Frequency translation

The present invention provides a serrodyne architecture that is capable of operating at high frequencies. The architecture generally includes a coupler, a delay line, and a switching system. The delay line is coupled to a reflective port of the coupler. The switching system is coupled to the delay line at different points and is configured to selectively shunt those points on the delay line to change the effective electrical length of the delay line. The effective electrical length is changed throughout the period, corresponding to the amount the frequency of the input signal is shifted. In operation, an input signal is provided to an input port of the coupler, and an output signal is provided at the output port, wherein the input signal and the output signal vary by a translation frequency, which is controlled based on how the effective electrical length of the delay line is changed.

FIELD OF THE INVENTION

The present invention relates to radio frequency communications, and in particular to translating signals received at one frequency to being centered about another frequency.

BACKGROUND OF THE INVENTION

In many communication technologies, there is a need to shift signals from one frequency to another. Such frequency translation often employs various types of mixers. Unfortunately, the inherent nature of mixing results in signals with a double sideband, which requires additional filtering or wastes available channel space in the allocated frequency spectrum. Further, the mixers often suffer from poor linearity and inject a loss in the signal path, which must often be compensated for with additional amplification. Compensating for linearity and loss shortcomings often results in additional complexity of the circuitry required for frequency translation.

An alternative to mixers has been serrodyne structures, which result in a pure frequency shift wherein the resultant signal has a single sideband. Unfortunately, traditional serrodyne architectures are based on ferrite-based phase shift elements and are not capable of operation at high frequencies due to limitation in switching speeds. In particular, the switching speeds for modern mobile communications is in the hundreds or thousands of Megahertz, which results in pulse transition times in the serrodyne that fall within the sub-nanosecond realm. Magnetically actuated devices simply cannot switch at these speeds. Accordingly, serrodyne architectures based on traditional magnetic architectures cannot operate at the required modulation frequencies for many wireless communication technologies. Thus, there is a need for a frequency translation architecture that relates in a single sideband output signal, and that can operate within a frequency range sufficient for modern wireless communication techniques.

SUMMARY OF THE INVENTION

The present invention provides a serrodyne architecture that is capable of operating at high frequencies. The architecture generally includes a coupler, a delay line, and a switching system. The delay line is coupled to a reflective port of the coupler, which may take the form of a circulator, hybrid coupler, or the like. The switching system is coupled to the delay line at different points and is configured to selectively shunt those points on the delay line to change the effective electrical length of the delay line. The effective electrical length is changed throughout the period, corresponding to the amount the frequency of the input signal is shifted. In operation, an input signal is provided to an input port of the coupler, and an output signal is provided at the output port, wherein the input signal and the output signal vary by a translation frequency, which is controlled based on how the effective electrical length of the delay line is changed.

If the effective electrical length is increased throughout the period corresponding to the translation frequency, the output signal is effectively downconverted and will be of a frequency less than the input signal. If the effective electrical length is decreased throughout the period corresponding to the translation frequency, the output signal will have a frequency greater than the frequency of the input signal. The amount of shift is equal to the translation frequency.

The switching system may take many forms, and may be created from discrete components or may be integrated within an integrated circuit. During operation, switching elements, such as transistors, within the switching system are controlled such that the simultaneous switching of multiple switching elements is not required in an effort to minimize complexity as well as avoid irregularities caused by the inability of multiple switching elements to switch in perfect unison.

The frequency translation architecture of the present invention is particularly applicable to a system in which the cabling requirements are reduced between a base station and a corresponding masthead, which supports the antennas associated with the base station. In this system, signals received from one or more main and diversity antennas are translated from one frequency to another, combined with signals from other antennas, and sent over a common cable between the base station and the masthead electronics. After being combined and delivered over a single cable, the signals can be separated and translated back to their original frequencies, as necessary, and processed accordingly. Those skilled in the art will recognize various other applications of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now toFIG. 1, a block representation of serrodyne-based frequency translation circuitry is illustrated according to a first embodiment of the present invention. In general, the frequency translation afforded by the present invention provides for single sideband frequency translation to essentially provide Doppler shifting of a radio frequency input (RFIN) at a first frequency, fRF, to a translated radio frequency (RFOUT) at a frequency of fRF±fSHIFT, wherein fSHIFTrepresents the amount RFINis being translated in frequency. The single sideband frequency translation minimizes waste of the available frequency spectrum.

