Abstract:
Method and apparatus for high speed wireless transmission involving selective PSK coding techniques are disclosed. In particular, to enhance effective throughput while maintaining a channel footprint backwards compatible with existing IEEE 802.11 and 802.11b (1999) standards, the payload of outbound data packets are selectively modulated using DQPSK techniques at the CCK symbol chipping rate. This results in a 22 Mbps effective throughput, a 2× improvement over the 802.11b standard. A complementary receiver includes a DQPSK demodulation pathway suitable for decoding inbound data packets at the 11 MHz CCK chipping rate but bypassing the conventional CCK encoder, resulting in a 22 Mbps data stream. Backwards compatibility with legacy 802.11 and 802.11b devices is partially accommodated using a data packet format similar to the conventional PLCP frame format but for the data transmission rate and use of heretofore reserved bits, causing legacy devices to ignore such packets.

Description:
TECHNICAL FIELD 
   This invention is generally directed to communications technology, and is particularly concerned with systems and techniques for high effective throughput wireless data communications using phase shift keying (“PSK”) modulation. 
   BACKGROUND OF THE INVENTION 
   The past few years has witnessed the ever-increasing availability of relatively cheap, low power wireless data communication services, networks and devices, promising near wire speed transmission and reliability. One technology in particular, described in the IEEE Standard 802.11b-1999 Supplement to the ANSI/IEEE Standard 802.11, 1999 edition, collectively incorporated herein fully by reference, and more commonly referred to as “802.11b” or “WiFi”, has become the darling of the information technology industry and computer enthusiasts alike as a wired LAN/WAN alternative because of its potential 11 Mbps effective throughput, ease of installation and use, and transceiver component costs make it a real and convenient alternative to wired 10 BaseT Ethernet and other cabled data networking alternatives. With 802.11b, workgroup-sized networks can now be deployed in a building in minutes, a campus in days instead of weeks since the demanding task of pulling cable and wiring existing structures is eliminated. Moreover, 802.11b compliant wireless networking equipment is backwards compatible with the earlier 802.11 1 M/2 Mbps throughput standard, thereby further reducing deployment costs in legacy wireless systems. 
   802.11b achieves relatively high payload data transmission rates or effective throughput via the use of orthogonal class modulation in general, and, more particularly, 8-chip complementary code keying (“CCK”) and a 11 MHz chipping rate to bear the payload. As such, previously whitened or scrambled bitstream data of interest is mapped into nearly orthogonal sequences (or CCK code symbols) to be transmitted, where each chip of the CCK code symbol is quaternary phase modulated using QPSK (“quadrature phase shift keying”) modulation techniques. Meanwhile the common phase of each CCK symbol is jointly determined by the current and previous symbols using differential QPSK or DQPSK modulation scheme. Subsequent conversion into the analog domain prepares these CCK symbols for delivery over a wireless medium RF modulated on a carrier frequency within the internationally recognized 2.4 GHz ISM band to form the payload or PLCP Service Data Unit of an 802.11b complaint Physical Layer Convergence Procedure (“PLCP”) frame. The high-rate physical layer PLCP preamble and header portions are still modulated using the 802.11 compliant Barker spreading sequence at an 11 MHz chipping rate. In particular, the preamble (long format—144 bits, short format—72 bits) is universally modulated using DBPSK (“differential binary phase shift keying”) modulation resulting in a 1 Mbps effective throughput, while the header portion may be modulated using either DBPSK (long preamble format) or DQPSK (short preamble format) to achieve a 2 Mbps effective throughput. 
   An IEEE 802.11b compliant receiver receives and downconverts an incident inbound RF signal to recover an analog baseband signal bearing the PLCP frame, and then digitizes and despreads this signal to recover the constituent PLCP preamble, header and payload portions in sequence. The preamble and header portions are Barker correlated and then either DBPSK or DQPSK demodulated based on the preamble format used to recover synchronization data and definitional information concerning the received PLCP frame, including the data rate (Signal field in the PLCP header) and octet length (Length field in the PLCP header) of the variable-length payload or PSDU portion. The CCK encoded symbols forming the PLCP payload portion are each correlated against 64 candidate waveforms in received per symbol sequence in combination with DQPSK demodulation to verify and reverse map each into the underlying bitstream data of interest, at either 4 bits per symbol (5.5 Mbps) or 8 bits per symbol (11 Mbps) increments. 
