Abstract:
Disclosed is a method of and apparatus for increasing the capacity of a wireless communication system. This is accomplished by having users that can support a higher than base modulation order be required to do so under predetermined conditions such as electrical distance from a base transceiver station (BTS) antenna to a user, the reception of data in a high speed burst (HSD) and the like. The same digital processor apparatus that may be used to provide a base order modulation scheme may be reprogrammed in a more complex fashion to provide signal processing at the higher modulation rate for any given user channel.

Description:
This Application is a continuation of U.S. application Ser. No. 09/218,220, filed 22 Dec. 1998 now abandoned, which is a continuation-in-part of and claims the benefit of U.S. Provisional Application No. 60/079,825, filed 30 Mar. 1998. 
    
    
     TECHNICAL FIELD 
     The present invention relates in general to variable modulation order and in particular to methods and systems for providing a wireless system which adjusts the modulation order of data being transmitted to the various users in a wireless system on an individual user basis. 
     BACKGROUND 
     Wireless communication systems have, in recent years, seen a tremendous growth surge. Advances in signal processing, driven by the demand for high speed data as well as improvements in spectral efficiency (such as for voice users), have made the balance between radio capacity and available user channels more complicated. For example, recent standards and equipment modification proposals relating to the use of high speed data (HSD) will further contribute to the problem. Such units will be capable of receive and/or transmit operations on multiple channels such as supplemental channels, a fundamental channels and dedicated control channels simultaneously. 
     In cellular systems, code channel availability as well as overall system capacity can be increased when cells are made ever smaller (more cells in a given area), however, various issues, including cost, efficiency and interference from transmissions in other cells, prevent such action from being a total solution. 
     In code division multiple access (CDMA) systems, the number of users that can be accommodated is a function of the available code channels. Typically, CDMA systems use Walsh codes, a set of orthogonal codes, where the number of codes available equals the chip rate divided by the data rate. Channels encoded with Walsh codes are called “Walsh channels” or “code channels.” It is desirable to use orthogonal codes in the forward link (FL) since much of the inter-channel interference cancels when orthogonal codes are used. As should be apparent, the FL comprises communication from the base transceiver station (BTS) to the mobile station (MS). 
     Typically, spread-spectrum communications systems such as CDMA, employ pseudo-random noise (PN) codes for spreading the communication signal to the desired bandwidth. As is well known in the art, a PN code is comprised of chips where a chip may be equated to a unit of time duration. A PN sequence of chips may be used in CDMA as a scrambling code. Following data modulation via a phase shift keyed output signal of a given modulation order, prior art CDMA, PCS and cellular communication systems, added Walsh codes and PN spreading combined in a well known manner. Known systems have used binary phase shift keyed (BPSK) and quadrature phase shift keyed (QPSK) modulation orders. 
     In general there are N orthogonal codes for a code of length N bits (chips). This also applies to Walsh codes, wherein there are N length N orthogonal Walsh codes. It may be noted that the PN chip rate is the same as the Walsh chip rate. In order to have consistent numerology the PN chip rate must equal the modulation symbol rate times the Walsh code length in chips. 
     In the design of some prior art systems using QPSK, a data rate into the encoder of 9,600 symbols per second, and a PN chip rate of 1.2288 Mcps (mega chips per second) allowed for 128 code channels to be transmitted simultaneously from a BTS antenna when the radio environment supports that many users (or user channels). 
     PCS and cellular communications systems often encounter various types of radio environments. For example, the radio signal may encounter various degrees of fading due to multipath and mobile velocities. Other factors such as shadowing may also cause a reduction of signal strength between transmitter and receiver. These same obstacles may also cause signal reflection which results in multipath signals that tend to confuse the receiver in determining what signal to detect. Some of these problems may be overcome by increasing the power of the transmitted signal. In view of the above, the radio environment may be such that the BTS (forward link) runs out of transmitter power before the number of code channels (Walsh codes) available are exhausted. It is generally deemed desirable for the radio environment to limit the system capacity rather than the number of available code channels. However, there may be situations in a given system when the available Walsh channels are exhausted before the BTS power limit is reached. In this case, the capacity of the system is artificially limited by the Walsh code channels rather than the radio environment. 
     BPSK (modulation order of 2) systems are simpler to implement than are QPSK systems since the signal processing complexity is greater for the latter. While an 8 or other higher order system might immediately come to ones mind as a way to solve the problem of having an adequate number of Walsh codes, other considerations must be addressed. If the transmissions employ higher order modularity (M&gt;4), then all users must purchase new equipment to use the system. Further, the transmissions must remain orthogonal in order for the system to be usable and/or practical, numerology must be accommodated and so must FEC coding. Just because BPSK and QPSK systems proved to be capable of providing orthogonality, does not mean that higher order modulation schemes are also orthogonal. With proper design, the result of which will be revealed below, these considerations can be satisfied. Therefore, it would be desirable to use a higher modulation order system when both the radio environment and the mobile capability can support a higher modulation order. 
