Abstract:
A multiuser direct sequence spread spectrum (DSSS) Orthogonal Frequency Division Multiplexing (OFDM) multiband of Ultra Wideband (UWB) communication system for short-distance wireless broadband communications is disclosed for indoor environment operations. Eleven frequency bands are employed, with each of the frequency bands having 650 MHz bandwidths. A 1024-point IFFT and FFT with 1,000 subcarriers are used to carry data and pilots within each of the frequency bands. The multiuser DSSS-OFDM multiband of the UWB communication system can transmit N different users at the same time by using a unique spreading sequence for each of the N different users. A QPSK modulation is used for a different data rate with scalability. The maximum transmitting data rate of the UWB communication system can achieve about 5.541 Gbps.

Description:
BACKGROUND 
   This invention is generally relative to a multiuser direct sequence spread spectrum (DSSS) Orthogonal Frequency Division Multiplexing (OFDM) multiband based Ultra Wideband (UWB) Communications for short-distance wireless broadband communications. 
   U.S. Federal Communications Commission (FCC) released a revision of Part 15 of Commission&#39;s rules regarding UWB transmission systems to allow marketing and operation of certain types of new products incorporating UWB technology on Apr. 22, 2002. Using spectrums occupied by existing radio service, an UWB device can operate without causing interference, thereby permitting scarce spectrum resources to be used more efficiently. Thus, it is feasible that the UWB technology is able to provide significant benefits for Government, public safety, businesses and consumers within an operation spectrum. 
   In general, FCC is adapting unwanted emission limits for UWB communication devices that are significantly more stringent than those imposed on other Part 15 devices. In indoor environments of UWB operations, FCC allows a wide variety of the UWB communication devices, such as high-speed home and business networking devices, subject to certain frequency and power limitations. An emission limitation is −10 dBm for indoor UWB operations. The UWB communication devices must operate in the frequency band from 3.1 GHz to 10.6 GHz. In addition, the UWB communication devices should satisfy the Part 15.209 emission mask limitations for the frequency band below 960 MHz and above 960 MHz. 
   For the UWB communication devices operating in indoor environments, Table 1 lists FCC restrictions of the emission masks (dBm) along with the frequency bands (GHz) as follows: 
   
     
       
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Frequency (MHz) 
               EIRP (dBm) 
             
             
                 
                 
             
           
           
             
                 
                0-960 
               −41.3 
             
             
                 
                960-1610 
               −75.3 
             
             
                 
               1610-1990 
               −53.3 
             
             
                 
               1990-3100 
               −51.3 
             
             
                 
                3100-10600 
               −41.3 
             
             
                 
               Above 10600 
               −51.3 
             
             
                 
                 
             
           
        
       
     
   
   FCC defines an UWB communication device where a fractional bandwidth is greater than 0.25 given by, 
                   FB   =     2   ⁢     (         f   H     -     f   L           f   H     +     f   L         )         ,           (   1   )               
where f H  is the upper frequency of −10 dBm emission point, and f L  is the lower frequency of −10 dBm emission point. The center frequency F c  of an UWB transmission system is obtained by using average of the upper and lower −10 dBm points as follows:
 
                   F   C     =           f   H     +     f   L       2     .             (   2   )               
Furthermore, a minimum frequency bandwidth of 500 MHz must be used for any indoor UWB communication devices regardless of the center frequency.
 
