Abstract:
Wireless communication methods and structures are provided that enhance communication robustness while reducing cost. They communicate downstream data with orthogonal frequency division multiplexing (OFDM) transmission processes and upstream data with single carrier transmission processes. This combination of transmission processes is configured with various signal modulations (e.g., quadrature phase shift keying (QPSK), m-ary phase shift keying (MPSK) and n-quadrature amplitude modulation (QAM)) to provide lower cost upstream communication from customer services equipments (CPEs) and more robust downstream communication from headends than has been achieved in conventional communication systems. Signal diversity is enhanced by receiving communication signals with multiple antennas that are spatially separated and have different polarizations to thereby enhance frequency diversity. Signal diversity is further enhanced by combining the received signals in ways that maximize the ratio of desired to undesired signals.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Serial No. 60/214,894 filed Jun 29, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     Wireless communication systems have typically selected like transmission processes for both upstream and downstream carrier signals. In these systems, therefore, a single selection is chosen as a trade-off between cost and effectiveness and such trade-offs have not generally realized optimum utilizations of existing communication technologies. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to wireless communication methods and structures that enhance communication robustness while reducing cost. These goals are realized by communicating downstream data with orthogonal frequency division multiplexing (OFDM) transmission processes and upstream data with single carrier transmission processes. 
     This combination of transmission processes is configured with various signal modulations (e.g., quadrature phase shift keying (QPSK), m-ary phase shift keying (MPSK) and n-quadrature amplitude modulation (QAM)) to provide lower cost upstream communication from customer services equipments (CPEs) and more robust downstream communication from headends than has been achieved in conventional communication systems. 
     In system embodiments, signal diversity is enhanced by receiving communication signals with multiple antennas that are spatially separated and potentially have different polarizations to thereby enhance signal diversity. Signal diversity is further enhanced by combining the received signals in ways that maximize the ratio of desired to undesired signals. 
     The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a wireless communication system of the present invention; 
     FIG. 2 is a block diagram of a headend in the communication system of FIG. 1, and 
     FIG. 3 is a block diagram of a CPE in the communication system of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to wireless communication methods and systems that combine OFDM transmission processes for downstream data communication and single carrier frequency hopping transmission processes for upstream data communication. OFDM is relatively expensive to realize but is particularly effective for non-line-of-sight communication applications because of its relative impunity to multipath phase and amplitude effects and its ability to realize cost effective methods for correcting these effects at the receiver. Although single carrier frequency hopping is more cost effective, it has often been limited to line-of-sight applications. The present invention employs these processes in embodiments that reduce costs and enhance communication robustness. A detailed investigation of these communication methods is preceded by the following description of a system embodiment. 
     A communication system  20  is shown in FIG.  1 . It includes a headend  22  and a plurality (e.g., M) of customer premises equipments  24  that are associated with each of a plurality (e.g., N) hub transceivers  26  that relay communication signals between the headend and the CPEs. The headend  22  receives program data  27  from program sources  28  (e.g., internet data via Ethernet protocol and television data via satellite and cable). 
     With its termination equipment and transceivers, the headend modulates the program data onto OFDM downstream carrier signals  30  which are generally relayed via the hub transceivers  26  to the CPEs  24 . The CPEs generate CPE data and modulate it onto upstream single carrier signals  32  which are relayed via the hub transceivers  26  to the headend  22 . 
     The headend  22  is illustrated in FIG. 2 which shows that it includes conventional termination equipment  38  and a headend transceiver  40  that receives the program data  27  into a media access controller  41  whose output is processed through an OFDM modulator  42 , a downstream upconverter  43  and a power amplifier  44 . The program data is thus modulated onto OFDM downstream carrier signals  30  which are radiated from an antenna  46 . 
     The headend also includes a plurality of receive antennas  48  that are spatially separated and which are configured to receive upstream single carrier signals  32  with different polarizations (e.g., vertical and horizontal polarizations) as indicated by + and − polarization symbols. The output of these receive antennas is downconverted in upstream downconverters  50  and coupled through an adaptive equalizer and combiner  52  to a demodulator  54  which passes upstream data  55  through a forward error corrector  56  to the media access controller  40 . As shown in FIG. 3, an exemplary CPE  24  includes a CPE transceiver  60 . The downstream carrier signals  30  are received within this transceiver by a plurality of receive antennas  61  which are spatially separated and configured to receive different signal polarizations as indicated by + and − polarization symbols. A selected receive antenna  61 S is preferably shared with upstream signals by steering its respective downstream signal through a diplexer  62  (as an example, the shared antenna  61 S is configured with two polarizations). 
     Signals from the antennas are downconverted in downstream downconverters  63  and combined in a diversity combiner  64  before being demodulated in an OFDM demodulator  66 . The demodulated downstream data signals  67  are processed through a media access controller  68  and a physical layer device (PHY)  70  to a CPE data interface  71  (e.g., a personal or network computer). 
