Abstract:
Techniques of channel correction and demodulation for wireless systems are enhanced so that higher effective data rates, lower error rates or both can be achieved with a minimal processing load. Pilots are adaptively moved and/or removed, and their positions are changed, to enhance the channel estimation, decoding, and demodulation processes at the receiver. Reception is also enhanced by adding, removing, or changing the positions, of information-carrying data bits.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/950,749 filed Dec. 5, 2007, which claims the benefit of U.S. Provisional Application No. 60/869,023 filed Dec. 7, 2006, the contents of which are hereby incorporated by reference herein. 
     
    
     BACKGROUND 
       [0002]    High-performance demodulation of orthogonal frequency division multiplexing (OFDM) signals requires accurate and sufficiently frequent characterization of the time-frequency channel that the signals experience. Once channels are fully characterized or, equivalently, “estimated”, such channel estimates can be used to maximize the effects of the channels in optimum demodulation of signals that underwent the channels. Typically, this process is called “channel equalization”. 
         [0003]    In modern wireless communication systems, where OFDM is the modulation technique of choice, such as the European Computer Manufacturers Association (ECMA)-368 ultra-wideband (UWB) personal area networks (PAN) systems, the characterization of the channels is typically performed by having two sets of signals. The first signal is called the preamble, and its pre-channel signal composition is fully known at the receiver on all time and frequency samples that comprise the preamble signal. Preambles are typically pre-pended before the data part of a packet. Using interpolation of channel estimates obtained from adjacent packets&#39; preambles, an estimate may be obtained of the time-frequency channel that the data part may experience. 
         [0004]    The second type of signal that is useful in further assisting channel characterization and eventual equalization are the pilots. The pilots are known signals that occupy a subset of the entire time-frequency sample space of the post-preamble part of a packet, and typically comprise multiple single or small-subset samples that are regularly dispersed in the time and frequency sample space of the post-preamble part of a packet. Again, using interpolation among the pilot samples, and/or using also the preamble parts, one can obtain an estimate of the channel.  FIG. 1  shows examples of distributing pilots across a typical time-frequency channel space. 
         [0005]    The problems addressed herein are related to the fact that UWB systems a typically subject to impairment by narrower-band signals and noises both natural and man-made, due to their large bandwidth and the abundance of interferences and noise sources that can impact only part of the spectral area of the UWB system. The UWB systems in the prior art do not provide methods where the transmitting side can ‘evade’ known sources of interferers in the time-frequency domain of operation. Nor does the prior art address mitigating the negative impacts of such interferers, as prior art methods use ‘fixed’ placement of reference signals in the time-frequency resource map of the transmitted signal. 
         [0006]    A typical UWB wireless communication channel, due to its large bandwidth, typically is not homogeneous across its operating frequency, even without the presence of explicit interference sources. For example, if two UWB devices move relative to each other, for example, by being carried around by users of UWB equipped mobile devices, the operating frequencies in the channel may experience very different degrees of distortion because the Doppler Effect would affect different sub-frequencies contained the large bandwidth of the channel in different degrees. 
         [0007]    Therefore, for UWB OFDM signals such as those compliant with the ECMA 368 UWB standards, there are multiple places in the frequency domain where the channel may experience drastically increased interference due to presence of narrowband interferers, both man-made and natural, and resulting degradation. The signal-to-noise ratio of such channel sub-spaces, are much lower than in the rest of the channel space. 
         [0008]    One problem of prior art UWB systems is that the transmitters of such systems have ‘fixed’ or ‘invariable’ placement of the training signals in the frequency and time domains. In UWB OFDM systems the fixed allocation or placement of the training-symbol resources that should be reliably detected to assist high-performance reception of the data symbols becomes problematic. 
         [0009]    Another problem of prior art systems is that prior art UWB OFDM systems do not exploit the lack of homogeneity of the channels for data transmission. These systems do not adaptively or variably allocate the data symbols in response to their knowledge of the channel conditions. This results in sub-optimum utilization of the capacity of the channel and is a problem commonly known to many different types of wireless communications systems. However, the effect of this problem is greater for UWB systems, as opposed to narrow band systems, due to their large operating bandwidths and ensuing larger chances of interferences and the larger degree of channel in-homogeneity. 
         [0010]    One way to deal with some of the problems described above is to use error correction coding on the control and data symbols in the transmitter. When channel conditions deteriorate locally and result in errors in symbol detection at the receiver, the presence of redundant, distributed information about the original signal in the transmitted waveform allows the receiver to recover the original symbols by using decoding techniques. 
         [0011]    Another way traditional methods used to alleviate the problems is to allocate a significant portion of time-frequency resources of the transmitted waveform to the known, training signal, so that, even when impairments take place for a subset of the training symbols, the rest of the training symbols may be sufficient to be useful in recovering channel information and assisting the demodulation of the data symbols. Typically in OFDM systems, training signals, comprised of preambles and pilots, can take up 10% or more of the time-frequency resources. However, this method can typically result in over allocation of training signal, since the design of the system would typically have to be done for the worst-case provisioning of the training symbols. The result of such over allocation would be under utilization of the true capacity of the channel, for the transmission of the actual, information-carrying data symbols. 
         [0012]    In order to compensate for the fact that the determined correction values are not perfect for various channel parameters, the parameters used to encode the data are adjusted. Thus, the prior art used adaptive modulation coding (AMC) to adaptively distribute information bits in order to optimally exploit available channel capacity that changes dynamically. In AMC, the transmitter adaptively selects one of many modulation and coding schemes, usually changeable per packet basis, depending on the quality of the channel which the transmitter anticipates its transmitted packet will go through. In the ECMA-358 UWB systems, for example, there are 8 different AMC modes that provide, on a per packet basis, an adaptive method to allocate bits across a packet. 
         [0013]    Table 1 below depicts the data rates available in the ECMA-368 AMC modes. In general, a lower coding rate (i.e., data/all symbols) and a lower coded bit per symbol rate improve the likelihood of correct decoding in the presence of signal distortion. The distortion could of course also be due to factors such as noise or interferers, which is not directly addressed by the compensating for the channel parameters. It is indirectly addressed in that the compensation is biased towards the desired signals and probably makes the undesired more randomized. The negative of using either approach is that the effective data rate of quadrature phase shift keying (QPSK) modulation and double carrier modulation (D CM) is impacted as shown in the left most column of Table 1. 
         [0000]    
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Coded Bits/ 
                 Info Bits/6 
               