   As can be seen, the UWB communication devices must be designed in such a way that the indoor UWB operations can only occur in the indoor environments according to indoor UWB emission masks given in Table 1. The UWB communication devices can be used for wireless broadband communications, particularly for a short-range high-speed data transmission that can be considered as broadband access to networks. 
   Given an frequency band from 3.1 GHz to 10.6 GHz as a single frequency band, an analog-to-digital (A/D) converter and a digital-to-analog (D/A) converter must operate at a very high sampling rate F s  so that an UWB communication receiver can be implemented in a digital domain. This leads to a high requirement for the A/D and D/A converters for UWB transmitter and receiver. Presently, developing such very high-speed A/D and D/A converters may not be possible with a reasonable cost. Thereby, it is a difficult problem to apply the A/D and the D/A converters directly for an UWB communication transceiver based on a single frequency band solution. On the other hand, a single frequency band-based UWB communication transceiver does not have a flexibility and scalability for transmitting and receiving a user data. In addition, the single frequency band-based UWB communication transceiver may have an interference with a wireless local area network (WLAN) 802.11a transceiver without using a special filter system or other approaches since the WLAN 802.11a transceiver operates at a lower U-NII frequency range from 5.15 GHz to 5.35 GHz and at an upper U-NII upper frequency range from 5.725 GHz to 5.825 GHz. 
   An OFDM is an orthogonal multicarrier modulation technique that has been extensively used in a digital audio and video broadcasting, and the WLAN 802.11a. The OFDM has its capability of multifold increasing symbol duration. With increasing the number of subcarriers, the frequency selectivity of a channel may be reduced so that each subcarrier experiences flat fading. Thus, an OFDM approach has been shown in a particular useful for wireless broadband communications over fading channels. 
   A DSSS approach is to use a pseudorandom (PN) sequence to spread a user signal. The PN sequence is a stream of binary ones and zeros referred to as chips rather than bits. The DSSS approach can be used to separate signals coming from multiusers. Multiple access interference (MAI) among multiusers can be avoided if a set of PN sequences is designed in such a way that a low crosscorrelation among the PN sequences is obtained. 
   The multiuser DSSS-OFDM multiband for UWB communications is disclosed herein according to some embodiments of the present invention. The present invention uses eleven frequency bands as a multiband, each of the frequency bands having 650 MHz bandwidths along with OFDM modulation for a multiuser UWB communication transceiver. A multiband OFDM solution allows using a low speed of the A/D and D/A converters. Moreover, a unique of the PN sequences is assigned to each user so that the multiusers can share the same each of the frequency bands to transmit and receive data based on OFDM multiband of UWB technologies. On the other hand, since the OFDM is an orthogonal multicarrier modulation, subcarriers within each of the frequency bands may be flexibility turned on or off. This can lead to avoid the interference with the WLAN 802.11a transceiver during the indoor UWB operations. In addition, the present invention of the multiuser DSSS-OFDM multiband for UWB communications has a scalability to transmit and receive from a data rate of 503.732 Mbps by using only one of the frequency bands to the data rate of 5.541 Gbps by using all of the eleven frequency bands (or a multiband). 
   Thus, there is a continuing need of the multiuser DSSS-OFDM multiband for an UWB communication transceiver employing an new architecture of the PN sequences, OFDM multicarrier multiband, and filtering for the indoor UWB operations. 
   SUMMARY 
   In accordance with one aspect, a multiuser DSSS-OFDM multiband of UWB communication transmitter may comprise a multiuser encoding and spreading unit, a multiband splitter, an inverse fast Fourier transform (IFFT) unit, a filtering unit, and a multiband multicarrier modulation. 
   Other aspects are set forth in the accompanying detailed description and claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a multiuser DSSS-OFDM multiband of UWB communication system with different user UWB mobile stations and a single UWB base station according to some embodiments. 
       FIG. 2  is a block diagram of a multiuser DSSS-OFDM multiband for an UWB base station communication transmitter according to some embodiments. 
       FIG. 3  is a detailed block diagram of a multiband splitter according to some embodiments. 
       FIG. 4  is a detailed block of a 1024-point IFFT employing 1000 subcarriers and 24 NULLs according to some embodiments. 
       FIG. 5  is a detailed block diagram of a filtering section according to some embodiments. 
       FIG. 6  is a detailed block diagram of a multiband multicarrier modulation according to some embodiments. 
       FIG. 7  is a detailed block diagram of a multiband quadrature phase shift keying (QPSK) modulation according to some embodiments. 
       FIG. 8  is a detailed QPSK constellation with a mapping relationship of bits and phases. 
       FIG. 9  is a frequency spectrum output of the multiuser DSSS-OFDM multiband of the UWB base station communication transmitter for the indoor UWB operations according to one embodiment. 
       FIG. 10  is a block diagram of a multiuser DSSS-OFDM multiband of an UWB mobile communication receiver for a single user according to some embodiments. 
       FIG. 11  is a detailed block diagram of a combination subsection including an analog bandpass filter, multiband QPSK down converters and demodulations, A/D converters, and digital receiver filters according to some embodiments. 
       FIG. 12  is a detailed block diagram of a multiband QPSK demodulation and down converter according to some embodiments. 
       FIG. 13  is a detailed block diagram of a combination subsection including a fast Fourier transform (FFT) and frequency-domain equalizers (FEQ) according to some embodiments. 
       FIG. 14  is a detailed block diagram of a multiband combination according to some embodiments. 
       FIG. 15  is a detailed block diagram of a despreading, deinterleaver, and decoding unit for a single user of the UWB mobile communication receiver according to some embodiments. 
   