     Upstream data signals  72  are originated by CPE users and are processed through the physical layer device  70  and the media access controller  68  to a modulator  74  where they are modulated onto an upstream single carrier signal that is upconverted in an upstream upconverter  76  and amplified in a power amplifier  78 . The single carrier signal  32  is steered through the diplexer  62  to be radiated from the shared antenna  60 S. 
     Having described the basic structures of the communication system  20  of FIGS. 1-3, attention is now directed to operation of the system. In downstream data communication, the media access controller  41  of FIG. 2 oversees and controls various communication functions, e.g., demodulation, modulation, frequency and bandwidth selection, power ranging, program source allocation and signal combinations in the CPE diversity combiner  64  of FIG.  3 . In addition, bundling of program data  27  at the headend  22  by the media access controller  41  presents data in a form that is usable by the CPE media access controller  68  of FIG.  3 . 
     Output signals from the media access controller  41  of FIG. 2 are processed in the OFDM modulator  42  with inverse Fourier transforms and error correction coding. The OFDM modulator provides various communication functions which include generating a variable number of subcarriers, sending continual scattered pilot signals that contain training sequences for channel estimation, providing variable guard bands and selecting various modulations (e.g., QPSK, MPSK and QAM) in each subcarrier. 
     Intermediate frequency data from the OFDM modulator is then upconverted (e.g., to the 2500 megahertz range) in the downstream upconverter  43  and amplified by the power amplifier  44  which is preferably backed off its maximum amplifying capability by a significant amount to enhance its linearity The upconverted and amplified downstream carrier signals  30  are then broadcast via the antenna  46 . 
     After relaying by hub transceivers ( 26  in FIG.  1 ), the downstream carrier signals are received, at each CPE  24  with the receive antennas  61  of FIG. 3 that are spatially separated and have different polarizations. The spatial differences provide reception time differences and, thereby, phase differences in the received downstream signals. These factors (i.e., polarization and phase) are the dominant elements in achieving decorrelation in received signals that enhances signal quality of the final received signal. 
     The downstream carrier signals are then downconverted in FIG. 3 by each antenna&#39;s respective downconverter  62  and combined in the diversity combiner  64  which scales (amplifies or attenuates), delays, and adds the downconverted signals in a way that maximizes the ratio of desired to undesired signals for the signal to be demodulated and thus minimizes the error rate. The demodulated signal will be passed to the media access controller  68  by the OFDM demodulator  66 . The diversity combiner  64  also processes continual and scattered pilot signals that contain training sequences for channel estimation. 
     The OFDM demodulator  66  processes the downconverted signals with fast Fourier transforms and forward error correction. Conventional OFDM modulators are capable of demodulating a variable number of subcarriers, working with variable channel guard bands and providing the necessary demodulation (e.g., QPSK, MPSK or QAM) in each subcarrier. Finally, the downstream data is channel processed in the media access controller  68  and output to an appropriate PHY  70  which further transforms the downstream data  67  into a format and voltage level that is usable by the CPE data interface  71 . The OFDM transmission processes in the downstream communication of FIGS. 1-3 essentially divide a given channel (e.g., a 6 megahertz channel) into a large number (e.g., 200 to 8000) of subchannels. Each subchannel contains a fraction of the original channel information and is isolated by temporal guard bands (e.g., several kilohertz or {fraction (1/32)} to ¼ of the data symbol rate) from subsequent symbols to thereby decrease multipath effects on the downstream communication. 
     Attention is now directed to the upstream data communication of the communication system  20  of FIGS. 1-3 which begins with user-generated CPE data  72  at a CPE data interface  71  of each CPE  24 . 
     This CPE data is directed through the PHY  70  to the media access controller  68  which bundles the data in a form that is usable by the head-end transceiver ( 22  in FIG.  2 ). In addition, the media access controller  68  performs other upstream data communication functions, e.g., overseeing and controlling demodulation, modulation, frequency and bandwidth selection, and power ranging. 
     The CPE data is then modulated (e.g., with QPSK or QAM) by the modulator  74  which preferably operates in a frequency hopping single carrier transmission mode. In particular, the frequency is hopped between channels under control of the media access controller  68  to reduce dispersive effects of the weather and terrain (e.g., rain and foliage). 
     After the data is modulated, the intermediate frequency signals are upconverted (e.g., to the 2500 megahertz range) in the upstream upconverter  76  and amplified in the power amplifier  78 . Because single frequency modulation is used for upstream communication, this power amplifier need not be as linear (and therefore not as expensive) as is preferred for the OFDM of the downstream communications. Finally, the diplexer  62  couples the power amplifier  78  to the shared antenna  61 S so that the upstream single carrier signal  32  is broadcast to the head end transceiver ( 22  in FIG. 2) via a respective hub transceiver ( 26  in FIG.  1 ). 