               
                 Data 
                   
                 Coding 
                   
                   
                 6 OFDM 
                 OFDM 
               
               
                 Rate 
                   
                 Rate 
                   
                   
                 Symbol 
                 Symbol 
               
               
                 (Mb/s) 
                 Modulation 
                 (R) 
                 FDS 
                 TDS 
                 (N CBP6S ) 
                 (N IBP6S ) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 53.3 
                 QPSK 
                 1/3 
                 YES 
                 YES 
                 300 
                 100 
               
               
                 80 
                 QPSK 
                 1/2 
                 YES 
                 YES 
                 300 
                 150 
               
               
                 106.7 
                 QPSK 
                 1/3 
                 NO 
                 YES 
                 600 
                 200 
               
               
                 160 
                 QPSK 
                 1/2 
                 NO 
                 YES 
                 600 
                 300 
               
               
                 200 
                 QPSK 
                 5/8 
                 NO 
                 YES 
                 600 
                 375 
               
               
                 320 
                 DCM 
                 1/2 
                 NO 
                 NO 
                 1 200   
                 600 
               
               
                 400 
                 DCM 
                 5/8 
                 NO 
                 NO 
                 1 200   
                 750 
               
               
                 480 
                 DCM 
                 3/4 
                 NO 
                 NO 
                 1 200   
                 900 
               
               
                   
               
             
          
         
       
     
