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Title:
FDMA/TDMA TRANSCEIVER EMPLOYING COMMON SIGNAL PROCESSING
Document Type and Number:
WIPO Patent Application WO/2000/059169
Kind Code:
A1
Abstract:
A FDMA/TDMA or TDCDMA modulated signal is received through an antenna (10, FIG. 1) and down converted through a down converter (20). The signal is converted to a digital format and through an analog to digital converter (30). The digital representation of the modulated signal is filtered and decimated by way of a polyphase filter (40). The decimated representations of TDCDMA modulated signals from the polyphase filter (40) are correlated with the appropriate pseudonoise code, while decimated FDMA/TDMA modulated signals are correlated frequency information corresponding to the center of the appropriate FDMA/TDMA channel. For transmission of FDMA/TDMA or TDCDMA baseband information signals, the signals are output from a communications processor (FIG. 2, 190), input to a transform (150) and interpolated using an inverse polyphase filter (140). The resulting modulated baseband information signals are input to a digital to analog converter (130), upconverted through up converter (120) , and transmitted.

Inventors:
Startup, James William (42 E. Maria Chandler, AZ, 85284, US)
Helm, Jim E. (16217 E. Park Avenue Gilbert, AZ, 85234, US)
Gross, Jonathan H. (1113 E. Betsy Lane Gilbert, AZ, 85296, US)
Application Number:
PCT/US2000/006291
Publication Date:
October 05, 2000
Filing Date:
March 10, 2000
Export Citation:
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Assignee:
MOTOROLA INC. (1303 East Algonquin Road Schaumburg, IL, 60196, US)
International Classes:
H04J4/00; H04L27/00; H04J13/00; (IPC1-7): H04L27/00; H04J4/00
Domestic Patent References:
WO1998044656A11998-10-08
WO1998024256A21998-06-04
Foreign References:
US5848097A1998-12-08
US5640416A1997-06-17
US5640385A1997-06-17
Attorney, Agent or Firm:
Ingrassia, Vincent B. (Motorola Inc. P.O. Box 10219 Scottsdale, AZ, 85271-0219, US)
Download PDF:
Claims:
CLAIMS What is claimed is:
1. In a communications receiver, a method for converting a received signal to a baseband signal, wherein said received signal is a frequency domain multiple access/time domain multiple access (FDMA/TDMA) or a time division, code division multiple access (TDCDMA) modulated signal, said method comprising the steps of: polyphase filtering a digital representation of said received signal to form a decimated representation of said received signal; and correlating said decimated representation of said received signal to create said baseband signal.
2. The method of claim 1 additionally comprising the step of converting said received signal to a digital representation of said received signal.
3. The method of claim 1, wherein said received signal is a TDCDMA modulated signal and said correlating step further comprises the step of multiplying said decimated representation of said received signal by a pseudonoise code.
4. The method of claim 1, additionally comprising the step of synchronizing timing between said communications receiver and a transmitter of said received signal.
5. The method of claim 4, wherein said received signal is a TDCDMA modulated signal and said synchronizing timing step additionally comprises the step of allocating a pseudonoise code to said transmitter of said received signal.
6. The method of claim 4, wherein said received signal is an FDMA/TDMA modulated signal and said synchronizing timing step additionally comprises the step of allocating a center frequency to said transmitter of said received signal.
7. The method of claim 1, wherein said polyphase filtering step further comprises the step of filtering said digital representation of said received signal using a root raised cosine filter response.
8. A receiver for converting received signals to baseband information signals, wherein said received signals are either FDMA/TDMA or TDCDMA modulated signals, comprising: a polyphase filter for decimating a digital representation of said received signals; and a transform coupled to said polyphase filter for correlating said decimated representation of said received signals to form said baseband information signals.
9. The receiver of claim 8, additionally comprising an analog to digital converter for converting said received signals to said digital representation of said received signals.
10. The receiver of claim 9, additionally comprising an antenna coupled to said analog to digital converter for receiving said received signals.
11. The receiver of claim 8, additionally comprising at least one modem coupled to said transform for extracting information from said baseband information signals.
12. The receiver of claim 8, additionally comprising a processor coupled to said transform for conveying at least one mixing coefficient to said transform.
13. The receiver of claim 8, wherein said polyphase filter includes a filter for limiting a frequency spectrum occupied by said decimated representation of said received signals.
14. The receiver of claim 13, wherein said filter included in said polyphase filter implements a root raised cosine response.
15. A transmitter which converts baseband information signals to modulated signals, wherein said modulated signals are either FDMA/TDMA or TDCDMA modulated signals, comprising: a transform for applying mixing coefficients to said baseband information signals to form modulated baseband signals; and an inverse polyphase filter coupled to said transform for interpolating said modulated baseband signals to form interpolated representations of said modulated baseband signals.
16. The transmitter of claim 15, wherein said transform performs an inverse Fourier transform.
17. The transmitter of claim 15, wherein said modulated signals are TDCDMA and said mixing coefficients represent a pseudonoise code.
18. The transmitter of claim 17, wherein said pseudonoise code is a Walsh code.
19. The transmitter of claim 18, wherein said transform performs an inverse Hadamard transform.
20. The transmitter of claim 17, wherein said pseudonoise code is a Gold code.
Description:
FDMA/TDMA TRANSCEIVER EMPLOYING COMMON SIGNAL PROCESSING Field of the Invention The invention relates to communication systems and, more particularly, to the use of signal processing equipment in communications transceivers.

