Wireless communications device allowing a soft handoff procedure in a mobile communications system

A wireless communications device includes an antenna that receives a first signal at a first frequency and a second signal at a second frequency and converts the first and second signals into a composite signal. A first oscillator outputs a first oscillator signal at a first frequency and a second oscillator outputs a second oscillator signal at a second frequency. A demodulator receives the composite signal and the first and second oscillator signals. The oscillator signals are selected so that the demodulator generates a low frequency signal with components of the first and second signals occupying a common frequency band. The wireless communications device allows executing a “Soft Handoff” even when the first and second frequencies are different.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to a communications system. More particularly, the invention relates to a wireless communications device and a method of receiving radio frequency signals within a communications system.

2. Description of the Related Art

One example of a communications system is a wireless communications system which can be a cellular mobile communications system. The cellular mobile communications system is implemented in a geographical area and logically divided into individual service cells. A fixed transceiver station such as a base station defines at least one cell and is connected to a base station controller. Mobile stations, such as hand-held or car-based cellular phones, move freely within the geographical area covered by a cell. The mobile stations not only move within a single cell, but also from one cell to a neighboring cell.

The base station handles all telephone traffic to and from those cellular phones which are currently located in the cell. The base station that serves a cellular phone is typically the one which is closest to the cellular phones and, thus, provides in many cases the best radio communications path to the cellular phones.

The cellular phones and the serving base station exchange radio signals in accordance with a communications protocol defined for a given communications system. The radio signals have frequencies within frequency bands that are assigned to the cells. In one example of a communications protocol, the radio signals can be structured in frames and channels.

In conventional Code Division Multiple Access (CDMA) systems, a pilot channel is defined for communications between the base stations and the cellular phones. The pilot channel carries no information, but provides the cellular phone, for example, with a reference for time, phase, and signal strength. The cellular phone constantly evaluates the strengths of the pilot channels of the serving and neighboring base stations to determine potential base stations. When the strength of the pilot channel of the serving base station falls below a predetermined threshold and the strength of the pilot channel of the neighboring base station exceeds a predetermined threshold, a handoff procedure is initiated. The procedure that transfers the mobile station from one cell to another cell, without dropping a call or losing information, is often called “Soft Handoff.”

In many conventional Soft Handoff procedures, the base stations of neighboring cells use the same frequencies. This requirement, however, limits the number of mobile stations that can be served by one base station. For example, if two neighboring base stations operate at different frequencies, a so-called “Hard Handoff” procedure typically takes place which causes a break in an existing connection and may result in a loss of information.

SUMMARY OF THE INVENTION

An embodiment of the invention involves a wireless communications device for a communications system. The wireless communications device includes an antenna, which receives a first signal at a first radio frequency and a second signal at a second radio frequency, and converts the first and second signals into a composite radio frequency (RF) signal. A first oscillator is operable to output a first oscillator signal at a first frequency, and a second oscillator is operable to output a second oscillator signal at a second frequency. A demodulator is coupled to receive the composite RF signal and the first and second oscillator signals. The oscillator signals are selected so that the demodulator generates a low frequency signal with components of the first and second signals occupying a common frequency band.

Another embodiment of the invention involves a wireless communications device having a first input configured to receive an input signal which comprises a first component allocated within a first frequency band and a second component allocated within a second frequency band. A first oscillator is configured to generate a first oscillator signal at a first oscillator frequency, and a second oscillator is configured to generate a second oscillator signal at a second oscillator frequency. A mixer is configured to receive the input signal, the first oscillator signal, and the second oscillator signal, and to convert at least a portion of the first component and at least a portion of the second component to a third frequency band.

A further embodiment of the invention involves a device having at least a first terminal which is configured to receive a first signal within a first frequency band from a first source and a second signal within a second frequency band from a second source. At least a second terminal is configured to receive at least a first reference signal and a second reference signal. A modulator in communication with the first and second terminals is configured to generate a first difference component within a third frequency band. The first difference component comprises the difference between a portion of the first signal within the first frequency band and the first reference signal. The modulator is further configured to generate a second difference component within the third frequency band, the second difference component comprising the difference between a portion of the second signal within the second frequency band and the second reference signal.

Another embodiment of the invention involves a wireless communications device having a first input to receive an input signal which comprises a first component having a first frequency allocated within a first frequency band and a second component having a second frequency allocated within a second frequency band. A first oscillator is configured to generate a first oscillator signal at a first oscillator frequency, and a second oscillator is configured to generate a second oscillator signal at a second oscillator frequency. A mixer is configured to receive the input signal, the first oscillator signal, and the second oscillator signal, and to convert at least a portion of the first component and at least a portion of the second component into a third frequency band. The portion of the first component has a first difference frequency corresponding to a difference between the first frequency and the first oscillator frequency, and the portion of the second component has a second difference frequency corresponding to a difference between the second frequency and the second oscillator frequency. The first difference frequency is approximately equal to the second difference frequency, both located within the third frequency band.

A further embodiment of the invention involves a method of receiving radio frequency (RF) signals with a wireless communications device that is operable in a communications system. The device receives a first signal within a first frequency band from a first source, and a second signal within a second frequency band from a second source. Further, the device transforms the first and second signals into a third frequency band, and processes the frequency-transformed first and second signals in order to maintain communications with the first and second sources.

Another embodiment of the invention involves a method of receiving radio frequency (RF) signals. A first RF signal has a first radio frequency and originates from a first transmitter station, and a second RF signal has a second radio frequency and originates from a second transmitter station. The first and second RF signals are received and converted into a composite signal. A first oscillator signal is generated having a first oscillator frequency, which is selected to have a first frequency difference to the first radio frequency. A second oscillator signal is generated having a second oscillator frequency, which is selected to have a second frequency difference to the first radio frequency. The composite signal is mixed with the first and second oscillator signals to generate an intermediate frequency signal. The intermediate frequency signal comprises a component of the first RF signal and a component of the second RF signal with the components being located within a common frequency band. The intermediate frequency signal is processed to generate a first baseband signal and a second baseband signal. The first baseband signal corresponds to the first RF signal and the second baseband signal corresponds to the second RF signal.

Another embodiment of the invention involves a wireless communications device having an antenna configured to receive a first signal at a first frequency and a second signal at a second frequency, and to output the first and second signals as a first composite signal. A first oscillator is operable to output a first oscillator signal at a first frequency, and a second oscillator is operable to output a second oscillator signal at a second frequency. A mixer receives the first composite signal, the first oscillator signal, and the second oscillator signal, and converts the first composite signal to a baseband signal with components of the first and second signals occupying at least a portion of a baseband frequency range.

A further embodiment of the invention involves a wireless communications device having a first input configured to receive an input signal which comprises a first component having a first frequency allocated within a first frequency band and a second component having a second frequency allocated within a second frequency band. A first oscillator is configured to generate a first oscillator signal at a first oscillator frequency, and a second oscillator is configured to generate a second oscillator signal at a second oscillator frequency. A mixer is configured to receive the input signal, the first oscillator signal, and the second oscillator signal, and to convert at least a portion of the first component and at least a portion of the second component into a baseband frequency range.

Another embodiment of the invention involves a device having at least a first terminal which is configured to receive a first signal within a first frequency band from a first source where the first signal is filtered to produce a filtered first signal within the first frequency band, and a second signal within a second frequency band from a second source where the second signal is filtered to produce a filtered second signal within the second frequency band. At least a second terminal is configured to receive at least a first reference signal and a second reference signal. A mixer, in communication with the filtered first and second signals and the second terminal, is configured to generate a first difference component within baseband frequencies and a second difference component within the baseband frequencies. The first difference component comprises the difference between a portion of the filtered first signal within the first frequency band and the first reference signal. The second difference component comprises the difference between a portion of the filtered second signal within the second frequency band and the second reference signal.

Another embodiment of the invention involves a method of receiving signals with a wireless communications device that is operable in a communications system. The device receives a first signal having a first frequency within a first frequency band from a first source and receives a second signal having a second frequency signal within a second frequency band from a second source. Further, the device transforms the first and second signals into baseband by mixing the first signal with a first oscillator signal at a first oscillator frequency, and mixing the second signal with a second oscillator signal at a second oscillator frequency. The difference between the first frequency and the first oscillator frequency, and the difference between the second frequency and the second oscillator frequency fall within the baseband. The device further processes the frequency-transformed first and second signals to maintain communications with the first and second sources.