   The major benefit CCK offers is strong inherent resistance to multipath interference, which is likely to be encountered in in-building transceiver deployment. However, in an effort to match occupied channel bandwidth of legacy base 802.11 systems, 802.11b compliant CCK modulation uses the same 11 MHz chipping rate, thereby limiting effective throughput to a maximum 11 Mbps effective data throughput. While acceptable for some applications, this data rate is deemed too slow for certain “broadband” applications such as full-screen streaming video and interactive gaming. 802.11b&#39;s sibling communications scheme, defined in the 1999 IEEE 802.11a Supplement to the ANSI/IEEE Standard 802.11, 1999 edition (“802.11a”) offers a higher effective throughput (up to 54 Mbps), but sacrifices backwards compatibility with 802.11b, and requires data transmission in the 5 GHz band which is not generally available outside North America. The forthcoming IEEE 802.11g high-rate PHY extension attempts to address 802.11 a&#39;s backwards compatibility issue through specifying dual 802.11b and 802.11a compliant transceivers, but this adds cost, complexity and power consumption to the very price sensitive, consumer-oriented mobile devices which stand to benefit most from high speed wireless data communications. 
   Accordingly, it would be advantageous to define a wireless data communications scheme which economically increases effective throughput over 802.11b compliant devices while maintaining backwards compatibility with such devices, thereby fully leveraging the worldwide benefits of ISM transmission and a large installed base of 802.11b systems. 
   SUMMARY OF THE INVENTION 
   The present invention is directed in part to a baseband processor and associated method suitable for use in a wireless transmitter. This processor and method utilizes packet header generation unit capable of generating a header portion of an outbound data packet, PSK modulation capable of PSK modulating the header portion at a first effective throughput and PSK modulating outbound data at a second effective throughput to form a PSK modulated payload (in which the second effective throughput is at least five times the first effective throughput), symbol modulation capable of symbol modulating the outbound data at a third effective throughput to form a symbol modulated payload (in which the third effective throughput being less than the second effective throughput), and modulation selection capable of selecting one of the PSK modulated payload and the symbol modulated payload to form a payload portion of the outbound data packet. 
   In accordance with an embodiment of the invention, a common PSK modulation arrangement may be provided to economically handle both header and payload modulation. In an alternative embodiment, dedicated modulation units may be provided, if for example, a single PSK modulation arrangement provides unacceptable performance. 
   Further, one or more disclosed embodiments make use of the DQPSK form of PSK modulation to increase effective throughput at least twice that of the third effective throughput where symbol modulation utilizes orthogonal class symbol modulation such as CCK modulation. As such, a DQPSK modulator may be selectively clocked at the CCK chipping rate to achieve such effective throughput. 
   The present invention is also directed in part to a baseband processor and associated method suitable for use in a wireless receiver. Such processor and method includes header demodulation capable of recovering of a header portion of an inbound data packet, along with packet demodulation capable of: 1) recovering first data at a first effective throughput from the payload portion of the data packet according to a symbol demodulation scheme; 2) recovering second data from this payload portion at a second effective throughput exceeding the first effective throughput according to a PSK demodulation scheme; and 3) selecting one of the first and second data as inbound data presented in the payload portion of the inbound data packet based on the recovered header portion thereof. Thus, for example, backwards compatibility may be maintained with legacy 802.11b devices while still providing enhanced throughput demodulation capabilities. 
   In accordance with an embodiment of the invention, first data recovery is performed using symbol correlation, including orthogonal class symbol correlation such as CCK correlation. Moreover, PSK demodulation here may include utilizing DQPSK demodulation. 
