Method for transmitting wideband signals via a communication system adapted for narrow-band signal transmission

[0002] This application claims priority from U.S. provisional application Ser. No. 60/050,884 which was filed on Jun. 26, 1997, now abandoned, and from U.S. provisional application Ser. No. 60/052,574 which was filed on Jul. 15, 1997, now abandoned, the entire contents of the both provisional applications being incorporated herein by reference.

[0003] The present invention relates generally to radio communication systems, and more particularly to a method for transmitting wideband signals via a communications system adapted for transmitting narrow-band signals.

[0004] Radio communication systems rely on modulating carrier frequencies in a finite portion of the electromagnetic spectrum to wirelessly transmit and receive signals. Modulation can be performed on the amplitude, frequency, and/or phase of the carrier frequency to separate the signal from unwanted noise. The signals typically convey information such as voice, video, and computer data to and from transceiving devices such as cellular base stations, cellular subscriber units, and personal computers.

[0005] The portion of the electromagnetic spectrum occupied by a particular transmission or communication system (i.e. bandwidth) may be wide or narrow. Wideband signals can be used to transmit large amounts of data in a relatively short period of time. For example, large computer data files and real-time video could benefit from a wideband signal. Narrow-band signals can be used to conserve the electromagnetic spectrum when transmitting signals with more modest requirements. For example, base stations and cellular subscriber units in most conventional cellular communication systems transmit and receive voice signals using a relatively narrow-band signal.

[0006] The amount of usable electromagnetic spectrum is limited by technology, environment, and cost. Extremely high frequency signals require expensive transceiving equipment. Accordingly, communication systems benefit by sharing desirable frequencies. Well-known multiple access techniques, such as code division multiple access (CDMA), time division multiple access (TDMA), and frequency division multiple access (FDMA) can be used by a communication system to share the electromagnetic spectrum available to that system. However, these techniques require the signals to be of substantially the same bandwidth. For example, a cellular base station may transmit to a plurality of cellular subscriber units by dividing a portion of the spectrum (e.g., 869 MHz-894 MHz) into a plurality of relatively narrow-band channels (e.g., 30 kHz). Similarly, a satellite communication system may transmit to a plurality of ground stations by dividing a portion of the spectrum (e.g., 3700 MHz-4200 MHz) into a plurality of relatively wideband channels (e.g., 36 MHz). The narrow-band signals and wideband signals occupy distinct portions of the electromagnetic spectrum in order to avoid interfering with each other.

[0007] Prior art approaches to bandwidth utilization suffer from certain drawbacks. For instance, prior art approaches do not allow wideband signals to occupy excess capacity in a narrow band system or narrow-band signals to occupy excess capacity in a wide band system. Further, prior art approaches require new communication systems infrastructure (e.g., base stations) to support new types of signals (i.e., signals using different bandwidths).

[0008] In accordance with a first aspect of the invention, a method of transmitting signals having a first bandwidth via a communication system adapted for communicating signals of a second bandwidth, wherein the second bandwidth is narrower than the first bandwidth , is provided. The method comprises the steps of modulating a first data signal with a particular orthogonal code to produce a first spreaded signal having the first bandwidth and transmitting simultaneously the first spreaded signal and the signals of the second bandwidth so that the first bandwidth overlays the second bandwidth. Preferably, multiple users may share the first bandwidth using a time division multiple access protocol.

[0009] In certain embodiments, the method further comprises a step of transmitting a pilot signal during transmission of the first bandwidth signals. Preferably, the pilot signal is terminated approximately when the transmission of the first bandwidth signals are terminated. Further, a step of transmitting control data for the first bandwidth signals on a control channel shared by the first bandwidth users may optionally be performed using a time division multiple access protocol.

[0010] In a preferred embodiment, the method further comprises a step of encoding the first bandwidth signals with forward error correction codes. In such an instance, the forward error correction codes may optionally be turbo codes. Preferably, the first bandwidth signals originate at a base station and are addressed to a predefined receiver. Also preferably, the second bandwidth signals comprises voice signals and the receivers of the second bandwidth signals comprise cellular subscriber units.

[0011] In accordance with another aspect of the invention, a method of adapting a data transmission rate, in response to a data error rate and a transmitter power level, to increase transmission range is provided. The method comprises the steps of increasing the transmitter power level in response to the data error rate and determining a predefined threshold for the transmitter power level. The method further comprises the step of decreasing the data transmission rate in response to the transmitter power level being substantially equal to the predefined threshold, until a predefined error rate is achieved.

