Abstract:
A digital transmitter includes a digital-to-digital sigma-delta modulator (SDM) receiving digital baseband data (f BB ) which sigma-delta modulates the baseband signal to generate a digital SDM signal. The digital SDM signal is digitally mixed with an oscillation signal (f LO ) to frequency shift the baseband data to f LO +/−f BB . The frequency shifted signal is filtered to remove either the upper or lower frequency band and the remaining signal is converted to an RF analog signal for eventual RF transmission. In another embodiment, the digital-to-digital SDM is a multi-level SDM that generates N parallel binary (digital) waveform signals which are each individually mixed with the oscillation signal. The resulting N frequency shifted SDM signals are summed and filtered, or in the alternative are each filtered and summed, to generate the RF analog signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 USC 119(e) to U.S. provisional Application Ser. No. 61/009,660, filed on Dec. 31, 2007, and which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to transmitters and, in particular, to a digital transmitter for use in communications. 
     BACKGROUND 
     In most current digital wireless systems, the traditional up-conversion chain (or significant portion thereof) is primarily analog and includes types such as super-heterodyne, low intermediate frequency (IF) and zero IF up-conversion technology. These technologies start with the conversion of inherently digital signals to analog signals through high performance digital-to-analog (D/A) converters, generally due to the higher frequencies involved. Once converted to the analog domain, various combinations of analog filters, amplifiers, mixers and modulators (and perhaps other analog elements) are cascaded to achieve the up-conversion from the output of the A/D converter(s) to the radio frequency (RF) band of interest (transmit RF signal). 
     Component variation, tolerances, and aging all affect the design requirements, costs, and manufacturability of the analog up-conversion (transmitter) and down-conversion (receiver) chains. One digital transmitter design, as described in U.S. Pat. No. 6,987,953 which is incorporated herein by reference, reduces or eliminates some or all of these problems by utilizing digital upconversion in the digital signal processing block. However, the output frequency is linked to the digital processing clock. 
     Accordingly, there is needed a digital transmitter that separates the digital signal processing from the RF translation. Utilization of digital frequency translation further allows for a transmitter design without the traditional need for high quality analog components. 
     SUMMARY 
     According to the present invention, there is provided a digital transmitter including a digital-to-digital sigma delta modulator for receiving a baseband digital signal and generating a modulated digital signal, a digital mixer for mixing the modulated digital signal with a digital oscillation signal to generate a frequency shifted modulated digital signal, and a filter for receiving and filtering the frequency shifted modulated digital signal and generating an analog radio frequency (RF) signal for RF transmission. 
     In another embodiment of the invention, there is provided a digital transmitter including a multi-level digital-to-digital sigma delta modulator for receiving a baseband digital signal and generating N modulated digital signals, a digital mixer for mixing each of the N modulated digital signals with a digital oscillation signal to generate N frequency shifted modulated digital signals, and a summation and filtering circuit for receiving, summing and filtering the N frequency shifted modulated digital signals to generate an analog radio frequency (RF) signal for RF transmission. 
     In yet another embodiment, there is provided a wireless communications device including a digital transmitter and an antenna. The digital transmitter includes a digital-to-digital sigma delta modulator for receiving a baseband digital signal and generating a modulated digital signal, a digital mixer for mixing the modulated digital signal with a digital oscillation signal to generate a frequency shifted modulated digital signal, and a filter for receiving and filtering the frequency shifted modulated digital signal and generating an analog radio frequency (RF) signal for RF transmission. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which: 
         FIG. 1  is a block diagram of a portion of a digital transmitter in accordance with the present disclosure; 
         FIG. 2  is a block diagram of another embodiment of a digital transmitter; and 
         FIG. 3  illustrates an exemplary wireless communications network, including communication devices incorporating the transmitter described in the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present disclosure describes a digital transmitter within a wireless communication device in a communications system. Numerous portions or aspects of the transmitter are omitted for brevity, and only those elements or devices necessary or relevant to an understanding of the present disclosure are described or shown herein. 