   The present invention is further directed in part to a data packet including a header portion defining first PSK modulated data having a first effective throughput, and a payload portion proceeding such header portion, in which the payload portion includes, based on said header portion, either second PSK modulated data having a second effective throughput at least five times the first effective throughput or symbol modulated data having a third effective throughput, the third effective throughput being less than the second effective throughput. In accordance with an embodiment, this data packet may further include a DBPSK modulated preamble, and the PSK modulated data may include DQPSK modulated data having an effective throughput of at least 22 Mbps. 
   Additional aspects and advantages of this invention will be apparent from the following detailed description of embodiments thereof, which proceeds with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified block diagram of a transceiver according to an embodiment of the present invention. 
       FIG. 2  is a more detailed block diagram of a transmit baseband processor shown in  FIG. 1 . 
       FIG. 3  is a more detailed block diagram of a receive baseband processor shown in  FIG. 1 . 
       FIG. 4  diagrammatically illustrates a data packet format in accordance with the embodiment of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
   Turning now to the FIGs.,  FIG. 1  illustrates a wireless communications transceiver  100  according to an embodiment of the invention. In this embodiment, inbound RF signals conveying a 802.11b compliant PLCP frame and/or a data packet formatted in accordance with the data packet format  400  shown in  FIG. 4  (“22 Mbps compatible data packet”) are picked up by the duplex antenna  110  and routed to the RF receiver unit  115  of a receiver  150  arranged in a manner consistent with the present invention. The RF receiver unit  115  performs routine downconversion and automatic gain control of the inbound RF signals, and presents an analog baseband signal containing the aforementioned 802.11b PLCP frame or data packet to the receive baseband processor  120 . The functions of the receive baseband processor  120  will be detailed below with reference to  FIG. 3 , including selective demodulation of the analog baseband signal to recover a 22 Mbps effective throughput inbound data bitstream from the payload of a received 22 Mbps compatible data packet. Generally speaking, however, the receive baseband processor  120  performs symbol correlation and/or demodulation of the preamble, header and payload portions of each inbound 802.11b PLCP frame or 22 Mbps compatible data packet to recover bitstream data for receiver synchronization (preamble), frame or packet definition (header), or the actual inbound data of interest (payload). 
   Once recovered by the receive baseband processor  120 , the inbound data contained in the PSDU of each received 802.11b PLCP frame or the payload of the 22 Mbps compatible data packet is delivered to a network interface such as the MAC layer interface  125  and then on to higher layer applications and devices being serviced by the transceiver  100 . Here, the MAC layer interface  125  differs from a conventional 802.11b MAC layer interface in that it can accommodate inbound data at a 22 Mbps maximum rate, and includes logic in its PMD sublayer  130  to recognize when the payload of an inbound 22 Mbps compatible data packet includes DQPSK modulated inbound data in its payload portion with an effective throughput of 22 Mbps. As will be discussed in greater detail below with reference to  FIG. 4 , the PMD sublayer may conveniently recognize this high rate DQPSK mode through interpretation of a 22 Mbps transmission rate in the Signal field  415  (e.g. 0×DC) as well setting a previously reserved bit in the Service field  420  of the inbound 22 Mbps compatible data packet which are recovered prior to the start of actual payload symbol demodulation. 
   Outbound data intended for wireless transmission originating from the device(s) or application(s) being serviced by the transceiver  100  are delivered to the transmit baseband processor  135  of the transmitter  160  from the MAC interface  125 . As before, the PMD sublayer  130  differs from a conventional PMD sublayer in that it can direct that outbound data be delivered to the transmit baseband processor  135  at a 22 Mbps transmission rate, and that the MAC interface  125  can selectively perform such transfer responsive to such directive or decision made by the PMD sublayer  130 . Directives from the PMD sublayer  130  expressing the desired transmission mode, including the 802.11b 1, 2, 5.5 and 11 Mbps effective throughput modes as well as the inventive 22 Mbps DQPSK mode are transferred to the transmit baseband processor as well for each PLCP frame/packet, including the HI_SPEED semaphore discussed in greater detail with respect to  FIG. 2 . The transmit baseband processor  135  formulates appropriate 802.11b PLCP frame or inventive data packet preamble and header information, and symbol encodes the outbound data as specified by the PMD  130  to generate a complete outbound 802.11b PLCP frame or data packet formatted in accordance with the data packet format  400  of  FIG. 4 . As the frame or packet is being developed, it is converted into analog form suitable for upconversion and RF transmission by the RF transmitter unit  140  consistent with 802.11b physical layer requirements. 