         [0014]    The disadvantages of the prior are in continuous channel estimation and signal extraction in UWB OFDM systems are: 
         [0015]    1) lack of adaptive allocation of training signals within a packet; and 
         [0016]    2) lack of adaptive allocation of information bits on the time-frequency channel plane within a packet. 
         [0017]    As to the lack of adaptive allocation of training signals within a packet, traditional UWB systems such as the ECMA-368 system use methods of fixed allocation of training signals such as the pilots or the preambles, regardless of the condition of the channel. Therefore, they can experience impairments to the training signals if the allotted training signals are placed in the parts of the time-frequency channel space where significant impairments take place. 
         [0018]    As to the lack of adaptive allocation of information bits on the time-frequency channel plane within a packet, due to the very wide bandwidths and also due to the fact that these devices are able to roam due to their small form-factors and usage models, (e.g., attached to a personal digital assistant (PDA) and carried around all the time, a WTRU, and the like), the UWB devices can easily face narrower-band interferences, both man-made and natural, that may vary in time and frequency. The transmitters in the current UWB OFDM systems do not allocate the information-carrying data bits within a packet, adaptively according to the changing or anticipated conditions of the communication channel. 
         [0019]    However, it is known that UWB channels may undergo impairments on certain time-frequency slots on a time-varying basis, even during the relatively short time period spanning individual packets. If such impairments take place on a large-scale basis, for example, in a significant portion of the time-frequency plane, and in a way that impairs the optimal use of the previously allocated preambles and pilots, large portions of data, even when strongly encrypted using channel codes, may not be recoverable at the receiver. 
       SUMMARY 
       [0020]    Techniques of channel correction and demodulation for OFDM systems are enhanced so that higher effective data rates and/or lower error rates can be achieved with a minimal processing load. Pilots are adaptively moved and/or removed, and their positions are changed, to enhance the channel estimation, decoding, and demodulation processes at the receiver. Reception is also enhanced by adding, removing, or changing the positions, of information-carrying data bits. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0021]    The foregoing summary and the following detailed description will be better understood when read with reference to the appended drawings, wherein: 
           [0022]      FIG. 1  shows an example of pilot distribution across a typical time-frequency channel space; 
           [0023]      FIG. 2  shows conventional ECMA-368 frame structure; 
           [0024]      FIG. 3  shows a simplified ECMA-368 frame structure; 
           [0025]      FIG. 4  shows a representation of moving pilots and data in the frequency domain; 
           [0026]      FIG. 5  is a representation of a physical layer convergence protocol (PLCP) header in ECMA-368 UWB orthogonal frequency domain multiplexed packet format; 
           [0027]      FIG. 6  is a representation of pilot removal and addition in the frequency domain; 
           [0028]      FIG. 7  depicts how a transmitter append and scrambling unit is used to format a scrambled physical layer service data unit (PSDU) in ECMA-368; 
           [0029]      FIG. 8  shows an example of a transmitter; 
           [0030]      FIG. 9  shows an example of a receiver; and 
           [0031]      FIG. 10  is an example of a block diagram of an OFDM transmitter. 
       
    
    