     SUMMARY OF THE INVENTION 
     The present invention comprises providing a wireless system, such as CDMA, which uses a base modulation order such as QPSK when the system user capacity is adequate and using a higher order modulation scheme for selected users when code channels are limited. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and its advantages, reference will now be made in the following Detailed Description to the accompanying drawings, in which: 
         FIG. 1  is a simplified diagram of a cellular system; 
         FIG. 2  is a block diagram illustrating how Walsh codes and PN spreading are typically combined with incoming data in a CDMA system; 
         FIG. 3  provides more detail for a portion of  FIG. 2  when QPSK modulation is used; 
         FIG. 4  illustrates one approach to performing 8-PSK modulation while using the data input of the 4-PSK of  FIG. 3  and utilizing longer Walsh codes, with respect to those used for 4-PSK, for a given data rate; 
         FIG. 5  illustrates a second approach to performing 8-PSK modulation while using the data input of  FIG. 3  (corresponding to 4-PSK) and utilizing longer Walsh codes, with respect to those used for 4-PSK, for a given data rate; 
         FIG. 6  illustrates a third approach to performing 8-PSK modulation while using the data input of  FIG. 3  (corresponding to 4-PSK) and utilizing longer Walsh codes, with respect to those used for 4-PSK, for a given data rate; and 
         FIG. 7  illustrates a fourth approach to performing 8-PSK modulation while utilizing longer Walsh codes, with respect to those used for 4-PSK, for a given data rate. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , a cellular network, represented by block  10 , communicates with a BTS  12 . In general, a BTS may also communicate with MSs in neighboring cells. The BTS  12  transmits signals to various MSs within a defined distance as represented by the outlines of a cell  14 . The BTS  12  transmits these signals via an antenna not shown. Within cell  14  are shown MSs  16 ,  18  and  20  with MS  16  being physically and electrically close to the BTS  12 . It is common knowledge that a user can be physically close to a BTS antenna and still not have a “good”, strong and/or clear signal. Thus further references to the distance between an MS and a transmitting antenna of a BTS such as  12  will by definition refer to “close” as having a strong and easily detectable signal when a small amount of power is used to transmit signals to that MS. In the alternative, an MS is “far” from an antenna when the signal received by the MS is weak in strength and/or hard to accurately detect even though a relatively large amount of power is used to transmit signals to that MS. In other words, many different factors such as terrain, atmospheric conditions, buildings and so forth could result in MS  18 , for example, being electrically farther away than MS  20  which is physically located near the extreme edge of cell  14 . An additional MS  22  is shown, where MS  22  is capable of receiving and/or transmitting HSD. 
     In  FIG. 2 , payload data is input to a Cyclic Redundancy Check (CRC) block  30  which adds to the total data transmitted. Additional data bits are inserted by a tail block  32 . In one prior art system, the data rate at the output of block  32  was 9600 bps (bits per second). The output of block  32  is applied to a forward error correction (FEC) circuit block  34 . For example, when code rate (R) is ½ (1/N=½), the output of block  34  is applied at a rate of 19,200 bps to a block interleaver  36  which performs a reordering of the bits comprising each frame of data passing through the block. The output of block  36  remains at the 19,200 bps rate and is applied to an inphase and quadrature phase (I/Q) mapping block  38  where 2 bits of data at a time are used to define in-phase and quad-phase output signals on leads  40  and  42  respectively. These outputs occur at a 9600 sps (symbols per second). The leads  40  and  42  are connected respectively to combining or multiplying means  44  and  46  respectively. A Walsh code is supplied on a lead  48  to a second input of each of the means  44  and  46 . The outputs of the multipliers  44  and  46  are supplied to a PN spreading block  50 , which has in- and quad-phase outputs on leads  52  and  54  respectively. 
       FIG. 3  repeats a portion of  FIG. 2  and shows blocks  34  through  46  utilizing the same designators.  FIG. 3  illustrates in addition that as the bit rate into the encoder is increased, the Walsh code length and accordingly the number of user channels that can be accommodated with a system decreases. 
