Abstract:
A system and method for the efficient transmission of information in a code division multiple access (CDMA) wireless telecommunication system. To increase the rate of reliable transmission, an orthogonal frequency-division multiplexing (OFDM) scheme is implemented in a direct-spread CDMA network, this combination sometimes being referred to as multi-carrier CDMA (MC-CDMA). Information (such as voice and data), interspersed with a known pilot signal, is encoded and spread across the frequency domain, rather than the time domain as in traditional CDMA; the allowable transmission bandwidth is divided into a number of carriers. To achieve even larger transmission rates, the guardband between carriers is eliminated. To prevent interference, the number of pilot transmissions is reduced and a corresponding number of frequency bins at the border of an OFDM block are loaded with zeros. The receiver simply ignores these subcarriers when reconstructing the transmitted signal.

Description:
[0001]    The present invention relates generally to radio telephony, and more specifically to a method and apparatus for increasing transmission capacity in a wideband MC-CDMA telecommunication system by implementing guardband elimination techniques.  
         BACKGROUND OF THE INVENTION  
         [0002]    The now ubiquitous telecommunication instruments commonly called cellular telephones (or simply “cell phones”) are actually mobile radios having a transmitter and a receiver, a power source, and some sort of user interface. They are referred to as cell phones because they are designed to operate within a cellular network. Despite being radios, they typically do not communicate directly with each other. Instead, these mobile telephones communicated over an air interface (radio link) with numerous base stations located throughout the network&#39;s coverage area. The network base stations are interconnected in order to route the calls to and from telephones operating within the network coverage area.  
           [0003]    [0003]FIG. 1 is a simplified block diagram illustrating the configuration of a typical cellular network  100 . As may be apparent from its name, the network coverage area (only a portion of which is shown in FIG. 1) is divided into a number of cells, such as cells  10  through  15  delineated by broken lines in FIG. 1. Although only six cells are shown, there are typically a great many. In the illustrated network, each cell has associated with it a base transceiver station (BTS). Generally speaking, BTS  20  is for transmitting and receiving messages to and from any mobile stations (MSs) in cell  10 ; illustrated here as MS  31 , MS  32 , and MS  33 , via radio frequency (RF) links  35 ,  36 , and  37 , respectively. Mobile stations MS  31  through MS  33  are usually (though not necessarily) mobile, and free to move in and out of cell  10 . Radio links  35 - 37  are therefore established only where necessary for communication. When the need for a particular radio link no longer exists, the associated radio channels are freed for use in other communications. (Certain channels, however, are dedicated for beacon transmissions and are therefore in continuous use.) BTS  21  through BTS  25 , located in cell  11  through cell  15 , respectively, are similarly equipped to establish radio contact with mobile stations in the cells they cover.  
           [0004]    BTS  20 , BTS  21 , and BTS  22  operate under the direction of a base station controller (BSC)  26 , which also manages communication with the remainder of network  100 . Similarly, BTS  23 , BTS  24 , and BTS  25  are controlled by BSC  27 . In the network  100  of FIG. 1, BSC  26  and  27  are directly connected and may therefore route calls directly to each other. Not all BSCs in network  100  are so connected, however, and must therefore communicate through a central switch. To this end, BSC  20  is in communication with mobile switching center MSC  29 . MSC  29  is operable to route communication traffic throughout network  100  by sending it to other BSCs with which it is in communication, or to another MSC (not shown) of network  100 . Where appropriate, MSC  29  may also have the capability to route traffic to other networks, such as a packet data network  50 . Packet data network  50  may be the Internet, an intranet, a local area network (LAN), or any of numerous other communication networks that transfer data via a packet-switching protocol. Data passing from one network to another will typically though not necessarily pass through some type of gateway  49 , which not only provides a connection, but converts the data from one format to another, as appropriate.  
           [0005]    Note that packet data network  50  is typically connected to the MSC  29 , as shown here, for low data rate applications. Where higher data rates are needed, such as in 1xEV-DO or 1 xEV-DV networks, the packet data network  50  is connected directly to the BSCs ( 26 ,  27 ), which in such networks are capable of processing the packet data.  
