Abstract:
An architecture for a receiver component in a wireless communications system is disclosed—one that supports both zero intermediate frequency (ZIF) and near-zero intermediate frequency (NZIF) operation. The architecture provides a down-conversion segment, and a local oscillator segment operatively associated with the down-conversion segment. An analog-to-digital conversion (ADC) segment is adapted to receive signals from the down-conversion segment and introduce the signals into a digital intermediate frequency (DIF) construct. The DIF construct performs a DC offset compensation or DC residue filtering on NZIF-based signals, and droop or mismatch compensation. Image removal is performed on NZIF-based signals, and DC offset compensation is performed on ZIF-based signals. Compensated signals are amplified to some nominal or desired level, and interpolation filtering of the amplified signals is performed prior to transmission thereof.

Description:
CROSS-REFERENCE TO RELATED APPLICATION AND CLAIM OF PRIORITY  
       [0001]     The present application is related to U.S. Provisional Patent No. 60/653,780, filed Feb. 17, 2005, entitled “Mobile Terminal Multi-Mode Common Core Receiver with Configurable Direct Downconversion and Near-ZIF Architecture”. U.S. Provisional Patent No. 60/653,780 is assigned to the assignee of the present application and is hereby incorporated by reference into the present disclosure as if fully set forth herein. The present application hereby claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent No. 60/653,780.  
     
    
     TECHNICAL FIELD OF THE INVENTION  
       [0002]     The present application relates generally to the field of wireless communications technologies and, more particularly, to apparatus and methods for providing a single receiver that supports both zero intermediate frequency (ZIF) and Near-ZIF operational modes.  
       BACKGROUND OF THE INVENTION  
       [0003]     Increasing demand for more powerful and convenient data and information communication has spawned a number of advancements in communications technologies, particularly in wireless communication technologies. A number of technologies have been developed to provide the convenience of wireless communication in a variety of applications, in various locations. This proliferation of wireless communication has given rise to a number of manufacturing and operational considerations.  
         [0004]     There are an increasing number of fixed and portable wireless applications that require, or can benefit from, operation in accordance with a plurality of communications standards or operational protocols. This is commonly referred to as multi-mode operation. Multi-mode capabilities in wireless communication products allow end-users to purchase a single product that may be used in a variety of locations for reasonable length of time—despite any proliferation of or changes in new technologies or standards. Multi-mode capabilities across wireless networks allow providers to offer new, advanced services to a broader range of customers, while fulfilling the needs of their legacy customer base. Thus, wireless base stations and mobile devices need to support portions of emerging standards, as well as revenue producing existing standards for backward compatibility.  
         [0005]     Although multi-mode support can be very desirable, it also presents a number of challenges when designing a multimode product—particularly when attempting to address the needs of disparate or competing communications standards or technologies. Commonly, the particular standard or technology associated with each “mode” of a multi-mode device requires substantially unique componentry or circuitry. As such, wireless system designers very rarely—if ever—provide a truly universal, single multi-mode device. Usually, conventional multi-mode systems comprise several devices—each designed to address one particular communications standard or technology—that are packaged together as a single multi-mode product.  
         [0006]     Consider, for example, two such technologies which are continuously growing in usage and deployment and, as such, are increasingly targeted for inclusion in multi-mode devices. Wideband Code Division Multiple Access (WCDMA) is a third-generation (3G), wideband, spread-spectrum mobile telecommunication air interface that utilizes code division multiple access multiplexing (CDMA). GSM (Global System for Mobile communications) is currently one of the most popular standards for mobile phones in the world. Although GSM continues to evolve, it is widely regarded as a narrow-band, second generation (2G) technology. Various improvements nonetheless seek to keep GSM viable—such as higher speed data transmission introduced with Enhanced Data rates for GSM Evolution (EDGE) technolgoy.  
         [0007]     WCDMA has a much more complex physical layer structure and operation than GSM—due, at least in part, to its much wider signal band. Along with additional complexity come a number of additional specifications and requirements. For example, WCDMA systems commonly utilize a ZIF (zero intermediate frequency) based architecture—due to their wideband operation—whereas GSM systems typically rely upon near-ZIF (NZIF) architectures compatible with their relatively narrow band operations.  