   Though not shown in  FIG. 1 , the transceiver  100  may form an operational part of a network interface apparatus such as a PC card or network interface card capable of interfacing with the CPU or information processor of an information processing apparatus such as a desktop or laptop computer, and may be integrated within and constitute a part of such information processing apparatus. This network interface apparatus may alternatively form an operational component of a wireless communications access point such as a base station as will be appreciated by these ordinarily skilled in the art. 
   Turning now to  FIG. 2 ,  FIG. 2  is a more detailed block diagram of the transmit baseband processor  135  shown in  FIG. 1 . So as not to obfuscate the teachings of the present invention, several 802.11b compliant directives and signals are not shown. Outbound data in the form of MPDU packets are sent by the MAC  125  at either 802.11b compliant 1, 2, 5.5 or 11 Mbps selectable transmission rates/modes matching the effective throughput of the outbound PLCP frame, as well as 22 Mbps in the DQPSK transmission mode of the present embodiment. The preamble and header portions of the outbound 802.11b frame are generated by the preamble generator  210  and the header generator  212 , which includes separate Length field  215 , Service field  220 , Signal field  225  and CRC  230  generation units corresponding to the fields constituting the header of the 802.11b PLCP frame or 22 Mbps compatible data packet. Once generated, the preamble, header and CRC (part of the header) are sequenced in frame order by the mux  235 , followed by the outbound MPDU data, which then undergoes routine scrambling/whitening using known techniques to optimize it for wireless transmission. 
   In contrast with a conventional 802.11b compliant header generation unit, the header generation unit  212  and the Signal generation unit  225  in particular includes logic capable of interpreting a ENABLE — 22 Mb_TX semaphore managed by the PMD  130  indicating whether or not the current frame/packet will be transmitted in 22 Mbps DQPSK mode in accordance with the present embodiment. If, for example, ENABLE — 22 Mb_TX is false, the Signal generation unit  225  will insert the appropriate 802.11b compliant data rate into the Signal field of the outbound PLCP frame. However, consistent with the data packet format  400  shown in  FIG. 4 , if ENABLE — 22 Mb_TX is true, the 22 Mbps DQPSK mode is selected and so the Signal generation unit  225  will insert 0×DC ( 220  in hexadecimal notation) into the Signal field  415  of the outbound data packet. Though not shown in  FIG. 2 , in addition to or in the alternative, the Service generation unit  220  may include logic capable of setting at least one heretofore reserved bit in the Service field  420  to indicate that the payload  435  arranged in accordance with the 22 Mbps DQPSK mode. More detail on the data packet format  400  will be provided below with reference to  FIG. 4 . 
   Once generated, the scrambled preamble, header and payload portions defining a current outbound PLCP frame or 22 Mbps compatible data packet are serially presented to the symbol encoding unit  251 , here shown containing a DBPSK modulation pathway (including DBPSK modulator  250 , mux  255  and Barker spreader  260 ) for preamble encoding and parallel DQPSK (DQPSK modulator  265 , mux  255  and the Barker spreader  260 , with the DQPSK modulator  265  being clocked at 1 MHz e.g. the CLK_SELECT semaphore is clear) and DBPSK modulation pathways for short/long header encoding respectively. These modulation pathways are used to encode the outbound data received from the MAC interface  125  when legacy base 802.11 1 Mbps (using the DBPSK modulator pathway) and 2 Mbps (using the DQPSK modulation pathway) transmission rates/modes are desired. For 5.5 and 11 Mbps rates compliant with 802.11b, a CCK modulator  270  is used clocked at the 11 MHz chipping rate. 