     DETAILED DESCRIPTION  
       [0032]    When referred to hereafter, the terminology “wireless transmit/receive unit (WTRU)” includes but is not limited to a user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, a cellular telephone, a personal digital assistant (PDA), a computer, or any other type of user device capable of operating in a wireless environment. When referred to hereafter, the terminology “base station” includes but is not limited to a Node-B, a site controller, an access point (AP), or any other type of interfacing device capable of operating in a wireless environment. 
         [0033]    For the purpose of illustration, the general structure of the ECMA-368 standard is presented. It will be recognized that this is just one implementation, and can be extended to other implementations with a change in values of certain parameters while still falling under the scope of this disclosure. 
         [0034]      FIG. 2  shows and example of a physical-layer frame structure of an ECMA-368 UWB OFDM system. The ECMA-368 frame consists of a physical-layer convergence protocol (PLCP) preamble 705, a PLCP header 710, and a PSDU 715.  FIG. 2  is not to scale, and, for illustrative purposes, the PLCP preamble 705 and the PLCP header 710 are exaggerated. The PLCP preamble 205 and the PLCP header 210 may be used as training sequences. 
         [0035]      FIG. 3  is a simplified view of the physical ECMA-368 UWB OFDM systems PSDU frame. Prior art methods consider the use of sliding windows that move in both time and frequency directions for best interpolation and seeding. It should be noted that the use of fixed-frequency interleaving (FFI) is assumed where all the frames from a UWB station are transmitted on the same 528 MHz frequency band. A system using time-frequency interleaving (TFI) may be used as well. 
         [0036]    The basic constituents of  FIG. 3  are: 
         [0037]    1) In the horizontal time domain, a frame is flanked by frames before and after it. The time-domain pattern is repeated in both positive (future) and negative (past) directions until the end of the frame boundary is reached. 
         [0038]    2) In the vertical frequency domain, pilot sub-channels flank nine data sub-channels. The frequency domain pattern described above is repeated in both positive and negative directions until the guard bands at the end of the frequency band are reached. 
         [0039]    3) Pilots occupy continuous time slots in the Pilot sub-channel in the frequency domain. The Data sub-channels use the rest of the sub-channels in the frequency domain. 
         [0040]    4) Frame preambles, consisting of the PLCP Preamble and PLCP Header in ECMA-368 systems, could be used for preparatory processing, i.e., where the very initial ‘seed’ for the channel matrix separation procedures, performed in a receiver, for the blind signal extraction in the data portion of the signals is obtained. 
         [0041]    5) The boundary between the payload and pad bits is variable depending on the actual size of the payload. The payload data therefore is varying in its average distance from the preamble for the frame, which always is present, and the preamble from the next frame, which may not be present if there is no next frame. 
         [0042]    In one embodiment, a method is provided whereby training signals, especially the pilots, are adaptively placed on the frequency domain of a given packet depending on the condition of the channel. 
         [0043]    In another embodiment, the condition and quality of the channel can be assessed by the frequency-domain channel estimates obtained by the transmitting UWB device&#39;s receiver, based on the previous packets received. From channel estimates thus obtained, the parts of the frequency spectrum can be analyzed and each such part can be determined whether it is suitable to include a pilot symbol within it or not. 
         [0044]    In another embodiment, the pilots can be removed in frequency regions where channel quality is excellent. In such frequency regions, good channel estimates may be obtained even without the pilots, since a prior channel estimate obtained from the preamble part, which covers the entire transmission bandwidth, may be sufficient and no pilots may be needed for better channel estimates around the frequency channels that are already excellent. The removed pilots, in turn, may be re-positioned to an area where channel quality is not good, to aid estimation of channels in and around frequency regions where existing pilots may not be sufficient to produce good channel estimates. 
         [0045]    In another embodiment, pilots can be removed in frequency regions where channel quality is very poor. This may take place if the transmitting UWB device decides either to remove data symbols from the poor-quality channels altogether, thereby negating the need to maintain pilots in and around these frequency channels, or if the transmitting UWB device does not quite remove all data symbols but if it applies more error-resistant modulation/coding schemes for the data symbols to be mapped onto these poor-quality frequency channels. In the latter case, the removal of the pilot channels may be tolerated because of the strengthened error protection. Again, the removed pilots can be re-positioned to other frequency channels to aid finer channel estimates around those frequency channels. 
         [0046]    In another embodiment, the pilots may be removed from both extremely excellent frequency channels and extremely poor frequency channels, and the removed pilots may be re-positioned to the remaining frequency channels. 
         [0047]    In order to determine which pilot channels are to be removed and/or re-positioned, the entire transmit bandwidth for a given packet can be partitioned into overlapping or non-overlapping sub-channel parts each consisting of one or multiple frequency bins. Each such sub-channel parts can then be analyzed for their suitability to retain or remove/reposition pilot channels. 
