Abstract:
A Time Division Duplex (TDD), Code Division Multiple Access (CDMA) communication system includes a plurality of Customer Premises Equipment (CPE) and an Access Point (AP) that communicate through RF links. A CPE contains a receiver baseband subsystem and a transmitter baseband subsystem, and further contains receiver circuitry operable during a receive period for receiving an RF carrier from the AP and for deriving a receiver tracking signal that is indicative of a frequency and phase shift between the received RF carrier and a reference signal. The receiver circuitry further includes a frequency to phase accumulator (FPA) and a digital phase shifter (DPS) for correcting the frequency and phase of a receiver baseband signal by an amount and in a direction indicated by the receiver tracking signal. CPE transmitter circuitry is operable during a next transmission period for operating the FPA and DPS circuitry to correct the frequency of a transmitter baseband signal by an amount indicated by the receiver tracking signal, and in a direction opposite to the direction indicated by the receiver tracking signal.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to Time Division Duplex (TDD) communication systems and, more particularly, relates to TDD communication systems wherein bidirectional digital data links are established between an Access Point (AP) or Hub and a plurality of customer premise equipment (CPE). 
   BACKGROUND OF THE INVENTION 
   In a TDD system two entities share a common carrier frequency channel for conducting bidirectional communications. In order to avoid interference and collisions, a first entity transmits during a first time period while a second entity receives the transmission from the first entity, and then the second entity transmits during a second time period while the first entity receives the transmission from the second entity. The first and second time periods are then repeated. In this manner it can be guaranteed that the two entities will not interfere with each others transmissions. Some amount of guard time can be inserted between the first and second periods, and between periods, to ensure that one entity does not begin transmitting before the other finishes. 
   At the start of a transmission the receiving entity must acquire the received signal, resolve any frequency uncertainty and lock to the carrier frequency and carrier phase, and then begin demodulating the modulated carrier. This can require some significant amount of time. Furthermore, the entire transmission might be missed if the receiver is unable to successfully acquire and lock to the received carrier during the transmission period. 
   As can be appreciated, it is desirable to provide an ability to make the received signal acquisition and carrier frequency lock processes occur as rapidly as possible. One approach provides a synchronous timing scheme wherein the basic frequency references of both entities are, ideally, identical. For example, the frequency reference of the AP or Hub is declared to be a master frequency reference, and the local frequency references of the CPEs are slaved to the AP frequency reference. Unfortunately, no frequency reference is ideal, and some drift can occur between the various frequency references of the CPEs and the AP. This frequency drift causes a phase and frequency shift between the AP and a receiving CPE, which can lead to received signal demodulation errors. 
   OBJECTS AND ADVANTAGES OF THE INVENTION 
   It is a first object and advantage of this invention to provide a method and apparatus for providing an improved received signal acquisition and carrier frequency lock process. 
   It is another object and advantage of this invention to provide a method and apparatus for use in a communication system that provides an improved received signal acquisition and carrier frequency lock process. 
   It is a further object and advantage of this invention to provide a method and apparatus for use in a TDD system that pre-corrects a frequency of a transmission from a transmitting entity so as to compensate for a detected amount of frequency error between the transmitting entity and the receiving entity, thereby improving the ability of receiving entity to acquire, lock to, and demodulate the received transmission. 
   SUMMARY OF THE INVENTION 
   The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention. 
   A Time Division Duplex (TDD), Code Division Multiple Access (CDMA) communication system includes a plurality of Customer Premises Equipment (CPE) and an Access Point (AP) that communicate through RF links. A CPE contains a receiver baseband subsystem and a transmitter baseband subsystem, and further contains receiver circuitry operable during a receive period for receiving an RF carrier from the AP and for deriving a receiver tracking signal that is indicative of a frequency and phase shift between the received RF carrier and a reference signal. The receiver circuitry further includes a digital phase shifter and a frequency to phase accumulator that together rotate the receiver baseband signal by some amount of frequency and phase and in a direction indicated by the receiver tracking signal. The CPE further contains, in accordance with an aspect of this invention, multiplexing circuitry for sharing the digital phase shifter and frequency to phase accumulator circuits between the receiver baseband subsystem and the transmitter baseband subsystem. The frequency to phase accumulator is loaded during a next transmission period for operating the digital phase shifter to correct a frequency of a transmitter baseband signal by an amount indicated by the receiver tracking signal, and in a direction opposite to the direction indicated by the receiver tracking signal. The effect is to pre-compensate an RF carrier that is transmitted to the AP from the CPE so as to reduce carrier acquisition time at a receiver of the AP. 