Another embodiment of the invention involves a method of receiving signals. A first signal has a first frequency and originates from a first transmitter station and a second signal has a second frequency and originates from a second transmitter station. The first and second signals are received and converted into a composite signal. A first oscillator signal is generated having a first phase and a second phase at a first oscillator frequency, which is selected to have a first frequency difference to the first frequency. A second oscillator signal is generated having a first phase and a second phase at a second oscillator frequency, which is selected to have a second frequency difference to the second frequency. The composite signal is mixed with the first oscillator signal at the first phase and the second oscillator signal at the first phase to generate a first baseband signal. The composite signal is also mixed with the first oscillator signal at the second phase and the second oscillator signal at the second phase to generate a second baseband signal. The first phases and the second phases are approximately 90° apart, and the first baseband signal corresponds to the in-phase signal and the second baseband signal corresponds to the quadrature signal.

A further embodiment of the invention involves a method of receiving signals with a wireless communications device, which is operable in a communications system. The device receives an input signal which comprises a first component allocated within a first frequency band and a second component allocated within a second frequency band. The device generates a first oscillator signal comprising a sine signal and a cosine signal at a first oscillator frequency, and a second oscillator signal comprising the sine signal and the cosine signal at a second oscillator frequency. Further, the device receives the input signal, the first oscillator signals, and the second oscillator signals. The device mixes the input signal with the sine signal and the cosine signal at the first oscillator frequency, and mixes the input signal with the sine signal an the cosine signal at the second oscillator frequency. Further, the device separates the input signal into a first baseband component and a second baseband component.

For purposes of summarizing the invention, certain embodiments, advantages and novel features of the invention have been described herein. Of course, it is to be understood that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention. Thus, the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1shows an illustration of a mobile communications system1manufactured by a public or private telephone company (“service provider”). The telephone company can provide access to a public switched telephone network (PSTN). The operating telephone company determines parameters of the mobile communications system1including, but not limited to, geographical coverage area, communications standards, frequency, system capacity, and the like.

In one embodiment, the mobile communications systems1is a cellular mobile communications system configured to operate as a Code Division Multiple Access (CDMA) system. Such an exemplary mobile communications system1is referred to as a cellular system. An embodiment of the invention is hereinafter described with reference to, but not limited to, such a cellular system1. It is contemplated that the invention is applicable in other mobile communications systems, such as systems known as a personal communications service using CDMA technology (PCS/CDMA) or other wireless systems.

The cellular system1ofFIG. 1includes a plurality of base stations B1, B2, each defining a cell. For instance, the base station B1defines a cell C1and the base station B2defines a cell C2. Neighboring cells C3, C4are shown for illustrative purposes. As indicated, the cells C1–C4overlap to a certain degree in the illustrated embodiment. It is contemplated that in other embodiments, the cells C1–C4can more or less overlap depending on the geographical area.

Communication lines L1, L2connect the base stations B1, B2to a base station controller BC1, which controls the base stations B1, B2and connects the cellular system1to a switching center in the domain of the service provider or to a (wire-based) public telephone system (PSTN). InFIG. 1, this connection is illustrated as “TO SWITCH.” The communications lines L1, L2are, for example, fiber-optic cables, twisted pair lines, coaxial cables, or combinations thereof typically used for communications lines. In certain embodiments, the communications lines L1, L2can represent wireless bi-directional radio connections and the like.

The cellular system1further includes at least one mobile station3which can freely move within the cellular system1. It is contemplated that a plurality of mobile stations3can be active or inactive within the cellular system1. The mobile station3can be, for example, a wireless phone, a handheld cellular phone, a cellular phone mounted in a vehicle, or any other wireless device (e.g., a pager) which can be used in a cellular system1. The mobile station3can move freely within each cell C1–C4and between the cells C1–C4. InFIG. 1, the mobile station3is indicated as a handheld cellular phone, which is located within the cell C1and served by the base station B1. The mobile station3is hereinafter referred to as the phone3.

As shown, the phone3is currently located within the cell C1and has a bi-directional radio connection with the base station B1. The bi-directional radio connection indicates that calls to and from the phone3are handled by the base station B1. The base station B1is therefore referred to as the serving base station B1. In one embodiment, the radio connection is established through a signal S1having a frequency band around a carrier frequency f1. In one embodiment, the carrier frequency f1is approximately 880 MHz.

When the phone3moves within the cellular system1, the phone3is handed off from one cell to another. This is referred to as an intra-system handoff. Before the handoff, the phone3communicates with the serving base station B1at a “pre-handoff” frequency f1, and after the handoff, the phone3communicates with the new base station B1at a “post-handoff frequency f2of a signal S2. Depending on the infrastructure of the cellular system1, the post-handoff frequency f2can be the same as the pre-handoff frequency f1, or the post-handoff frequency f2can be different from the pre-handoff frequency f2.

FIG. 2illustrates a scenario in which the geographical area covered by the cellular system1is also covered by a second mobile communications system100. The second mobile communications system100is under the control of a different service provider whose infrastructure is in one embodiment generally similar to the infrastructure of the cellular system1. InFIG. 2, the infrastructure of the cellular system1is as shown inFIG. 1and the phone3is in communication with the serving base station B1.

An exemplary cell structure of the second mobile communications system100is indicated through dashed lines. The second mobile communications system100has a plurality of base stations BS1, BS2, which are connected to a controller BC2. The base station BS1serves a cell C5and the base station BS2serves a cell C6. The second mobile communications system100has assigned frequencies (e.g., f3) for the cells that can be different from the frequencies assigned to the cellular system1. For example, the second mobile communications system100can be a PCS/CDMA system operating at a frequency band around approximately 1800 MHz and the cellular system1can be a cellular CDMA system operating at a frequency band between approximately 800 MHz and approximately 900 MHz.

In addition to the intra-system handoff described with reference toFIG. 1, in some applications, the phone3can be configured to move (roam) freely between the cellular system1and the mobile communications system100(e.g., a PCS/CDMA system) as indicated inFIG. 2. That is, the phone3has the capability of seamless roaming, for example, from a cellular CDMA system to a PCS/CDMA system. This is referred to as an “inter-system handoff.” Under these circumstances, the phone3is handed off from the cellular CDMA system to the PCS/CDMA system and the pre-handoff frequency f1and the post-handoff frequency f3are different.

Referring toFIGS. 1 and 2, independent of whether an intra-system handoff or an inter-system handoff occurs, the capability of the desired “Soft Handoff” is maintained within the systems1,100. As described below in greater detail, the phone3includes two local oscillators which can be tuned to appropriate frequencies so that a radio connection with the base station of a “target” cell can be made before the radio connection with the (previous) serving base station is broken. The first local oscillator is tuned to a frequency fLO1, and the second local oscillator can be tuned to a frequency fLO2.

The frequencies fLO1, fLO2are selected so that the frequencies f1, f2of the signals S1, S2are down converted to frequencies within a common frequency band. If the frequencies f1, f2are approximately the same, the frequencies fLO1, fLO2are also approximately the same. Correspondingly, if the frequencies f1, f2are different, the frequencies fLO1, fLO2are different. The latter case occurs, for example, when the signal S1originates from a cellular CDMA system (f1=880 MHz) and the signal S2originates from a PCS system (f2=1960 MHz). In this example, the frequency fLO1can be approximately 680 MHz and the frequency fLO2can be approximately 1760 MHz so that after the down conversion resulting differences frequencies (880 MHz−680 MHz, and 1960 MHz−1760 MHz) are within the same frequency band of about 200 MHz.

While the phone3is active or in a stand-by mode, the phone3constantly evaluates the signal strengths received in the pilot channels of the serving base station B1and the neighboring base stations, such as the base station B2, to determine potential base stations for an upcoming handoff. When the signal strength of the pilot channel of the serving base station B1falls below a predetermined threshold and the signal strength of the pilot channel of another base station B2exceeds a predetermined threshold, the handoff procedure is started. In case the phone3is in the stand-by mode, the evaluation of the signal strengths of the pilot channels serves to determine which base station B1, B2, BS1will be the serving base station if the phone3becomes active.

Focusing on an embodiment of a cellular CDMA system which has an infrastructure as shown inFIG. 1, the base station B1transmits and receives radio signals within a frequency band around the carrier frequency f1assigned to the cell C1. For instance, the base station B1transmits at a frequency of approximately 880 MHz and receives at a frequency of approximately 835 MHz. Similarly, the base station B2transmits at 1960 MHz and receives radio signals within a frequency band around a carrier frequency f2of approximately 1880 MHz assigned to the cell C2. It is contemplated that in another embodiment, the base stations B1, B2can operate within the same frequency band, which is assigned to neighboring cells.

FIG. 3schematically illustrates one embodiment of the phone3. The phone3includes an antenna11, a display, and a keypad. A portion of the case of the phone3is cut away to show a motherboard5of the phone3with an integrated circuit10which includes an RF receiver, or a portion thereof, as described below. The integrated circuit10is hereinafter generally referred to as the RF receiver10. Although not shown inFIG. 1, those skilled in the art will appreciate that the phone3comprises a central processor unit (CPU) and plurality of other components and functional modules of conventional phones.