         [0048]      FIG. 4  illustrates two pilot channels, Cp[0] and Cp[2], that were originally placed on the frequency sub-channels -55 and -35 respectively. They are removed and repositioned to different frequency sub-channels -54 and -34, respectively. Note that the positions of the data channels also change, due to the change in the pilot channel&#39;s positions. However, the new positions of the data channel can be easily determined if the changes in the positions of the pilot channels are known and indicated, since the sequence of the data channel positions is really not changed except for the interspersed poisoning of the pilot channels. Thus, only the changes in the positioning of the pilot channels has to be conveyed to the receiving UWB device or WTRU, for it to be able to determine which frequency channels contain the new positions of the pilots. In the structures of the pilot placement in the original ECMA-368 specification, the frequency channels are grouped by 10 frequency bins, or sub-channels, comprised of one pilot channel and 9 data channels. These 10-bin intervals are indicated by the rectangular boundaries in the  FIG. 4 . Frequency bins -56 to -47 comprise the first 10-bin interval, and the bins -46 to -37 comprise the next 10-bin interval, and so on. 
         [0049]    Since in most situations the portion of the UWB spectrum where significant interference exists is likely to be small compared to the whole spectrum, it is likely that the number of frequency parts, or bins, that are affected by such interference is proportionately very small compared to the whole number of such parts, or bins. Therefore, if a UWB device uses a method where pilots are removed and repositioned from severely impaired frequency channels only, then once such bins are determined, the transmitting UWB device may only need to indicate to the receiving UWB device the locations or indices of the relatively few frequency bins that are affected. 
         [0050]    For the particular application to the ECMA-368 UWB OFDM packet format, in one embodiment, some of the available reserved bits in the physical layer (PHY) PLCP header are used to indicate the indices of the frequency bins where pilots are either removed or added. 
         [0051]      FIG. 5  illustrates that in the currently proposed ECMA-368 UWB PLCP header format, a total of 15 reserved bits are reserved in 4 different reserved bit fields. 
         [0052]    In one embodiment, some of the ‘reserved’ bits are used for various indications proposed. Since there are 12 pilot channels in ECMA-368, 12 bits are required to indicate whether any of the pilot channels are removed or repositioned. For example, a bit string of 000010010001 would indicate that the 5th, 8th, and the 12th pilots are removed or repositioned. In addition, for each pilot that is re-positioned, one may need up to 4 bits to indicate where that pilot is re-positioned if the pilot can re-position only within the same 10-bin frequency interval where the pilot was originally placed. 
         [0053]    One embodiment of a method to use these reserved bits for the indication of the changed positions of the frequency bins is to apply additional constraints on the positions that the pilots can be removed/re-positioned, as there are only 15 bits to use only the reserved bits are used. 
         [0054]    By way of example, it is possible to use a constraint where first, only one out of every three pilots can be removed or repositioned, and secondly, a pilot can be removed from one position to another that is within 8 frequency bins away from the original position within the same 10 frequency bins that included the original position of the displaced pilot. The first constraint means that there can be at most 4 pilots that can be removed/repositioned, requiring 2 of the reserved bits. The second constraint means another 3 bits are required, per repositioned pilots. If the maximum of 4 pilots are removable/repositionable, then at most 15 bits are required to indicate which pilot positions are removed, and where they are repositioned, under the stated constraint. Since the ECMA-368 PLCP header already has 15 reserved bits, one could just use these bits for this indication. 
         [0055]    Turning back to  FIG. 4 , there is shown a signal where only one pilot is removed and replaced to a new position within the same 10-bin interval where the original pilot was positioned. 
         [0056]    By way of another example, it is possible not to constrain where the new positions can be located. It is possible to remove a pilot position within one 10-bin interval, and re-position this pilot symbol to another 10-bin interval where there was already a pilot. That other pilot could be repositioned.  FIG. 6  illustrates such a situation. 
         [0057]    In  FIG. 6 , the original pilot symbols Cp[0] and Cp[1] on frequency bins -55 and -45, respectively, are re-positioned to frequency bins -56 and -47, respectively. Shown after re-positioning, the first 10-bin interval now contains two pilot bins. Also shown is that the second 10-bin interval does not contain a pilot after the re-positioning. Again, it is possible to apply some constraints in order to use the 15 reserved bits to indicate the repositioning of the pilots in different ways. 
         [0058]    If the pilot removal and repositioning is done with little or no constraints on where they can be removed and repositioned, using just the 15 reserved bits may not be adequate. Therefore, it may be possible to use the reserved bits in more than one packet to convey the change in the positions of the pilots. For example, if PLCP headers in two successive packets are to be used, a total of 30 reserved bits will be available to indicate the positional changes in the pilots. The cost to use multiple PLCP headers, on the other hand, is increased delays, (latencies) in the indication of the change in the pilots, from the transmitting UWB device to its intended recipient. 
         [0059]    Another possible method to use for the indication of the changed positions of the pilots and data channels is by way of a new type of control packet. This method would require a new definition of a special control packet that is currently not in the ECMA-368 specification. 