As illustrated, a multi-port frequency coupler2, such as a circulator, is associated with a delay line4having a defined electrical length. The delay line4is divided into a number of sections, and in the present example is divided into three sections, wherein nodes defining the boundaries of each section are selectively coupled to ground through a corresponding switching element6(A–C). The switching element6may take the form of any high-speed switching device, such as a field effect transistor (FET) or bipolar transistor. The switching elements6are controlled by control circuitry8in a manner that will be described in further detail below.

The coupler2as illustrated inFIG. 1Aincludes at least three ports: an input port (I), a reflection port (R), and an output port (O). The input port I receives the signal RFINto be translated and the output port O provides the translated signal RFOUT. The delay line4is coupled between the reflection port R and electrical ground. The switching elements6are coupled between their respective nodes on the delay line4and ground, and when active, will short the corresponding node on the delay line4to ground. As such, the effective electrical length of the delay line4will change depending on which switching elements6are active and shunting their corresponding nodes on the delay line4to ground. In essence, the control circuitry8controls the switching elements6to provide an electronically controlled short circuit having a controllable electrical length at the reflection port R of the coupler2. In the present embodiment, the delay line4has an electrical length of 135 degrees and the three illustrated nodes represent 0, 45, and 90 degrees in electrical length. In one embodiment, the switching elements6are sufficiently proximate to their corresponding nodes on the delay line4to have minimal impact on the effective electrical length.

Depending on how the switching elements6are controlled, the signal RFINcan be translated to a higher or lower frequency. In general, a translation to a higher frequency is referred to as upconversion, and a translation to a lower frequency is referred to as downconversion. In operation, the energy associated with the input signal RFINis radiated out of the reflection port R along the delay line4, reflected back into the reflection port R, and provided to the output port O. Preferably, the input signal RFINis isolated from the reflected signal, which represents the translated signal RFOUT. The translation of the input signal RFINis controlled by the sequential changing of the effective electrical length of the delay line4. If the switching elements6are configured to operate to effectively increase the electrical length, the increasing electrical length effectively increases the wavelength, and thus decreases the frequency of the signal. If the electrical length of the delay line4is decreased, the wavelength is compressed and the frequency is increased. The amount of the frequency shift is a function of the cycle associated with the changing electrical length provided at the reflection port (R).

With reference toFIG. 1B, an example is provided wherein RFINis translated from a frequency fRFto a frequency of fRF−fSHIFT. The state table ofFIG. 1Bcorresponds to how each of the switching elements6A–6C is controlled throughout a period corresponding to fSHIFT(1/fSHIFT). Further, a logic 1 indicates that the switching element6is active, and the corresponding node on the delay line4is coupled to ground. A logic 0 indicates that the switching element6is open. Initially assume that all of the switching elements6A–6C are active. At this point, the delay line4is shunted to ground at the 0° node, and any instant radiation exiting the reflection port R is reflected back to the reflection port R from the 0° node. For the second state in the period, assume the control circuitry8operates to deactivate switching element6A while switching elements6B and6C remain active. At this point, the instant radiation reflected out of the reflection port R is reflected from the 45° node, and thus the effective electrical length is increased. For state three, switching element6B is deactivated, and the instant radiation is reflected from the 90° node, thus further extending the electrical length associated with the reflection port R. For the final state, all of the switching elements6are deactivated, and the instant radiation from the reflection port R is reflected from the point at which the delay line4is grounded (the 135° node). The systematic increase of the electrical length associated with the reflection port R effectively decreases the frequency of the input signal RFINand results in the signal RFOUTbeing provided at the output port O at a frequency equal to fRF–fSHIFT. Notably, the switching of the switching elements6is continuously repeated from one period to the next.

In one embodiment, transition from state four back to the initial state one is controlled by initially activating switching element6A, such that the switching of switching elements6B and6C to an active state has no effect on the provided electrical length, since the 0° node is shunted to ground. By the time the transition to state two is required, switching elements6A–6C are all activated. Thus, for high-frequency operation, any difficulties normally associated with simultaneously switching transistors or other like switching elements are avoided.