         [0008]     In order to design a multi-mode product that conforms to both architecture and operational schemes, conventional devices and systems usually combine many different components and modules, each targeted to support operation and processing in either a ZIF or NZIF protocol. Most such conventional components and modules are designed to function only in one operational context—either the wideband (ZIF) or the narrow band (NZIF)—not in both. As a result, a large number of inefficiencies are introduced to the production and operation of conventional multi-mode devices, which increase device and system costs and introduce a greater potential for system reliability and performance problems.  
         [0009]     As a result, there is a need for a system that provides a single receiver or transceiver architecture that efficiently supports both wideband and narrow band operational modes (i.e., ZIF and NZIF)—obviating the need for multiple, specialized components within a single multi-mode device—while providing efficient and dependable wireless communications, in an easy and cost-effective manner.  
       SUMMARY OF THE INVENTION  
       [0010]     A versatile system, comprising various apparatus and methods, is provided for a single receiver or transceiver architecture that efficiently supports both wideband and narrow band operational modes—particularly ZIF and near-ZIF (NZIF). The system of the present disclosure provides a reconfigurable digital IF (DIF) construct, within a multi-mode receiver architecture. The DIF construct provides an optimal receiver down conversion technique for operations in a given mode. The system of the present disclosure provides for the optimization of performance for various modes, while at the same time easing receiver processing functions. The system of the present disclosure thus provides a single, common-core receiver that performs equivalent to multiple, dedicated receivers, with no compromise in signal processing quality.  
         [0011]     Specifically, an architecture for a receiver component in a wireless communications system is disclosed—one that supports both zero intermediate frequency (ZIF) and near-zero intermediate frequency (NZIF) operation. The architecture provides a down-conversion segment, and a local oscillator segment operatively associated with the down-conversion segment. An analog-to-digital conversion (ADC) segment is adapted to receive signals from the down-conversion segment and introduce the signals into a digital intermediate frequency (DIF) construct. The DIF construct performs DC offset compensation or DC residue filtering on NZIF-based signals—as well as droop or mismatch compensation. Image filtering is performed on NZIF-based signals, and DC offset compensation is performed on ZIF-based signals. Compensated signals are amplified to some nominal or desired level, and interpolation filtering of the amplified signals is performed prior to transmission thereof.  
         [0012]     Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, 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, may 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; and the terms “construct”, “element” or “component” mean any device, system or part thereof that control or perform at least one operation, and may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular construct or element may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts:  
         [0014]      FIG. 1  illustrates one embodiment of wireless communications receiver system in accordance with the present disclosure; and  
         [0015]      FIG. 2  illustrates an embodiment of a multi-mode signal processing segment in accordance with the present disclosure.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]      FIGS. 1 and 2 , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged receiver structure, whether such structure relies upon ZIF, NZIF or some other desired operational protocols.  
         [0017]     The following discloses a versatile system, comprising various architectures, apparatus and methods for providing a single multimode receiver—whether as a stand-alone receiver, or as part of a transceiver device. This multimode architecture efficiently supports both wideband and narrow band operational modes—especially where such operations are based upon ZIF and NZIF protocols.  
         [0018]     The system of the present disclosure provides a reconfigurable digital IF (DIF) construct, implemented within a multi-mode receiver. The DIF construct provides—regardless of whether a ZIF or NZIF based operational mode is active—an optimal receiver down conversion for efficient signal processing. Utilizing the DIF construct of the present disclosure, a single, common-core receiver performs equivalent to multiple, dedicated receivers, with no compromise in signal processing quality. The architecture of the present disclosure provides for optimization of performance for various modes and, at the same time, reduces the volume of receiver processing functions needed to realize such optimization. This invention thus provides a single common core receiver that functions as effectively, and more efficiently, than multiple receiver multi-mode systems—with no compromise in signal processing quality. This greatly reduces system costs and inefficiencies while improving operational reliability.  
         [0019]     The system of the present disclosure provides a mobile terminal receiver architecture that can be conveniently reconfigured by software to perform either a direct down conversion—commonly preferred for wide band systems and referred to as ZIF—or a near-ZIF (NZIF) down conversion, having certain advantages for narrow band systems. The system of the present disclosure recognizes that the selection of a one down conversion technique over another is based upon a wide variety of factors—particularly any receiver test specifications required by an applicable standard.  