   DETAILED DESCRIPTION 
   Some embodiments described herein are directed to the multiuser DSSS-OFDM multiband of an UWB communication system for the indoor UWB operations. The multiuser DSSS-OFDM multiband of UWB communication system may be implemented in hardware, such as in an Application Specific Integrated Circuits (ASIC), digital signal processor, field programmable gate array (FPGA), software, or a combination of hardware and software. 
   Multiuser DSSS-OFDM Multiband of UWB System 
   A multiuser DSSS-OFDM multiband of UWB communication system  100  for the indoor UWB operations is illustrated in  FIG. 1  in accordance with one embodiment of the present invention. UWB mobile stations from  110   a  to  110   p  can communicate with an UWB base station  140  to transmit and receive information data through the frequency bands in an indoor environment simultaneously. An UWB mobile station  110   a  transmits and receives the data through its antenna  120   a  into air, and communicates with the UWB base station  140  through an antenna  130 . In a similar way, other UWB mobile stations from  110   b  to  110   p  also transmits and receives the data through their antennas from  120   b  to  120   p , respectively, and communicate with the UWB base station  140  through the antenna  130  as well. The UWB base station  140  is coupled to an UWB network interface section  142  that is connected with an UWB network  144 . 
   Each of the UWB mobile stations from  110   a  to  110   p  uses a unique PN sequence to spread and despread a user source signal. Knowing all of the PN sequences of the UWB mobile stations from  110   a  to  110   p , the UWB base station  140  can transmit and receive all of the data from all of the UWB mobile stations from  110   a  to  110   p  based on an OFDM multiband solution by spreading and despreading of user PN sequences, respectively. The multiuser DSSS-OFDM multiband of the UWB communication system uses a QPSK modulation and multicarrier within each of the frequency bands to transmit and receive a data rate of 503.732 Mbps on one single frequency band up to the data rate of 5.541 Gbps on all of the eleven frequency bands. As a result, the multiuser DSSS-OFDM multiband of the UWB communication system  100  can transmit and receive a maximum data rate at 5.541 Gbps by using all of the eleven frequency bands simultaneously. 
   UWB Base Station Transmitter Architecture 
     FIG. 2  is a block diagram of the multiuser DSSS-OFDM multiband of UWB base station transmitter architecture  200  for the indoor UWB operations according to some embodiments. There are a number of p users from a user-1 bitstream  210   a  to a users bitstream  210   p , respectively. The user-1 bitstream  210   a  is coupled to a ½-rate convolution encoder  212   a , which is connected to an interleaver  214   a . Using the unique PN sequence of a user-1 key  218   a  spreads the output sequence of the interleaver  214   a . In a similar way, the users bitstream  210   p  is coupled to a ½-rate convolution encoder  212   p  that is connected to an interleaver  214   p . Using the unique PN sequence of a user-p key  218   p  spreads the output sequence of the interleaver  214   p . In addition, all of the PN sequences are orthogonal each other. This means that a cross-correlation between one PN sequence and other PN sequences is almost zero, while a self-correlation of a user PN sequence is almost equal to one. Then, the p output sequences from the interleaver  214   a  to the interleaver  214   p  in a parallel operation are added together to form a serial sequence output by using a sum over block duration  220 . The serial output of the sum over block duration  220  is converted into eleven parallel sequences by using a multiband splitter  230  (see the detail illustration of the multiband splitter  230  in  FIG. 3 ). Thus, the first of the output sequence from the multiband splitter is converted into a 512-parallel sequence by suing an serial-to-parallel (S/P)  240   a . The 512-parallel sequence is formed to 512-parallel complex sequence with a