With reference toFIG. 1C, an upconversion process is described. For the upconversion process, the previously described downconversion process is effectively reversed. Initially, assume that switching elements6A–6C are deactivated, such that the beginning of the period starts with the longest possible electrical length, and the electrical length decreases throughout the period. By decreasing the electrical length, the frequency of the input signal RFINis increased. Again, the period associated with decreasing the electrical length associated with the delay line4directly corresponds to the amount by which the input signal RFINis translated in frequency. When transitioning to state two, the control circuitry8will activate switching element6C to shunt the 90° node to ground to reduce the effective electrical length associated with the reflection port R. For state three, switching element6B is activated to shunt the 45° node of the delay line4to ground and further reduce the electrical length associated with the reflection port R. During the fourth state, the control circuitry8activates switching element6A to shunt the 0° node of the delay line4to ground and further reduces the electrical length associated with the reflection port R. Thus, throughout the period (states 1–4), the electrical length for reflecting the instant radiation associated with the input signal RFINis decreased, which results in an increase in the frequency associated with RFOUTby a frequency equal to fSHIFT.

When transitioning back from state four to state one, the control circuitry8will deactivate switching elements6B and6C during state four, such that the actual transition to state one from state four will correspond to switching element6A being deactivated. Since the 0° node is shunted to ground while switching elements6B and6C are deactivated, the deactivation of switching element6B and6C has no impact on the electrical length associated with the reflection port R. Again, simultaneous switching of multiple switching elements6is not required.

In an exemplary upconversion scenario, RFINmay have a carrier frequency of 1850 MHz, wherein communication channels are divided into a number of 1.2288 MHz bands. To upconvert the signal RFINby 162 MHz to 2012 MHz (RFOUT), fSHIFTequals 162 MHz. The period during which the four switching states occur is 1/162 MHz or 6.17 nanoseconds. As such, each state is approximately 1.53 ns when fSHIFTequals 162 MHz.

The concepts of the present invention may be implemented in different ways. With reference back toFIG. 1A, the circulator embodiment of the coupler2may have a fourth port through which a matching network10is coupled to ground. The output port O and the port to which the matching network10is coupled are associated such that the matching network10corresponds to the output impedance and operates to improve isolation of the signal RFOUTfrom the signal RFIN, as will be appreciated by those skilled in the art. Other potential embodiments include but are not limited to those illustrated inFIGS. 2 and 3.

The embodiment ofFIG. 2uses a circulator as the coupler2, yet varies the implementation of the delay line4in association with the switching elements6A–6C. In this embodiment, a switching network6′ is connected at the 0°, 45°, and 90° nodes through ¼ wavelength connections. The quarter wave lines operate to transform the switch impedance from a short circuit at the switch node to an open circuit at the delay line node. Introduction of the quarter wave transformer enables the switching network6′ to be located at one physical location. As such, the effective electrical length is selectively increased or decreased throughout a given period by connecting select nodes to each other or to ground. Those skilled in the art will recognize various techniques for sequentially lengthening or shortening the effective electrical length of the delay line4using various switching techniques, architectures, or implementations over the period corresponding to fSHIFT. The embodiment ofFIG. 2is particularly beneficial when the invention is being implemented in an integrated fashion. A single physical location for the switching network enables use of a single integrated switch matrix for the switching network. Use of a single integrated circuit switch network reduces the manufacturing cost and improves product reliability over use of three separate switch circuits for example.

With reference toFIG. 3, the coupler2takes the form of a hybrid coupler, which effectively includes two reflective ports, referenced as 0° and 90° ports. Essentially, half of the radiated power associated with RFINis reflected along delay lines4A and4B and the reflected waves are combined in-phase and provided to the output port O. Application of half the radiated power to each switch element improves the linearity of this implementation by 3 dB over those in which a single delay line is used. In operation, the reflections from port O are shifted by 90 degrees and those from port90are not shifted, such that the resultant reflections are in-phase with one another. InFIG. 3, quarter wave lines are used as inFIG. 2to enable collocation of the switch network. The embodiment of the invention illustrated inFIG. 3may not require the quarter wave lines in the event that the switch network consists of a series of discrete switch elements or in the event that the delay line is meandered so as to enable connection to the switching network without recourse to use of quarter wave lines.

The present invention may be implemented in a system to reduce cabling required in a base station environment. In the system, signals that were normally transmitted over separate cables are frequency shifted about different center frequencies, combined, and sent over a single cable. The frequency shifting may be provided by the previously described frequency translation architectures. At a receiving end of the cable, the combined signals are recovered and processed in traditional fashion. The system is particularly useful in a diversity environment, wherein multiple antennas are used to receive a common signal. In such an environment, certain of the signals received from the main and diversity antennas are shifted in frequency, combined with one another, and transmitted over a common cable. Accordingly, each sector, which includes a main and one or more diversity antennas, will need only one cable for transmitting the received signals from the antennas to electronics in a base housing.