         [0060]    Another similar method is to extend the definition of some existing packet types, such as the ACK packet, to include sufficiently many bits to indicate the new positions of the pilots. 
         [0061]    It is left to determine how the transmitting UWB device can determine which pilot frequency channels are under the influence of severe channel impairment and need to repositioned. One method for such determination is to use frequency-domain channel estimate results from the reverse-link. By way of example, assume UWB Device A wishes to assess which of the current pilot positions need to be replaced/repositioned while communicating with another Device B. Device A can first obtain the channel estimate from its reception of reverse-link packets that it has received from Device B. Then, Device A can compute an estimate of the reciprocal channel, i.e., the channel from the Device A to Device B. This estimate is then analyzed to assess whether any portions of the frequency spectrum, i.e., any frequency bins, are likely to have been severely impaired. After the impaired frequency bins are identified, and if it is determined that some of the impaired frequency bins are on or very near the frequency bins that are previously assigned to have pilot symbols, then the pilots on those bins could be removed and replaced with data symbols instead. Similarly, a pilot channel can be re-positioned to frequency bins that are different from the ones where the pilot channel has been originally assigned to. One option of doing it may be to reposition the pilot channel to a frequency bin half-way between its strongest nearest-neighbor pilot-channel frequency bin and the frequency bin where its pilot channel is removed. 
         [0062]    Another method to determine which pilot channels are to be removed and/or repositioned may be by direct indication from the receiving UWB device. This can be done, again, either by using some of the reserved bits in the PLCP header of packets from the receiving UWB device. This is similar to the method for designating the pilot position change on the transmitter device, but using different reserved bits than is used in the transmitter device. Alternatively, a newly defined control packet may be used, or a new extension of a positive acknowledgement (ACK) or request-to-send (RTS) packet from the receiving device may be employed. 
         [0063]    Set forth below are a number of embodiments of a method whereby information carrying data symbols can be adaptively allocated using adaptive modulation coding on different frequency channels (bins) within a UWB OFDM packet. 
         [0064]    In one embodiment of such a method, estimates of the frequency channel, obtained for example by the several methods that were already described in the previous section, can be used to identify which frequency bins are of good quality , for example, high signal-to-noise ratio and which are of poorer quality. The transmitting UWB device then can allocate more information bits, using higher-order modulation schemes and/or lower-rate channel coding scheme, in or around the frequency bins with high channel quality. The device can also allocate fewer information bits, using lower-order modulation schemes and/or higher-rate channel coding scheme, in or around the frequency bins with low channel quality. Adaptive bit allocation over frequency bins is done within a packet, unlike the schemes used in existing UWB OFDM systems such as the ECMA-368 systems. 
         [0065]    Assume that the transmitting UWB device uses an adaptive bit allocation scheme over different frequency bins within a packet. In order for the receiving UWB device to demodulate the signal correctly, it will need to know which of the frequency bins used what kind of modulation or channel coding schemes. Multiple embodiments of a method for solving this problem are set forth below. 
         [0066]    Initially, some of the reserved bits in the PLCP header can be used to indicate the variation of the modulation and/or channel coding according to different frequency bins. Since there are only 15 reserved bits in the ECMA-368 standard, one may need to use more than one packet, and thus more than one PLCP header and its associated reserved bits to be able to indicate a detailed partitioning of the bit allocation over frequency bins. There are 100 data channels within the 128 available frequency channels. If two different bit allocation schemes for each of the data channels are used, and the ability to adaptively allocate bits to all of the data channels are required, 100 indication bits are required. Further constraints may be in place to reduce the number of the required reserved bits for the indication. For example, the 100 data channels can be divided to 10 consecutive, non-overlapping bin-intervals, each with 10 data channels within it. Then, one only needs 10 bits to indicate which bit-allocation scheme (out of the possible two) any one 10-bin interval would use. 
         [0067]    Secondly, similar to the method for indication of pilot removal/re-position set forth above, more than one PLCP header and its available reserved bits can be used to indicate adaptive bit allocation on different frequency bins (or bin intervals). 
         [0068]    Thirdly, similar to the methods set forth above, a new control packet (frame) type may be used wherein the frame has control fields defined to indicate the adaptive allocation of bits. 
         [0069]    Set forth below is a method for adaptively allocating a different number of information bits to the different frequency bins. 