     With reference to  FIG. 1 , there is shown a relevant portion of a digital transmitter  100  in accordance with the present disclosure. The transmitter  100  includes a digital signal processing or processor (DSP) core  102 . The DSP core  102  generates coded I (in-phase) and Q (quadrature) digital baseband signals (in other embodiments, I and Q may not be utilized). The I and Q digital baseband signals each typically comprise a stream of samples or bits representing a digital value, or word having n bits. The baseband data signals have a baseband frequency f BB . 
     As will be appreciated, the processing, generation and functionality utilized to generate the I and Q digital baseband signals that are output from the DSP core  102  are not shown or described. This is known to those of ordinary skill in the art. In general terms, the digital data is processed by encoding, interleaving, converting, and perhaps spreading (using orthogonal codes and psuedo-random (PN codes)) to generate the I and Q digital baseband signals (often referred to as samples at a particular sampling rate). 
     It will be understood that the modulation and/or coding scheme utilized in the present disclosure is not limited to quadrature (I and Q) modulation or coding, and other modulation or coding techniques may be utilized with modifications to the present disclosure. In addition, the I and Q baseband signals may relate to a single carrier or multiple (1 to N) carriers. Utilization of I and Q baseband signals permit zero IF upconversion which reduces distortion. 
     The baseband signal(s) output from the DSP core  102  is input to a sigma-delta modulator (SDM)  104 . The SDM  104  is implemented as a digital-to-digital SDM that receives digital data as an input and generates a digitally coded output at a higher data rate. The sigma-delta modulated output signal is mixed digitally with a local oscillator signal at f LO  by a digital mixer  106 . A local oscillator  108  generates the oscillator signal (in binary form, referred to as “digital”). The digital mixer  106  outputs a digitally mixed output signal with the baseband data information at f LO +/−f BB . A filter  110  selects one frequency range for the output signal at f out . The filter  110  may be analog or digitally based, such as a finite impulse response (FIR) filter. The signal f out  from the filter  110  has the form of a high quality radio frequency (RF) signal ready for RF transmission (e.g., carrier frequency mixing, power amplification and transmission). The baseband information has been digitally translated or shifted in frequency. 
     It will be understood that the structure and implementation of a sigma-delta modulator (SDM) is well-known to those skilled in the art, and therefore, no specific design or implementation is described herein. The local oscillator  108  may be external (off-chip) which allows the other integrated components in the transmitter  100  to be frequency agnostic. Only the final oscillator (carrier frequency) determines the final output frequency. Therefore, the integrated circuit(s) implementing the digital transmitter  100  may be useful for radio systems at different frequencies. Further, the separation of the DSP core  102  operation and the frequency translation (digital upconversion) permits SDM signal characteristics to be optimizable independently from the RF translation. 
     The design of the transmitter  100  has several benefits and advantages. The entire signal chain operates at binary levels (digital). The filter  100  provides a digital to analog conversion process. The oscillator signal f LO  is not limited to the same clock frequency used in the DSP core  102  or the SDM  104 . Since the signals are binary, the digital mixer  106  does not require high linearity in order to keep distortion products minimized. Timing alignment of the signal may still be controlled by the DSP core  102 . The local oscillating signal and mixer  106  are only used to frequency translate the baseband information. 
     As will be appreciated, the transmitter  100  shown in  FIG. 1  illustrates a single-bit implementation, i.e., the SDM  104  output is a single-bit stream. Now referring to  FIG. 2 , there is illustrated a digital transmitter  100   a  in accordance with the present disclosure utilizing multi-level or multi-bit SDM signals. The transmitter  100   a  includes a DSP core  102   a . The baseband signal(s) output from the DSP core  102   a  is input to a multi-level sigma-delta modulator (SDM)  104   a . The SDM  104   a  is implemented as a multi-level (or multi-bit output) digital-to-digital SDM that receives digital data as an input and generates N parallel binary (e.g., +1/−1, 1/0, HMO) digitally coded outputs at a higher data rate. Each of the sigma-delta modulated output signals are individually mixed digitally with a local oscillator signal at f LO  by a digital mixer  106   a . A local oscillator  108   a  generates the oscillator signal (in binary form, referred to as “digital”). The digital mixer  106   a  outputs N frequency translated parallel binary digital signals each with baseband data information at f LO +/−f BB . 