symmetric conjugate. The 512-parallel complex sequence is passed through an IFFT  242   a  to produce a 1024-parallel real sequence. The IFFT  242   a  is coupled to a guard  244   a  to insert 256 samples as a guard interval for the output sequence of the IFFT  242   a . As a result, the output of the guard  244   a  is a 1280-parallel real sequence. Then, the 1280-parallel real sequences are passed through a filtering section  246   a  to produce even and odd modulated signal sequences. Carriers multiply the even and odd modulated signal sequences of the filtering section  246   a  by using a multiband multicarrier modulation  250 . In the same way, the eleventh of the output sequence from the multiband splitter  230  is converted into a 512-parallel sequence by suing an S/P  240   k . The 512-parallel sequence is formed to 512-parallel complex sequence with the symmetric conjugate. The 512-parallel complex sequence is passed through an IFFT  242   k  to produce a 1024-parallel real sequence. The IFFT  242   k  is coupled to a guard  244   k  to insert 256 samples as a guard interval for the output sequence of the IFFT  242   k . Thus, the output of the guard  244   k  is a 1280-parallel real sequence. The guard interval is used to avoid an intersymbol interference (ISI) between IFFT frames. Then, the 1280-parallel real sequences are passed through a filtering section  246   k  to produce even and odd modulated signal sequences. Carriers multiply the even and odd modulated signal sequences of the filtering section  246   k  by using a multiband multicarrier modulation  250 . Finally, the eleven paralleled output signal sequences of the multiband multicarrier modulation  250  are added together and passed through a power amplifier (PA)  260  into air. 
   Referring to  FIG. 3  is a detailed block diagram  300  of a multiband splitter ( 230 ) architecture according to some embodiments. The multiband splitter ( 230 ) architecture includes ten sample delay units from  310   a  to  310   k , eleven down sample units from  320   a  to  320   k , eleven random access memory (RAM) units from  330   a  to  330   k , and a modular counter  340 . An input sequence of a length of N data is divided into eleven parallel sequences with a length of N/11 data by using the sample delays from  310   a  to  310   j  and the down samples of  320   a  to  320   k . The eleven output sequences of the down sample units from  320   a  to  320   k  are stored into RAM memories of  330   a  to  330   k . A row size of each of the RAM units from  330   a  to  330   k  is  512  and the number of bits in each row can be programmed. A modular counter is used to control an address of the RAM units from  330   a  to  330   k  for storing input sequence and sending out output sequence. 
   Referring to  FIG. 4  is a detailed block diagram  400  of a 1024-point IFFT  410  ( 242 ) according to some embodiments. There are 24 Nulls including # 0  (DC), and # 501  to # 523 . The rest of the input # 0  (DC) and # 501  to # 523  are set to zero. The coefficients from 1 to 500 are mapped to the same numbered IFFT inputs from # 1  to # 500 , while the coefficients from 500 to 1 are also copied into IFFT inputs from # 524  to # 1023  to form a complex conjugate. Thus, there are a total of 1,000 subcarriers for transmitting data and pilot information. In order to make a coherent detection robust against frequency offsets and phase noise, eight of the 1,000 subcarriers are dedicated to pilot signals that are assigned into the subcarriers of # 100 , # 200 , # 300 , # 400 , and # 624 , # 724 , # 824 , and # 924 . These pilots are binary phase-shift keying (BPSK) modulated by a pseudo binary sequence to prevent a generation of spectral lines. In this case, other  992  subcarriers of each OFDM are dedicated to assign for transmitting data information. After performing a 1024-point IFFT, an output of the 1024-point IFFT is cyclically extended to a desired length in each of the multiband. 
   Table 2 lists data rate-dependent parameters of the 1024-point IFFT operation for each of the frequency bands. 
   