Prior to delving into the details of the system, an overview of a base station environment10is illustrated inFIG. 4according to one embodiment. The illustrated base station environment is exemplary of the primary components in a cellular access network. A base housing12is provided in a secure location in association with a mast14, which may be a tower or other structure near the top of which is mounted a masthead16. Communications for the base station environment10are distributed between the masthead16and the base housing12. In particular, the base housing12will include base housing electronics18, which include the primary transceiver and power amplification circuitry required for cellular communications. The masthead16will include masthead electronics20, which generally comprise the limited amount of electronics necessary to operatively connect with multiple antennas22, which are mounted on the masthead16. The masthead electronics20and the base housing electronics18are coupled together with one or more feeder cables24. For the illustrated embodiment, there are six antennas22divided into three sectors having two antennas22each. For each sector, one feeder cable24is provided between the masthead electronics20and the base housing electronics18. Accordingly, there are three feeder cables24illustrated inFIG. 4. In traditional base station environments, each antenna would be associated with one feeder cable24.

Turning now toFIG. 5, a block representation of the base housing electronics18and one sector of the masthead electronics20is provided according to one embodiment of the cabling reduction system. Notably, there are two antennas22illustrated. A first antenna is referred to as a main antenna22M, and the second antennas is referred to as a diversity antenna22D. For signals transmitted from the main antenna22M, a signal to be transmitted will be provided over the feeder cable24to a duplexer26in the masthead electronics20. The signal to be transmitted (MAIN TX) is sent to another duplexer28and transmitted via the main antenna22M.

For receiving, signals transmitted from remote devices will be received at both the main antenna22M and the diversity antenna22D. The signals received at the main antenna22M are referred to as the main receive signals (MAIN RX), and the signals received at the diversity antenna22D are referred to as the diversity receive signals (DIVERSITY RX). In operation, the main receive signal received at the main antenna22M is routed by the duplexer28to a low noise amplifier (LNA)30, which will amplify the main receive signal and present it to main frequency translation circuitry32, such as that described above. The main frequency translation circuitry32will effect a frequency translation, which is essentially a shift of the main receive signal from being centered about a first center frequency to being centered around a second center frequency. The main frequency translation circuitry32may also take the form of a mixer or the like, instead of the serrodyne architectures described above. The mixer is capable of shifting the center frequency of the main receive signal.

Similarly, the diversity receive signal received at the diversity antenna22D may be filtered via a filter34and amplified using an LNA36before being presented to diversity frequency translation circuitry38. The diversity frequency translation circuitry38will effect a frequency translation of the diversity receive signal from being centered about the first center frequency to being centered about a third center frequency. Preferably, the first, second, and third center frequencies are sufficiently different as to allow signals being transmitted or received at those frequencies to be combined without interfering with one another.

With reference toFIG. 6, a graphical illustration of the frequency translation process is provided. As illustrated, the main and diversity receive signals are centered about the first center frequency fC1, wherein the translated main receive signal is centered about center frequency fC2and the translated diversity receive signal is centered about center frequency fC3. The center frequencies are sufficiently spaced along the frequency continuum to avoid any interference between the signals transmitted on those center frequencies.

Returning toFIG. 5, the translated main receive signal and the translated diversity receive signal provided by the main and diversity frequency translation circuitries32and38are then combined with combining circuitry40and presented to the duplexer26. The duplexer26will then transmit the composite signal to the base housing electronics18.

The composite signal will be received by a duplexer40and provided to separation circuitry42, which will effectively separate the translated main receive signal and the translated diversity receive signal and provide them to main frequency translation circuitry44and diversity frequency translation circuitry46, respectively. The translated main and diversity receive signals will be shifted back to being centered about the first center frequency fC1, which was originally used for transmitting the main and diversity receive signals from the remote device. Accordingly, the main and diversity receive signals are recovered by the main and diversity frequency translation circuitries44and46and provided to transceiver circuitry48, wherein the receive signals are processed in traditional fashion and forwarded to a mobile switching center (MSC) or other device via an MSC interface50.

For transmitted signals, the base housing electronics18will generate a main transmit signal (MAIN TX) using the transceiver circuitry48and provide the main transmit signal to a power amplifier (PA)52. The amplified main transmit signal will then be transmitted to the duplexer40, which will send the amplifier main transmit signal over the feeder cable24toward the masthead electronics20, which will route the main transmit signal to the main antenna22M as described above.

