level of detail shown in FIG. 4, mobile station MS is conventional including a receive/transmit switch or diplexer  100  coupling antenna A to the input of RF receiving apparatus, including a cascade of a RF section receive processing block  200   a  and a baseband section receive processing block  300   a , and also coupling antenna A to the output of RF transmitting apparatus, including a cascade of a baseband section transmit processing block  300   b  and a RF section transmit processing block  200   b.    
     RF section receive processing block  200   a  includes a cascade of a low noise amplifier  210  and frequency downconverter  220  for conversion from RF to baseband, e.g. a direct conversion quadrature mixer (not shown), and baseband section receive processing block  300   a  includes a cascade of an analog to digital converter  310 , a complex despreader  320  for applying a despreading code, a channel estimation and de-rotation decision feedback loop  330 , a channel decoder  340  having a digital output for received decoded data signals, and a digital to analog converter  350  for producing an analog output e.g. representing received decoded voice signals. These digital and analog outputs are provided to a user interface  400  for use by and/or sensory stimulation of a user, and the user interface provides to analog and digital inputs of baseband section transmit processing block  300   b  user responsive voice and/or data signals, respectively. 
     Baseband section transmit processing block  300   b  digitally encodes and applies a spreading code to the voice signals, after an analog to digital conversion, and also encodes and spreads the data signals, converts the encoded and spread signals to digital form, and supplies these encoded and spread signals, after a digital to analog conversion, to RF section transmit processing block  200   b  for power amplification and frequency upconversion to RF. 
     For further orientation, reference is made to FIG. 5 of the drawing which shows the mobile station MS as including the antenna A, an RF section  500  (which implements the receive transmit switch or diplexer  100  and RF section receive and transmit processing  200   a ,  200   b  of FIG.  4 ), a baseband section (which implements baseband section receive and transmit processing  300   a ,  300   b  of FIG.  4 ), and a user interface section (which implements user interface  400  of FIG.  4 ). Baseband section  600  includes digital signal processor (DSP)  610 , microprocessor (lP)  120 , read only memory (ROM)  630 , random access memory (RAM) 640 , analog to digital converter (A/D)  650 , and digital to analog converter (D/A)  660 . User interface section  700  includes microphone  710 , speaker  720 , keypad  730 , and a display driver  740  which drives an LCD display  750 . 
     The present invention pertains particularly to channel estimation and de-rotation decision feedback loop  330  of FIG. 4 which is specially configured for slot by slot re-initialization and re-training. As is conventional, the channel estimation and derotation functionality is implemented by DSP operating on program instructions stored in ROM  630 . 
     The relevant slot structure, as shown in FIG. 2, is seen to comprise a series of time slots, e.g. i−2, 1−1, i, i+1, i+2, each consisting of a predetermined sequence of N pilot  pilot symbols during a first portion of the slot followed by a larger number, N data , of data symbols during a second portion of the slot. 
     Channel estimation and de-rotation decision feedback loop  330  is shown in simplified form in FIG. 1 in conjunction with complex despreader  320  which supplies complex input symbols to a first inputs of first and second multipliers  332 ,  333  of feedback loop  330 . First multiplier  332  also receives at a second input estimated complex conjugate channel coefficients E, and produces at its output complex soft symbols supplied to channel decoder  340  (FIG. 5) and also to a hard decision device  335  which converts the complex soft symbols to complex hard decisions. The output of hard decision device  335  is applied to one input of a selector device  336 , shown as a switch, whose other input is fed by the output of complex pilot symbols generator  339 , and whose output is coupled to a second input of second multiplier  333  via a complex conjugate device  338 . Selector switch  336  is controlled by a timing device  337  such that the signal coupled to the second input of second multiplier  333  is derived from the complex pilot symbols output from generator  339  during the first portion of the slot and from the complex hard decisions output from hard decision device  335  during the second portion of the slot. 
     Optionally, complex conjugate device  338   a  may be utilized which is located in the path between the output of complex despreading device  320  and the first input of second multiplier  333  rather than locating complex conjugate device  338  in the path between the output of selector device  336  and the second input of multiplier  333 . In either event, the output of second multiplier  333  is a feedback signal F which is applied to a low pass filter  334  whose output constitutes the estimated complex conjugate channel coefficients E applied to the second input of first multiplier  332 . 
     FIG. 3 illustrates channel estimation and de-rotation decision feedback loop  330  in more detail, utilizing the alternative in which the complex conjugate device appears intermediate the output of despreader  320  and the first input of second multiplier  337  after a one sampling interval delay  337 . Hard decision device  334  is seen to comprise a complex to real/imaginary device having real and imaginary component outputs which are compared with a threshold of zero in comparators  330   b  and  330   c  respectively, to produce binary hard decisions. These binary hard decisions for the real and imaginary components are applied to binary to numeric converters  330   d  and  330   e , respectively, the outputs of which feed device  330   f  for forming a complex numeric therefrom. The output of device  330   f  is applied to one input of selector device  336  via one sampling interval delay  330   g , and the output of selector device feeds the second input of multiplier  333 . 
     Complex pilot symbols generator  339  comprises a pilot vector generator  339   a  which generates at its output the known sequence of pilot symbols in the form of a complex vector. The output of generator  339   a  is applied to the input of a complex vector to scalar converter  339   b  via one sampling interval delay  339   b , and the output of converter  339   c  forms the output of complex pilot symbols generator  339  which the other input of selector device  336 . 
     Further, the low pass filter  334  between the output of second multiplier  333  and the input of first multiplier  332  is seen to be implemented by an infinite impulse response (iir) digital filter. 
     It should now be appreciated that the objects of the present invention have been satisfied by the present invention since the decision feedback loop will effectively retrain and reinitialize due to the introduction of the generated pilot sequence into the feedback loop during each slot for correlation with the received pilot sequence. 
     While the present invention has been described in particular detail, it should also be appreciated that numerous modifications are possible within the intended spirit and scope of the invention. In interpreting the appended claims it should be understood that: 
     a) the word “comprising” does not exclude the presence of other elements or acts than those listed in a claim; 
     b) the word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. 
     c) any reference signs in the claims do not limit their scope; and 
     d) several “means” may be represented by the same item of hardware or software implemented structure or function.