Background of the Invention In a satellite communications system, an orbiting satellite communications node maintains a communications link with a terrestrial-based subscriber. These subscribers may communicate voice or data signals to and from the satellite communications node depending on the needs of the individual subscribers and their associated subscriber unit equipment. When the satellite is in contact with a terrestrial based subscriber using a voice channel, communications between the satellite and the subscriber can be accommodated using a fairly narrow channel bandwidth, such as 5 KHz or less. However, when the satellite is in contact with a terrestrial based subscriber using a data channel, a much larger channel bandwidth, such as 100 KHz or greater, may be required. Additionally, the data rate can vary by a factor of greater than 10. In a typical satellite communication system, a variable channel bandwidth of 9.6 KBPS of up to 144 KBPS can be allotted to a particular subscriber unit. In these systems, the satellite must accommodate a wide variety of data channel capacities according to the needs of the particular subscribers and their associated subscriber unit equipment.

In order to establish high-quality voice communication channels, a frequency domain multiple access (FDMA) technique is used to allocate channels to the subscriber unit. This can be combined with a time domain multiple access (TDMA) technique so that each subscriber unit transmits and receives voice information during a specified period of time. In practice, a FDMA/TDMA scheme such as this is a preferable technique for providing high-quality voice channels to subscriber units.

For those subscriber units which communicate data with the satellite communications node, a time division, code division multiple access (TDCDMA) technique can be used to allocate channels to these subscriber units. In a TDCDMA system, the timing of each subscriber unit is adjusted so that all signals reach the

satellite at the same time. The use of a TDCDMA channel provides an efficient technique for conveying digital data to a satellite communications node. Thus, in order for the satellite communications node to receive signals from both types of subscriber units, the satellite should be capable of receiving voice signals modulated using FDMA combined with TDMA, as well as data signals modulated using a TDCDMA technique.

In a typical satellite communications system, the satellite communications node may employ separate receiver and transmitter subsystems for each type of communications channel. Therefore, the satellite electronics package generally includes a hardware suite customized for use in receiving and transmitting transmissions using FDMA/TDMA channels, while a separate hardware suite is used to receive and transmit signals using TDCDMA channels. The use of separate receiver and transmitter subsystems designed to receive and transmit both signal types substantially increases the weight and size of the satellite communications node, as well as increasing the power consumed by the satellite electronics package.