[0012] In some embodiments, the predefined threshold is a maximum power level or a predefined percentage of a maximum power level. Preferably, the predefined error rate is adjusted according to a type of data being transmitted. In such an instance, when the type of data being transmitted is substantially encoded voice signals, a first error rate is used and when the type of data being transmitted is substantially computer data, a second error rate is used.

[0013] In accordance with yet another aspect of the invention, a method of adapting a data transmission rate, in response to a data error rate and a transmitter power level, to increase data transmission rate is provided. The method comprises the steps of decreasing the transmitter power level in response to the data error rate and determining a predefined threshold for the transmitter power level. The method further comprises the step of increasing the data transmission rate in response to the transmitter power level being substantially equal to the predefined threshold, until a predefined error rate is achieved.

[0014] In some embodiments, the predefined threshold is a minimum power level or a predefined percentage of a maximum power level. Preferably, the predefined error rate is adjusted according to a type of data being transmitted. In such an instance, when the type of data being transmitted is substantially encoded voice signals, a first error rate is used and when the type of data being transmitted is substantially computer data, a second error rate is used.

[0015] In accordance with still another aspect of the invention, a method of transmitting a message intended for transmission on a first bandwidth via a communication system adapted for communicating signals of a second bandwidth, wherein the second bandwidth is narrower than the first bandwidth is provided. The method comprises the steps of determining a length for the message and determining a predefined threshold for message length. The method further comprises the step of transmitting the message using the first bandwidth signal via an access channel when the length is less than the predefined threshold.

[0016] These and other features and advantages of the present invention will become more apparent from a detailed consideration of the following detailed description of certain preferred embodiments when taken in conjunction with the drawings in which:

[0017] FIG. 1 is a schematic illustration of a communication system capable of utilizing the teachings of the present invention;

[0018] FIG. 2 is a spectral diagram of a wideband signal and narrow-band signals transmitted in accordance with the teachings of the invention;

[0019] FIG. 3 is a spectral diagram of a wideband signal and narrow-band signals transmitted in accordance with the teachings of the invention;

[0020] FIG. 4 is a diagram of Walsh tree;

[0021] FIG. 5 is a block diagram of a typical code-division multiple access transmitter and receiver;

[0022] FIG. 6 is a more detailed diagram of the transmitter and receiver of FIG. 5.

[0023] FIG. 7 is a flow chart of a program that can be implemented by the transceivers of FIG. 1 to increase transmission range; and,

[0024] FIG. 8 is a flow chart of a program that can be implemented by the transceivers of FIG. 1 to increase data transmission rate.

[0025] Although the following description focuses on methods for transmitting wideband signals via a radio communication system adapted for transmitting narrow-band signals, persons of ordinary skill in the art will readily appreciate that the techniques of the present invention are in no way limited to radio communication systems, systems transmitting signals with only two distinct bandwidths, or to systems adapted for transmitting narrow-band signals. On the contrary, any communication system which might benefit from shared access to a plurality of frequencies by two or more transceivers transmitting and/or receiving signals at two or more bandwidths may employ the techniques shown herein. Such systems might include systems employing methods for transmitting narrow-band signals via a radio communication system adapted for transmitting wideband signals. Further, wired systems such as computer networks could employ the techniques provided herein without departing from the scope of the invention.

[0026] A representative communication system capable of utilizing the teachings of the present invention is shown in FIG. 1. A base station 10, such as a cellular communications base station, is used to transmit and receive a plurality of relatively narrow-band signals 12, such as voice signals, to and from a plurality of narrow-band transceivers (transmitter/receiver) 14, such as cellular subscriber units. The base station 10 is typically coupled to a network 20, such as a public switched telephone network (PSTN), the Internet, and/or any other analog or digital network. Radio signals (narrow-band signals 12 and/or wideband signals 16) transmitted to the transceivers (narrow-band transceivers 14 and/or wideband transceivers 18) by the base station 10 could originate from a device connected to the network 20. Similarly, signals 12, 16 transmitted to the base station 10 by the transceivers 14, 18 could be destined for a device connected to the network 20.

[0027] The narrow-band signals 12 are transmitted by modulating one or more carrier frequencies 15 (shown in FIGS. 2 and 3) with relatively narrow bandwidths (e.g., 1.25 MHZ as is used for a conventional voice cellular channel) employing code division spread spectrum techniques. The narrow-band signals 12 could be analog voice signals, digitally encoded voice signals, short data messages, and/or any other narrow-band signals or combinations of signals. Cooperation among the narrow-band transceivers 14 to share the available electromagnetic spectrum can be achieved using well known multiple access techniques such as code-division multiple access (CDMA) and frequency-division multiple access (FDMA).