         [0020]     According to the present disclosure, and as illustrated now in reference to  FIG. 1 , a multimode receiver component  100  comprises an analog RF front-end segment  102 , having a low noise amplifier segment  104  and a down-conversion segment  106 . In order for segment  106  to successfully accommodate two or more different down-conversion schemes, a local oscillator (LO) for down-converter segment  106  needs to be highly versatile and adaptable.  
         [0021]     In response, the system of the present disclosure provides a programmable synthesizer component  108  (e.g., a phase-locked loop (PLL) based device, either fractional or integer based) in conjunction with a widely tunable oscillator element  110 . Element  110  then feeds a mixer/demodulation element  112 . Element  112  may be somewhat more complex than a comparable function in conventional single mode system, however, the present system recognizes that since the difference in ZIF and NZIF operation is usually relatively small (e.g., about 100 kHz˜200 kHz), this is not an inhibiting design challenge for such an element.  
         [0022]     The rest of analog segment  102  comprises an analog variable gain amplifier (VGA) block  114 , and an analog-to-digital converter (ADC) block  116  to digitize signals being processed. Programmable low-pass filters  118  (LPFs) are implemented at the output portion of both the down converter block  106  and VGA block  114 , to provide blocking and anti-aliasing functions. From these segments, signals pass to the DIF construct  120 , which processes those signals in digital domain (as described hereinafter) before outputting the signals to a baseband modem (not shown) via digital to analog converter (DAC) elements  122 .  
         [0023]     Referring now to  FIG. 2 , one illustrative embodiment of a DIF construct  200  in accord with the present system is depicted. Construct  200  may be provided, for example, for multimode utilization with WCDMA ZIF and GSM/EDGE NZIF based systems. Construct  200  may be provided such that it uses the same data bus and width for both ZIF and NZIF modes. Depending upon the communication and processing technologies of a given application, construct  200  may comprise separate but parallel paths for processing different signal components. For example, in the embodiment depicted in  FIG. 2 , construct  200  illustratively comprises signal processing paths  202  and  204  for parallel processing of quadrature components (I) and (Q), respectively.  
         [0024]     Construct  200  further comprises an IF to baseband digital mixer element  206 . Mixer element  206  is utilized for NZIF operation, as driven by a numerically controlled oscillator (NCO)  208 , and is bypassed or disabled for ZIF operation. Mixer  206  provides complex down-conversion necessary for image filtering when operating at NZIF frequency.  
         [0025]     A DC offset correction element  210  is also provided. DC offset correction  210  is utilized in NZIF operation but may not be needed at an IF frequency of 170 kHz or above. ZIF operation will utilize a DC residual correction element  212 —provided at some point after processing by FIR filters  218 ,  222  and  226 .  
         [0026]     As signals are introduced to construct  200  via inputs  214 , a signal may first be processed by a first filtering element  216 , prior to any offset compensation performed by element  210 . As depicted in  FIG. 2 , filtering element  216  comprises a cascaded integrator-comb (CIC) type of filter. The specific topology and magnitude of element  216  may be varied to match design requirements of a given application. As depicted in  FIG. 2 , element  216  comprises a 5-stage CIC filter. Filter  216  has a programmable decimation rate (M) that may be provided or determined based on the incoming ADC rate.  
         [0027]     From element  216 , signal proceeds through offset compensation  210 , and may then be filtered again by second filtering element  218  before proceeding to a mismatch compensation element  220 . As depicted in  FIG. 2 , element  218  comprises a symmetric finite impulse response (FIR) type filter, providing droop compensation of prior analog LPFs  118  or CIC filter  216 . After compensation by element  218 , signal proceeds through mismatch compensation  200  to mixer element  206 . After processing by element  206 , signal may then proceed through a channel filtering element  222  before processing by a gain adjust element  224 . As depicted in  FIG. 2 , element  222  comprises a symmetric FIR type filter.  