FIG. 4shows a schematic illustration of a receive path and a transmit path. Both paths are associated with the antenna11to receive and transmit signals. In the illustrated embodiment, the transmit path includes a conventional transmitter for RF signals, and the receive path comprises the RF receiver10(hereinafter referred to as the receiver10), a signal processing module7and a speaker9. The receiver10is interconnected between the antenna11and the signal processing module7which is connected to the speaker9.

The receiver10includes several groups of amplifiers which are separated by frequency-changing circuits (e.g., mixers, modulators or demodulators) to extract information carried by a weak signal voltage that appears at terminals of the antenna11. The antenna11receives the signals S1, S2, for example, from the serving base station B1of the cell C1and the target base station B2of the cell C2, and converts the signals S1, S2to a composite electrical signal. The composite electrical signal includes the frequencies f1, f2which can have same or different values depending on the infrastructure of the systems1,100. As the frequencies f1, f2are in the radio frequency range (e.g., 880 MHz, or 1960 MHz), the composite electrical signal is hereinafter referred to as the “composite RF signal.”

As described below in greater detail, the receiver10converts the composite RF signal, which includes the signals S1, S2, from an initial high frequency (RF) range down to a lower frequency range, the baseband.

In one embodiment, the down conversion process includes two stages. A first stage down converts the composite RF signal from the RF range to an intermediate frequency range, and a second stage down converts the composite RF signal from the intermediate frequency range to the baseband. The down conversion process is also known as “heterodyning.” A receiver using the two stage down converting process is known as a super-heterodyning receiver.

In another embodiment, the down conversion process includes one stage. A single stage down converts the composite RF signal from the RF frequency range to a baseband frequency range. A receiver using a single down converting process is known as a direct conversion receiver.

Therefore, the receiver10outputs the signals S1, S2as baseband signals, which are input to the signal processing module7for further processing.

In one embodiment, the receiver10is implemented as an integrated circuit and configured to operate at a voltage between 2.7 volts and 5 volts. The voltage can be provided by a re-chargeable battery, or if the phone3is mounted to a car, from the car battery. However, those skilled in the art will appreciate that the receiver10can be configured to operated at lower or higher voltages. Further, it is contemplated that not all components of the receiver10are necessarily integrated in the integrated circuit. That is, a specific implementation of the receiver10may have discrete and isolated components in combination with integrated circuits.

The illustrated embodiments of the receiver10show the receiver10in a single-ended embodiment. In another embodiment, the receiver10can be implemented in a differential embodiment. In some applications, the differential embodiment is preferred to differentiate the actual signal from noise and, thus, to improve the signal-to-noise ratio. If the receiver10is implemented in the differential embodiment, the components of the receiver10are connected between two differential lines which are typically referred to as “positive” and “negative”, or “+” and “−.” Compared to the single-ended embodiment, the components are duplicated for each differential line in the differential embodiment. The principal operation, however, corresponds to the operation of the single-ended embodiment.

Focusing on the single-ended implementation,FIG. 5shows a schematic illustration of an embodiment of the receiver10. The receiver10includes a mixer module12, which down converts the composite RF signal to the baseband, and a baseband processor38. The mixer module12has an input13and outputs15a,15bto connect the mixer module12to the antenna11and the baseband processor38, respectively. The baseband processor38has an output19, which is connectable to the signal processing module7.

In one embodiment, the mixer module12comprises a combination of an amplifier14and a mixer18for signal amplification and frequency down conversion. The amplifier14is, for example, a low-noise amplifier (LNA) that receives the composite RF signal, amplifies the composite RF signal, and feeds the amplified RF signal to the mixer18. In addition, the mixer18receives oscillator signals LO1, LO2generated by two separate local oscillators34,36. The oscillator signals LO1, LO2are, for example, sinusoidal signals each having a constant amplitude and frequency.

The mixer18multiplies the composite RF signal and the oscillator signals LO1, LO2, and the various signal components mix with each other. The oscillator signal LO1mixes with the signals S1, S2of the composite RF signal and the oscillator signal LO2mixes with the signals S1, S2. As is known in the art, this mixing process results in a signal that includes a variety of different frequencies. These different frequencies include the original frequencies f1, f2, fLO1, fLO2, their harmonics, for example, 2f1, 2f2, 2fLO1, 2fLO2, and their sums and differences, for example, f1±fLO1, f2±fLO2.

In one embodiment, the difference frequencies −f1+fLO1, −f2+fLO2are of interest. The oscillator frequencies fLO1, fLO2are selected so that the difference frequencies −f1+fLO1, −f2+fLO2fall within the same frequency band and have approximately the same value, i.e., (−f1+fLO1)≈(−f2+fLO2). This frequency value is hereinafter referred to as the “intermediate frequency,” which is lower than the initial frequencies f1, f2, and written as “f1˜fLO1, f2˜fLO2.” The local oscillators34,36can be tuned to appropriate oscillator frequencies fLO1, fLO2that fulfill the requirement of (f1˜fLO1)≈(f2˜fLO2). It is contemplated that this requirement generally indicates that the differences (f1˜fLO1; f2˜fLO2) fall within the same frequency band and that the differences (f1˜fLO1; f2˜fLO2) can be in the MHz range.

Because the mixer18generates an output signal that comprises a variety of different frequencies, a filter20is connected to the mixer18in order to block frequencies other than the intermediate frequency f1˜fLO1, f2˜fLO2. The signal output from the filter20is referred to as the intermediate frequency (IF) signal.

In the illustrated embodiment, the mixer module12further includes a filter16, an amplifier22, and two mixers26,28. The filter16is connected between the mixer18and the amplifier14connected to the input13. The mixer18is connected to the filter16to receive the bandlimited composite RF signal and to the local oscillators34,36. As shown, the filter16is a bandpass filter which limits the bandwidth of the composite RF signal received from the amplifier14to block undesired frequency components and to reduce noise in the composite RF signal. The undesired frequency components can be caused, for example, by nonlinearities of the amplifier14that result in intermodulation products. In one embodiment, the passband of the filter16is about 25 MHz to allow passage of a receive band between about 850 MHz and 900 MHz, more precisely between 869 MHz and 894 MHz, and to block frequencies outside of this receive band.

The local oscillators34,36are in one embodiment conventional local oscillators configured to operate at the different oscillator frequencies fLO1, fLO2. The oscillator signals LO1, LO2can be sinusoidal signals each having a frequency between 500 MHz and 2.5 GHz. In one embodiment, the oscillator signal LO1has a frequency fLO1of approximately 955 MHz and the oscillator signal LO2has a frequency fLO2of approximately 960 MHz. These values for the frequencies fLO1, fLO2correspond to radio frequencies of 875 MHz and 879 MHz, respectively.

The oscillator signals LO1, LO2are tunable to adapt to other phone systems which operate, for example, at carrier frequencies of about 1800 MHz or 1900 MHz. Alternatively, the phone3can be a dual band cellular phone which can operate within different frequency bands, for example, 800 MHz, 900 MHz, 1800 MHz, or 1900 MHz. Independent of what carrier frequencies the signals S1, S2have, the frequencies of the signals LO1, LO2are generally selected so that the difference (f1˜fLO1) is approximately in the same frequency band as the difference (f2˜fLO2). An exemplary signal output from the filter20, in which the down converted signals S1, S2fall within the same frequency band, is shown inFIG. 6and described below.

AlthoughFIG. 5shows the local oscillators34,36as belonging to the mixer module12, it is contemplated that the local oscillators34,36may be located outside the mixer module12and at other locations within the phone3. If the mixer module12is implemented as an integrated circuit, the local oscillators34,36are typically located off-chip. In one embodiment, the local oscillators34,36are conventional frequency synthesizers whose frequencies are referenced to piezoelectric crystals. The synthesizers are tunable within a predetermined range. It is contemplated that other types of local oscillators, such as voltage controlled oscillators (VCO), can be used to generate the desired IF signal.

An output of the mixer18is connected to the filter20, which is in the illustrated embodiment a bandpass filter. The filter20has a passband between approximately 1.25 MHz and approximately 85 MHz. In another embodiment, for example, in direct conversion receivers, the filter20is implemented as a low-pass filter, which has, for example, a cut-off frequency of approximately 0.63 MHz. The filter20selects the desired frequency band around the intermediate frequency f1˜fLO1, f2˜fLO2, and blocks frequencies, which are located outside the passband, or are higher than the cut-off frequency. It is contemplated that other values for the passband or the cut-off frequency can be chosen.

