Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J13/00—Code division multiplex systems
  • H04J13/10—Code generation
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03L—AUTOMATIC CONTROL, STARTING, SYNCHRONISATION OR STABILISATION OF GENERATORS OF ELECTRONIC OSCILLATIONS OR PULSES
  • H03L7/00—Automatic control of frequency or phase; Synchronisation
  • H03L7/06—Automatic control of frequency or phase; Synchronisation using a reference signal applied to a frequency- or phase-locked loop
  • H03L7/08—Details of the phase-locked loop
  • H03L7/085—Details of the phase-locked loop concerning mainly the frequency- or phase-detection arrangement including the filtering or amplification of its output signal
  • H03L7/091—Details of the phase-locked loop concerning mainly the frequency- or phase-detection arrangement including the filtering or amplification of its output signal the phase or frequency detector using a sampling device
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03L—AUTOMATIC CONTROL, STARTING, SYNCHRONISATION OR STABILISATION OF GENERATORS OF ELECTRONIC OSCILLATIONS OR PULSES
  • H03L7/00—Automatic control of frequency or phase; Synchronisation
  • H03L7/06—Automatic control of frequency or phase; Synchronisation using a reference signal applied to a frequency- or phase-locked loop
  • H03L7/16—Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop
  • H03L7/18—Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop using a frequency divider or counter in the loop
  • H03L7/181—Indirect frequency synthesis, i.e. generating a desired one of a number of predetermined frequencies using a frequency- or phase-locked loop using a frequency divider or counter in the loop a numerical count result being used for locking the loop, the counter counting during fixed time intervals
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/69—Spread spectrum techniques
  • H04B1/707—Spread spectrum techniques using direct sequence modulation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J13/00—Code division multiplex systems
  • H04J13/0007—Code type
  • H04J13/0022—PN, e.g. Kronecker
  • H04J13/0029—Gold
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/18—Phase-modulated carrier systems, i.e. using phase-shift keying
  • H04L27/20—Modulator circuits; Transmitter circuits
  • H04L27/2003—Modulator circuits; Transmitter circuits for continuous phase modulation
  • H04L27/2007—Modulator circuits; Transmitter circuits for continuous phase modulation in which the phase change within each symbol period is constrained
  • H04L27/2017—Modulator circuits; Transmitter circuits for continuous phase modulation in which the phase change within each symbol period is constrained in which the phase changes are non-linear, e.g. generalized and Gaussian minimum shift keying, tamed frequency modulation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/18—Phase-modulated carrier systems, i.e. using phase-shift keying
  • H04L27/22—Demodulator circuits; Receiver circuits
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/0014—Carrier regulation
  • H04L2027/0024—Carrier regulation at the receiver end
  • H04L2027/0026—Correction of carrier offset
  • H04L2027/0028—Correction of carrier offset at passband only
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/0014—Carrier regulation
  • H04L2027/0044—Control loops for carrier regulation
  • H04L2027/0053—Closed loops
  • H04L2027/0055—Closed loops single phase

Definitions

• This invention relates to spread-spectrum communications systems that utilize Gold-code sequence generator in combination with an M-ary encoding scheme.
• a conventional mobile communication system comprises communication networks each of which includes a base station and a number of mobile radios that communicate with the base station.
• a communication signal from one mobile radio should not interfere with communication signals from other mobile radios within a network, and communication signals from one network should be free of interference from communication signals of other networks.
• TDMA Time Division Multiple Access
• FDMA Frequency Division Multiple Access
• TDMA time that cannot be used for communication.
• non-coherent modulation techniques are preferable since they do not require additional time to synchronize, and since they are more tolerant of Rayleigh-fading channels.
• non-coherent modulation schemes are more susceptible to interference than coherent modulation schemes.
• M-ary signaling is one non-coherent modulation scheme that minimizes the susceptibility to interference.
• 64-ary orthogonal signaling is used in the IS-95 CDMA cellular system.
• the present invention allows a wireless communication system to maximize the number of co-existing networks within a frequency band while achieving a large far-near ratio, and while using inexpensive class-C transmitters. This is achieved in part by using spreading codes that retain their near-orthogonality when transmitted through multipath channels. Morever, the present invention permits a plurality of radios to access the network by time-sharing the transmission channel in such a manner that a receiver can synchronize to each transmission with a minimum of time, which in turn allows simultaneous communication of voice and data information.