         [0070]      FIG. 7  shows a transmitter append and scrambling unit  700  of an ECMA-368 system, which formats PSDU frames, i.e., the packets that contain the information-carrying data symbols in the physical layer (PHY) level. The transmitter append and scrambling unit  700  scrambles a frame payload  705  that carries information-carrying data bits, with a 32-bit frame check sequence (FCS)  710 , and packet assembler/disassembler (PAD) bits  715 . Six (6) “zero”-valued tail bits  720  are also input to the transmitter append and scrambling unit  700 . The transmitter append and scrambling unit  700  appends together a scrambled frame payload  730 ,  32  scrambled FCS bits  735 , scrambled PAD bits  740  and unscrambled zero-valued tail bits  740  to form a scrambled PSDU frame  750 . 
         [0071]      FIG. 8  shows an example of transmitter  800  that includes the transmitter append and scrambling unit  700 . Additionally, the transmitter may further include a convolutional encoder/puncturer  805 , a bit interleaver  815 , a modulation mapper  825 , an OFDM modulator  835  and a transmit antenna  845 . The modulation mapper  825  may be a QPSK modulation mapper or a DCM mapper. The transmitter  800  may be incorporated into a WTRU and/or a base station. 
         [0072]    Referring to  FIG. 8 , the scrambled PSDU frame  750  output by the transmitter append and scrambling unit  700  is fed into the convolutional encoder/puncturer  805 , which outputs encoded and punctured bits  810  that are then bit-interleaved by the bit interleaver  815 . Then, using knowledge on the transmit channel characteristics, the modulation mapper  825  selects either QPSK modulation or the DCM as the mapping for a particular frequency bin, and then maps the interleaved bits  820  to the selected modulation mapping (QPSK or DCM). The mapped bits  830  output by the modulation mapper  825  are then fed into the OFDM modulator  835 , which modulates the mapped bits  830  to produce OFDM-modulated output bits  840  that is transmitted via the transmit antenna  845 . 
         [0073]      FIG. 9  shows an example of a receiver  900  that re-constructs the scrambled PSDU  750 . The receiver  900  may include a receive antenna  905 , an OFDM demodulator  915 , a de-mapper  925 , a bit de-interleaver  935  and a Viterbi decoder  945 . The de-mapper  925  may be a QPSK modulation de-mapper or a DCM de-mapper. The receiver  900  may be incorporated into a WTRU and/or a base station. 
         [0074]    Referring to  FIG. 9 , a baseband signal  910  received by the receive antenna  905  is demodulated by the OFDM demodulator  915 . The resultant demodulated signal  920  is fed into the de-mapper  925 , which first selects, using information on the channel characteristics, either a QPSK modulation de-mapping or a DCM de-mapping, and then de-maps the demodulated signal  920  accordingly. The de-mapped signal  930  is then fed into the bit de-interleaver  935 , which de-interleaves the de-mapped signal  930 . The bit de-interleaver  935  outputs de-interleaved bits  940 , which are fed into the Viterbi decoder  945 . The Viterbi decoder  945  outputs the scrambled PSDU  750 , which may be further processed (scrambled and de-appended) to produce a PSDU frame payload (not shown). 
         [0075]    Set forth below are alternative embodiments of a method for adaptive allocation of the bits onto different frequency bins can be performed in many different ways for slightly modified versions of ECMA-368 PSDU. 
         [0076]    One embodiment includes adaptive allocation by variable use of QPSK versus DCM. The transmitting UWB device allocates information bits to different channel frequency bins by using different modulation mapping per different frequency bins. For example, where frequency channels are excellent and superior to the average of all the frequency channels, information bits that would have been QPSK modulated may be up-modulated to DCM. Likewise, where frequency channel quality is very poor, information bits that would have been DCM modulated may be down-modulated to QPSK. The signals then are mapped to an OFDM modulator using IFFT. 
         [0077]    At the receiving UWB device, the receiver first converts the OFDM signal back to the frequency domain by applying FFT. Then, using an indication, obtained by, for example, reading the PLCP header reserved bits, as explained in the previous paragraphs of this section, regarding which modulation scheme correspond to which frequency bins, the receiver can then collect the received bits into two partitions, one corresponding to a set of frequency bins and containing the bits that were modulation-mapped to QPSK by the transmitting UWB device, and the other partition corresponding to the other set of frequency bins and containing bits that were modulation-mapped to DCM by the transmitting UWB device. The receiver then can de-map the modulated signals separately from the two partitions. After the two sets of signals are demodulated, one by QPSK, and the other by DCM demodulation, the demodulated bits are collated again, in the correct order reflecting the order of the frequency bins, and proceeds to further receiver processing such as bit interleaving, convolutional decoding, and descrambling, as depicted  FIG. 9 . 
         [0078]    This particular solution requires only a modest change to the existing transmitter and receiver procedures of ECMA-368 systems. The transmitter would need to maintain two sets of signals, one to use DCM and the other to use QPSK. The receiver, in order to maintain two sets of signals, also would need to be designed to be able to perform both QPSK and DCM demodulations. The only other change to the ECMA specification would be the indication methods set forth above. 
         [0079]    Another embodiment of the method includes adaptive allocation by variable use of channel coding rates. In this embodiment different channel coding rates per different frequency bins with different channel qualities are used. The ECMA-368 specification, for example, provides 4 different channel coding rates, 1/3, 1/2, 5/8, and 3/4, by using a base 1/3 convolutional encoder coupled with four different puncturing schemes. One, therefore, could identify up to 4 different levels of channel quality, partition the frequency bins according to the 4 quality-levels, and use different channel coding rates on symbols to be carried on different frequency bins. This requires a change of interleaving and QPSK/DCM mapper, as well as OFDM modulator, in the transmit side, in which that the transmitter would have to maintain four different sets of information bits, and separately channel code, interleave, modulation map (to QPSK or DCM), and then separately modulate to the OFDM, and then combine the up to four different OFDM signals in the time domain by superposing them. 