       FIG. 4  shows a single user channel having identical bit rate inputs as presented in  FIG. 3  while employing a higher order modulation in order to increase the Walsh code lengths, as compared to  FIG. 3 , and thus increase the number of user channels available in the system. The approach presented in this figure, and the following  FIGS. 5 and 6 , is unique and was originated to utilize the same bit rate as presently used in the prior art. In the following explanation, the input will be assumed to be 9.6 kilo bits per second (kbps). The example of  FIG. 4  shows 8-PSK but similar techniques would allow the use of even higher order modulation, for example, 16-PSK or 16-QAM, or higher. An encoder  60 , having a code rate of ⅔, in contrast to the code rate ⅓ as in  FIG. 3 , passes its 14.4 kbps output signal to a puncture block  62 , where a selected one of each 4 bits is removed before passing the resulting 10.2 kbps bit stream to an 8-PSK mapping block  64 . The 8 PSK mapping uses 3 bits per symbol to provide an output of only 3.6 ksps (kilo symbols per second). The blocks  60 ,  62  and  64  are enclosed with a dash line to indicate the TCM Encoder portion of this circuitry where TCM is an acronym for Trellis Coded Modulation. In order to get the rate up to an amount necessary to obtain the proper Walsh code, each symbol is duplicated in a symbol repetition block  68  to obtain a symbol rate of 7.2 ksps. It is noted that the symbol repetition is optional. Alternately, if the symbols are not repeated then the Walsh code lengths can be doubled for each of the respective data rates. Of course, the longer Walsh code lengths support more user channels, however, there are other considerations which might impact this choice. For example, due to the coherence time of the channel, a shorter or longer Walsh code may be desired. A symbol block interleaver  70  then interleaves the symbols, followed by Walsh coding via multipliers  72  and  74 . A simple mathematical examination will prove that this process allows at least twice the number of user channels for a given payload data input rate as obtained with 4-PSK (QPSK). Note that without the symbol repetition, block  70 , four times the user channels are allowed. It may also be shown, either by testing or mathematics, that the summation of all the channels are orthogonal and provide the desired cancellation effect. 
     For QPSK, such as set forth in  FIG. 3 , the quadrature bit stream (i.e., in-phase and quadrature channels), is equivalent to two BPSK bit streams, one on the in-phase and one on the quadrature channel. Thus, in such a case, at the receiver, inverse mapping, with respect to the I/Q mapping shown in  FIG. 3 , is employed followed by decoding of the FEC encoded data stream. In contrast, however, for higher modulation orders (i.e., M&gt;4), the inverse of I/Q mapping is not as straightforward with respect to optimal detection of the FEC code. Therefore, the channel coding and modulation are combined (as is well understood in the art), whereby a Trellis code is employed at the transmitter, as shown in  FIG. 4 . 
     It is noted that a rate n/(n+1) (e.g., rate ⅔) Trellis code is readily available in the literature. In the subsequent description, various methods are identified for obtaining a desired code rate for the trellis codes, other than R=n/(n+1), which is compatible with the present invention. The optimal method for rate matching of the TCM encoding, however, will be a function of the Trellis Code design and thus the preferred method among those methods described herein (i.e.,  FIGS. 4-7 ) should be chosen accordingly. The rate matching methods described herein are symbol repetition, bit repetition, and puncturing. Puncturing is a method by which 1/m (m a positive integer larger than zero) bits or symbols are removed from the information stream in a prescribed fashion. For example, suppose the bit rate into an FEC block is n/(n+1), repetition of each bit (original plus one copy), followed by puncturing 1 of m bits. Then, the bit rate following the puncturing is given by 
             2   ·     1   R     ·         m   -   1     m     .           
In contrast, a code rate R=1/n FEC code for BPSK or QPSK modulation (M≦4) are readily available, as well as puncturing patterns.
 
     In  FIG. 5 , an encoder  80 , a bit repetition block  82 , a puncture block  84 , and an 8-PSK mapping block  86  form a TCM encoder  88 . In this figure, the bits, prior to puncturing, are repeated rather than the symbols such that the output bit rate of block  82 , assuming a bit rate into encoder  80  is 9.6 Kbps, is 28.8 Kbps. Removing ¼ of these bits results in a bit rate of 21.6 ksps at the input of block  86 . In block  86 , combining 3 bits per symbol produces the indicated output symbol rate of 7.2 Ksps. A block  90  and the associated multipliers  92  and  94  perform in the same manner as shown in  FIG. 4 . 
     In  FIG. 6 , an encoder  100 , a puncture block  102  and an 8-PSK mapping block  104  comprise the TCM encoder  106 . Since the code rate in block  100  is ⅓, the output of the encoder  100  is already at 28.8 kbps and thus repetition is not required (i.e., to get the correct bit rate to block  104 ), as occurred in  FIG. 5 . A block interleaver  108  and multipliers  110  and  112  operate as did similar blocks in  FIG. 5 . It may be noted that  FIG. 6  is based on a ⅓ rate FEC code, so therefore, the resulting TCM code may require development beyond that which is available in the current literature. If such development is required, it is believed straightforward for those skilled in the art along the lines of established mathematical methods. 