• the present invention also allows multiple private conversations between mobile radios while a plurality of the mobile radios receive broadcast data.
• the present invention provides a communication device that includes a transmitter and receiver.
• the transmitter includes an M-ary encoder configured to generate an M-l number of distinctive symbols, each comprising k bits. M is equal to 2 k and k is a positive integer.
• the transmitter also includes a code generator configured to produce spread spectrum codeword sequences based on the symbols generated by the M-ary encoder and based on first and second Gold code polynomials.
• the transmitter is configured to send a radio signal based on the spread spectrum codeword sequences.
• the receiver is configured to receive the radio signal.
• the receiver includes a first shift register configured to receive an input signal generated based on the received radio signal and a second shift register configured to receive and circularly shift a locally generated codeword sequence, identical to codeword sequence used to encode symbols.
• the receiver also includes an accumulator coupled to the first and second shift registers and configured to multiply and accumulate stored values in the first and second shift registers each time the second shift register is circularly shifted and a selecting device coupled to the accumulator and configured to identify one symbol from the plurality of symbols based on outputs from the accumulator.
• a method corresponding to the above described communication device is also provided.
• the base station and its mobile radios share the carrier frequency assignment by dividing transmission time into segments called time-slots.
• the time-slots are organized into frames such that mobile units and the base station can be assigned one or more time-slots within a frame, during which time they may transmit their modulated signal as a message segment.
• Each time-slot assignment can be used to convey voice or data information.
• the intended recipient, or recipients, for each message segment are identified within each message segment by its "destination address.” This enables the base station to broadcast messages to a plurality of mobile units while several mobile-base-mobile transmissions occur.
• Each mobile-base-mobile communication consists of the mobile unit transmitting a message segment to the base station, which then re -transmits the message segment to another mobile unit.
• the re-transmission from the base station may be addressed to a plurality of mobile units.
• Message segments may contain either voice or data information.
• Time-slots may also be assigned to a plurality of mobile units for infrequent transmissions.
• the mobile units share the assignment using a slotted- ALOHA protocol. If two mobile units transmit simultaneously, their signals will not be correctly received and both units retransmit the message segments after a random delay if they are not acknowledged by the base station.
• the base station and mobile units demodulate the spread-spectrum message segments by first synchronizing the receiver's time reference to a preamble spreading code. Using the synchronized time reference, the receiver demodulates M-ary symbols by finding the one-of-M spreading sequence which correlates best with the received signal. The M-ary symbols are then decoded into binary data conveying either voice or data information.
• the present invention includes a simplified method of finding the sequence with highest correlation.
• the mobile radios of the present invention can transmit and receive digitized voice data or Internet communication packets.
• the mobile radio of the present invention can send and receive messages to and from the Internet. This capability is in addition to operating as a regular mobile telephone.
• mobile radios which are capable of synchronizing the frequency of their respective oscillators to their respective base-stations to reduce the likelihood of introducing demodulation errors in both the base-to-mobile and mobile-to-base communication links, are also discussed.
• FIG. 1 is an exemplary illustration of two networks operating in the same geophysical area
• FIG. 2 is a block diagram shows the operations to modulate and transmit a stream of binary-encoded information using the GMSK M-ary spread-spectrum method;
• FIG. 3 shows a Gold-code sequence generator;
• FIG. 3A, 3B, 3C, 3D and 3E illustrate various embodiments of the Gold-code sequence generator
• FIG. 4 shows a preferred embodiment of GMSK modulation with 64-ary Gold-code sequences
• FIG. 5 shows the division of time into frames and time-slot assignments
• FIG. 6 shows the operations to synchronize and demodulate the received GMSK/M-ary spread-spectrum signal into binary information
• FIG. 7 shows a preferred embodiment of GMSK demodulation of 64-ary Gold-code sequences
• FIG. 8 shows a preferred embodiment of a gl/g2-code correlator
• FIG. 9 shows a preferred method of calculating soft decisions
• FIG. 10 is a block diagram of the base and mobile radio
• FIG. 11 shows the sequential order that bits are written into the interleaver, and the sequential order that 6-bit symbols are read from the interleaver;
• FIG. 12 shows a block diagram of a frequency synchronization circuit of the present invention
• FIG. 13 shows a block diagram of a frequency discriminator of the present invention
• FIG. 14 shows a flow chart of a loop filter of the present invention.