[0028] The base station 10 could also be used to transmit and receive a plurality of relatively wideband signals 16, such as data signals, to and from a plurality of wideband transceivers 18, such as computers. The wideband signal(s) 16 are transmitted by modulating a carrier frequency 15 with a relatively wide bandwidth employing code division spread spectrum techniques with higher capacity than the narrow-band signals described above (e.g., a bandwidth sufficient for transmitting a data signal such as 3.75 MHz). The wideband signals 16 could be analog video signals, digitally encoded video signals, long data messages, and/or any other wideband signals or combinations of signals. The wideband signals employ CDMA techniques. However, cooperation among the wideband transceivers 18 to share the available electromagnetic spectrum can be achieved using well known multiple access techniques such as time-division multiple access (TDMA). As is known in the art, CDMA techniques spread a signal over a larger bandwidth than that which is required for its transmission. As a result of this spreading gain, a wideband signal (consisting of one or more spread signals) may frequency overlay the narrow-band signals described above.

[0029] Preferably, the wideband signal 16 is encoded with forward error correction codes such as turbo codes, or any convolutional code. Also preferably, compensation is provided for interference cancellation during reception of the wideband signal 16 where the interference is caused by other wideband transmitters, such as an adjacent cell base station. For example, a well-known TDMA protocol could be used within the wideband system. Using a TDMA protocol within the wideband system allows multiple users to share a single code channel, thereby minimizing the number of codes and simplifying interference cancellation. In addition, it is preferred that a spread spectrum pilot signal be transmitted using the same carrier during transmission of the wideband signal 16 and terminated approximately when the transmission of the wideband signal is terminated, thus providing coherent detection, but without causing undesirable intracell interference. Further, control data for the wideband signal 16 could be transmitted on the common control channel which utilizes a single Walsh code and is shared by the wideband transceivers 18 in a TDMA protocol.

[0030] In conventional wireless communication systems, these two signal types (wideband and narrow-band) would occupy distinct regions of the electromagnetic spectrum to avoid interference. However, in a preferred embodiment of the present invention, the electromagnetic spectrum available to the plurality of narrow-band signals 12 overlays the electromagnetic spectrum available to the wideband signals 16. As a consequence of the well-known spreading gain provided by the spread spectrum signals, this overlay is possible. In accordance with the present invention, while the narrow-band signals are transmitted, the wideband signals can share the same spectrum using CDMA techniques generally, and more particularly, by using an orthogonal modulation method that 1) selectively aligns or spaces the narrow band carrier frequency/frequencies with respect to the wideband carrier frequency, 2) spreads wideband signals with orthogonal codes particularly selected to minimize correlation between wideband and narrow-band signals. and 3) spreads the wideband signals by dividing them into a plurality of data streams, spreading the individual data streams, and then recombining the data streams before transmission.

[0031] Illustrated in FIG. 2 and FIG. 3 are spectral diagrams of a wideband signal 16 (i.e., an overlay signal) overlapping a plurality of narrow-band signals 12 (i.e., underlay signals). Although a limited number of signals 12, 16 are shown, any number of narrow-band signals 12 and wideband signals 16 may be accommodated using the teachings of the present invention. Namely, one or many wideband signals 16 may overlay one or many narrow-band signals 12. In FIG. 2, the wideband signal 16 and one of the narrow-band signals 12buse substantially the same carrier frequency 15b. However, another narrow-band signal 12auses a different carrier frequency 15aseparated by an offset 23a. In FIG. 3, the wideband signal 16 uses one carrier frequency 15band a narrow-band signal 12cuses a different carrier frequency 15cseparated by a different offset 23b. In a typical CDMA communication system orthogonal codes (e.g., Walsh codes) are used to spread a data stream and differentiate transmissions. In an alternate embodiment, by systematically separating the orthogonal codes and the carrier frequencies 15 used by base stations 10 and transceivers 14, 18 in a CDMA system, wideband signals 16 can be transmitted substantially simultaneously with narrow-band signals 12.