Therefore, it is highly desirable for the satellite to make use of common receiver and transmitter hardware which employs signal processing equipment capable of and transmitting both FDMA/TDMA signals as well as TDCDMA signals. It is also highly desirable for the power consumption of such equipment to be reduced to a minimum level.

Brief Description of the Drawings The invention is pointed out with particularity in the appended claims. However, a more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the figures, wherein like reference numbers refer to similar items throughout the figures, and: FIG. 1 is a block diagram of an FDMA/TDMA and TDCDMA receiver employing common signal processing equipment in accordance with a preferred embodiment of the invention; FIG. 2 is a block diagram of an FDMA/TDMA and TDCDMA transmitter employing common signal processing equipment in accordance with a preferred embodiment of the invention;

FIG. 3 is a flow chart of a method employed by an FDMA/TDMA and TDCDMA receiver employing common signal processing equipment in accordance with a preferred embodiment of the invention; and FIG. 4 is a flow chart of a method employed by an FDMA/TDMA and TDCDMA transmitter employing common signal processing equipment in accordance with a preferred embodiment of the invention.

Description of the Preferred Embodiments A FDMA/TDMA and TDCDMA receiver employing common signal processing equipment enables a communications node, such as a communications satellite, to receive both FDMA/TDMA and TDCDMA without the need for separate receiver equipment customized for both signal types. Additionally a FDMA/TDMA and TDCDMA transmitter allows a communications node to transmit FDMA/TDMA and TDCDMA without the need for separate receiver equipment customized for both signal types. This allows the satellite communications node to receive high quality voice signals through an FDMA/TDMA channel using the same equipment as used to receive data transmissions through a TDCDMA channel. Further, the use of common transmitter hardware allows the satellite communications node to transmit high quality voice signals through an FDMA/TDMA channel using the same equipment as used to transmit data through a TDCDMA channel. By obviating the need for separate equipment, the size, weight, and power consumption of the receiver equipment can be substantially reduced. This results in less costly satellite assets which, in turn, results in lower cost communications services for individual subscribers.

FIG. 1 is a block diagram of an FDMA/TDMA and TDCDMA receiver employing common signal processing equipment in accordance with a preferred embodiment of the invention. The receiver of FIG. 1 is desirably included in a moving communications node, such as a satellite or airborne communications platform. In an alternate embodiment, the receiver of FIG. 1 is included in a terrestrial base station or other centralized receiving station.

In FIG. 1, antenna 10 receives communication signals transmitted from one or more external sources. In a preferred embodiment, the signals received through antenna 10 can be either TDCDMA modulated signals, or FDMA/TDMA modulated signals which convey digitized voice or general-purpose data. Antenna 10 can be any

type of receiving structure such as a dipole, monopole above a ground plane, or any other type of receiving antenna in which an electrical current is induced on a conductive material resulting from an impinging electromagnetic wave. Additionally, antenna 10 can be representative of an array of antennas which receive electromagnetic signals using a focused receive antenna beam.

The incoming TDCDMA and FDMA/TDMA modulated signals from antenna 10 are conveyed to down converter 20. Down converter 20 serves to reduce the carrier frequency of the received signals so that further signal conditioning can occur using lower frequency components. Down converter 20 may also include low noise amplifiers which serve to boost the received signal strength so that the down conversion process does not result in an excessive lost of received signal strength.

Further, down converter 20 may also include analog low pass or band pass filtering in order to filter out any unwanted signals which may be outside the particular communications band used by the incoming modulated signals.

Down converted signals from down converter 20 are input to analog to digital converter 30. Analog to digital converter 30 functions to convert the received TDCDMA or FDMA/TDMA modulated signals to digital representations. Analog to digital converter 30 preferably includes sufficient dynamic range and sampling rate to allow accurate quantization of the incoming analog signal from down converter 20.