   When it is desired to transmit the outbound data at an effective throughput 22 Mbps, thereby doubling the 802.11b 11 Mbps maximum, in accordance with this embodiment, the 11 MHz chipping clock is applied to the DQPSK modulator  265 . Since each DQPSK sample can encode 2 bits of outbound data, this results in a 22 Mbps throughput. The 11 MHz clock is applied through the modulation control unit perceiving the HI_SPEED semaphore from the PMD sublayer  130  as being set and consequently setting the CLK_SELECT signal to a logic level (e.g. true) directing the mux  245  to direct the 11 MHz clock to the clock input of the DQPSK modulator  265  as it received the scrambled outbound data. Line  267  communicatively couples the output of the DQPSK modulator to the modulation selection mux  275  to complete the 22 Mbps mode DQPSK modulation pathway. 
   The modulation control unit  290  drives the MOD_SELECT signal, which is used in conjunction with the conventional long/short signal to have mux  255  and mux  275  select which of the four possible modulation pathways described above should be used to develop the symbol encoded preamble, header and payload of the outbound 802.11b PLCP frame or 22 Mbps compatible data packet. For example, when generating an 11 Mbps mode 802.11b PLCP frame, the modulation control unit asserts e.g. “00” to select the DBPSK pathway to generate the symbol encoded preamble  410  and the PLCP header  440  (if a short preamble is desired, the DBPSK pathway is used only for the short preamble, and the mux  255  selects the DQPSK modulator  265  output when clocked to 1 MHz to develop the symbol encoded PLCP header). As such, the modulation control unit  290  will then assert e.g. “10” on the MOD_SELECT line during the payload portion part of the PLCP frame generation sequence to deliver CCK modulated outbound data to the FIR  280 , digital-to-analog converter or DAC  285 , and onto the RF transmitter unit  140 . 
   If, however, a 22 Mbps compatible data packet is to be transmitted, the modulation control unit  290  instead asserts e.g. “01” on the MOD_SELECT line during the payload portion part of the data packet generation sequence to select output from the 22 Mbps DQPSK modulation pathway to deliver DQPSK modulated symbols at 11 MHz to the FIR  280  and DAC  285 . 
   Though the present embodiment contemplates a dual role, dual clocked DQPSK modulator  265 , in fact in an alternative embodiment, separate DQPSK modulators (1 clocked at 1 MHz, the other clocked at 11 MHz) may be provided to undertake the necessary DQPSK modulation activities specified herein. 
   Turning now to  FIG. 3 ,  FIG. 3  is a more detailed block diagram of the receive baseband processor  120  shown in  FIG. 1 . As such, an analog baseband signal conveying an inbound 802.11b PLCP frame or 22 Mbps compatible data packet recovered by the RF receiver unit  110  of the receiver  150  is fed to the input of the analog to digital converter  310  to convert it into digital form. With the aid of the 44 MHz clock, the ADC produces a corresponding digital signal sampled at 44 MHz. Next, this digital signal passes through the digital FIR LPF  315  to reject out-of-band interference, and the down sampler  320  to provide a 22 Mhz digital signal bearing the frame/packet of interest. 
   This 22 MHz signal next encounters two parallel baseband demodulation pathways. The first demodulation pathway, including the Barker correlator  330 , down sampler  335 , Rake  340  and the down sampler  345  is used to recover a despread 1 MHz signal representing the preamble and header portions of the inbound frame or data packet for symbol demodulation by the combination DBPSK/DQPSK demodulator  375 . This first demodulation pathway—demodulator combination  375  is also used for symbol decoding a base 802.11 PLCP frame payload in 1 Mbps/2 Mbps modes. The second demodulation pathway is used to symbol demodulate a high rate 802.11b payload portion of the inbound frame as well as the 22 Mbps DQPSK mode modulated payload for an inbound 22 Mbps compatible data packet, and includes a 22 MHz to 11 MHz down sampler  350  following by a decision feedback equalizer  355 . To begin the CCK symbol decode process for 802.11b compliant payloads at 11 Mbps or 5.5 Mbps transmission modes, a CCK correlator  360  is provided. It should be noted that, in addition, a parallel pathway  357  that bypasses this correlator  360  is provided to deliver a 22 Mbps DQPSK modulated payload directly to the DBPSK/DQPSK demodulator  375 . 