[0032] Typically, each user is assigned one or more orthogonal codes from a set of mutually orthogonal codes (e.g., Walsh codes) during the setup of a call. Each input data stream is mapped to an orthogonal code (e.g., a logical one maps to the orthogonal code itself and logical zero maps to its binary complement or vice versa). The set of Walsh codes can be generated using Hadamard matrix. A Hadamard matrix of order n can be defined recursively as: $W_{2n}=\left[\begin{array}{cc}{W}_n& W_n\\ W_n& \stackrel{\_}{W_n}\end{array}\right]$

[0033] Where W₁is defined as W₁[O] and {overscore (W)}₁ is the binary complement of W₁. A Walsh code of length 2ncorresponds to a row of the matrix W_2n.

[0034] An alternative way to describe the Walsh codes is illustrated in FIG. 4. The unique Walsh code of length one sits at the root of the tree. Each node of the tree represents a valid Walsh code. The two nodes at the second level represents the two Walsh codes of length 2, etc. Nodes at the same level represents the Walsh codes of the same length. For any given two Walsh codes (two nodes in the tree, not necessarily at the same level), we can trace the tree toward the root until the traced back path first meets at a node. This common node along the traced back paths of two different Walsh codes is defined as the first common node. Note that the Walsh code represented by the first common node of two Walsh codes is the prefix of both Walsh codes and it is the largest common prefix.

[0035] A typical CDMA communication system is shown in FIG. 5. A transmitter 21 (one or more of which is integral to the base station 10 and/or transceivers 14, 18) spreads a data signal 22, representing a message to be sent, with a spread signal 26 and mixes the result with a carrier frequency 15 to produce a radio signal (narrow-band signal 12 or wideband signal 16). The radio signal 12, 16 is captured by a receiver 28 (integral to the base station 10 and/or transceivers 14, 18) which recovers an estimate 30 of the data signal 22 by mixing the radio signal 12, 16 with a local carrier frequency 17 which downconverts the radio signal to an intermediate frequency or baseband and despreads 34 the signal.

[0036] A more detailed diagram of the transmitter/spreader 21 and receiver/despreader 28 of FIG. 5 is shown in FIG. 6. The data signal 22 to be transmitted is preferably encoded with a forward error correction code by a conventional convolutional encoder 36, typically comprising shift registers and modulo-2 adders. Since convolutional encoding only works most efficiently on random (non-consecutive) errors, and since burst errors are common in wireless environments, the encoded signal is typically interleaved by an interleaver 38. The interleaver 38 shuffles the bits into a random sequence, thus making burst errors appear to be random errors after deinterleaving, and outputs the random sequence to an orthogonal encoder 40.

[0037] As can be appreciated from FIG. 6, the orthogonal encoder 40 includes a demultiplexer 41 and a plurality of modulators 42. The demultiplexer 41 demultiplexes the interleaved data stream is split into a plurality of paths, for example, if three narrow-band underlay signals are expected, then the interleaved data stream is split into three parallel streams, and input to the modulators 42. Each data stream is spread by a Walsh code, such that the prefix of this orthogonal Walsh code being used is preferably different from all other orthogonal codes currently being used to encode transmitted signals in the same geographical area and frequency band used by the underlay narrow-band signals. The multiple outputs of the Walsh modulators 42 are provided to transmitters 44, and are serially combined and ultimately transmitted as a single wideband overlay signal from antenna 46 as further shown in FIG. 6.

[0038] If more than one carrier frequency 15 is used by the narrow-band underlay, each is preferably separated by an offset 23 substantially equal to an integer multiple of the narrow-band signal's 12 chip rate. For example, if three narrow-band signals 12 and one wideband signal 16 are transmitted simultaneously (see FIG. 2), the wideband signal 16 and the first narrow-band signal 12bcould be transmitted at a first carrier frequency 15b. The second narrow-band signal 12acould be transmitted at a second carrier frequency 15aoffset 23abelow the first carrier frequency 15bby an amount substantially equal to the narrow-band signals' 12 chip rate. Similarly, the third narrow-band signal 12ncould be transmitted at a third carrier frequency 15noffset above the first carrier frequency 15bby an amount substantially equal to the narrow-band signals' 12 chip rate.

[0039] As further shown in FIG. 6, the wideband overlay signal can be received at antenna 48 of a rake receiver 50. The rake receiver 50 outputs the received signal to an orthogonal decoder 51, which includes demodulators 52 that demodulate the signal and output the demodulated signal to a multiplexer 54. The multiplexer 54 multiplexes the demodulated signal and outputs the multiplexed decoded demodulated signal to a deinterleaver 56. The deinterleaver 56 outputs the multiplexed decoded demodulated deinterleaved signal to a convolutional decoder 58 which further decodes the signal to produce the recovered data signal 30.