     A summation circuit  109   a  sums together the N frequency translated parallel binary digital signals to produce a multi-level SDM signal that includes the original multi-level SDM baseband information, but with the baseband information shifted in frequency. A filter  110   a  selects one frequency range for the output signal at f out . In one embodiment, the summation and filtering functions are implemented in a summation/filter circuit. 
     In another embodiment (not shown), the summation circuit  109   a  and filter  110   a  are replaced with a filter bank (independently filtering each of the N signals) followed by a summation circuit. 
     RF Communications Network 
     Now referring to  FIG. 3 , there is illustrated a block diagram of an exemplary wireless communications network  1120 . The wireless communications network  1120  includes a first wireless communications device  1100  and a second wireless communications device  1104 . The first wireless communications device  1100  is shown including the transmitter  100  as described above and in accordance with the present disclosure. Similarly, the second wireless communications device  1104  includes the transmitter  100  as described above and in accordance with the present disclosure. Each of the devices  1102  and  1120  include a receiver  700  (these may be similar or different). It will be understood it is not necessary for both of the devices  1100  and  1104  to include the transmitter  100 —either one or both may include the transmitter  100 . 
     The two communications devices  1100  and  1104  communicate via RF signals utilizing an antenna  1102  and an antenna  1106 , respectively, as shown. 
     The exemplary wireless communications network  1120  may operate in accordance with one or more wireless protocols or technologies, such as CDMA, TDMA, FDMA, UMTS, etc. (and versions thereof). Further, the network  1120  may support circuit-switched, and packet-switched or packet data communications. 
     In the embodiment in  FIG. 3 , the first communications device  1100  is illustrated as a mobile station or mobile terminal (or possibly fixed), such as a wireless handset, while the second communications device  1104  is illustrated as a base station, though not limited to such embodiment. The devices  1100 ,  1104  may be any device having wireless communications capabilities. As shown, the base station  1104  includes a base transceiver subsystem (BTS)  1108  that includes the transmitter  100 . The BTS  1108  is connected to a base station controller (BSC)  1110 . Collectively, the BTS  1108  and the BSC  1110  are logically referred to as the “base station”  1104 . Multiple BTS  1108  sometimes share one BSC  1110 . The BSC  1110  manages resource allocation among the several BTSs. More generally, the terms “base station” and “access network” refer to any entity (or collection of entities) that communicates wirelessly with mobile stations for communications sessions (e.g., circuit-switched or packet-switched). The base station  1104  is coupled to the public switched telephone network (PSTN) or other data or switched network. This path may include additional elements such as a mobile switching center (MSC) (not shown) coupled to the BSC  1110 . 
     In some embodiments, some or all of the functions or processes of the one or more of the devices are implemented or supported by a computer program that is formed from computer readable program code and that is embodied in a computer readable medium. The phrase “computer readable program code” includes any type of computer code, including source code, object code, and executable code. The phrase “computer readable medium” includes any type of medium capable of being accessed by a computer, such as read only memory (ROM), random access memory (RAM), a hard disk drive, a compact disc (CD), a digital video disc (DVD), or any other type of memory. 
     It may be advantageous to set forth definitions of certain words and phrases used throughout this patent document. The terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. The term “or” is inclusive, meaning and/or. The phrases “associated with” and “associated therewith,” as well as derivatives thereof, mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like. 
     While this disclosure has described certain embodiments and generally associated methods, alterations and permutations of these embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure, as defined by the following claims.