           [0006]    The cellular network  100  of FIG. 1 has several advantages. As the cells are relatively small, the telephone transmitters do not need a great deal of power. This is particularly important where the power source, usually a battery, is housed and carried in the cell phone itself. In addition, the use of low-power transmitters means that the mobile stations are less apt to interfere with others operating nearby. In some networks, this even enables frequency reuse, that is, the same communication frequencies can be used in non-adjacent cells at the same time without interference. This permits the addition of a larger number of network subscribers. In other systems, codes used for privacy or signal processing may be reused in a similar manner.  
           [0007]    At this point, it should also be noted that as the terms for radio telephones, such as “cellular (or cell) phone” and “mobile phone” are often used interchangeably, they will be treated as equivalent herein. Both, however, are a sub-group of a larger family of devices that also includes, for example, certain computers and personal digital assistants (PDAs) that are also capable of wireless radio communication in a radio network. This family of devices will for convenience be referred to as “mobile stations” (regardless of whether a particular device is actually moved about in normal operation).  
           [0008]    In addition to the cellular architecture itself, certain multiple access schemes may also be employed to increase the number of mobile stations that may operate at the same time in a given area. In frequency-division multiple access (FDMA), the available transmission bandwidth is divided into a number of channels, each for use by a different caller (or for a different non-traffic use). A disadvantage of FDMA, however, is that each frequency channel used for traffic is captured for the duration of each call and cannot be used for others. Time-division multiple access (TDMA) improves upon the FDMA scheme by dividing each frequency channel into time slots. Any given call is assigned one or more of these time slots on which to send information. More than one voice caller may therefore use each frequency channel. Although the channel is not continuously dedicated to them, the resulting discontinuity is usually imperceptible to the user. For data transmissions, of course, the discontinuity is not normally a factor.  
           [0009]    Code-division multiple access (CDMA) operates somewhat differently. Rather than divide the available transmission bandwidth into individual channels, individual transmissions are spread over a frequency band and encoded. By encoding each transmission in a different way, each receiver (i.e. mobile station) decodes only information intended for it and ignores other transmissions. The number of mobile stations that can operate in a given area is therefore limited by the number of encoding sequences available, rather than the number of frequency bands. The operation of a CDMA network is normally performed in accordance with a protocol referred to as IS-95 (interim standard-95) or, increasingly, according to its third generation (3G) successors, such as those sometimes referred to as 1xEV-DO and 1xEV-DV, the latter of which provides for the transport of both data and voice information.  
           [0010]    [0010]FIG. 2 is a flow diagram illustrating the basic steps involved in sending a CDMA transmission according to the prior art. At S TART  it is assumed that information from an information source (such as a caller&#39;s voice) is available and that a connection has been established with a receiving node. At step  205 , the audible voice information is sampled and digitally encoded. The encoded information is then organized into frames (step  210 ). Error detection bits are then added (step  215 ) so that the receiver can evaluate the integrity of the received data. The resulting signal is then convolutionally encoded (step  220 ). Block interleaving is then performed (step  225 ) on the resulting signal to further enhance the receiver&#39;s ability to reconstruct the bit stream with a minimum of error. The interleaved signal is then spread by a pseudonoise (PN) code (step  230 ), a long code is applied (step  235 ) and a Walsh code is used to spread the wave form and provide channelization (step  240 ). I and Q short codes are added (step  245 ) and the results filtered (step  250 ) before being combined and spread (step  255 ), then amplified (step  260 ) for transmission.  
           [0011]    As alluded to above, mobile stations and the network they are a part of are presently being used to carry an increasingly large amount of traffic. Not only is the number of ordinary voice calls increasing, but so is the number of other uses to which mobile stations can be put. Short message service (SMS) messaging and instant messaging are becoming more popular, faxes and emails can be sent through mobile stations, and World Wide Web pages can be downloaded. Portable personal computers can be equipped to send through the network data files such as spreadsheets, word processing documents, and slide presentations. All of this information may enter and leave the network infrastructure through the air interface, meaning that more efficient methods of radio transmission are constantly in demand. The present invention presents a solution that addresses this growing need.  