• FIG. 15 shows a system in which a computer is connected to a mobile radio of the present invention.
• An exemplary communication system of the present invention includes a number of communication networks, two of which are illustrated in FIG. 1.
• a first network includes a base station 1 and mobile radios 2, 3, and 4.
• a second network comprises a base station 5 and mobile radios 6, 7, and 8.
• the two networks are preferably interconnected through external interfaces 9 and 10, such as the Public Switched Telephone Network (PSTN) or a direct wired connection.
• PSTN Public Switched Telephone Network
• the external interfaces also connect the networks to other data and voice communication systems including the Internet.
• the mobile radios may communicate with the base station, or with other mobile radios belonging to the same network by establishing communication links to the respective base station.
• a mobile radio communicates with other mobile radios of its network by transmitting to the base station, which then relays the transmission to the receiving mobile radio, or radios.
• Each mobile radio and base station includes a transmitter and a receiver. The transmitter and receiver in each mobile radio or base station can be also combined into a transceiver.
• a transmitter of the present invention includes an M-ary encoder 23, a spread spectrum encoder 25, a Gaussian minimum shift keying (GMSK) modulator 27, an up-converter 29 and a class-C transmitter 31.
• the M-ary encoder 23 receives binary encoded data or voice bit stream 21 as input.
• the input data are then encoded into k-bit waveform symbols.
• k is preferably equal to 6 and M is preferably equal to 64.
• output data of the M-ary encoder in the present invention are 6-bit symbols that are input to the spread spectrum encoder 25.
• the M-ary encoder can also be a 32-ary, 128-ary or any other M-ary encoder available to one of ordinary skill in the art.
• the spread spectrum encoder 25 preferably includes a Gold-code sequence generator.
• Gold-code sequence generator A general description of Gold codes is provided in "Optimal Binary Sequences for Spread Spectrum Multiplexing", Robert Gold, IEEE Trans. Info. Theory; Oct. 1967, pp 619- 621, which is incorporated herein by reference.
• Each shift register is composed of a number of stages (e.g., 6) and includes a feedback arrangement 53, 54 which comprises one or more exclusive-OR gates for each set of the shift registers 50, 51.
• the M-ary symbols i.e., 64-ary symbol 56
• the shift registers 50 and 51 are clocked M-l times to generate maximal-length sequences.
• outputs of certain stages are fed back through the respective feedback arrangements 53, 54.
• the output sequence is a set of M-l zeroes.
• the two maximal-length sequences generated by the shift registers 50, 51 are input to the exclusive-OR 52 to generate a Gold sequence as output.
• the shift register 51 is loaded with all- zeroes symbol (i.e., initialization 55) and register 50 is loaded with a non-zero symbol, preferably all-ones.
• the resulting sequence is thus unique to synchronization to protect against mistaking the preamble for data.
• FIGs. 3, 3A, 3B, 3C, 3D and 3E The various embodiments of feedback arrangements 53,54 in accordance with the gl and g2 polynomials in the above table are illustrated in FIGs. 3, 3A, 3B, 3C, 3D and 3E.
• the Gold-code sequence generator and its polynomials are changed appropriately in accordance with what is known in the art.
• the code sequences from the Gold cold sequence 49 are pre-coded with a one-chip delay element 60 and an exclusive-OR 61.
• the output of the exclusive-OR 61 undergoes GMSK modulation.
• the output of the exclusive OR 61 is filtered by a low-pass filter 62 having an impulse response of
• B is the bandwidth of the lowpass filter 62 having a Gaussian shape and 7 is the chip interval of the code sequence.
• the impulse response of the filter 62 is preferably a finite impulse response (FIR) clocked at a multiple of the chip rate.
• the filter output is integrated by an integrator 63 to yield modulation angles.