In one embodiment, the amplifier22is connected to a control line24to receive an automatic gain control signal AGC from a central controller (not shown) of the phone3. The control signal AGC controls the amplifier22to amplify the IF signal with a desired gain. The amplifier22is operable at a gain between +45 dB and −45 dB to amplify the IF signal to a predetermined level over the entire dynamic range of the receiver

In the illustrated embodiment, the mixers26,28form a conversion module located within the mixer module12, and connect to an output of the amplifier22. Those skilled in the art, however, will appreciate that in another embodiment the mixers26,28can be located within the baseband processor38. An output of the mixer26is connected to the output15aand an output of the mixer28is connected to the output15b. A local oscillator32generates an oscillator signal LO3that is, for example, a sinusoidal signal having an oscillator frequency fLO3. The oscillator signal LO3is input to the mixer26and, with a 90 degrees phase shift, to the mixer28. That is, in one embodiment, the mixers26,28receive signals having a sine function and a cosine function.

The oscillator frequency fLO3is selected so that the IF signal, having a frequency with (f1˜fLO1)≈(f2˜fLO2), is down converted to the baseband at a frequency fBof approximately 0–630 kHz. Similar to the first down conversion stage implemented through the mixer18, the oscillator frequency fLO3is selected so that the IF signal is downconverted to baseband “In phase” (I) and “Quadrature” (Q) outputs. The second down conversion stage, implemented by the mixers26,28, splits the IF signal into the two components I, Q which correspond to I/Q components containing information transmitted by the base stations B1, B2. The components I, Q are input to the baseband processor38which performs the processing necessary to convert the received CDMA signal back to an uncoded (“de-spread”) signal and extracts the voice/data signals.

As is known to the person skilled in the art, CDMA is a spread spectrum technique for multiple access. The CDMA technique is sometimes explained with reference to a situation encountered at a cocktail party. Like in a cellular CDMA system, all guests are talking in the same room simultaneously, but every conversation occurs in a different language. If one guest does not understand these languages, they would all sound like “noise” from the guest's perspective. However, if the guest would know the “code,” i.e., the appropriate language, the guest could “filter out” the unknown languages (noise) and listen only to the conversation in the language the guest understands.

Besides the language (code) problem, the guest may encounter another problem. Even with knowledge of the appropriate language, the guest may not hear the complete conversation because either the speaker does not speak loud enough, or the other speakers speak too loud. The guest can signal to the speaker to speak louder, but can also signal to the other guests to speak more softly. The cellular CDMA system applies a corresponding “power control” process and filter function.

Referring to a cellular CDMA system, multiple telephone conversations are spread across a wide segment of a (broadcast) frequency spectrum at a transmitter and “de-spread” at the receiver. Each user (telephone call) is assigned a unique code to modulate transmitted data. The code is unique and distinguishes a specific call from the multitude of other calls simultaneously transmitted over the same broadcast spectrum. The code is a long sequence of ones and zeros similar to the output of a random number generator of a computer. The computer generates the code using a specific algorithm and the numbers appear to be random. Because the codes are nearly random, there is very little correlation between the different codes. In addition, there is very little correlation between a specific code and any time shift of that same code.

Thus, the distinct codes can be transmitted over the same time and the same frequencies and the signals can be decoded at the receiver by correlating the received signal which is the sum of all transmitted signals with each code. As the receiver has the correct code, it can decode the received signal, i.e., the receiver can select “its” conversation from all the others. With CDMA, all users on a 1.25 MHz-wide channel can share the same frequency spectrum because each user's conversation is differentiated utilizing CDMA's unique digital codes. That same 1.25 MHz of frequency spectrum is re-used in each cell in the network.

In one embodiment, the base station B1, B2, BS1, BS2communicates with each phone every 1.25 milliseconds to control its power level. Every 1.25 milliseconds, the base station B1, B2, BS1, BS2instructs the phone3to increase or decrease its power, depending upon its distance from the base station B1, B2, BS1, BS2. The CDMA phone3transmits only the minimum power required to maintain a communications link. If the phone3is too far away from the serving base station B1, and the phone's transmitted power can not be increased, or if a neighboring base station B2, BS1, BS2provides for a better radio connection, the phone3is handed off to one of the neighboring cell/base stations B2, BS1, BS2.

The receiver10illustrated inFIG. 5monitors the pilot channels received at the frequencies f1, f2. The pilot channels are down converted to the baseband as described above and the signal strength of the pilot channels is determined independently. The signal strengths of the pilot channels are compared to a threshold value. If the signal strength of the target cell's pilot channel is above the threshold value, the controller BC1(FIG. 1) initiates the handoff procedure.

FIG. 6is a graph illustrating an exemplary spectrum of the IF signal, wherein the amplitude of the IF signal is shown as a function of the frequency f. For example, a spectrum analyzer is connected to an output of the filter20to measure the spectrum. As the IF signal passes through the filter20, the spectrum of the IF signal is band limited having a bandwidth B of approximately ±630 kHz.

As described above, the IF signal is a composite signal comprising the signals S1, S2that originate from two different base stations, for example, the serving base station B1and the target base station B2. In the illustrated embodiment, the amplitude of the signal S2is higher than the amplitude of the signal S1. The signals S1, S2can be separated through correlation with the respective codes as described above. When the signals S1, S2are separated, the signal strengths in the pilot channels can be determined.

FIG. 7is a flow chart illustrating the operation of the phone3when it receives RF signals originating from, for example, two different base stations B1, B2, BS1, BS2. Referring toFIG. 1, for the following description it is assumed the phone3moves from the serving cell C1to the target cell C2. The procedure is initialized at state800.

Proceeding to state802, the receiver10receives the signals S1, S2from the serving base station B1of the cell C1, and the target base station B2of the cell C2. The signal S1has the frequency f1and the signal S2has the frequency f2. As discussed above, the frequency f1can be equal to the frequency f2or different from the frequency f2. The antenna11receives the signals S1, S2simultaneously and, thus, converts the signals S1, S2to the composite RF signal.

Proceeding to state804, the amplifier14amplifies the relative weak composite RF signal to a level sufficient for further processing. As the amplifier14may cause undesired modulation products in addition to other potentially present noise components, the serially connected filter16serves to block these modulation products and noise components in order to minimize noise within the composite RF signal. In one embodiment, the filter16is a band pass filter that limits the bandwidth of the composite signal.

Proceeding to state806, the mixer18receives the amplified and band limited composite RF signal. In one scenario, for example, while the phone3is in the very proximity of the base station B1and thus has only a radio connection (signal S1) with the base station B1, the phone3operates the local oscillator34so that the oscillator signal LO1mixes with the signal S1to generate the IF signal having the desired intermediate frequency f1˜fLO1. The oscillator36can be tuned to approximately the same frequency, i.e., fLO1≈fLO2, so that the oscillator signal LO2leads to the same IF signal, or the oscillator36scans across a predetermined frequency range which allows the phone3to detect if another signal is present.

In another scenario, the phone3starts to move away from the base station B1and closer to the base station B2. While the phone3processes the signal S1, for example, to decode the signal S1and to detect if another signal is present, the phone3tunes the local oscillator36to the frequency fLO2so that the frequency difference f2˜fLO2is in the same frequency band as described above.

Proceeding to state808, the phone3has moved closer to the base station B2and the local oscillators34,36are appropriately tuned to generate the oscillator signals LO1, LO2. The mixer18is part of the first down conversion stage, which converts the signals S1, S2to the lower intermediate frequency. The mixer18mixes the composite RF signal, including the signals S1, S2, and the oscillator signals LO1, LO2to generate an output signal that includes the desired IF signal with (f1˜fLO1)≈(f2˜fLO2) as explained above.

Proceeding to state810, the phone3processes the signal output from the first down conversion stage. The filter20separates the IF signal from the output signal in that it passes only the IF signal. The amplifier22amplifies the IF signal to compensate for losses that occurred through separating the IF signal from the output signal.

The processing further includes separating the IF signal in the second down conversion stage into the components I, Q. The IF signal is split. One part of the IF signal is multiplied with a sine signal and the other part of the IF signal is multiplied with a cosine signal. The sine signal and the cosine signal are derived from the oscillator signal LO3having the oscillator frequency fLO3. The second down conversion stage outputs the components I, Q which have the baseband frequency fB.

Proceeding to state812, the baseband processor38receives the components I, Q and applies the pseudo-noise codes. The application of the pseudo-noise codes results in two separate signals in the baseband. These signals are further processed in the subsequent signal processing module7. The signal processing module7, for example, extracts the traffic channel to convert the signal S2into an analog speech signal, and analyzes the signal strength of the pilot channel. The procedure ends at state814.

In the above embodiment, the frequencies f1, f2are allocated within the same frequency band and the signals S1, S2from the antenna11share a common receive path up to the mixer18. Both signals S1, S2pass through the filter16. However, in another embodiment of the systems1,100, the frequencies f1, f2can be in different frequency bands. In this case, the receive path of the mixer module12is modified, as shown inFIG. 8, because under these circumstances one of the signals S1, S2could be blocked by the filter16.

