Non-Orthogonal Mobile Subscriber Multiple Access System

This invention relates generally to noise code communications systems and more particularly to a non-orthogonal code division multiple access communications system.

Multiple access communications systems utilizing noise coded signals are well known. Typical examples of such multiple accessing communications systems are shown and described in: U.S. Pat. No. 3,908,088, entitled, "Time Division Multiple Access Communications System", which issued to Frank S. Gutleber, the present inventor, on Sept. 23, 1975; U.S. Pat. No. 4,215,244, entitled, "Self-Adaptive Mobile Subscriber Access System Employing Time Division Multiple Accessing", which issued to Frank S. Gutleber on July 29, 1980; and U.S. Pat. No. 4,301,530, entitled "Orthogonal Spread Spectrum Time Division Multiple Accessing Mobile Subscriber Access System", which issued to Frank S. Gutleber on Nov. 17, 1981.

What is significant about the systems disclosed in the aforementioned patents is that they employ pairs of multiplexed noise codes of the type termed code mates having autocorrelation functions which upon detection provide an impulse autocorrelation function and wherein the configurations described enable random access or direct call-up to be accomplished with minimal interference between users. The codes utilized are called noise codes because the information to be communicated is coded with a digital code that is "noise like" in that it will compress to an impulse when detected with a matched filter.

The utilization of different noise codes for a plurality of users in a nonsynchronous mobile subscriber multiple access system has been found to be heretofore less than adequate because of self-interference between users. Such a system is referred to as a non-orthogonal system. Although each interference signal is reduced by the time-bandwidth product, interfering users in close proximity make for intolerable system operation.

Accordingly, it is an object of the present invention to provide an improved non-orthogonal noise code communications system.

Another object of the present invention is to provide a code division multiple access noise code communications system.

Still another object of the present invention is to provide means for cancelling noise coded interference in a nonorthogonal code division multiple access system.

And yet another object of the present invention is to provide a code division multiple access system which enables a large quantity of users to operate in the same confined geographical area without incurring unacceptable mutual interference.

Accordingly, these and other objects are achieved in a code division multiple access communications system including a plurality of users wherein each user employs a different noise code which is modulated on an RF carrier. Each user utilizes radio apparatus which is adapted to generate, transmit, receive and detect multi-bit digital noise codes while operating within the same RF frequency band. The receiver portion of each user apparatus is adapted to receive a desired noise coded signal from another user but additionally includes an adaptive interference cancelling system comprised of a plurality of matched filters coupled to the front end or RF demodulator section which operate to detect and identify any undesired and thus interfering noise codes coming from other close proximity users along with the desired signal. The matched filters are coupled to respective noise code generators which provide a coherent replica of the respective interference code but one devoid of the desired signal. The replica signal is then fed to signal subtractor means along with a composite input received signal which includes both the desired signal and any interference signal. The subtractor circuitry operates to cancel the interference signal(s) leaving the desired noise coded signal to be matched filter detected free of interference.

FIG. 1 is a simplified illustration of a non-orthogonal ground mobile access system employing code division multiple accessing;

FIG. 2 is a functional block diagram of the transmitter portion of apparatus for use with the communications system shown in FIG. 1; and

FIG. 3 is a functional block diagram of the receiver portion of apparatus for use with the communications system shown in FIG. 1 and illustrating the preferred embodiment of the subject invention.

Referring now to the drawings and more particularly to FIG. 1, there is illustrated a multiple access system such as a mobile subscriber access noise coded communications system employing code division multiple access (CDMA). Each user Nos. 1, 2, . . . n-1 and n operates with and accordingly generates a different respective multi-bit binary digital noise code 1 . . . n of the same number of bits of the spread spectrum type. Moreover, the codes generated are utilized to modulate RF frequency carriers in the same frequency band and as a consequence self-interference between the various users normally exists for those users which are geographically relatively close to one another. Since the rms value of the crosscorrelation function of different noise codes will be suppressed by the time-bandwidth product, users of the system that are not in close proximity to one another would receive or cause a negligible degree of interference to any other specific user having a receiver responsive to more codes operable in the aforementioned same frequency band. Those users that are in close proximity, however, will interfere with each other since the large increase in the amplitude or level of the interference due to the short range difference between a desired and interfering source of noise code communications signals would overcome the interference suppression associated with the margin of an n bit noise code. As an example, if an interfering user has a 10/1 range advantage, the received signal of the interfering user would be 100 times or 20 db stronger than a desired received signal which is received in a composite signal including the interfering signal. If the system shown in FIG. is implemented with radio apparatus, e.g. transceivers wherein each transceiver employed a 100 bit noise code, then the gain margin advantage of 100/1 or 20 db would be completely nullified and the output signal to interference ratio would be unity or Odb which is totally inadequate.

Further as shown in FIG. 1, in the deployment of user communications apparatus 10.sub.1, 10.sub.2, . . . 10.sub.n, user No. 2 communicating with user No. 1 would be interfered with by code 3 from user No. 3 as well as code 5 from user No. 5 in a standard non-orthogonal CDMA system. In a like manner, users 3 and 5 would be interfered with by the code from user No. 2 and codes 3 or 5 as the case may be. The interfering regions are indicated by the loops identified by reference numerals 12, 14 and 16.