• a modulator 64 quadrature phase modulates an RF carrier based on the modulation angles.
• modulation pre-coding 60 and 61, preserves the characteristics of code sequence after the GMSK modulation discussed above.
• MSK modulation with a code sequence, p k , having values of ⁇ 1.
• the phase sequence can be written as
• sequence/ ⁇ is constructed such that
• GMSK modulation For GMSK, the pre-coded sequence/ ⁇ is filtered with impulse response h(t), and truncated to finite length LT, to produce the modulation phase sequence
• n k-L/2
• the GMSK modulated carrier signal is then up-converted and transmitted by the transmitter, which is preferably a class-C transmitter 31 , over a communication link.
• the communication link between the base station and its mobile radios is time-division multiple accessed (TDMA).
• TDMA time-division multiple accessed
• the structure of TDMA, shown in FIG. 5, includes a continuous stream of frames 33.
• each frame includes a number of slots 35. More specifically, each 120- millisecond TDMA frame is divided into 72 time slots.
• the base station always transmits during three consecutive slots and every other three slots of the frame.
• the base station transmits during slots 0, 1, 2, 6, 7, 8, ... 66, 67, and 68.
• the base station assigns the remaining slots as either "reserved slots” or "ALOHA slots” for its mobile radios to transmit radio signals.
• the reserved slots are assigned for use by a specific mobile radio only.
• the ALOHA slots are available for any mobile radio to use.
• the ALOHA transmissions are always one slot long; however, transmissions in reserved slots may be either one, two, or three slots long.
• Each transmission burst, whether one, two, or three slots long begins with a synchronization preamble and ends with a guard interval.
• the synchronization preamble allows the receiver to determine the correct time-alignment of the transmitter's spreading sequence.
• a unique word follows the synchronization preamble to signify the start of data, which follows the unique-word pattern.
• the data segment varies in length depending upon the number of slots being used.
• a preferred embodiment of a receiver of the present invention includes an antenna 37, a down-converter 39, a Gold code correlator 41, magnitude selection circuitry 43 and an M-ary decoder 45.
• the antenna 37 and down-converter 39 are implemented by using standard components and devices known in the art in order to receive the transmitted signal and to translate the received signal to baseband signals, respectively.
• the Gold code correlator 41 uses the property that the Gold-code sequence is the product of two maximal-length sequences.
• the Gold- code correlator 41 includes a receiver filter 70, a pair of mixers 71, 72 in series and a circular correlator 73.
• the receiver filter 70 rejects out-of-band noise and interference while passing the GMSK signal.
• the mixer 71 removes the (-j) k sequence, or 90° /chip frequency offset produced by the MSK modulation. Further, the mixer 72 removes the gl sequence, one of the two maximal-length sequences, during demodulation.
• the circular correlator 73 is therefore simplified to compute the correlation against the M-l different shifts of a single maximal-length code sequence and the all-zero sequence. In particular, the correlator 73 preferably determines when the time that each
• the 63-chip sequence starts by using the synchronization preamble, transmitted at the beginning of each transmission burst.
• the synchronization preamble includes the all-zero gl sequence and the g2 generator is initialized to a predetermined non-zero value, preferably all-ones.
• the largest g2-sequence correlation magnitude indicates the most likely time-of- arrival of the received signal.
• the correlation magnitudes are averaged with an exponentially-decaying filter over several 63-chip intervals before deciding the most likely time-of-arrival.
• the data segment When the data segment is transmitted, gl ⁇ g2 is received. During this mode, the received signal is correlated with the gl sequence by the mixer 72. Subsequently, the circular correlator 73 correlates the residual signal with M-l different shifts of the g2 sequence and the all-zero sequence. The magnitudes of the M correlator outputs are compared by the magnitude-selection circuitry 43 to determine the most likely M-ary symbol that was transmitted.
• the correlator 73 also includes a parallel input shift register 82 having 2(M-1) by k number of stages (e.g., 126 by 6) configured to receive in parallel the 2(M-1) bits of input data from the serial input shift register 81.
• the correlator 73 further includes a g2-code shift register 83 having 2(M-1) number of stages.