     
       
             
             
             
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
                 
                 
                 
               Coded bits 
               Coded bits 
               Data bits 
             
             
               Data rate 
               Modula- 
               Coding 
               per sub- 
               per OFDM 
               per OFDM 
             
             
               (Mbits/s) 
               tion 
               rate 
               carrier 
               symbol 
               symbol 
             
             
                 
             
           
           
             
               503.732 
               QPSK 
               ½ 
               2 
               1983.998 
               991.999 
             
             
                 
             
           
        
       
     
   
   Table 3 shows the 1024-point IFFT of timing-related parameters for each of the frequency bands. 
   
     
       
             
             
             
           
             
             
             
             
           
         
             
               TABLE 3 
             
             
                 
             
             
               Parameters 
               Descriptions 
               Value 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               N ds   
               Number of data subcarriers 
               992 
                 
             
             
               N ps   
               Number of pilot subcarriers 
               8 
             
             
               N ts   
               Number of total subcarriers 
               1000 
             
             
               D fs   
               Frequency spacing for subcarrier 
               0.6347 
               MHz 
             
             
                 
               (650 MHz/1024) 
             
             
               T FFT   
               IFFT/FFT period (1/D fs ) 
               1.5755 
               μs 
             
             
               T gd   
               Guard duration (T FFT /4) 
               0.3938 
               μs 
             
             
               T signal   
               Duration of the signal BPSK-OFDM 
               1.9693 
               μs 
             
             
                 
               symbol (T FFT  + T gd ) 
             
             
               T sym   
               Symbol interval (T FFT  + T gd ) 
               1.9693 
               μs 
             
             
               T short   
               Short duration of training sequence 
               3.938 
               μs 
             
             
                 
               (10 × T FFT /4) 
             
             
               T gd2   
               Training symbol guard duration (T FFT /2) 
               0.7877 
               μs 
             
             
               T long   
               Long duration of training sequence 
               3.938 
               μs 
             
             
                 
               (2 × T FFT  + T gd2 ) 
             
             
               T preamble   
               Physical layer convergence procedure 
               7.876 
               μs 
             
             
                 
               preamble duration (T short  + T long ) 
             
             
                 
             
           
        
       
     
   
     FIG. 5  is a detailed block diagram  500  of a filtering section ( 246 ) according to some embodiments. A switch unit  510  including two switches of  520   a  and  520   b  is used to split a serial data sequence into two parallel data sequences with an even and an odd number, respectively. The switch  520   a  rotates to the even number of data (for example, b 2 , b 4 , b 6 , . . . ) to form a serial even data sequence, and the switch  520   b  rotates to the odd number of data (for example, b 1 , b 3 , b 5 , . . . ) to form a serial odd data sequence. Using a transmitter shaped filter  540   a  to shape a transmitter spectrum and limit the frequency band filters serial even sequences of the switch  520   a  output. The output of the transmitter shaped filter  540   a  is passed through a D/A converter  550   a , which is coupled to an analog reconstruction-filter  560   a . The analog reconstruction-filter  560   a  does a smooth of signal of the D/A converter  550   a  output. In the same way, using a transmitter shaped filter  540   b  to shape the transmitter spectrum and limit the frequency band filters output of the serial odd sequences of the switch  520   b . The output of the transmitter shaped filter  540   b  is passed through a D/A converter  550   b  that is coupled to an analog reconstruction-filter  560   b . The analog reconstruction-filter  560   b  does smooth of the signal of the D/A converter  550   b . A bit detector  530  identifies a value data either “0” or “1” from the output of the switch  520   a  and the switch  520   b . The bit detector  530  is used to control a multiband QPSK modulation. 