FIG. 8shows a section of a mixer module12′ which is a further embodiment of the mixer module12. The illustrated section includes the receive path between the input13and the mixer18′. The remaining section of the mixer module12′, i.e., between the mixer18′ and the outputs15a,15b, is as shown inFIG. 5.

The receive path between the mixer18′ and the input13includes a first path having a serial arrangement of an amplifier14′ and a low-pass filter16′, and a second path having a serial arrangement of an amplifier14″ and a filter16″. The filters16′,16″ are connected to the mixer18′, and the amplifiers14′,14″ are connected to a duplexer40which is further connected to the input13and, thus, to the antenna11.

The amplifiers14′,14″, like the amplifier14shown inFIG. 5, amplify the composite RF signal that includes the signals S1, S2. As the filter16inFIG. 5, the filters16′,16″ can be bandpass filters or low pass filters, each filter16′,16″ passing only the desired signal frequency f1or f2. For example, the filter16′ is configured to pass only the signal S1, and the filter16″ passes only the signal S2. In a cellular CDMA system, the filters16′ and16″ are tuned to pass signals in a frequency band between about 869 MHz and about 894 MHz. In a PCS/CDMA system, the filters16′ and16″ are tuned to pass signals in a frequency band between about 1930 MHz and about 1960 MHz

The mixer18′ receives the signals S1, S2and oscillator signals LO1′, LO2′ generated by the local oscillators34′,36′. The oscillator signals LO1′, LO2′ have oscillator frequencies fLO1′, fLO2′, respectively. The oscillator signals LO1′, LO2′ and the signals S1, S2mix as described above. The oscillator frequencies fLO1′, fLO2′are selected so that the output signal from the mixer18′ has signal components with f1˜fLO1′≈f2˜fLO2′.

FIG. 9shows an illustration of an embodiment of the mixer18′ shown inFIG. 8. The mixer18′ includes a mixer18a′ connected to the filter16′ and receiving the oscillator signal LO1′, and a mixer18b′ connected to the filter16″ and receiving the oscillator signal LO2′. Each mixer18a′,18b′ is connected to a signal combiner42that combines the output signals (intermediate frequency signals) of the mixers18a′,18b′ to the IF signal input to the filter20.

FIG. 10shows an illustration of an embodiment of the mixer18shown inFIG. 5. The mixer18includes a mixer18aconnected to the filter16and receiving the oscillator signal LO1, and a mixer18bconnected to the filter16and receiving the oscillator signal LO2. Each mixer18a,18bis connected to a signal combiner43that combines the output signals (intermediate frequency signals) of the mixers18a,18bto the IF signal input to the filter20.

The phone3allows a soft handoff between neighboring cells that operate at different carrier frequencies. The phone3has two local oscillators34,36and at least one of them is tunable over a predetermined frequency range to cover the frequencies used in neighboring cells or even cells of a different system.

In one embodiment, the phone3moves exclusively within the system1which is a cellular CDMA system. When the phone3moves from one cell to another, the system1is configured to perform intra-system handoffs. In case the neighboring cells C1–C4have the same assigned frequency (i.e., f1≈f2), the phone3operates like a conventional cellular phone. However, if the neighboring cells C1–C4have different assigned frequencies (i.e., f1≠f2), in accordance with the present invention, the phone3still allows performance of the “Soft Handoff.”

While the phone3has an active traffic connection with the base station B1, the phone continuously monitors the signal strength of the pilot channel of this traffic connection. During the traffic connection, the local oscillator34is tuned so that the difference frequency f1˜fLO1is the intermediate frequency. In addition, the phone3“listens” if it receives pilot channels from neighboring cells C2–C4. For that purpose, the phone3scans a predetermined frequency range by tuning the local oscillator36correspondingly. As soon as a (neighboring) pilot channel, for example, within the signal S2at the frequency f2, is present and the oscillator frequency fLO2is set so that the difference f1˜fLO1falls within the same frequency band as the difference f2˜fLO2, components of both signals S1, S2fall within the band of the intermediate frequency defined by the filter20. In this case, the phone3detects the presence of the neighboring pilot channel.

Once detected, the phone3continues to monitor the signal strength of the neighboring pilot channel. When the signal strength of the neighboring pilot channel exceeds the predetermined threshold, the system1initiates the hand off from the cell C1to the cell C2. At the time this hand off occurs, the phone3is tuned to receive simultaneously the signals S1, S2. That is, when the previous connection (signal S1) is broken, the new connection (signal S2) already exists. Although the neighboring frequencies are different, the soft handoff and its advantages are maintained. The user of the phone3does not notice the hand off, because the new connection is made before the old connection is broken.

In another embodiment, the phone3moves between the systems1,100, for example, from the cell C1to the cell C5, and the systems1,100allow inter-system handoffs. Such an inter-system handoff could be necessary, for example, if the user of the phone3reaches a limit of the coverage area of the system1during a phone call, but continues to travel and to talk. Without an inter-system handoff, the phone call would be terminated, eventually without a warning, because the radio connection suddenly breaks.

The system1can be a conventional cellular CDMA system in which the neighboring cells C1–C4operate at the same assigned frequency f1. The system100can be a conventional PCS system in which the neighboring cells C5, C6operate at the same assigned frequency f3which is different from the frequency f1.

While the phone3has an active traffic connection with the base station B1, the phone continuously monitors the signal strength of the pilot channel of this traffic connection. The phone3also monitors the signal strengths of neighboring pilot channels of the system1, to determine when a handoff within the system1is necessary. During the traffic connection, the local oscillator34is tuned so that the difference frequency f1˜fLO1is the intermediate frequency.

In addition, the phone3“listens” if it receives pilot channels from neighboring cells C5of the system100. For that purpose, the phone3scans a predetermined frequency range defined by the system100by tuning the local oscillator36correspondingly. As soon as a (neighboring) pilot channel, for example, at the frequency f3, is present and the oscillator frequency fLO2is set so that the requirement (f1˜fLO1)≈(f3˜fLO2) is fulfilled, components of both signals fall within the band of the intermediate frequency defined by the filter20. In this case, the phone3detects the presence of the neighboring pilot channel. The subsequent procedure, including the soft handoff between the cell C1(system1) and the cell C5(system100) is as described above.

Referring again toFIG. 4, the receiver10converts the composite RF signal, which includes the signals S1, S2, from an initial high frequency (RF) range down to a lower frequency range, the baseband. In the embodiment described above, the down conversion process includes two stages. A first stage down converts the composite RF signal from the RF range to an intermediate frequency range, and a second stage down converts the composite RF signal from the intermediate frequency range to the baseband. As described above, a receiver using the two stage down converting process is known as a super-heterodyning receiver.

FIGS. 11–16illustrate another embodiment of the receiver10, which uses a down conversion process comprising one stage. A single stage down converts the composite RF signal from the RF frequency range to a baseband frequency range. A receiver using a single down converting process is known as a direct conversion receiver.

FIG. 11shows a schematic illustration of a direct conversion embodiment of the receiver10. Focusing on the single-ended implementation of the receiver10, the receiver10includes a mixer module112, which down converts the composite RF signal to the baseband, and a baseband processor38. The mixer module112has an input13and outputs15a,15bto connect the mixer module112to the antenna11and the baseband processor38, respectively. The baseband processor38has an output19, which connects to the signal processing module7.

In one embodiment, the mixer module112comprises a combination of an amplifier114and a mixer118for signal amplification and frequency down conversion. The amplifier114is, for example, a low-noise amplifier (LNA) that receives the composite RF signal, amplifies the composite RF signal, and feeds the amplified RF signal to the mixer118. In addition, the mixer118receives oscillator signals LO1, LO2generated by two separate local oscillators134,136. The oscillator signals LO1, LO2are, for example, sinusoidal signals each having a constant amplitude and frequency. It is contemplated that a single local oscillator generates oscillator signals LO1, LO2. In other embodiments, the local oscillator signals are periodic signals having varying amplitude and frequency.

The mixer118multiplies the composite RF signal and the oscillator signals LO1, LO2, and the various signal components mix with each other. The oscillator signal LO1mixes with the signals S1, S2of the composite RF signal and the oscillator signal LO2mixes with the signals S1, S2. As is known in the art, this mixing process results in a signal that includes a variety of different frequencies. These different frequencies include the original frequencies f1, f2, fLO1, fLO2, their harmonics, for example, 2f1, 2f2, 2fLO1, 2fLO2, and their sums and differences, for example, f1±fLO1, f2±fLO2.