         [0028]     Element  224  provides a coarse gain adjustment (i.e., switchable step gain), from which signal may then proceed through another channel filtering element  226 , before proceeding to variable gain amplification (VGA) element  228 . As depicted in  FIG. 2 , element  226  also comprises a symmetric FIR type filter. Once signal has been processed through VGA element  228 , it may then proceed through one or more forms of interpolation filter elements  230 ,  232 , before being output  234  from construct  200 . As depicted in  FIG. 2 , element  230  comprises a symmetric FIR type interpolation filter, while element  232  comprises a 5-stage CIC interpolation component. Element  232  has a programmable interpolation rate (N) that may be determined or provided based upon the rate of a DAC to which signals are output  234 .  
         [0029]     VGA element  228  may be provided to maintain some nominal signal level into a baseband modem from output  234 . Digital channel filtering elements  222  and  226  may be provided in a programmable format or configuration—enabling those elements to be reconfigurable or optimizable for signals in different modes with various bandwidths. The FIR filters of those elements may be designed to attenuate close-in blockers—including adjacent channel interferers, in GSM/EDGE, as well as any up-converted residual DC spurious noise from use of an NCO. These filters may also be designed or configured to perform amplitude equalization on frequency responses from, for example, analog low pass filters at the output of down-converter and VGA  228  output—to address any amplitude droop effect on a signal from being at a non-zero IF frequency.  
         [0030]     In most embodiments, both ZIF and NZIF configurations are provided without any image rejection filtering. As such, image rejection is of particular concern in NZIF operation. For example, a minimum of about 35 dB of image rejection may be required when operating in GSM/EDGE mode, with an NZIF configuration. Given certain tolerances in I/Q mismatch, complex filtering in the digital domain is a preferable approach, and more deterministic in image reduction, which would otherwise be very challenging to provide in the analog domain.  
         [0031]     Given the versatility of the present system, a number of application-specific or general-purpose adaptations may be readily implemented. For example, a receiver synthesizer may be exploited to adopt an NZIF configuration at greater than 110 kHz for GSM/EDGE—providing room for the FIR filters of construct  200  to block out any upcoverted DC spurs that may occur at 110 kHz, since baseband require signal bandwidth for EDGE processing may be greater than 100 kHz. In some instances, it may be possible that analog filter bandwidth may be constrained by amplitude droop or group delay effects on a signal, while at the same time maintaining selectivity. Advantageously, NZIF arrangement of construct  200  is not required to perform any digital DC correction if the IF is high enough (e.g., 170 kHz or above)—since any upcoverted residual DC component may be sufficiently filtered out, and digital functional blocks can be conceptualized as being perfectly linear.  
         [0032]     Another advantage of an NZIF arrangement of the present invention is that—in a narrowband system like GSM/EDGE—a high-pass transfer function of an analog DC correction loop will induce little to no degradation of a signal, especially if an IF of greater than 135 kHz is used. This provides more accurate corrections with the receiver fully turned on and the dynamics of high signals and blockers, allowing for IM2 product reduction during active burst for better AM suppression. In certain ZIF operation instance, such as a broadband system like WCDMA, any high-pass effect on a signal is relatively insignificant. Primarily, a wider overall bandwidth, in conjunction with specifications that require close-in blockers that can become challenging to reduce using analog filters with high cut-off corners, prohibit WCDMA from operating in an NZIF mode.  
         [0033]     It should now be easily appreciated by one of skill in the art that the system of the present disclosure provides and comprehends a wide array of variations and combinations easily adapted to a number of multi-mode, ZIF/NZIF applications. The relative arrangement and orientations of certain filtering or compensation elements may be provided in any manner suitable for a particular application. All such variations and modifications are hereby comprehended. It should also be appreciated that the system of the present disclosure may be readily implemented in any desired design or fabrication processes. The constituent members or components of this system may be produced or provided using any suitable hardware, software, or combination of hardware and software.  
         [0034]     The embodiments and examples set forth herein are therefore presented to best explain the present invention and its practical application, and to thereby enable those skilled in the art to make and utilize the system of the present disclosure. The description as set forth herein is therefore not intended to be exhaustive or to limit any invention to a precise form disclosed. As stated throughout, many modifications and variations are possible in light of the above teaching without departing from the spirit and scope of the following claims.