     In  FIG. 7 , an encoder  120  and an 8-PSK mapping block  122  form a TCM encoder  106 . In this figure, no puncturing is required since the code rate in block  120  is ⅔ and thus its output is 21.6 kbps with an input bit rate of 14.4 kbps. In block  122 , combining 3 bits per symbol produces the indicated output symbol rate of 7.2 Ksps. An interleaver block  126  and its associated multipliers  128  and  130  perform in the same manner as shown in  FIG. 5 . Although the bit rate input shown in  FIG. 7  varies from that presently used in QPSK CDMA systems, this approach has definite advantages in not requiring the puncturing of  FIGS. 4-6  or the repetition action of  FIGS. 4 and 5 .  FIG. 7  further uses presently available technology in that encoder  120  uses R=⅔. Finally, the straightforward architecture of  FIG. 7  is able to accommodate a given number of codes with a higher input data rate than occurs in  FIGS. 4-6 . It should be noted that a single digital processor chip may be programmed or configured to perform the functions required by the circuitry blocks shown in each of the  FIGS. 3 ,  4 ,  5 ,  6  and  7 . In other words, a digital processor may be programmed (or reprogrammed) to create either a base modulation order such as BPSK, 4-PSK or a higher modulation order such as 8 or 16 and provide the required orthogonal output. 
     With the above in mind, it should be apparent that a wireless network can be designed such that any given user channel may operate at either some system base modulation rate such as QPSK or at a higher modulation order. For example, the BTS unit may output several channels, where each of the individual code channels employ any of the aforementioned modulation orders, while all the code channels still maintain orthogonality with respect to one another. This alternate operational mode may be obtained, when circumstances require and/or the radio environment permits, by reprogramming the appropriate digital processor performing the function illustrated in any of the  FIGS. 3-7 . 
     A BTS has data available to the BTS as to how close electrically any given MS is to an antenna. For example, the power transmitted to an MS in a CDMA system may be adjusted to a level necessary to obtain good reception by an MS in accordance with data (or some indicator) returned to the BTS from the MS. This tends to optimize the system for power radiated by a BTS antenna as well as helping minimize interference between user channels. For example, depending on the complexity of the Trellis codes (i.e., complexity with respect to TCM code states), if the power required in supplying signals to a given MS is low compared to other MSs, it should be a good candidate for receiving signals using a higher modulation order. Such a determination is even more important when a given MS is provided data in the form of a high data rate since a large number of channels may be required for such an action. 
     While it is believed that the use of a higher order modulation for even some of the MS users in a system will allow more user channels to be active, some numerical examples will be set forth. 
     It may be assumed that a HSD user in a single order 4-PSK system such as presented in  FIG. 3  is assigned a Walsh code length of 4 where the base Walsh length is 256 as shown in the first line of  FIG. 3 . Although the user communicates at a very high rate, that single user consumes ¼ of the total Walsh codes. In such a situation, the system supports 1 HSD user +¾ of 256 other code channels for a total of 193 users (for the purpose of this explanation, a code channel is equated to a user). 
     If this HSD user operates in a system as set forth in the present invention where one or more channels may employ a higher modulation order than a base modulation order, then significantly more users may be accommodated. It may be assumed that the HSD user is electrically close enough to the BTS that the user may readily support a modulation order of 8 based on a Walsh length of 512. For a given total data rate, a Walsh length of 8 in such a system is equivalent to a Walsh length of 4 when the modulation order is 4. Thus a HSD user that can support 8-PSK would only consume ⅛ of the total 512 codes available. It may be noted that a “normal” (base modulation order—QPSK) user, effectively uses two 512 codes. Based upon the above description this system may support 1 HSD user +⅞ (512/2) for a total of 1+224 or 225 users. This increases the code channels by 32, where the only user of the higher modulation order is a single HSD unit. 
     Some of the regular users may also support a higher order transmission rate. It should thus be apparent that the maximum number of available orthogonal code channels can be significantly increased over that obtainable from prior art systems that supported only a single modulation order. 
     The present invention has been described primarily with respect to CDMA using 4-PSK as a base modulation order and 8-PSK as an alternate modulation order for some or all of the channels when the radio environment supports the higher modulation order. However, the invention is believed to cover all wireless systems, which may use different modulation orders in accordance with various factors including, but not limited to, the radio environment. 
     Although the invention has been described with reference to specific embodiments, these descriptions are not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore, contemplated that the claims will cover any such modifications or embodiments that fall within the true scope and spirit of the invention.