The previous embodiment is configured to minimize the impact on the existing transceiver circuitry48in the base housing electronics18. In an alternative embodiment, the translated main and diversity receive signals may be presented directly to the transceiver circuitry48, which may be modified to be able to process the signals directly, instead of requiring them to be translated back to being centered about their original center frequency, fC1. Further, the receive signals that are translated may be shifted up or down in frequency to varying degrees. For example, the receive signals may be shifted down to an intermediate frequency, to a very low intermediate frequency, or to a near DC frequency, such as that used in Zero IF architectures.

Although not shown, power may be fed from the base housing electronics18to the masthead Electronics20via the antenna feeder. Power would be coupled to the feeder cable24and off of the feeder cable24using a conventional Bias-T as is typically done for masthead electronics20. Furthermore, a communication link between the base housing electronics18and masthead electronics20may also be desirable and implemented. The communication link could be implemented at baseband or at an RF frequency other than those frequencies of interest to the wireless operator, using a low power RF transceiver.

If it is desirable to control the frequency translation to a high level of precision, a local oscillator (LO) signal in the form of a sine wave could be fed up the feeder cable24from the base housing electronics18and be extracted by the masthead electronics20. The LO signal could be a sine wave in the range of 100 to 200 MHz to facilitate separation from the RX and TX signals.

Redundancy is often an issue for the masthead electronics20. It is therefore desirable that a minimum amount of functionality be maintained in the event of a hardware failure with either the LNAs or frequency translation circuitry. It would therefore be advantageous in both the main and diversity receive paths be equipped with frequency translation circuitry. If one frequency translation circuit32should fail, the main signal would pass through the redundant circuitry unshifted and remain at its original frequency. In such an event the main receive signal could propagate downwards to the base housing electronics18at its original RF frequency and the diversity receive signal would continue to be propagated as described.

Turning now toFIG. 7, a second embodiment of the cabling reduction system is illustrated. In this embodiment, the main receive signal is not translated, while the diversity receive signal is translated. Thus, the main receive signal and a translated diversity receive signal are combined in the masthead electronics20and sent over the feeder cable24to the base housing electronics18. In particular, the main receive signal is received at main antenna22M, and forwarded to combining circuitry40via the duplexer28, and through an LNA30. The diversity receive signal is received at diversity antenna22D, filtered by the filter34, amplified by the LNA36, and translated from the first center frequency fC1to a second center frequency fC2by the diversity frequency translation circuitry38. The main receive signal and the translated diversity receive signal are combined by combining circuitry40and sent to duplexer26for delivery to the base housing electronics18over the feeder cable24. Upon receipt, the duplexer40at the base housing electronics18will send a composite receive signal to the separation circuitry42, which will provide the main receive signal to the transceiver circuitry48, and the translated diversity receive signal to the diversity frequency translation circuitry46, which will translate the translated diversity receive signal back to being centered about center frequency fC1to effectively recover the diversity receive signal, which is then provided to the transceiver circuitry48for processing. The main transmit signal is transmitted from the main antenna22M as described in association withFIG. 5.

With reference toFIG. 8, a graphical illustration of the translation of the diversity receive signal is shown, as processed in the embodiment ofFIG. 7. As illustrated, the translated diversity receive signal is shifted to be centered about center frequency fC2, wherein both the main and the original diversity receive signals are centered about center frequency fC1.

If a masthead LNA is not desired or needed for the main receive signal, the invention can be further simplified by removing the LNA30, duplexer26, and combining circuitry40. In such a case, both the transmit and main receive signals can be fed directly to the duplexer26, where they will be combined with a translated diversity receive signal. The duplexer26would be designed such that the main filter encompass both the main transmit and main receive frequencies, and the other filter would encompass a shifted diversity receive frequency. This implementation would provide a simpler and less costly module while minimizing transmit path loss.

The advantages of this embodiment are twofold. Firstly, the main receive path can be composed of only passive components, thereby improving reliability. Alternatively, if an LNA30is desired at the masthead16for both the main and diversity receive signals, this embodiment remains simpler since only the diversity receive frequency needs to be translated at the masthead16, simplifying the electronics and frequency plan.