• a pair of accumulators 84, 85 are provided to correlate the 2(M-1) bits of data in the parallel input shift register 82 with the 2(M-1) bits of data in the g2-code shift register 83.
• the output signal from mixer 72 represented byI R +jQ R , is sampled an integer number of times per chip interval.
• the signal from the mixer 72 is preferably sampled twice per chip, or 126 times per GMSK symbol interval.
• the correlator 73 operates in two different modes: synchronization and demodulation.
• the correlator 73 operated in the synchronization mode to correctly align locally generated gl and g2 code sequences to the received signal. After the locally generated code sequences are aligned in time to the received signal, the correlator 73 operates in the demodulation mode to find the most likely sequence of M-ary symbols that is being received.
• the incoming I R +jQ R signal is shifted into the register 81 at each sampling instant.
• the register 82 is enabled, via
• IQLoadEnable signal to load the shifted signals at each sampling instant to be correlated against the g2 code that has been loaded into register 83.
• the accumulators, multiply-and- add circuits, 84 and 85 compute the complex correlation at each sampling instant.
• envelope computation 86 yields a real-valued correlation at each instant by taking, or approximating, the square root of a squared real term added to a squared imaginary term. This process is performed for at least 63 chip intervals, and the sampling instant for which the real-valued correlation is maximized indicates the end, or time alignment, of the gl and g2 codes.
• the incoming I R +jQ R signal is also shifted into register 81 at each sampling instant.
• the register 82 is only loaded at the end of each g2 code sequence.
• the incoming signal is also accumulated in 87 for the duration of the g2 code interval.
• the envelope of the accumulation is computed by 88.
• the envelope value is calculated by taking, or approximating, the square root of a squared real term added to a squared imaginary term. This envelope is the correlation with the all-zero, g2 sequence.
• a real-valued correlation is computed by 84, 85, and 86. Every other correlation yields sixty-three values.
• the largest of these output values, 86, and the output of 88 is identified as the most likely g2 code sequence that was transmitted.
• the most likely 64-ary symbol can be represented by 6 bits, corresponding to the modulator input at the transmitter.
• the likelihood of each of the 6 bits is required to decode the convolutionally-encoded user-data bit stream using a maximum likelihood decoder as described in the article "Convolutional Codes and Their Performance in Communications Systems", A. J. Viterbi, IEEE Transactions on Communications Technology, October 1971, pp 751-771, which is incorporated herein by reference.
• the largest envelope can convey this likelihood information for all six bits, also called soft decisions, as described in the article "Performance of Power-Controlled Wideband Terrestrial Digital Communication", A. J. Viterbi, A. M. Viterbi, and E. Zehavi, IEEE Transactions on Communications Technology, April 1993, pp. 559-568, which is incorporated herein by reference.
• FIG. 9 shows the preferred embodiment of formulating six soft decisions for the 64-ary demodulator output.
• Each of the 64 codeword symbol can be uniquely numbered by a six-bit number; i.e. bits “0" through “5". Thirty-two of the codewords are numbered such that bit “0" is zero valued, and the remaining thirty-two codewords have bit “0” equal to "1".
• envelope computation 86 or 88 For each codeword symbol, envelope computation 86 or 88 sequentially computes the envelope. These envelope values are applied to peak detectors 90 and 91.
• the peak detector 90 identifies the largest envelope of the thirty- two codewords with bit "0" equal to "0”
• the peak detector 91 identifies the largest envelope of the thirty-two codewords with bit "0" equal to "1".
• a difference calculator 92 calculates the difference of the identified peak envelopes. This difference is used as the soft decision for bit "0", with the magnitude of the difference conveying the relative likelihood of the correct bit decision being "0" instead of "1 ". Similar processing is performed to calculate soft decisions for bits "1" through "5".
• Both base stations and mobile radios can include the functional components shown in FIG. 10.
• the antenna 100 is used to collect electromagnetic radiation for reception and launch electromagnetic radiation for transmission.
• the transmitter-receiver (T-R) switch 101 directs the signal flow for transmission and reception.
• the receiver down-converter 102 translates the received radio- frequency signals to baseband in-phase (I) and quadrature -phase (Q) signals. Either a bandpass filter at an intermediate frequency or lowpass filters at baseband restrict the bandwidth of the received signals.