In one embodiment, the difference frequencies f1−fLO1, f2−fLO2are of interest. The oscillator frequencies fLO1, fLO2are selected so that the difference frequencies f1−fLO1, f2−fLO2fall within the baseband frequency range and have approximately the same baseband value, i.e., (f1−fLO1)≈(f2−fLO2). This frequency value is hereinafter referred to as the “baseband frequency,” which is lower than the initial frequencies f1, f2, and written as “fB.” The local oscillators134,136can be tuned to appropriate oscillator frequencies fLO1, fLO2such that the differences f1−fLO1and f2−fLO2fall within the baseband frequency range. In an embodiment, the baseband frequency range is from approximately 0 Hz to approximately 630 kHz. In another embodiment, the baseband frequency range is from approximately 0 Hz to greater than 630 kHz. In a further embodiment, the baseband frequency range is from greater than 0 Hz to less than 630 kHz. In another embodiment, the baseband frequency range is from greater than 0 Hz to greater than 630 kHz.

Because the mixer118generates an output signal that comprises a variety of different frequencies, filters120,121are connected to the mixer118in order to block frequencies other than the baseband frequencies f1−fLO1, f2−fLO2. The signals output from the filters120,121are referred to as the baseband signals.

In the illustrated embodiment, the mixer module112further includes a filter116, and amplifiers122,123. The filter116is connected between the mixer118and the amplifier114connected to the input13. The mixer118is connected to the filter116to receive the band limited composite RF signal and to the local oscillators134,136. As shown, the filter116is a bandpass filter which limits the bandwidth of the composite RF signal received from the amplifier114to block undesired frequency components and to reduce noise in the composite RF signal. The undesired frequency components can be caused, for example, by nonlinearities of the amplifier114that result in intermodulation products. In one embodiment, the passband of the filter116is about 25 MHz to allow passage of a receive band between about 850 MHz and 900 MHz, more precisely between 869 MHz and 894 MHz, and to block frequencies outside of this receive band.

The local oscillators134,136are in one embodiment conventional local oscillators configured to operate at the different oscillator frequencies fLO1, fLO2. The oscillator signals LO1, LO2can be sinusoidal signals each having a frequency between 500 MHz and 2.5 GHz. In one embodiment, the oscillator signal LO1has a frequency fLO1which is approximately equal to the radio frequency f1and the oscillator signal LO2has a frequency fLO2which is approximately equal to the radio frequency f2.

Typical frequency ranges of fLO1and fLO2for exemplary frequency bands are approximately 869 MHz to approximately 894 MHz for the U.S. Cellular frequency band, approximately 832 MHz to approximately 870 MHz for the Japanese Cellular frequency band, approximately 1930 MHz to approximately 1990 MHz for the U.S. PCS frequency band, approximately 1840 MHz to approximately 1870 MHz for the Korean PCS frequency band, and approximately 2110 MHz to approximately 2170 MHz for the Wideband CDMA frequency range. Additional frequency ranges of fLO1and fLO2for other frequency bands are, for example, approximately 2400 MHz to approximately 2497 MHz, approximately 5150 MHz to approximately 5350 MHz, approximately 2400 MHz to approximately 2480 MHz, and approximately 1575.42 MHz.

The oscillator signals LO1, LO2are tunable to adapt to other phone systems which operate, for example, at carrier frequencies of about 1800 MHz, 1900 MHz, or 2100 MHz. In other embodiments, the oscillator signals LO1, LO2are tunable to adapt to other phone systems, which operate, for example, at carrier frequencies of about 800 MHz, or 900 MHz. In yet other embodiments, the oscillator signals LO1, LO2are tunable to adapt to other phone systems which operate, for example, at carrier frequencies of about 2400 MHz, 5200 MHz, or 1575 MHz.

Alternatively, the phone3can be a dual band cellular phone which can operate within different frequency bands, for example, 800 MHz, 900 MHz, 1575 MHz, 1800 MHz, 1900 MHz, 2100 MHz, 2400 MHz, or 5200 MHz. Independent of what carrier frequencies the signals S1, S2have, the frequencies of the signals LO1, LO2are generally selected so that the difference f1−fLO1and the difference f2−fLO2are in the baseband frequency range.

AlthoughFIG. 11shows the local oscillators134,136as belonging to the mixer module112, it is contemplated that the local oscillators134,136may be located outside the mixer module112and at other locations within the phone3. In one embodiment, the local oscillators134,136are located in the mixer module112. If the mixer module112is implemented as an integrated circuit, the local oscillators134,136are typically located off-chip. In another embodiment, the local oscillators134,136are located on the mixer module integrated circuit.

In one embodiment, the local oscillators134,136are conventional frequency synthesizers whose frequencies are referenced to piezoelectric crystals. The synthesizers are tunable within a predetermined range.

In another embodiment, voltage controlled oscillators (VCO) can be used to generate the desired local oscillator frequency. In one embodiment, a multiplier multiplies the output of the voltage controlled oscillator to produce the local oscillator frequency. In another embodiment, a divider divides the output of the voltage controlled oscillator to produce the local oscillator frequency. Thus, the local oscillator frequency, which is related to the radio frequency, can be a harmonic or a sub-harmonic of the voltage controlled oscillator frequency.

In other embodiments, other devices, such as, for example, hybrid crystal oscillators, temperature compensated crystal oscillators, and the like, can be used to generate the desired local oscillator frequency.

It is contemplated that a wide variety of frequencies can be used to generate local oscillator frequencies fLO1, fLO2.

A first output of the mixer118connects to the filter120, and a second output of the mixer118connects to the filter121. In the illustrated embodiment, the filters120and121are implemented as low-pass filters, which have, for example, a cut-off frequency of approximately 630 kHz. The filters120,121select the desired frequency band around the baseband frequency f1−fLO1, f2−fLO2, and block frequencies, which are higher than the cut-off frequency. It is contemplated that other values for the cut-off frequency can be chosen.

In one embodiment, the amplifiers122,123connect to control lines124,125, respectively, to receive automatic gain control signals AGC from a central controller (not shown) of the phone3. The control signals AGC control the amplifiers122,123to amplify the baseband signal with a desired gain. The amplifiers122,123operate at gains between approximately +45 dB and approximately −45 dB to amplify the baseband signal to a predetermined level over the entire dynamic range of the receiver10. An output of the amplifier122connects to the output15aand an output of the amplifier123connects to the output15b.

As shown inFIG. 11, the amplifiers122,123belong to the mixer module112. It is contemplated that the amplifiers122,123may be located outside the mixer module112and at other locations within the phone3.

In one embodiment, the amplifiers122,123can be implemented in the analog domain as part of the mixer module112. In another embodiment, the amplifiers122,123can be implemented in the digital domain as part of the baseband processing module38. In a further embodiment, if the mixer module112is implemented as an integrated circuit, the amplifiers112,123can be implemented in the digital domain as part of the mixer module112.

FIG. 12shows an illustration of an embodiment of the mixer118shown inFIG. 11. The mixer118includes mixers118a,118bconnecting to the filter116and receiving the oscillator signal LO1, and mixers118c,118dconnecting to the filter116and receiving the oscillator signal LO2.

A phase shifter130receives the oscillator signal LO1and a phase shifter131receives the oscillator signal LO2. The mixer118areceives the oscillator signal LO1and the mixer118breceives the oscillator signal LO1with a 90 degrees phase shift. Similarly, the mixer118breceives the oscillator signal LO2and the mixer118dreceives the oscillator signal LO2with a 90 degrees phase shift. That is, in one embodiment, the mixers118a,118creceive signals having a sin function and the mixers118b,118dreceive signals having a cosine function.

In another embodiment, the mixer118areceives the oscillator signal LO1with a +45 degrees phase shift, and the mixer118breceives the oscillator signal LO1with a 45 degrees phase shift. Likewise, the mixer118creceives the oscillator signal LO2with a +45 degrees phase shift, and the mixer118dreceives the oscillator signal LO2with a −45 degrees phase shift. That is, in one embodiment, mixers118a,118creceive oscillator signals that are 90 degrees out of phase from the oscillator signals received by mixers118b,118d, respectively.

Each mixer118a,118cconnects to a signal combiner143that combines the output signals (baseband signals) of the mixers118a,118cto the baseband signal input of the filter120. Likewise, each mixer118b,118dconnects to a signal combiner144that combines the output signals (baseband signals) of the mixers118b,118dto the baseband signal input of the filter121.

The oscillator frequency fLO1is selected so that the composite RF signal at the output of filter116is down converted to the baseband at a frequency fB. The down conversion stage, implemented by the mixers118a,118bsplits the composite RF signal into baseband “In phase” (I) and “Quadrature” (Q) outputs, which correspond to the in phase and quadrature components containing information transmitted by the base station B1.

Likewise, the oscillator frequency fLO2is selected so that the composite RF signal at the output of filter116is down converted to the baseband at a frequency fB. The down conversion stage implemented by mixers118c,118dsplits the composite RF signal into baseband in phase and quadrature outputs, which correspond to the in phase and quadrature components containing information transmitted by the base station B2.