   A demodulation controller  365  is used to control which demodulation pathway should be used via the DEMOD_SELECT control line coupled to the control element mux  370 , based on which portion of the inbound frame or packet is being demodulated, as well whether 5.5 Mbps+ payload modulation modes are specified. To this end, the demodulation controller bases its DEMOD_SELECT decision based on the conventional HI_RATE PSDU semaphore, generated by the known rx state machine  372  in conjunction with appropriate signalling issued by the MAC interface  125 , which determines whether the 802.11b high rate PHY transmission modes are specified, along with the inventive ENABLE — 22 Mb_RX semaphore (set if 22 Mbps DQPSK mode according to the present invention is specified). Though not required, in this embodiment, the ENABLE — 22 Mb_RX semaphore is managed as a register within the MAC I/F  125  by the PMD sublayer  130 . The combination DBPSK/DQPSK demodulator  375  is used to recover the symbol encoded inbound data presented in the preamble, header and payload portions, including the 22 Mbps payload mode consistent with the present invention. The DBPSK/DQPSK demodulator is clocked at the symbol rate; i.e., 1 MHz for 1 Mb and 2 Mb modes, and 1.375 MHz for 5.5 Mb and 11 Mb modes, and 11 MHz for 22 Mb mode. 
   After symbol demodulation, the recovered inbound data is descrambled by the descrambler  380  in a known fashion, and delivered to the MAC interface  125  ( FIG. 1 ). 
     FIG. 4  illustrates the data packet format  400  used by the transceiver  100  of  FIG. 1  to support the 22 Mbps DQPSK transmission mode, in addition to 802.11b and base 802.11 modes. To this end, the data packet format  400  specifies an 802.11b compliant PLCP preamble portion  410  and can support either long or short formats. The next portion, the PLCP header  440 , includes an 8 bit Signal field  415 , 8 bit Service field  420 , 16 bit Length field  425 , and 8 bit Frame check sequence  430 . Like the 802.11b compliant PLCP header, the header portion  440  can be transmitted at 1 Mbps (DBPSK modulation at 1 MHz) or 2 Mbps (DQPSK modulation at 1 MHz) effective throughput. Moreover, in this embodiment, the Signal field  415  differs from an 802.11b compliant Signal field only in that it is additionally capable of specifying the 22 Mbps data rate 0×DC or 220 in hexadecimal notation. Though not required, one or more heretofore reserved bits in the Service field  420  are used in this embodiment to indicate that the payload  435  is modulated in accordance with the above-described 22 Mbps mode. 
   The Length  425  and Frame Check Sequence  430  fields are formatted in accordance with corresponding fields in the 802.11b compliant PLCP header. The PSDU  435  contains the payload modulated in accordance with one of the legacy 802.11b modes (1/2/5.5/11 Mbps) or the 22 Mbps mode consistent with the present invention as specified in the Signal  415  and/or Service  420  fields. 
   Thus, this data packet format  400  is backwards compatible with the base 802.11 and 802.11b standards, and would only deviate from the standard 802.11 and 802.11b PLCP frame formats only where the 22 Mbps DQPSK transmission mode according to the present embodiment is selected. Furthermore, a legacy 802.11b device would, upon decoding and interpretation of the Signal  415  and Service  420  fields, consider an inbound packet formatted in accordance with packet format  400  as an unrecognizable, illegal or garbled 802.11b packet, or possibly even as interference and would thus ignore the packet. The 22 Mbps transmitter would then back-off to legacy 802.11b CCK 11/5.5 Mbps modes in a known manner to retransmit the data of interest. 
   It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments of this invention without departing from the underlying principles thereof. The scope of the present invention should, therefore, be determined only by the following claims.