   Referring to  FIG. 6  is a detailed block diagram  600  of a multiband multicarrier modulation ( 250 ) according to some embodiments. Eleven analog signals of the output of the analog reconstruction-filters in parallel are passed through eleven multiband QPSK modulations from  610   a  to  610   k  in parallel. The bit detectors from  530   a  to  530   k  are used to control the multiband QPSK modulations from  610   a  to  610   k , respectively. The output signals of the multiband QPSK modulations from  610   a  to  610   k  are coherently added together by using a sum unit  620 . Then, the output of the sum unit  620  is passed through an analog bandpass filter  630  to produce bandlimited signals for an UWB communication transmitter. 
   Referring to  FIG. 7  is a detailed block diagram  700  of a multiband QPSK modulation ( 610 ) according to some embodiments. The analog signals from the even and odd sequences in parallel are multiplied with carriers from an output of a multi-oscillator  710  by using multiplier units from  730   a  and  730   b . The multi-oscillator  710  contains four carriers: sin(2πf i t), −sin(2πf i t), cos(2πf i t), and −cos(2πf i t). A switch  720   a  is used to connect with either a position of  712   a  or a position of  712   b . In the same way, a switch  720   b  is used to connect with either a position of  714   a  or a position of  714   b . Using the bit detector  530  (as shown in  FIG. 6 ) controls both of the switches  720   a  and  720   b . The switch  720   a  connects to the position of  712   a  when the bit detector  530  identifies “00” bits from the output of the switches  520   a  and  520   b  as shown in  FIG. 5 . The switch  720   a  connects to the position of  712   b  when the bits detector  530  identifies “10” bits from the output of the switches  520   a  and  520   b  in  FIG. 5 . In a similar way, the switch  720   b  connects to the position of  714   b  if the bit detector  530  identifies “01” bits from the output of the switches  520   a  and  520   b  in  FIG. 5 . The switch  720   b  connects to the position of  714   a  if the bit detector  530  identifies “11” bits from the output of the switches  520   a  and  520   b  in  FIG. 5 . Then, a switch  740  rotates either a position of  730   a  or a position of  730   b . The bit detector  530  also controls the switch  740 . When the bit detector  530  identifies either “00” or “10” bits from the output of the switches  520   a  and  520   b , the switch  740  connects to the position of  730   a . When the bit detector  530  identifies either “01” or “11” bits from the output of the switches  520   a  and  520   b , the switch  740  connects to the position of  730   b . In this case, the outputs of the switch  740  are a QPSK modulation. 
   Referring to  FIG. 8  is a detailed QPSK mapping relationship  800  according to two-bit information. A QPSK constellation  810  contains four mapping points, two points on the I-axis and tow points on the Q-axis. A mapping relationship of a bit pattern and a phase  820  contains four bit patterns along with corresponding four-phase information. The bit patterns of “00”, “01”, “10”, and “11” represent “0”, “π/2”, “π”, and “3π/2” phases, respectively. 
   Output Spectrum of UWB Base Station Transmitter 
     FIG. 9  is an output frequency spectrum  900  of the multiuser DSSS-OFDM multiband of UWB base station communication transmitter, including eleven frequency band spectrums from  920 A to  920 K according to some embodiments. A FCC emission limitation  910  of the indoor UWB operations is also shown in  FIG. 9 . Each frequency bandwidth of the eleven frequency band spectrums from  920 A to  920 K for a transmitter is 650 MHz with different carrier frequencies under the FCC emission limitation  910 . The detail positions of the frequency band spectrums (dBm) for the UWB communication transmitter along with the center, lower and upper frequencies (GHz) as well as corresponding channel frequency bandwidth (MHz) are listed in Table 4: 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 4 
             