   The CPE may further contain sample and hold circuitry that is responsive to an end of the receive period for storing the receiver tracking frequency error. This stored frequency error is inverted and loaded into the frequency to phase accumulator which drives the digital phase shifter at the compensated frequency during the next transmission period. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above set forth and other features of the invention are made more apparent in the ensuing Detailed Description of the Invention when read in conjunction with the attached Drawings, wherein: 
       FIG. 1  is simplified block diagram of a TDD communication system having an AP and a plurality of CPEs that is suitable for practicing the teachings of this invention; 
       FIG. 2  is a circuit block diagram of a portion of one of the CPEs of  FIG. 1 , showing circuit components for pre-rotating a phase of a transmission to the AP in accordance with these teachings; and 
       FIG. 3  shows a TDD timing diagram that is useful in explaining the teachings of this invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  depicts a simplified block diagram of a TDD communication system  1  having an AP  12  and at least one, but preferably a plurality of CPEs  10 , and that is suitable for practicing the teachings of this invention. The illustrated and presently preferred embodiment of the TDD communication system  1  operates as a Code Division Multiple Access (CDMA) type system that employs orthogonal codes, such as, but not limited to, Walsh-Hadamard codes, for spreading and despreading the communication signals transmitted between the CPEs  10  and the AP  12  over wireless links (WL)  11 . However, other types of access systems may also benefit from the teachings of this invention, including Time Division Multiple Access (TDMA) and combined CDMA/TDMA type systems. In essence, the TDD communication system  10  may be considered to be a variant of a CDMA/TDMA system, as individual ones of the respective time divided transmissions between a given CPE  10  and the AP  12  are made using the CDMA spreading technique, wherein the transmitted waveform is spread over the available bandwidth using one or more properly selected spreading codes. Other types of communications systems may also benefit from the teachings of this invention. 
   Each CPE  10  is preferably comprised of a CDMA system  10 A, assumed to generally include spreaders and despreaders, as well as data phase modulators (transmit), data phase demodulators (receive), frequency reference(s) such as local oscillators (LOs), carrier loop tracking circuits, possibly signal interleavers and deinterleavers, and other circuitry known to those skilled in this art. The CDMA system  10 A is, however, modified in accordance with the teachings of this invention in a manner described below. The CDMA system  10 A is coupled between a transceiver  10 B and a suitable data processor  10 C. The transceiver  10 B is preferably an RF transceiver providing wireless RF link  11  communication capabilities, preferably but not limited to line of sight (LOS) communication, with a similar RF transceiver  12 B of the AP  12 . However, in other embodiments the transceivers  10 B,  12 B may be implemented as optical transceivers. 
   The data processor  10 C could have the functionality of a conventional single user personal computer or workstation, and/or it may serve as a front-end for a local area network (LAN)  18  having a plurality of data processors (DPs)  20  coupled thereto. The exact nature and use of the CPE  10  is not germane to an understanding of the teachings of this invention. It is noted that some or all of the CPEs may also have voice capability (e.g., a microphone and a speaker), and that voice connections can also be made over the wireless links  11 . 
   The Hub or Access Point (AP)  12  is similarly constructed to include a CDMA section  12 A, preferably a multi-channel CDMA system for accommodating the plurality of CPEs  10 , at least one but preferably a plurality of the transceivers  12 B for establishing the wireless links  11  with the CPEs  10 , and an Access Point processor  12 C for controlling the overall operation of the AP  12 . The AP processor  12 C is preferably bidirectionally coupled to at least one data communication network  14  which, in turn, can provide access to the Internet and World Wide Web (WWW). In this manner, and by example, one of the DPs  20  can be used to access the WWW via the LAN  18 , CPE  10 , wireless link  11 , AP  12  and the data communication network  14 . 
   During operation, a local oscillator of the AP  12  is preferably frequency and phase synchronous with a local oscillator of the CPE  10 . However, some amount of frequency and phase shift between the two LOs will typically be present. A phase shift may also be introduced due to RF channel impairments between the AP  12  and the CPE  10 . As such, at least the CPE  10  is provided with carrier loop tracking circuitry (not shown) for frequency and phase locking the local oscillator of the CPE  10  to the incoming received carrier signal from the AP  12 . 
   Referring to  FIG. 2 , there is shown a portion of the CDMA system  10 A of one of the CPEs  10 . The upper part of the drawing shows a portion of the conventional receiver baseband circuitry or subsystem while the lower part shows a portion of the transmitter baseband circuitry or subsystem, in accordance with these teachings. 
   Referring first to the upper part of  FIG. 2 , the receiver (RX) includes Inphase (I) and a Quadrature (Q) baseband channels that are obtained from the RF transceiver  10 B. The I and Q channels are essentially mirror images and include an analog to digital (A/D) converter  22 A,  22 B, DC offset removal circuitry  24 A,  24 B and (optional) filters  26 A,  26 B. Conventionally, the outputs of the filters  26 A and  26 B would be applied to the corresponding I and Q inputs of a digital phase shifter (DPS)  28 . The DPS  28 , during the receive period, operates to rotate the frequency and phase of the I and Q signals by an amount indicated by the output of a frequency to phase accumulator (FPA)  44 . The FPA  44  is driven by an output of a carrier loop tracking circuit. The output of the DPS  28  are signals representing rotated I and Q signals that are applied to a despreader  30  and subsequent baseband circuitry. The despreader  30  is used to extract the CPE&#39;s signal from the received signal and to then pass the received signal on to a data decoder and related circuitry. 