The outputs of analog to digital converter 30 are then input to polyphase filter 40. As known to those skilled in the art, a polyphase filter, such as polyphase filter 40, introduces deliberate aliasing by down sampling the incoming digital representation of the received modulated signal. Polyphase filter 40 operates on the output from analog to digital converter 30 and introduces separate paths through the filter as illustrated by filter branches 45 of polyphase filter 40. Each of the separate paths through polyphase filter 40 is decimated by way of commutator 43, which operates by sending the first sample to from analog to digital converter 30 to the first of filter branches 45.

Commutator 43 then sends the second sample from analog to digital converter 30 to the second of filter branches 45. This process is repeated until all of filter branches 45 have received a sample from analog to digital converter 30. Commutator 43 then returns to the first of filter branches 45 and repeats the process using the next set of samples.

In a preferred embodiment, the number of filter branches 45, M, is equal to 32, however, a greater or lesser number of branches may be used according to the

requirements of the particular application. Desirably, the number of filter branches 45 is equal to 2", such as 8,16,32, and so on. As previously mentioned, polyphase filter 40 decimates each output by a factor equal to the number of filter branches (M) present in polyphase filter 40. As an example, if the input sampling rate to polyphase filter 40 is 160 MHz, representing 32 channels of equal bandwidth, then polyphase filter 40 preferably includes 32 filter branches. For this example, the output sampling rate of each branch would be 5 MHz. Thus, an additional effect of the use of polyphase filter 40 is that the filter performs a decimation function. Additionally, the decimation factor can be varied according to the channel bandwidth of the TDCDMA or FDMA/TDMA modulated channels received through antenna 10. This allows the receiver to process higher capacity channels through a change in the sampling rate of the filter branches 45 of polyphase filter 40, or through a change in the number of filter branches 45 of polyphase filter 40 Through the action of commutator 43, each of filter branches 45 in polyphase filter 40 receives an aliased version of the output of analog to digital converter 30. It is noteworthy that commutator 43 also introduces delay in each of filter branches 45.

The delay causes each aliased frequency to appear at the baseband output of filter branches 45 with a different phase.

Filter branches 45 of polyphase filter 40 operate in concert to filter each modulated signal received by antenna 10 whether the signal is FDMA/TDMA or TDCDMA modulated. Note that this filter function is not evident at the output of polyphase filter 40 since the signals are aliased through the action of commutator 43.

In a preferred embodiment, the filter function implemented by each of filter branches 45 is a root raised cosine filter response. A root raised cosine filter response is desirable since it provides low inter-symbol interference, thus minimizing noise from adjacent channels. However, other filter responses, such as Gaussian, may be used according to the particular inter-symbol interference tolerances of the particular application. In alternate embodiments, filtering by filter branches 45 to reduce inter- symbol interference may not be required at all.

Polyphase filter 40 functions as a group of digitally implemented band pass filters. An advantage of the use of polyphase filter 40 over a digitally implemented band pass filter, is that a polyphase filter is computationally more efficient. This results in minimizing the number of mathematical operations which are performed by polyphase filter 40 which, in turn, reduces the demand on computing and primary

power resources.

The outputs of each of filter branches 45 of the polyphase filter 40 are then input to transform 50. Transform 50 comprises a group of complex multipliers which use mixing coefficients to unwrap the aliased signals output from each branch of polyphase filter 40. In a preferred embodiment, each mixing coefficient is supplied by processor 60. For TDCDMA channels, the mixing coefficients supplied by processor 60 are either +1, according to the particular pseudonoise code sequence used for the particular channel. The particular code has the effect of unwrapping the aliased signals from the output of each filter branch by performing a fast Hadamard transform.

Processor 60 selects a particular Walsh, Gold, or other code sequence used in conventional CDMA correlation to perform the correlation. In this manner, incoming TDCDMA signals can be"correlated"to extract the baseband information included in each signal.

For FDMA channels, processor 60 conveys frequency information to those complex multipliers within transform 50 which correspond to FDMA/TDMA channels.