             
                 
             
             
                 
               Center 
               Lower 
               Upper 
               Frequency 
             
             
                 
               Frequency 
               Frequency 
               Frequency 
               Bandwidth 
             
             
               Multichannel Label 
               (GHz) 
               (GHz) 
               (GHz) 
               (MHz) 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               920A 
               3.45 
               3.125 
               3.775 
               650 
             
             
               920B 
               4.10 
               3.775 
               4.425 
               650 
             
             
               920C 
               4.75 
               4.425 
               5.075 
               650 
             
             
               920D 
               5.40 
               5.075 
               5.725 
               650 
             
             
               920E 
               6.05 
               5.725 
               6.375 
               650 
             
             
               920F 
               6.70 
               6.375 
               7.025 
               650 
             
             
               920G 
               7.35 
               7.025 
               7.675 
               650 
             
             
               920H 
               8.00 
               7.675 
               8.325 
               650 
             
             
               920I 
               8.65 
               8.325 
               8.975 
               650 
             
             
               920J 
               9.30 
               8.975 
               9.625 
               650 
             
             
               920K 
               9.95 
               9.625 
               10.275 
               650 
             
             
                 
             
           
        
       
     
   
   During the indoor UWB operation, the fourth and/or fifth frequency band (labeled with  920 D and/or  920 E in  FIG. 9 ) of the multiuser DSSS-OFDM multiband of UWB base station transmitters can be turned off in order to avoid an interference with a WLAN 802.11a lower U-NII frequency band and/or upper U-NII frequency band. In some cases, the multiuser DSSS-OFDM of the UWB base station and mobile transmitters can further turn off some subcarriers within the OFDM in the fourth and/or fifth frequency band if the WLAN 802.11a only uses certain subchannels in the lower U-NII or in the upper U-NII frequency bands. 
   UWB Mobile Receiver Architecture 
     FIG. 10  is a block diagram of a DSSS-OFDM multiband of UWB mobile communication receiver  1000  for the indoor UWB operations according to some embodiments. A low noise amplifier (LNA)  1010 , which is coupled to an automatic gain control (AGC)  1012 , receives the multiuser DSSS-OFDM multiband-based UWB signals from an antenna  130  (as shown in  FIG. 1 ). The output of the LNA  1010  is passed through the AGC  1012  to adjust amplitude of the multiuser DSSS-OFDM multiband-based UWB signals for a multiband multicarrier down converter and demodulation  1020 . The eleven bandlimited multiuser DSSS-OFDM multiband of UWB analog signals of an output multiband multicarrier down converter and demodulation  1220  are in parallel sampled and quantized by using an A/D converter unit  1022 , with a sampling rate at 720 MHz. A software and time control  1070  is used to control the AGC  1012 , the multiband multicarrier down converter and demodulation  1020 , and the A/D converter unit  1022 . Using a digital receiver filter unit  1024  to remove out of band signals filters the digital signals of output of the A/D converter unit  1022 . The output digital signals of the digital receiver filter unit  1024  are passed through a time-domain equalizer (TEQ)  1026 . The TEQ  1026  is used to reduce the length of cyclic prefix to a more manageable number without reducing performance significantly. In other words, the TEQ  1026  can produce a new target channel with a much smaller effective constraint length when concatenated with the channel. Thus, the outputs of the TEQ  1026  in parallel are passed through a set of S/Ps from  1030   a  to  1030   k  to produce parallel digital sequences. Each of the S/Ps from  1030   a  to  1030   k  produces 1280 parallel digital sequences for each of guard removing units from  1032   a  to  1032   k . The guard removing units from  1032   a  to  1032   k  remove 256 samples from the 1280 parallel digital sequences of the S/Ps from  1030   a  to  1030   k  to produce 1024 parallel digital sequences, which are used as inputs for FFT units from  1034   a  to  1034   k . Each of the FFT units from  1034   a  to  1034   k  produces 512 frequency-domain signals that are used for frequency-domain equalizer (FEQ) units from  1036   a  to  1036   k . The FEQ units from  1036   a  to  1036   k  are used to compensate for phase distortions, which are a result of phase offsets between sampling clocks in the transmitter and the receiver of the multiuser DSSS-OFDM multiband of UWB communication transceiver. This is because phases of the received outputs of the multiband FFT units from  1034   a  to  1034   k  are unlikely to be exactly the same as the phases of the transmitter symbols at the input to the IFFT units from  242   a  to  242   k  of the multiuser DSSS-OFDM multiband of base station UWB transmitter (as shown in  FIG. 2 ). Thus, the outputs of the FEQ units from  1038   a  to  1038   k  are passed through a set of parallel-to-serial (P/S) units from  1038   a  to  1038   k  to produce a serial sequence for all of the eleven frequency bands. All of the serial sequences of the output of the P/S units from  1038   a  to  1038   k , with each sequence length of M, are added together to produce a sequence length of 11M by using a multiband combination  1040  (detailed architecture as shown in  FIG. 14 ). The output sequence of the multiband combination  1040  is passed through a despreading, deinterleaver, and decoding unit  1050 . The despreading, deinterleaver, and decoding unit  1050  performs despreading, deinterleaving and decoding for the multiuser DSSS-OFDM multiband of UWB mobile communication receiver. 