         SUMMARY OF THE INVENTION  
         [0012]    In one aspect, the present invention is a method of transmitting a radio signal over the air interface in a multi-carrier code division multiple access (MC-CDMA) communication network. The method is intended to make more efficient use of the available spectrum, and includes the steps of encoding and spreading the information stream or streams, then converting the information stream into a plurality of parallel information streams and, after interleaving, mapping the streams into a plurality of subcarriers, the subcarriers in turn forming adjacent carriers of approximately equal size that together span the available bandwidth, wherein at least one and preferably a plurality of subcarriers at each carrier boundary are dummy bins containing no traffic information or pilot signal such that the carriers may be immediately adjacent to each other without experiencing a loss of data due to overlapping subcarriers at their boundary. The method may further include the steps of passing the mapped data through a pulse-shaping filter and amplifying it for transmission.  
           [0013]    In another aspect, the present invention is a system radio telecommunication including a orthogonal frequency division modulation (OFDM) modulator for applying an inverse fast Fourier transform to a symbol stream to form a plurality of frequency-block carriers, each carrier divided into a plurality of subcarriers across which the information is spread and interspersed with a plurality of pilot symbols to be used by the receiver for channel estimation, wherein at least one subcarrier at an end of at least one carrier carries only a dummy symbol such that the at least one carrier can be situated in the frequency spectrum adjacent to another of the plurality of carriers without the need for a guardband to separate them. The system may further include a receiver for presenting the received radio signal to an OFDM demodulator for applying a FFT algorithm to create block streams that can be deinterleaved and padded through a detector to reconstruct the transmitted symbol stream.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    For a more complete understanding of the present invention, and the advantages thereof, reference is made to the following drawings in the detailed description below:  
         [0015]    [0015]FIG. 1 is functional block diagram illustrating the relationship of selected components of a typical CDMA telecommunication network, such as one that might advantageously employ the apparatus and method of the present invention.  
         [0016]    [0016]FIG. 2 is a flow diagram illustrating a method of transmitting a CDMA signal according to the prior art.  
         [0017]    [0017]FIG. 3 is a waveform diagram illustrating a waveform distribution of OFDM subcarriers spread in a single block across a 5 MHz frequency band.  
         [0018]    [0018]FIG. 4 is a waveform diagram illustrating a waveform occupying the same 5 MHz band as the waveform of FIG. 3, but divided into three OFDM carriers separated by guardbands.  
         [0019]    [0019]FIG. 5 is a functional block diagram illustrating a system for sending and receiving radio signals in a telecommunication network according to an embodiment of the present invention.  
         [0020]    [0020]FIG. 6 is a waveform diagram illustrating a waveform formed according to an embodiment of the present invention.  
         [0021]    [0021]FIG. 7 is a flow chart illustrating a method of processing a radio signal according to an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0022]    [0022]FIGS. 1 through 7, discussed herein, and the various embodiments used to describe the present invention are by way of illustration only, and should not be construed to limit the scope of the invention. Those skilled in the art will understand the principles of the present invention may be implemented in any similar radio-communication device, in addition to those specifically discussed herein.  
         [0023]    The present invention is an improvement on existing CDMA transmission schemes, and has been found to provide higher data rates without sacrificing performance, even when compared to newer CDMA applications such as 1xEV-DV. As mentioned above, code division multiple access (CDMA) is a successful if still imperfect multiple access scheme. In order to address its shortcomings, a number of solutions have been proposed. One solution involves the use together of CDMA techniques and orthogonal frequency division multiplexing (OFDM). OFDM is a modulation method in which multiple user symbols are transmitted in parallel using a large number of different subcarriers. These subcarriers, sometimes called frequency bins, are used to spread transmitted information with respect to frequency rather than time (as with conventional CDMA).  