For these channels, processor 60 conveys coefficients such as e+iQn where fn represents the phase necessary to unwrap aliased channels at the output of the filter branches 45 of polyphase filter 40. Through the complex multiplication of the digital representation of modulated signals from polyphase filter 40, the baseband information present in each FDMA/TDMA channel can be extracted. Those skilled in the art will recognize the transform function implemented for FDMA/TDMA as a discrete Fourier Transform. In a preferred embodiment, processor 60 plans and allocates FDMA/TDMA and TDCDMA channels to each of the external sources transmitting to the receiver of FIG. 1.

The output of transform 50 is conveyed to one of modems 80 so that the baseband information signal can be further processed in order to remove error correction and other channel coding information. Additionally, each of modems 80 maintains the required framing information so that each received FDMA/TDMA frame and TDCDMA message can be mapped to a particular subscriber unit using communications processor 90.

The additional elements of FIG. 1 include transmitter 95 and antenna 105 which are coupled to processor 60 in order to transmit timing information used to synchronize timing between processor 60 and any subscriber units which transmit TDCDMA or FDMA/TDMA signals. These timing signals are generated within processor 60 and

transmitted through transmitter 95 and antenna 105. The receiving subscriber units preferably make use of these timing signals in order to advance their respective timing elements to account for propagation delays from the subscriber unit to the satellite.

This ensures that signals transmitted from the subscriber units reach the satellite synchronously.

FIG. 2 is a block diagram of an FDMA/TDMA and TDCDMA transmitter employing common signal processing equipment in accordance with a preferred embodiment of the invention. In FIG. 2, baseband information signals intended for a plurality of external subscriber units are generated within communications processor 190. These signals may include digitized voice or general purpose data. Additionally, communications processor 190 informs processor 160 of the desired modulation technique (FDMA/TDMA or TDCDMA) to be applied to each channel which the baseband information signals are expected to use in order to transmit the signal each subscriber unit. The baseband information signals are conveyed to modems 180 where any required overhead, such as forward error correction and channel coding, is applied to the signal. In a preferred embodiment, modems 180 perform substantially the reverse function as that performed by modems 80 or FIG. 1. The baseband information signals are output from modems 180 to transform 150, which is coupled to the outputs of modems 180.

Transform 150 incorporates complex multipliers which wrap the baseband information signals with an appropriate mixing coefficient for use with the particular channel. For those baseband information signals which are to be formatted for transmission using FDMA/TDMA channels, the mixing coefficients conveyed from processor 160 include phase information. In a preferred embodiment, the phase information allows transform 150 to perform an inverse fast Fourier transform on each baseband information signal. For FDMA/TDMA signals, processor 60 conveys coefficients such as eiXn, where n represents the phase necessary to wrap each baseband information signal for use with FDMA/TDMA channels.

For those baseband information signals which are to be formatted for use with a TDCDMA channel, processor 160 conveys an appropriate pseudo-noise code, such as a Walsh code or a Gold code. For the case of a Walsh code, transform 150 performs an inverse fast Hadamard transform. For TDCDMA channels, the mixing coefficients supplied by processor 160 are either +1, according to the particular pseudonoise code sequence used for a particular channel.

The FDMA/TDMA or TDCDMA modulated baseband information signal is output from transform 150 to inverse polyphase filter 140. Inverse polyphase filter 140 incorporates filter branches 145 which preferably apply a root raised cosine filter to the signals from transform 150. Each output of filter branches 145 is commutated by way of commutator 143 and output to digital to analog converter 130. In a preferred embodiment, inverse polyphase filter 140 performs substantially the reverse function of polyphase filter 40 of FIG. 1. Thus, whereas polyphase filter 40 of FIG. 1 performs a decimation function, inverse polyphase filter 140 performs an interpolation function as known to those skilled in the art.

The modulated baseband information signals output from inverse polyphase filter 140 are input to digital to analog converter 130 which converts the modulated baseband information signals from inverse polyphase filter 140 to an analog format which is output to upconverter 120. Upconverter 120 then transmits the signal through antenna 110 to the receiving subscriber units.