     In the headend transceiver  22  of FIG. 2, the upstream single carrier signals  32  are received in the receive antennas  48  that are spatially separated and configured to receive signals having different polarizations. The upstream single carrier signals are then combined in the adaptive equalizer and combiner  52  which varies the order of equalizing and combining to that which is the most effective for error reduction and which will scale (amplify or attenuate), delay, and/or add the downconverted signals to maximize the ratio of desired to undesired signals. 
     In particular, the adaptive equalization methods of the adaptive equalizer and combiner  52  are directed by the media access controller  41  to minimize the probability of data errors by the use of different algorithms (e.g., decision-directed equalization, filter output computation based upon training and transversal filter storage, transversal filter coefficient adaptation, zero-forcing equalization which starts with the sinc function (sinπt/πt) and solves n simultaneous equations, least mean squares in which transversal filters are gradually adjusted to converge to a filter that minimizes the error between the equalized data word and a stored reference header word, decision feedback equalization and recursive least squares). To facilitate this algorithm processing, the adaptive equalizer is preferably programmed in a digital signal processor or similar flexible state machine architecture that is optimized for implementing algorithms. 
     After equalization, the signal is demodulated (e.g., with QPSK or QAM demodulation) to baseband by the single carrier demodulator  54 . If frequency hopped, the frequency selection for the frequency hopped carrier signal  32  is controlled by the media access controller  41  to minimize dispersive effects of the weather and terrain (e.g., rain and foliage). 
     The CPE data is then processed with various algorithms (e.g., Viterbi and Reed-Solomon) in the forward error corrector  56  to further reduce errors and is then coupled to the media access controller  41  for realizing various customer needs. 
     In operation of the communication system  20  of FIGS. 1-3, therefore, the downstream data is communicated with the aid of OFDM transmission processes and the upstream data is communicated with the aid of single carrier transmission processes. The present invention thus utilizes the better non line-of-sight capabilities of OFDM processes for headend and hub transmissions where relatively few equipment installations are required and single carrier transmission processes for the far more numerous CPE installations. 
     The system thus limits the number of expensive installations because it has only one headend ( 22  in FIG. 1) and its hub transceivers ( 26  in FIG. 1) are also of limited quantity. In an exemplary cost reduction, the expensive linear power amplifiers ( 44  in FIG. 2) that are required for OFDM processes are used only in the headend and the hubs. The receiver portion of the headend transceiver can also be configured to be more sensitive (and thus more expensive) because it is only used once in the communication system  20 . 
     The single carrier for upstream communication is preferably frequency hopped for each transmission burst and received at the headend with multiple antennae that differ spatially and have different polarizations to thereby compensate, for the conventional lack of robustness of this method. The degrading effects of various link characteristics (e.g., multipath and frequency fading) are thereby mitigated. 
     These link characteristics determine how well a particular communication signal is received. In a method of the invention, the CPE transceiver ( 60  in FIG. 3) communicates to the headend (e.g., via acknowledgement handshaking protocols) which of a set of substantially separated frequency channels will receive the downstream data of various bandwidth requirements. 
     The CPE transceiver also communicates which of a set of substantially separated frequency hopping channels it will use to transmit the upstream data (which may also have various bandwidth requirements) to thereby achieve frequency diversity by means of frequency hopping across a substantially separated set of available frequency channels. Thus frequency diversity is realized downstream with channel selection across a set of substantially separated frequency channels and is realized upstream by frequency hopping across a substantially separated set of available channels. 
     In a sectorized or cellular embodiment of the invention, the available frequency is divided such that the downstream and upstream channels assigned to each CPE are interleaved with those assigned to CPEs in other sectors or cells. In this system embodiment, some communication channels are not available to the CPEs of a particular sector or cell. 
     In a method of the invention, therefore, initial communication contact begins with a predetermined order of available channels, so that communication links can be established for initial contact between the headend and the CPEs. The order and sequencing is preferably part of a look-up table that is programmed into signal computers of the headend and CPE media access controllers ( 41  in FIG. 2 and 68 in FIG.  3 ). 
     Communication components that have been described above (e.g., media access controllers, diversity combiners and physical layer devices) are conventional and easily obtained in varying degrees of complexity. Modulation methods of the invention have been disclosed above to include n-quadrature amplitude modulation. Exemplary values for n are 4, 16, 32, 64, 128, 256, 512 and 1024. 
     The preferred embodiments of the invention described herein are exemplary and numerous modifications, variations and rearrangements can be readily envisioned to achieve substantially equivalent results, all of which are intended to be embraced within the spirit and scope of the invention as defined in the appended claims.