• the I and Q signal components are demodulated into soft decisions that are deinterleaved and used to decode the digital data messages.
• Demodulation is performed by first synchronizing 103 the receiver timing to the incoming signal, and then correlating 104 the incoming signal with the M Gold-code sequences.
• the data processor 108 parses the digital messages, and either stores the received data for later transmission, outputs the data to the external interface, or uses the data for internal processing. During transmission, the data processor 108 outputs digital data messages to the radio.
• the radio encodes and interleaves the data for error correction prior to Gold-code encoding 10b and GMSK modulation 10c.
• the GMSK signal is then up- converted lOd to the RF carrier for amplification.
• the transmitter power is controlled by the data processor to assure link quality while minimizing interference to other networks.
• the amplified carrier is then conducted through the T-R switch 101 to be radiated by the antenna 100.
• the base station uses three consecutive time slots every other three slots of a frame for transmission.
• Each slot duration is 5/3 milliseconds which contains 96 symbols at the symbol rate of 57,600 baud.
• Each base- to-mobile transmission starts with a 32-symbol synchronization preamble which is followed by an 8-symbol unique word.
• 232 symbols of user data which includes addressing, cyclic redundancy check (CRC) coding, and at least two symbols of forward error correction (FEC) flush.
• CRC cyclic redundancy check
• FEC forward error correction
• a 16-symbol guard interval during which no data is transmitted, follows the user data.
• the convolutional encoder generates 1392 bits for each three time slot transmission.
• the 1392 bits are interleaved by writing six rows of L bits, and then reading L columns of 6-bit symbols.
• FIG. 11 illustrates the prefened interleaver embodiment.
• the bits are read in groups of 6 bits with each group forming a 6-bit symbol.
• the first symbol, or group, read is the leftmost column consisting of bits 1, 57, 113, 169, 225, and 281.
• the second symbol is the next column consisting of bits 2, 58, 114, 170, 226, and 282.
• the reading operation continues in this fashion until the last column consisting of bits 56, 112, 168, 224, 280, and 336 is read.
• the first 56 symbols are interleaved and output to the Gold encoder 9b before the second block of 96 symbols are interleaved.
• the second block of 96 symbols are interleaved before the final block of 80 symbols is interleaved.
• the 6-bit symbols that are read from the interleaver output are used to initialize the g2-sequence generator 50 to the beginning of each 63-chip Gold-code sequence.
• the gl -sequence generator 51 is initialized to a non-zero value, such as all-ones, during the unique-word and user-data intervals.
• the product of the gl and g2 sequences is used to GMSK modulate the RF carrier for transmission.
• the prefened BT product is 0.25, thus confining 99.9% of the power of the modulated carrier to be within a bandwidth of less than 5 MHz.
• the BT product is the product of the Gaussian lowpass filter bandwidth, B, and the spreading-code chip interval, T, as defined earlier.
• mobile radios share the three consecutive time slots that the base radio is not using.
• the slot duration and symbol rate of the mobile-to-base transmissions are the same as the base-to-mobile transmissions.
• the mobile-to-base time slots are either assigned to specific mobile radios for their exclusive use, or assigned to a group of radios for their mutually shared used.
• Mobile-to-base transmissions are one, two, or three slots long. In all cases, the transmission starts with a 32-symbol synchronization preamble and an 8-symbol unique word.
• User data which includes addressing, CRC coding, and at least two symbols of FEC flush, is transmitted following the unique word, and its length depends upon the number of slots being used.
• the base radio may be connected to another entity via the external interface at the base station.
• the base radio may be connected to the PSTN via the external interface at the base station.
• the mobile radios synchronize the frequency of their respective oscillators to their respective base-stations to reduce the likelihood of introducing demodulation errors in both the base-to-mobile and mobile-to-base communication links.
• Traditional methods of frequency synchronization utilize the carrier signal or the modulating symbol rate.
• the present invention uses the rate of message bursts from the base station.
• the time interval between unique words (UW) transmitted from the base station is N milliseconds.
• Each mobile radio detects the occunences of the unique words from the base-stations and adjusts its oscillator so that it can conectly predict the time interval between unique word detections.