         [0024]    [0024]FIG. 3 is a waveform diagram illustrating a wavefrom  300  distribution of OFDM subcarriers spread in a single block across a 5 MHz frequency band. (The 5 MHz band is exemplary; others may be suitable as well.) The subcarriers used in OFDM have overlapping spectra, but their signal waveforms are specifically chosen to be orthogonal so as to reduce interference between them. Each of the subcarriers, or ‘frequency bins’, may carry either information (traffic) or pilot symbols. Note in this context that the terms ‘traffic’, ‘information’, and ‘data’ may sometimes used interchangeably even though a distinction is sometimes made between voice information and data information. Pilot symbols also represent information, but not information that is associated with a user. Rather, they make up a signal known to the receiver that can be used for channel estimation so that the user traffic can be reproduced more faithfully.  
         [0025]    A given information stream need not be spread across the entire available frequency band. FIG. 4 is a waveform diagram illustrating a waveform  400  occupying the same 5 MHz band as the waveform  300  of FIG. 3, but divided into three OFDM carriers (or ‘blocks’), enumerated  401 ,  402 , and  403 . Each carrier occupies approximately 1.25 MHz of the frequency band. Note that each carrier still includes a relatively large number of subcarriers. As the use of a number of carriers is typical, the combination of OFDM with CDMA is often referred to as a multi-carrier CDMA (MC-CDMA) technology.  
         [0026]    The OFDM carriers are separated from each other by a guardband. In FIG. 4, guardband  405  separates carrier  401  and carrier  402 , and guardband  410  separates carrier  402  and carrier  403 . The purpose of the guardbands is to reduce performance degradation caused by increased bandwidth due to the addition of the cyclic prefix and natural imperfection of the pulse-shaping filters. That is, some interference will occur notwithstanding the fact that the subcarriers are formed of mutually orthogonal waveforms. Nevertheless, the presence of these guardbands, results in an inefficient use of the available bandwidth. To make the MC-CDMA system more efficient, the present invention proposes a technique for eliminating the guardbands while maintaining or even improving performance levels.  
         [0027]    [0027]FIG. 5 is a simplified block diagram illustrating an exemplary system  500  for sending information over an air interface using MC-CDMA in accordance with an embodiment of the present invention. The portion of the Figure above the broken line represents a transmitter  501 , such as one that might be found in a telecommunication network base station, and below is illustrated a receiver  551  for example one operating in a mobile station. The broken line itself represents a multipath channel over the air interface of the radio telecommunication network.  
         [0028]    In transmitter  501 , serial-to-parallel (S/P) converter  505  splits the modulated symbol streams (of all K users) into K blocks of J streams (S 0,0  to S K-1,J-1 ). Each of these streams s is spread by multiplication with a Walsh-Hadamard code (c 0  to c J-1 ), and then presented to a summer ( 510   0 . . .    510   k . . .    510   K-1 ), which sums the streams associated with each block  0  through K−1 into a single spread stream (S 0  to S K-1 ). The spread streams S k  are then passed through S/P converters  515   0 . . .    515   k . . .    515   K-1  before being presented to interleaver  520  for block interleaving. OFDM modulator (IFFT)  525  is coupled to interleaver  520  and maps the interleaved signal into frequency bins (subcarriers) and adds a cyclic prefix.  
         [0029]    In accordance with the present invention, the OFDM modulator  525  creates dummy bins of the subcarrier frequency bins at the boundary of each carrier, the dummy bins preferably holding a logical zero that is simply ignored by the receiver. Alternately, the dummy bins can be situated at only one side of the carrier, meaning that the immediately adjacent subcarriers of the adjacent carrier hold information. The dummy bins allow this information to be read notwithstanding the overlap of subcarriers at the carrier boundary. The resulting signal is then passed through a pulse-shaping filter  530  and transmitted over a radio channel using antenna  535 .  
         [0030]    Receiver  551  includes the antenna  553  for receiving the transmitted radio signal. The received signal is first passed through a matched band-pass receive filter  555  to suppress out-of-band noise and interference. The filtered signal is then passed through an OFDM demodulator (FFT)  560  and demodulated into frequency-domain signal Z k  (signals of other blocks may be present as well, but for simplicity only one is shown). Deinterleaver  565  deinterleaves signal Z k  and is coupled to parallel-to-serial (P/S) converter  570   k , which creates a bit stream Y k  (again, there may be one associated with each block, even though only one stream is shown in FIG. 5). A detector  575   k  generates soft or hard decision outputs for each original symbol or bit stream (Ŝ k,0  to Ŝ K,J-1 )  
         [0031]    [0031]FIG. 6 is a waveform diagram illustrating a waveform  600  formed according to an embodiment of the present invention. Waveform  600 , as formed by OFDM modulator  525  (shown in FIG. 5), consists of carrier  601  and carrier  602 . Note that in contrast to the waveform  400  of FIG. 4, however, there is no guardband present between the carriers  601  and  602 . Instead, overlapping subcarriers  610  occupy the boundary between the adjacent carriers.  