Signal combiner143combines the in phase component containing information transmitted by base station B1and the in phase component containing information transmitted by the base station B2to produce the in phase (I) component of the baseband signal.

Signal combiner144combines the quadrature component containing information transmitted by base station B1and the quadrature component containing information transmitted by the base station B2to produce the quadrature (Q) component of the baseband signal.

The baseband processor38receives the components I, Q, performs the processing to convert the received CDMA signal back to an uncoded (“de-spread”) signal, and extracts the voice/data signals.

The receiver10illustrated inFIG. 11monitors the pilot channels received at the frequencies f1, f2. The pilot channels are down converted to the baseband as described above and the signal strength of the pilot channels is determined independently. The signal strengths of the pilot channels are compared to a threshold value. If the signal strength of the target cell's pilot channel is above the threshold value, the controller BC1(FIG. 1) initiates the handoff procedure.

FIG. 15is a graph illustrating an exemplary spectrum of either the in phase or quadrature component of the baseband signal, wherein the amplitude of the component of the baseband signal is shown as a function of the frequency f. For example, a spectrum analyzer connects to an output of the filter120or the filter121to measure the spectrum. For example, as the baseband signal passes through the filter120or the filter121, the spectrum of the baseband signal is pass band limited having a bandwidth B of approximately 0 kHz to approximately 630 kHz.

As described above, the baseband signal is a composite signal comprising the signals S1, S2that originate from two different base stations, for example, the serving base station B1and the target base station B2. In the illustrated embodiment, the amplitude of the signal S2is higher than the amplitude of the signal S1. The signals S1, S2can be separated through correlation with the respective codes. When the signals S1, S2are separated, the signal strengths in the pilot channels can be determined.

FIG. 16is a flow chart illustrating the operation of the phone3when it receives RF signals originating from, for example, two different base stations B1, B2, BS1, BS2. Referring toFIG. 1, for the following description it is assumed the phone3moves from the serving cell C1to the target cell C2. The procedure is initialized at state1600.

Proceeding to state1602, the receiver10receives the signals S1, S2from the serving base station B1of the cell C1, and the target base station B2of the cell C2. The signal S1has the frequency f1and the signal S2has the frequency f2. As discussed above, the frequency f1can be equal to the frequency f2or different from the frequency f2. The antenna11receives the signals S1, S2simultaneously and, thus, converts the signals S1, S2to the composite RF signal.

Proceeding to state1604, the amplifier114amplifies the relatively weak composite RF signal to a level sufficient for further processing. As the amplifier114may cause undesired modulation products in addition to other potentially present noise components, the serially connected filter116serves to block these modulation products and noise components in order to reduce noise within the composite RF signal. In one embodiment, the filter116is a band pass filter that limits the bandwidth of the composite signal. It is contemplated that other filters116, such as, for example, low pass filters, high pass filters, and the like, can limit the bandwidth of the composite signal.

Proceeding to state1606, the mixer118receives the amplified and band limited composite RF signal. In one scenario, for example, while the phone3is in the very proximity of the base station B1and thus has a wireless connection (signal S1) with the base station B1, the phone3operates the local oscillator134so that the oscillator signal LO1mixes with the signal S1to generate the baseband signal having the desired baseband frequency f1−fLO1. The oscillator136can be tuned to approximately the same frequency, i.e., fLO1≈fLO2, so that the oscillator signal LO2leads to the same baseband signal, or the oscillator136scans across a predetermined frequency range which allows the phone3to detect if another signal is present.

In another scenario, the phone3starts to move away from the base station B1and closer to the base station B2. While the phone3processes the signal S1, for example, to decode the signal S2and to detect if another signal is present, the phone3tunes the local oscillator136to the frequency fLO2so that the frequency difference f2−fLO2is in the baseband frequency range as described above.

Proceeding to state1608, the phone3has moved closer to the base station B2and the local oscillators134,136are appropriately tuned to generate the oscillator signals LO1, LO2. The mixer118down converts the signals S1, S2to signals having frequencies in the baseband frequency range. The mixer118mixes the composite RF signal, including the signals S1, S2, and the oscillator signals LO1, LO2to generate an output signal that includes the desired baseband signal with (f1−fLO1)≈(f2−fLO2).

Proceeding to state1610, the processing further includes separating the signals S1, S2into the baseband components I, Q. The mixer118amultiplies the RF signal with a first sine signal and the mixer118bmultiplies the RF signal with a first cosine signal. The first sine and cosine signal derive from the oscillator signal LO1having the oscillator frequency fLO1.

The mixer118cmultiplies the RF signal with a second sine signal and the mixer118dmultiplies the RF signal with a second cosine signal. The second sine and cosine signal derive from the oscillator signal LO2having the oscillator frequency fLO2. Mixer118further combines the in phase baseband components corresponding to RF signals S1, S2, and combines the quadrature baseband components corresponding to the RF signals S1, S2. The down conversion stage outputs the components I, Q which have the base band frequency fB.

Proceeding to state1612, the baseband processor38receives the components I, Q and applies the pseudo-noise codes. The application of the pseudo-noise codes results in two separate signals in the baseband. These signals are further processed in the subsequent signal processing module7.

The signal processing module7, for example, extracts the traffic channel to convert the signal S2into an analog speech signal, and analyzes the signal strength of the pilot channel. In another embodiment, the signal processing module7extracts the traffic channel to convert the signal S2into a digital or an analog signal, such as, for example, a speech signal, an audio signal, or the like. The procedure ends at state1614.

In the above embodiment, the frequencies f1, f2are allocated within the same frequency band and the signals S1, S2from the antenna11share a common receive path up to the mixer18. Both signals S1, S2pass through the filter116.

However, in another embodiment of the systems1,100, the frequencies f1, f2can be in different frequency bands. In this case, the receive path of the mixer module112is modified, as shown inFIG. 13, because under these circumstances one of the signals S1, S2could be blocked by the filter116.

FIG. 13shows a schematic illustration of an embodiment of the receiver10. The receiver10includes a mixer module112′, which down converts the composite RF signal to the baseband, and a baseband processor38. The mixer module112′ has an input13and outputs15a,15bto connect the mixer module112′ to the antenna11and the baseband processor38, respectively. The baseband processor38has an output19, which connects to the signal processing module7.

In one embodiment, the mixer module112′ comprises a mixer118′ for frequency down conversion. The receive path between the mixer118′ and the input13includes a first path having a serial arrangement of an amplifier114′ and a low-pass filter116′, and a second path having a serial arrangement of an amplifier114” and a filter116″. The filters116′,116″ connect to the mixer118′, and the amplifiers114′,114″ connect to duplexers140,141, respectively, which further connect to a diplexer or a switch139. The diplexer or switch139connects to the input13and, thus, to the antenna11.

The amplifiers114′,114″, are, for example, low noise amplifiers that receive and amplify the composite RF signal that includes the signals S1, S2.

The filters116′,116″ can be bandpass filters or low pass filters, each filter116′,116″ passing the desired signal frequency f1or f2. For example, the filter116′ is configured to pass the signal S1, and the filter116″ passes the signal S2. For example, in a cellular CDMA system, the filters116′ and116″ are tuned to pass signals in a frequency band between about 869 MHz and about 894 MHz. In a PCS/CDMA system, for example, the filters116′ and116″ are tuned to pass signals in a frequency band between about 1930 MHz and about 1960 MHz.

The mixer118′ receives the signals S1, S2from filters116′,116″, and oscillator signals LO1′, LO2′ generated by the local oscillators134′,136′. The oscillator signals LO1′, LO2′ have oscillator frequencies fLO1′, fLO2′, respectively.

The local oscillators134′,136′ are, in one embodiment, conventional oscillators configured to operate at the different oscillator frequencies fLO1′, fLO2′. For example, the oscillator signals LO1′, LO2′ can be sinusoidal signals each having a frequency between 500 MHz and 2.5 GHz.

Independent of what carrier frequencies the signals S1, S2have, the frequencies of the signals LO1′, LO2′ are generally selected so that the difference frequency f1−fLO1′and the difference frequency f2−fLO2′are in the baseband frequency range.

AlthoughFIG. 13shows the local oscillators134′,136′ as belonging to the mixer module112′, it is contemplated that the local oscillators134′,136′ may be located outside the mixer module112′ and at other locations within the phone3. If the mixer module112′ is implemented as an integrated circuit, in one embodiment, the local oscillators134′,136′ are typically located on-chip. In another embodiment, if the mixer module112′ is implemented as an integrated circuit, the local oscillators134′,136′ are located off-chip. In one embodiment, the local oscillators134′,136′ are conventional frequency synthesizers whose frequencies are referenced to piezoelectric crystals. The synthesizers are tunable within a predetermined range. It is contemplated that other types of local oscillators, such as voltage controlled oscillators (VCO), can be used to generate the desired baseband signal.