   The FPA  44  basically functions as an accumulator that adds the value output by a sample and hold (S/H)  42  each clock period, thereby changing the phase that the DPS  28  receives each clock to produce a frequency correction. The combination thus corrects for a small frequency error. The frequency generated is given by:
 
 fo =((clock frequency)× S/H  value)/Length of accumulator,
 
e.g., 2 32  if a 32-bit accumulator is used.
 
   In accordance with this invention an input multiplexer (MUX)  32  is provided at the frontend of the DPS  28 . The MUX  32  is controlled with a transmit/receive (XMIT/RCV) signal so as to couple, during the TDD receive period and in a conventional fashion, the received baseband I and Q signals to the input of the DPS  28 . From the output of the DPS  28  the rotated signals are applied to the despreader  30 . 
   A frequency/phase loop filter that forms a part of the carrier loop tracking circuit contains a measure of the receive frequency error and also the phase error of the received signal. At the end of a receive burst the receive frequency error is held by the sample and hold (S/H)  42 , inverted and applied to the frequency to phase accumulator  44 . This signal then drives the digital phase shifter  28  to generate a frequency opposite to that of the receive frequency error, thus pre-correcting a transmitted frequency error. As such, when the AP  12  receives the CPE  10  transmission the CDMA system  12 A can more rapidly and reliably carrier lock to the received CPE signal, and then despread and demodulate the received signal. 
   The output of the DPS  28  is provided to I and Q filters  40 A,  40 B, respectively, and then to transmit path I and Q digital to analog converters (D/As)  40 C,  40 D, respectively. These signals are subsequently upconverted to the desired transmission carrier frequency. 
   Referring now as well to  FIG. 3 , it can be seen that the TDD waveform contains a plurality of transmit periods interspersed with a plurality of receive periods. The transmit and receive periods may or may not be of the same duration. During the receive period the carrier loop tracking circuit is operative to maintain the CPE  10  LO locked to the received carrier. The amount and direction of the frequency shift that is required to be corrected (the amount required to maintain lock) can be indicated by an analog or a digital signal. This signal is then used by the FPA  44  and the DPS  28  as described above. 
   In accordance with the teachings of this invention the disclosed circuitry of the MUX  32  cooperates with the DPS  28 , the frequency to phase accumulator  44 , and the output signal (receiver tracking signal) of the carrier loop tracking circuit to correct the frequency of the transmitter baseband I and Q channels. 
   To facilitate this operation the sample and hold (S/H) circuit  42  can be provided if the output signal of the carrier loop tracking circuit is an analog signal. If the output signal of the carrier loop tracking circuit is instead a digital signal, then the sample and hold function can be implemented with a simple register or with a memory location for storing the last value of the receiver tracking signal during the next transmission period. 
   In operation, the output signal of the carrier loop tracking circuit is sampled during the receive period and passed through to the frequency to phase accumulator  44 . The last frequency-only component of the loop filter is then inverted and held at the end of the receive period or burst. While being held the inverted signal is applied to the frequency to phase accumulator  44 , which drives the DPS  28  to create a correction frequency, for the duration of the transmit period. In this manner the amount of frequency correction applied by the DPS  28  to the transmitter baseband I and Q channels, during a single transmission period, is a function of the amount of frequency error detected by the carrier loop tracking circuit at the end of the previous receive period. The use of a single value for the frequency correction during the transmission period is possible since the transmit and receive periods are typically much shorter than the time over which the signal propagation of the RF channel changes, and are also typically shorter than the time over which the CPE  10  LO drifts by a significant amount with respect to the AP  12  LO. In the preferred embodiment of this invention the transmit and receive periods each have a typical duration in the range of about 0.5 milliseconds to about 4 milliseconds, a period of time that is typically much smaller than signal propagation time constants experienced in the RF channel, such as slow fades, which can be seconds and tens of seconds, and is also smaller than the typical (short term) LO drift rate. 
   The use of this invention enables the AP  12  to more quickly acquire the carrier phase from the CPE  10 , as the frequency error has been substantially removed. This enables the transmitted signal to arrive at the AP  12  with little or no frequency ambiguity due to RF channel impairments and frequency-influencing conditions that may exist between the CPE  10  and the AP  12 , such as frequency shifts between the CPE  10  and AP  12  LOs. As such, non-coherent clocks may be employed between the CPE  10  and the AP  12 , thus simplifying the overall system design and operation. 
   While the invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the scope and spirit of the invention. For example, while  FIG. 2  shows the preferred embodiment wherein the DPS  28  is time multiplexed between the transmit and receive channels using the MUX  32 , in other embodiments of this invention the MUX  32  could be eliminated, and a second DPS  32  provided in the transmit path. Thus, this invention should not be construed to be limited by only the illustrated and above-described embodiments, but should be given a scope commensurate with the scope of the following claims.