FIG. 3 is a flow chart of a method employed by an FDMA/CDMA receiver employing common signal processing equipment in accordance with a preferred embodiment of the invention. The receiver of FIG. 1 is suitable for performing the method. At step 205, the transceiver synchronizes timing elements of preferably all FDMA/TDMA or TDCDMA external transmitters. Step 205 also includes allocating pseudonoise codes to TDCDMA external transmitters as well as allocating a center frequency to FDMA/TDMA channels. In step 210, an incoming modulated signal from external transmitters is received through an antenna. The incoming modulated signals are converted from a carrier frequency to a lower frequency in step 220. In step 230, the modulated signals are converted to a digital representation of the received modulated signals. In step 240, the digital representation of the received modulated signals are polyphase filtered in order to create a decimated representation of the received modulated signals and to filter the incoming signals with an appropriate filter response, such as a root raised cosine, in order limit the frequency spectrum occupied by the resulting signal and to reduce inter-symbol interference. At step 250, a processor determines whether to demodulate the signals as an FDMA/TDMA signal or as a TDCDMA signal. In a preferred embodiment, the processor makes this decision in accordance with a previous allocation of FDMA/TDMA or TDCDMA channels.

If the decision of step 250 indicates that the received modulated signals are TDCDMA signals, step 270 is executed where a transform is loaded with the

appropriate pseudonoise codes, such as a Walsh or Gold code. This information takes the form of +1 or-1 according to the particular pseudonoise code sequence used for a particular channel. If the decision of step 250 indicates that the incoming signals are FDMA/TDMA, step 260 is executed where a transform is loaded with the appropriate phase information such as e+iXn where cPn represents the phase necessary to unwrap aliased channels at the output of the filter branches 45 of polyphase filter 40. In step 280, the incoming modulated signals are multiplied by the complex multiplication factor loaded in either step 270 (+1 or-1) or step 280 (e''"). The signals are demodulated in step 300 in order to extract the baseband information.

FIG. 4 is a flow chart of a method employed by an FDMA/TDMA and TDCDMA transmitter employing common signal processing equipment in accordance with a preferred embodiment of the invention. The transmitter of FIG. 2 is suitable for performing the method. At step 310, a baseband information signal is generated. This signal may be either digitized voice, or general purpose data. Step 310 additionally includes selecting either a FDMA/TDMA or TDCDMA modulation technique. At step 320, any required overhead, such as forward error correction, is added to the base band information signal. At step 330, a suitable mixing coefficient is applied to the base band information signals. When the baseband information signals are to be transmitted using a FDMA/TDMA channel, the mixing coefficients desirably represent phase information such as e+iXn where sDn represents the phase necessary to modulate the baseband information signals as FDMA/TDMA signals. For FDMA/TDMA signals, step 330 represents applying an inverse Fourier transform.

When the baseband information signals are to be transmitted using a TDCDMA channels, the mixing coefficients include + 1 according to the particular pseudonoise code chosen for use with a particular channel. For TDCDMA signals, step 330 represents applying an inverse Hadamard transform. At step 340, the modulated signals are filtered using a root raised cosine filter in order to reduce inter-symbol interference. At step 370, the modulated base band information signals are upconverted and radiated in step 380.

An FDMA/TDMA and TDCDMA receiver employing common signal processing equipment enables a communications node, such as a communications satellite, to receive and process either FDMA/TDMA or TDCDMA modulated signals without the need for receiver equipment dedicated to each signal type. This allows the communications node to receive high quality voice signals through an FDMA/TDMA

channel using the same equipment as used to receive data transmissions through a TDCDMA channel. Through the use of signal processing equipment the required size, weight, and power consumption of the receiver equipment can be substantially reduced. This results in less costly satellite assets which consequently lowers the cost of communications services provided to individual subscribers.