Turning now toFIG. 9, a multi-band implementation of the cabling reduction system is illustrated. A multi-band communication environment is one in which the same or different cellular communication techniques are supported by a base station environment. As illustrated, a single base housing12is used, but different base housings12may be used for the different frequency bands. In many instances, the different modes of communication, whether incorporating the same or different underlying communication technologies, are centered about different center frequencies. Two common frequencies about which cellular communications are centered are 800 MHz and 1900 MHz. Accordingly, the base station environment must be able to transmit and receive signals at both 800 MHz and 1900 MHz, and may require diversity antennas22D to assist in receiving signals. In operation, received signals in the 800 or 1900 MHz bands (BAND1and BAND2, respectively) may be received at diversity antenna22D, wherein a duplexer54will send 800 MHz receive signals (800RXD) through LNA56to BAND1frequency translation circuitry58, which will translate the 800 MHz receive signal about a different center frequency. In this example, assume the BAND1frequency translation circuitry downconverts the 800 MHz receive signal to a first intermediate frequency (IF1), wherein the downconverted signal is generally referred to as800RXD@IF1. Similarly, 1900 MHz receive signals (1900RXD) will be provided through LNA60to BAND2frequency translation circuitry62, which will downconvert the 1900 MHz receive signal to a second intermediate frequency (IF2), wherein the downconverted signal is represented as1900RXD @ IF2.

The800RXD @ IF1and1900RXD@IF2signals are combined using combining circuitry64to form a composite signal IF1+IF2, which is provided to combining circuitry26′, which will combine the composite signal IF1+IF2with any signals received at the main antenna22M, and in particular, 800 MHz and 1900 MHz receive signals (800RX and1900RX). Thus, the combining circuitry26′ may combine the 800 and 1900 MHz receive signals with the composite IF1+IF2signal and present them over the feeder cable24to separation circuitry42provided in the base housing electronics18. The separation circuitry42will provide the 800 and 1900 MHz signals to the transceiver circuitry48, as well as send the800RXD @ IF1and1900RXD @ IF2(translated) signals to respective BAND1and BAND2frequency translation circuitry66and68. The BAND1frequency translation circuitry66may upconvert the800RXD @ IF1signal to recover the original800RXD signal, and the BAND2frequency translation circuitry68will process the1900RXD @ IF2signal to recover the original1900RXD signal. The800RXD and1900RXD signals are then provided to the transceiver circuitry48for processing in traditional fashion. As noted for the previous embodiment, the transceiver circuitry48may be modified to process the downconverted or otherwise translated signals without requiring retranslations back to the original center frequencies, as provided by the BAND1and BAND2frequency translation circuitry66and68.

Accordingly, the system provides for translating signals from one or more antennas22in a base station environment in a manner allowing the translated signals to be combined with one another and other untranslated signals for transmission over a common antenna feeder24. The present invention is applicable to single and multi-band communication environments, and is not limited to communication technologies or particular operating frequencies. In general, the translation of received signals need only operate such that when the signals are combined with other signals, there is no interference or the interference is otherwise minimal or manageable. Further, the receive signals may be from any spatially diverse array of antennas for one or more sectors. As noted, two base housings12that operate in different bands may share the same feeder cables24and masthead16.

Redundancy is a key issue for masthead electronics20. Active components which are used in the LNA30and frequency translation circuitry32,38are less reliable than passive components used to implement the duplexers26, combining circuitry40, and filters34. As such, it may be necessary to bypass the LNAs30within the module. An LNA bypass is standard practice for masthead LNAs30.

More important is redundancy in the frequency translation circuitry32,38. Since the objective is to transmit two receive signals, main and diversity, down the same antenna feeder24to the base housing electronics18, loss of the frequency translation function means that only one of the receive signals can be relayed to the base housing electronics18. It is therefore important to consider redundancy schemes in practice.

One approach is to simply include multiple levels of redundancy within each circuit block. A more sophisticated scheme would be to further use frequency translation circuitry on both the main receive and diversity receive signals as shown inFIG. 5. However, the frequency translation circuitry32,38should be designed as to allow a signal to pass through with relatively little attenuation in the event of a hardware failure. Such would be the case with a serrodyne implemented using exclusively shunt or reflection type switches. The combining circuitry40could be designed to accept a signal at the translated receive frequency or original receive frequency on either port. The frequency translation circuitry32,38would only be used in one branch at any given time, and in the other branch the signal would be passed through the frequency translation circuitry with little or no effect. In the event that the active frequency translation circuitry32,38should fail, the unused frequency translation circuitry32,38could be turned on to implement the frequency translation on this branch, and the failed frequency translation circuitry32,38would then allow the signal to pass through untranslated.

Finally, in cases where four-branch receive diversity is used, it is conceivable that each sector contain one transmit signal and four receive signals. In such a case the system could easily be expanded to translate the frequency of all receive signals or alternately on the three diversity receive signals to separate frequencies and combine them all onto one feeder cable24, where they would be separated by another circuit at the base station housing12.