   Referring to  FIG. 11  is a detailed block diagram  1100  of one combination subsection  1028  including an analog bandpass filter  1110 , eleven multiband QPSK down converters and demodulations from  1120   a  to  1120   k , twenty-two A/D converters from  1130   a  to  1130   v , and twenty-two digital receiver filters from  1140   a  to  1140   v  according to some embodiments. The input signal of the AGC  1012  output (as shown in  FIG. 10 ) is passed through the analog bandpass filter  1110 , which is used to eliminate the out of band images. The output of analog signals of the analog bandpass filter  1110  is in parallel passed through the eleven multiband QPSK down converters and demodulations from  1120   a  to  1120   k . Each of the multibands QPSK down converters and demodulations from  1120   a  to  1120   k  produces two analog signals as input signals for each of the A/D converters from  1130   a  to  1130   v . The output digital signals of the A/D converters from  1130   a  to  1130   v  are in parallel passed through the digital receiver filters from  1140   a  to  1140   k  to produce the desired digital signals for a multiuser DSSS-OFDM multiband of UWB mobile receiver. All of the A/D converters from  1130   a  to  1130   v  use the same bit resolution and the same sampling rate. In a similar way, all of the digital receiver filters from  1140   a  to  1140   v  have the same filter attenuations and filter bandwidths with the same filter coefficients and a linear phase. 
   Referring to  FIG. 12  is a detailed block diagram  1200  showing the multiband QPSK down converter and demodulation  1120  according to some embodiments. The input signal r(t) of the analog bandpass filter  1110  output is passed through two multipliers  1210   a  and  1210   b  at the same time. The analog signal r(t) is multiplied with cos(2πf i t) by using the multiplier  1210   a  to produce an analog baseband signal r 1 (t). In the same way, the analog signal r(t) is multiplied with sin(2πf i t) by using the multiplier  1210   b  to produce an analog baseband signal r 2 (t). Then anti-aliasing analog filters  1220   a  and  1220   b  sort both of the analog baseband signals r 1 (t) and r 2 (t) to produce the bandlimited analog signals for the A/D converters. 
     FIG. 13  is a detailed block diagram  1300  of a combination subsection including the FFT  1034  and the FEQ  1036  according to some embodiments. The FFT  1034  has a 1024-point input of a real-value and produces a 512-point complex data with labels from 0 to 511, while a 512-point complex data with labels from 511 to 1023 is disable. The FFT  1034  with labels from 0 to 511 also contains 12 Nulls. So, the FFT  1034  produces a 500-point complex data for the FEQ  1036 . The FEQ  1036  contains 500 equalizers from  1310   a  to  1310   z , 500 decision detectors from  1320   a  to  1320   z , and 500 subtractions from  1330   a  to  1330   z  that operate in parallel. Each of the equalizers from  1310   a  to  1310   z  has a N-tap with an adaptive capability. Each of the decision detectors from  1320   a  to  1320   z  is a multi-level threshold decision. Each of the subtractions from  1330   a  to  1330   z  performs subtracting between the output of each of the equalizers from  1320   a  to  1320   z  and the output of each of the decision detectors from  1320   a  to  1320   z . The output of each of the subtraction from  1330   a  to  1330   z  is referred to as an error signal, which is used to adjust the N-tap of the each of the equalizers from  1310   a  to  1310   z  by using an adaptive algorithm  1330 . 
   The phases of the received outputs of the FFT  1034  do not have exactly the same as the phases of the transmitter symbols at the input to the IFFT units from  242   a  to  242   k  of the multiuser DSSS-OFDM multiband of UWB base station transmitter (as shown in  FIG. 2 ). In addition, the phase responses have to be considered with the channel, which is coped with the TEQ  1026  (as shown in  FIG. 10 ). Thus, the FEQ  1036  in  FIG. 13  is used to compensate for the phase distortion that is a result of the phase offset between the sampling clocks in the transmitter and the receiver of the multiuser DSSS-OFDM multiband of UWB communication transceiver. The FEQ  1036  also offers an additional benefit of scaling the received signal before decoding. This is because the FEQ  1036  can be used to adjust a gain of the FFT  1034  output so that the decision detectors from  1320   a  to  1320   z  can be set the same parameters for all subchannels regardless of different subchannel attenuations. 
     FIG. 14  is a detailed block diagram  1400  of multiband combination ( 1040 ) according to some embodiments. The multiband combination ( 1040 ) includes a modular counter of  1410 , eleven RAM memories from  1420   a  to  1420   k , eleven up samples from  1430   a  to  1430   k , and ten sample delays from  1440   a  to  1440   j . Eleven input sequences in parallel are stored into the RAM memories from  1420   a  to  1240   k . A row size of each of the RAM memories from  1420   a  to  1420   k  is 512 and the number of bits in each row can be programmed. The modular counter  1410  is used to control an address of the RAM memories from  1420   a  to  1420   k  for storing input sequences and sending out output sequences. The outputs of the RAM memories from  1420   a  to  1420   k  are interleaved each other to form a serial output sequence. The length size of the serial output sequence is 5,632 per segment, which is used for the despreading, deinterleaving, and decoding unit  1050  (as shown in  FIG. 10 ). 
   Referring to  FIG. 15  is a detailed block diagram  1500  of the despreading, deinterleaving, and decoding unit ( 1050 ) including a despreading  1510 , an user-i key  1520 , deinterleaver  1530 , a decoding  1540 , and a user-i bitstream  1550  according to one embodiment. The output sequences of the multiband combination ( 1040 ) are passed into the despreading  1510  by multiplying a spreading sequence of the user-i key  1520 , which provides a unique key sequence. Cross correlations of the output sequences of the multiband combination ( 1040 ) and the unique key spreading sequence of the user-i key  1520  produce an encoded user-i data bitstream. This encoded user-i data bitstream is then deinterleaved by using the deinterleaver  1530  that is also coupled to the decoding  1540 . The decoding  1540  decodes the encoded user-i data bitstream to produce an original transmitted user-i data bitstream that is stored in the user-i bitstream  1550 . 
   While the present invention has been explained with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. The appended claims cover all such modifications and variations as fall within the true spirit and scope of the present invention.