[0040] In yet another alternative, each underlay narrow-band carrier frequency 15 is preferably separated by an offset 23 substantially equal to an odd multiple of one half the chip rate of the narrow-band signal 12. For example, if two narrow-band signals 12 and one wideband signal 16 are transmitted simultaneously (see FIG. 3), the wideband signal 16 could be transmitted at a first carrier frequency 15b. The first narrow-band signal 12ccould be transmitted at a second carrier frequency 15coffset below the first carrier frequency 15bby an amount substantially equal to one half the narrow-band signals' 12 chip rate. Similarly, the second narrow-band signal 12dcould be transmitted at a third carrier frequency 15doffset above the first carrier frequency 15bby an amount substantially equal to one half the narrow-band signals' 12 chip rate.

[0041] In a further embodiment of the present invention, the length of a message intended for transmission by a wideband transceiver 18 using the wideband signal 16 (see FIG. 1) is compared to a predefined threshold for the message length by a comparator 19 (See FIG. 5). If the length of the message to be transmitted is below the threshold, the message is transmitted using the common access channel which is shared by the users of the wideband signal.

[0042] In another embodiment of the present invention, the range at which the transceivers 14, 18 are able to transmit signals 12, 16 is increased. FIG. 7 illustrates a flow chart of a program that can be implemented by the transceivers 14, 18 to increase transmission range. The programmed steps are performed by a control circuit such as a microprocessor or application specific integrated circuit (ASIC) as is conventional. Once the program is initiated by a transceiver 14, 18 (e.g., a cellular subscriber unit is turned on or a computer needs to transmit data), the control circuit determines if the current error rate is greater than a predetermined error rate (block 180). The predetermined error rate could vary based on the type of data being transmitted. For example, voice signals may tolerate errors more readily than computer data and may therefore be transmitted with a higher error rate. If the current error rate is not greater than the predetermined error rate, the control circuit continues to monitor the error rate as long as the program is executing (block 180). However, if the current error rate is greater than the predetermined error rate, the control circuit determines if the current transmitter power level is substantially equal to a predefined threshold for the transmitter power level (block 182). The predefined threshold for the transmitter power level could be a maximum power level or a predefined percentage of a maximum power level (e.g., the most power allowed by a standard or regulation or the most power the transceiver can produce based on design constraints or current battery level). If the current transmitter power level is not substantially equal to the predefined threshold for the transmitter power level, the control circuit increases the transmitter power level (block 184). However, if the current transmitter power level is substantially equal to the predefined threshold for the transmitter power level, the control circuit decreases the data transmission rate (block 186). By adapting the data transmission rate, in response to the data error rate and transmitter power level, the transmission range can be increased.

[0043] In yet another embodiment of the present invention, the data transmission rate at which the transceivers 14, 18 are able to transmit signals 12, 16 is increased. FIG. 8 illustrates a flow chart of a program that can be implemented by the transceivers 14, 18 to increase the data transmission rate. The programmed steps are performed by a control circuit such as a microprocessor or application specific integrated circuit (ASIC) as is conventional. Once the program is initiated by a transceiver 14, 18 (e.g., a cellular subscriber unit is turned on or a computer needs to transmit data), the control circuit determines if the current error rate is less than a predetermined error rate (block 190). Again, the predetermined error rate could vary based on the type of data being transmitted. If the current error rate is not less than the predetermined error rate, the control circuit continues to monitor the error rate as long as the program is executing (block 190). However, if the current error rate is less than the predetermined error rate, the control circuit determines if the current transmitter power level is substantially equal to a predefined threshold for the transmitter power level (block 192). The predefined threshold for the transmitter power level could be a minimum power level or a predefined percentage of a maximum power level. If the current transmitter power level is not substantially equal to the predefined threshold for the transmitter power level, the control circuit decreases the transmitter power level (block 194). However, if the current transmitter power level is substantially equal to the predefined threshold for the transmitter power level, the control circuit increases the data transmission rate (block 196). By adapting the data transmission rate, in response to the data error rate and transmitter power level, the data transmission rate can be increased.

[0044] In summary, persons of ordinary skill in the art will readily appreciate that a method for transmitting wideband signals via a communications system adapted for transmitting narrow-band signals has been provided. Systems and apparatus implementing the teachings of the invention can enjoy increased efficiency in bandwidth utilization.

[0045] The foregoing description has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teachings. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.