         [0032]    Ordinarily, this may render the pilot or traffic bits carried in these subcarriers unusable, and any data lost would have to be recovered by error correction techniques. As mentioned above, however, in accordance with the present invention the overlapping bins  610  are dummy bins, preferably filled only with logical zeros. These dummy bins  610  may include one or more boundary subcarriers from each of the carriers  601  and  602 , or may be formed only on one side of the carrier in such a manner that boundary subcarriers of an adjacent carriers my carry usable information.  
         [0033]    With respect to FIG. 6, note also that each illustrated carrier  601  and  602  is shown to occupy 1.25 MHz of the frequency spectrum. As compared with an allowable bandwidth of, for example, the waveforms of FIGS. 3 and 4, this would permit the inclusion of four carriers of this dimension, even though only two are shown in FIG. 6. The result is a greater data rate. In this regard, it should be pointed out that the use of four carriers in a 5 MHz band is exemplary only, and that larger or smaller carriers may be used over a larger or smaller allowable bandwidth. The number of carriers may even be adjustable to accommodate, for example, a temporary increase in the amount of traffic.  
         [0034]    [0034]FIG. 7 is a flow diagram illustrating a method  700  of receiving a radio signal according to an embodiment of the present invention. Initially, (S TART ), it is presumed that the system of FIG. 5 is being utilized, although the operation of various other embodiments of the present invention should be apparent in light of this disclosure and the accompanying drawings. In addition, it is assumed that the information to be transmitted (including pilot signals) has been encoded and modulated into symbol streams. The process then begins at step  705 , where the modulated symbols are divided into K blocks of J streams (step  705 ). Each of these J streams is then spread with a Walsh-Hadamard code (step  710 ), then the streams of each block are summed (step  715 ) to form K streams (for example, S 0  to S K-1  shown in FIG. 5). Each of the K streams is then separated into streams through S/P converters (step  720 ) and interleaved (step  725 ).  
         [0035]    The interleaved signals are then mapped into bins (or subcarriers) (step  730 ) in an OFDM modulator applying an inverse fast Fourier transform (IFFT). As mentioned above, in accordance with the present invention a number of bins at the boundary of each carrier in the multi-carrier signal are left as dummy bins so that no data will be lost due to interference between overlapping subcarriers. The exact number and location of the individual dummy bins may vary according to system design preferences. In some cases dummy bins may overlap with other dummy bins, in others they may overlap with subcarriers holding useable information. In systems where the number of pilot signals interspersed in each carrier is specified by system protocol, the dummy bins may be accommodated by reducing this number, thereby not affecting the actual data capacity of the carrier. The OFDM modulator may also add a cyclic prefix (step  735 ), and then presents the signal to a pulse-shaping filter ( 740 ) before the signal is amplified for transmission (step  745 ) over a multipath channel.  
         [0036]    When an MC-CDMA receiver receives the signal (step  750 ), it first passes it through a receive filter (step  755 ), preferably one matched to the pulse-shaping filter of the transmitter. The filtered signal is then presented to an OFDM demodulator and a fast Fourier Transform (FFT) is applied (step  760 ) to create (for example) signals Z 0  to Z K-1 . The signal for each block is then deinterleaved (step  765 ) and the block streams Y 0  to Y K-1  reconstructed using a P/S converter (step  770 ). Each of the resulting streams are them presented to a detector (step  775 ) for channel estimation and signal detection.  
         [0037]    The preferred descriptions are of preferred examples for implementing the invention, and the scope of the invention should not necessarily be limited by this description. Rather, the scope of the present invention is defined by the following claims.