• FIG. 12 preferably includes a burst demodulator 121, a frequency discriminator 123, loop filter 125, digital-to-analog converter (DAC) 127 and a controllable oscillator 129.
• DAC digital-to-analog converter
• the burst demodulator 121 includes the down-converter 102, synchronization 103, and conelator 104 blocks of FIG. 10.
• the controllable oscillator 129 is preferably a voltage-controlled crystal oscillator (VCXO).
• the frequency discriminator 123 is preferably implemented by counting clock cycles between unique words.
• the unique words (UWs) are received from the base station once every 10 milliseconds, which is exactly 4x36,288 clocks when the mobile radio clock is at the conect frequency.
• the frequency discriminator 123 includes a modulo-36,288 up counter 131 and a 16-bit latch 133 enabled when a UW is detected.
• the modulo counter 131 continuously counts while the latch 133 stores the value of the modulo counter into a memory location (or a register) " Latched _Count" each time a UW is detected.
• the frequency of the VCXO is decreased when the Latched _Count exceeds the previous value of Latched '_Count, and the frequency of the VCXO is increased when the
• Latched_Count less than the previous value of Latched _Count.
• the no change is introduced when the Latched_Count is equal to the previous value of Latched _Count.
• One benefit of using a modulo counter is that even if one or two UWs are failed to be detected this does not cause a major failure in frequency synchronization. For instance, if one UW is missed, when the next UW is detected, the rate of increase or decrease is simply divided by two in order to account for the missing UW.
• ⁇ f ' Latched _Count - Previous _Latched_Count
• the software loop filter 125 operates in two different modes: an acquisition/re- acquisition mode during which the frequency of the base oscillator is searched; and normal operation during which the frequency of the base oscillator is tracked.
• the search aperture to search the frequency preferably covers the total range of frequency error due to temperature and/or aging oscillator enors.
• the frequency step size is preferably small enough to guarantee that at least one unique word will be detected.
• search strategies may be used. The simplest example is a simple incremental search that rolls over from F Aperture /2 to -F Aperture /2. A better strategy is to increment by one step, decrement by two steps, increment by three steps, and so on until the entire range is tested.
• the software outputs a number to the DAC to generate a control voltage for the VCXO.
• step 135 UW is sought for each base-to-mobile slot interrupt for a predetermined length of a "TimeOut.”
• the length of a "TimeOut” is preferably between 3-5 UWs. If UW is not detected, the loop filter operates in the acquisition/re-acquisition mode. In this mode of operation, the VCXO is set to Next(VCXOJEstimate) which computes the next "search frequency," VCXOJZontrol is set to VCXO _Estimate and process is returned from the interrupt. (Step 137.)
• step 141 whether the input is the first input since the initialization or not is determined. If it is the first, Delta _F is set to zero. If it is not, Delta_F is set to Latched _Count - Previous _Latched_Count. (Step 143) Following the above steps, Previous _Latched_Count is set to Latched_Count and VCXO ⁇ Estimate is set to VCXO stimate - LOOP GAIN* Delta _F. (Step 145) Further, in step 147, an enor checking function is executed.
• Step 149) The Saved _VCXO XEstimate is used when there is a loss-of-synchronization or power shut-down. It should be noted that the above described synchronization scheme can be utilized in any radio signal communication system. Referring back to FIG. 11, the data processor 108 also digitizes voice and formats Internet communication packets into similar digital data streams so that either can be transmitted and received by the transceiver. This aspect of the present invention is utilized in a voice/Internet dual communication system illustrated in FIG. 15.
• the voice/Internet dual system includes a computer 151 configured to receive and send Internet communication packets.
• the computer is connected to the mobile radio 2 which can establish a communication link with the base station 1.
• the mobile radio 2 and the base station 1 are implemented as discussed above.
• the external interfaces 9, discussed in connection with FIG. 1, can be a phone jack for providing the Internet connection to the base station 1.
• the voice/Internet dual system operates similarly to a conventional mobile phone except that it offers the additional capability of handling both voice and binary data, e.g., Internet communication packets.

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• Digital Transmission Methods That Use Modulated Carrier Waves (AREA)

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