The oscillator signals LO1′, LO2′ and the signals S1, S2mix as described above. The oscillator frequencies fLO1′, fLO2′are selected so that the output signal from the mixer118′ has signal components with the difference frequencies f1−fLO1′, f2−fLO2′. For example, difference frequencies f1−fLO1′, f2−fLO2′fall within the baseband frequency range.

Because the mixer118′ generates an output signal that comprises a variety of different frequencies, filters120,121are connected to the mixer118′ in order to block frequencies other than the baseband frequencies f1−fLO1′, f2−fLO2′. A first output of the mixer118′ connects to the filter120, and a second output of the mixer118′ connects to the filter121. The signals output from the filters120,121are referred to as the baseband signals.

In the illustrated embodiment, the filters120and121are implemented as low-pass filters, which have, for example, a cut-off frequency of approximately 630 kHz. The filters120,121select the desired frequency band around the baseband frequency f1−fLO1′, f2−fLO2′, and block frequencies, which are higher than the cut-off frequency. It is contemplated that other values for the cut-off frequency can be chosen.

In the illustrated embodiment, the mixer module112′ further includes amplifiers122,123. The output of filter120connects to the input of amplifier122, and the output of filter121connects to the input of amplifier123.

In one embodiment, the amplifiers122,123connect to control lines124,125, respectively, to receive automatic gain control signals AGC from a central controller (not shown) of the phone3. The control signals AGC control the amplifiers122,123to amplify the baseband signal with a desired gain. In one embodiment, the amplifiers122,123are operable at gains between +45 dB and −45 dB to amplify the baseband signal to a predetermined level over the entire dynamic range of the receiver. It is contemplated that the amplifiers122,123are operable at other gains to amplify the baseband signal to a predetermined level over the entire dynamic range of the receiver. An output of the amplifier122connects to the output15aand an output of the amplifier123connects to the output15b.

FIG. 14shows an illustration of an embodiment of the mixer118′ shown inFIG. 13. The mixer118′ includes mixers118a′,118b′ connecting to the filter116′ and receiving the oscillator signal LO1′, and mixers118c′,118d′ connecting to the filter116″ and receiving the oscillator signal LO2′. The mixer118a′ receives the oscillator signal LO1,′ and the mixer118b′ receives the oscillator signal LO1′ with a 90 degrees phase shift. The mixer118c′ receives the oscillator signal LO2′, and the mixer118d′ receives the oscillator signal LO2′ with a 90 degrees phase shift. That is, in one embodiment, the mixers118a′,118c′ receive signals having a sine function and the mixers118b′,118d′ receive signals having a cosine function.

In another embodiment, the mixer118areceives the oscillator signal LO1′ with a +45 degrees phase shift, and the mixer118breceives the oscillator signal LO1′ with a −45 degrees phase shift. Likewise, the mixer118creceives the oscillator signal LO2′ with a +45 degrees phase shift, and the mixer118dreceives the oscillator signal LO2′ with a −45 degrees phase shift. That is, in one embodiment, mixers118a′,118c′ receive oscillator signals that are 90 degrees out of phase from the oscillator signals received by mixers118b′,118d′.

Each mixer118a′,118c′ connects to a signal combiner143′ that combines the output signals (baseband signals) of the mixers118a′,118c′ to the baseband signal input of the filter120. Likewise, each mixer118b′,118d′ connects to a signal combiner144′ that combines the output signals (baseband signals) of the mixers118b′,118d′ to the baseband signal input of the filter121.

The phone3allows a soft handoff between neighboring cells that operate at different carrier frequencies. The phone3has two local oscillators and at least one of them is tunable over a predetermined frequency range to cover the frequencies used in neighboring cells or even cells of a different system.

In one scenario, the phone3moves exclusively within the system1which is a cellular CDMA system. When the phone3moves from one cell to another, the system1is configured to perform intra-system handoffs. In case the neighboring cells C1–C4have the same assigned frequency (i.e., f1≈f2), the phone3operates like a conventional cellular phone. However, if the neighboring cells C1–C4have different assigned frequencies (i.e., f1≠f2), the phone3still allows performance of the “Soft Handoff.”

While the phone3has an active traffic connection with the base station B1, the phone continuously monitors the signal strength of the pilot channel of this traffic connection.

In a super-heterodyning embodiment, during the traffic connection, the local oscillator34is tuned so that the difference frequency f1˜fLO1is the intermediate frequency. In addition, the phone3“listens” if it receives pilot channels from neighboring cells C2–C4. For that purpose, the phone3scans a predetermined frequency range by tuning the local oscillator36correspondingly. As soon as a (neighboring) pilot channel, for example, within the signal S2at the frequency f2, is present and the oscillator frequency fLO2is set so that the difference f1˜fLO1falls within the same frequency band as the difference f2˜fLO2, components of both signals S1, S2fall within the band of the intermediate frequency defined by the filter20.

In a direct conversion embodiment, during the traffic connection, the local oscillator134is tuned so that the difference frequency f1−fLO1is in the baseband frequency range. In addition, the phone3“listens” if it receives pilot channels from neighboring cells C2–C4. For that purpose, the phone3scans a predetermined frequency range by tuning the local oscillator136correspondingly. As soon as a (neighboring) pilot channel, for example, within the signal S2at the frequency f2, is present and the oscillator frequency fLO2is set so that the difference f1−fLO1falls within the same baseband frequency range as the difference f2−fLO2, components of both signals S1, S2fall within the band of the baseband frequency range defined by the filters120,121.

In both embodiments, the phone3detects the presence of the neighboring pilot channel.

Once detected, the phone3continues to monitor the signal strength of the neighboring pilot channel. When the signal strength of the neighboring pilot channel exceeds the predetermined threshold, the system1initiates the hand off from the cell C1to the cell C2. At the time this hand off occurs, the phone3is tuned to receive simultaneously the signals S1, S2. That is, when the previous connection (signal S1) is broken, the new connection (signal S2) already exists. Although the neighboring frequencies are different, the soft handoff and its advantages are maintained. The user of the phone3does not notice the hand off, because the new connection is made before the old connection is broken.

In another scenario, the phone3moves between the systems1,100, for example, from the cell C1to the cell C5, and the systems1,100allow inter-system handoffs. Such an inter-system handoff could be useful, for example, if the user of the phone3reaches a limit of the coverage area of the system1during a phone call, but continues to travel and to talk. Without an inter-system handoff, the phone call would be terminated, eventually without a warning, because the radio connection suddenly breaks.

The system1can be a conventional cellular CDMA system in which the neighboring cells C1–C4operate at the same assigned frequency f1. The system100can be a conventional PCS system in which the neighboring cells C5, C6operate at the same assigned frequency f3which is different from the frequency f1.

While the phone3has an active traffic connection with the base station B1, the phone continuously monitors the signal strength of the pilot channel of this traffic connection. The phone3also monitors the signal strengths of neighboring pilot channels of the system1, to determine when a handoff within the system1should be performed.

In a super-heterodyning embodiment, during the traffic connection, the local oscillator34is tuned so that the difference frequency f1˜fLO1is the intermediate frequency. In addition, the phone3“listens” if it receives pilot channels from neighboring cells C5of the system100.

For that purpose, the phone3scans a predetermined frequency range defined by the system100by tuning the local oscillator36correspondingly. As soon as a (neighboring) pilot channel, for example, at the frequency f3, is present and the oscillator frequency fLO2is set so that the requirement (f1˜fLO1)≈(f3fLO2) is fulfilled, components of both signals fall within the band of the intermediate frequency defined by the filter20.

In a direct conversion embodiment, during the traffic connection, the local oscillator134is tuned so that the difference frequency f1−fLO1is in the baseband frequency range.

In addition, the phone3“listens” if it receives pilot channels from neighboring cells C5of the system100.

For that purpose, the phone3scans a predetermined frequency range defined by the system100by tuning the local oscillator136correspondingly. As soon as a (neighboring) pilot channel, for example, at the frequency f3, is present and the oscillator frequency fLO2is set so that the requirement (f1−fLO1)≈(f3−fLO2) is fulfilled, and components of both signals fall within the baseband defined by the filters120,121.

In both of these embodiments, the phone3detects the presence of the neighboring pilot channel. The subsequent procedure, including the soft handoff between the cell C1(system1) and the cell C5(system100) is as described above.

While the above detailed description has shown, described and identified several novel features of the invention as applied to different embodiments, it will be understood that various omissions, substitutions and changes in the form and details of the described embodiments may be made by those skilled in the art without departing from the spirit of the invention. Accordingly, the scope of the invention should not be limited to the foregoing discussion, but should be defined by the appended claims.