         [0080]    At the receiver, one would also need to have essentially four different receivers as depicted in  FIG. 9 , one each for each of the bit streams that in the transmitter had been encoded with one of the four different channel coding rates. 
         [0081]    Another embodiment includes using a mix of variable modulation and channel coding rates on the different frequency bins. This would be hybrid combination of the two embodiments set forth above, and requires more modification of the ECMA-368 specification. However, this method would enable up to 8 different levels of bit allocation on to different frequency channels with different qualities. 
         [0082]      FIG. 10  is an example of a multiple-input multiple-output (MIMO) UWB-OFDM transmitter  1000  with N antennas. Note that for the current state of art in the UWB-OFDM standard, such as ECMA-368, is based on a single-input single-output (SISO) system where N=1. The transmitter  100  may include a serial-to-parallel (S/P) converter  1010 , adaptive pilot allocation units  1015   1 - 1015   N , frequency interleavers  1020   1 - 1020   N , adaptive mapping units  1025   1 - 1025   N , a quantizer  1030 , processing units  1035   1 - 1035   N , baseband to radio frequency (RF) converters  1040   1 - 1040   N  and transmit antennas  1045   1 - 1045   N . The adaptive mapping units  1015   1 - 1015   N  may be adaptive QPSK modulation mapping units or adaptive DCM mapping units. The transmitter  1000  may be incorporated into a WTRU and/or a base station. 
         [0083]    Referring to  FIG. 10 , the S/P converter  1010  converts a baseband data input signal  1005  into N parallel bit streams. Each of the adaptive pilot allocation units  1015   1 - 1015   N  adaptively allocates pilots in the frequency bi N , and generate pilot allocation indication bits  1017   1 - 1017   N , which the frequency interleavers  1020   1 - 1020   N  use to determine the frequency bins to interleave the parallelized input signals  1018   1 - 1018   N . The adaptive pilot allocation units  1015   1 - 1015   N  also output data bits  1018   1 - 1018   N , which have been multiplexed to respective ones of the N parallel bit streams output by the S/P converter  1010 . The output data bits  1018   1 - 1018   N  are generated in the adaptive pilot allocation units  1015   1 - 1015   N , using the knowledge of the channel, and then carried by some or all of the reserved bits in the PSDU header ( 215  in  FIG. 2 ). The pilot allocation indication bits  1017   1 - 1017   N  are carried on some or all of the reserved bits (shown in  FIG. 5 ) in the  5 -Octet PHY header portion of the PLCP header ( 210  in  FIG. 2 ). After frequency interleaving, the adaptive mapping units  1025   1 - 1025   N  map the outputs of the frequency interleavers  1020   1 - 1020   N  to either QPSK modulation or DCM, which selection is adaptively made based on pilot allocation and information about the frequency channel corresponding to the frequency bins. The QPSK modulation or DCM-mapped signals are then quantized by the quantizer  1030 , and the resulting signals are transformed into time-domain signals and appended with guard band bits by the processing units  1035   1 - 1035   N . The N , the resulting signals  1038   1 - 1038   N  are converted to analog RF signals by the baseband to RF units  1040   1 - 1040   N . Finally, the RF signals are transmitted from the antennas  1045   1  to  1045   N . 
         [0084]    Note that  FIG. 10  depicts a MIMO transmitter. For a SISO transmitter with  1  antenna, the S/P converter  1010  is removed and there will be one sequence of units  1015  to  1045 , without the indices. However, the rest of the operations would be similar to those as described for  FIG. 10 . 
         [0085]    Although the features and elements are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements. The methods or flow charts may be implemented in a computer program, software, or firmware tangibly embodied in a computer-readable storage medium for execution by a general purpose computer or a processor. Examples of computer-readable storage mediums include a read only memory (ROM), a random access memory (RAM), a register, cache memory, semiconductor memory devices, magnetic media such as internal hard disks and removable disks, magneto-optical media, and optical media such as CD-ROM disks, and digital versatile disks (DVDs). 
         [0086]    Suitable processors include, by way of example, a general purpose processor, a special purpose processor, a conventional processor, a digital signal processor (DSP), a plurality of microprocessors, one or more microprocessors in association with a DSP core, a controller, a microcontroller, Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs) circuits, any other type of integrated circuit (IC), and/or a state machine. 
         [0087]    A processor in association with software may be used to implement a radio frequency transceiver for use in a wireless transmit receive unit (WTRU), user equipment (UE), terminal, base station, radio network controller (RNC), or any host computer. The WTRU may be used in conjunction with modules, implemented in hardware and/or software, such as a camera, a video camera module, a videophone, a speakerphone, a vibration device, a speaker, a microphone, a television transceiver, a hands free headset, a keyboard, a Bluetooth® module, a frequency modulated (FM) radio unit, a liquid crystal display (LCD) display unit, an organic light-emitting diode (OLED) display unit, a digital music player, a media player, a video game player module, an Internet browser, and/or any wireless local area network (WLAN) module.