The present invention is directed to a CDMA system involving a plurality of user communications apparatus operating with spread spectrum noise codes. The receiver portion of each apparatus includes an adaptive interference cancelling system which is responsive to all the codes of the other users for cancelling undesired close proximity user interference which affects the desired reception from another user.

Referring now to FIG. 2, each user communications apparatus also includes a transmitter portion 18 which in its broadest aspects is comprised of a noise code generator 20 coupled to a 1 through n code selector 22. The noise code selected and generated is fed to transmitter apparatus 24 which is coupled to a radiating antenna 26 which is adapted to radiate a noise code modulated RF signal to another user apparatus which includes a receiver portion 28, the details of which are shown in FIG. 3.

Referring now to FIG. 3, a receiving antenna 30 is coupled to a receiver front end section 32 which is adapted to demodulate the RF signal received from the transmitter 18 from another communicating user and provide an output at circuit node 34 which is a composite noise coded signal including not only the desired signal, but also any other interfering noise coded signals received from other users in close proximity. Adaptive interference cancelling circuitry 33 follows the receiver RF demodulator section 32 and is comprised of a bank i.e. a plurality 1, 2 . . . n of like circuits 36.sub.1, 36.sub.2 . . . 36.sub.n which are respectively adapted to detect a mutually different unwanted interfering noise code signal and regenerate it again free of the desired signal. More particularly, each circuit 36.sub.1, 36.sub.2, 36.sub.n is comprised of a matched filter 38 which is coupled to circuit node 34. Each matched filter 38 is operable to detect a particular noise code of codes 1 through n and establish the precise time of its arrival. Since the codes utilized by the various users 1 . . . n are comprised of spread spectrum noise codes, the matched filter 38 will compress the code and provide a high amplitude impulse output in a well known fashion which is fed to a video detector circuit 40 which is adapted to enhance the interfering signal by the time bandwidth product or equivalently the number of noise code bits so that positive detection and identification of strong interference signals is provided. The detected compressed signal from the detector 40 is next fed to a noise code generator 42 which is operable to coherently regenerate the same code structure detected; however, it is totally free of any other interfering code signal as well as the desired signal. The code generated in each of the circuits 36.sub.1, 36.sub.2 . . . 36.sub.n is next fed to one of a plurality of cascaded or tandemly connected signal subtractor circuits 44.sub.1, 44.sub.2 . . . 44.sub.n wherein a particular regenerated noise code is subtracted from a composite signal appearing at circuit node 34. The composite signal appearing at circuit node 34 is coupled to the first signal subtractor 44.sub.1 shown in FIG. 3 by means of a time delay circuit 46 which provides a time delay of .tau. to compensate for the delay introduced when compressing noise coded signals in the appropriate matched filter 38 and regenerating the respective interfering codes in the code generator 42. A variable gain amplifier 46.sub.1, 46.sub.2, 46.sub.n is furthermore coupled between the noise codes circuits 36.sub.1 , 36.sub.2 and 36.sub.n and their respective subtractor circuits 44.sub.1, 44.sub.2 and 44.sub.n to set the amplitude of the generated code equal to its respective value at the output of the front end section of the receiver 32.

It can be seen, therefore, that a coherent replica of any unwanted interference code is generated and appropriately fed to one of the subtractor circuits 44.sub.1, 44.sub.2 . . . 44.sub.n which also receives a composite signal consisting of both the desired signal and the interference signal, but due to the subtractive process, the output of the last subtractor circuit 44.sub.n comprises a signal free of interference signals which is then fed to a matched filter 48 that is matched to the code structure of the desired signal, which is compressed thereby and fed to a detector 50. The detector 50 provides an output signal that is n times greater than any residual unwanted interference signal that may still exist after subtracting all of the relatively strong interference signals out of the received composite signal appearing at circuit node 34.

While FIG. 3 discloses a bank of matched filters and code generators commonly connected to circuit node 34, it may be desirable to additionally include means, not shown, at the output of the receiver front end section 32 to sequentially search most or all of the codes utilized by the system to provide a correct match for a matched filter.

While the adaptive interference cancelling system shown in FIG. 3 can utilize any class of noise code, the use of multiplexed noise codes such as disclosed in the aforementioned U.S Pat. Nos. 3,908,088; 4,215,244; and 4,301,530 offer substantial advantages in that they not only compress to a lobeless impulse, but the quantity of these unique codes is extremely large for moderate time-bandwidth products. Furthermore, they are relatively easy to generate and compress and the apparatus required is relatively small in size, negligible in weight and low in cost. In an application of multiplexed codes involving relatively strong multipath interference signals, each interfering signal will be isolated when the composite signal is detected in the matched filter since the codes compress to a lobeless impulse when linearly added together. The composite compressed signal plus multipath interference signals would therefore automatically regenerate the original coded signal plus the multipath signal in a single code generator and provide cancellation of the composite interfering signal with a single amplitude adjustment.

Having thus shown and described what is at present considered to be the preferred embodiment of the invention, it should be understood that the same has been made by way of illustration and not of limitation and accordingly, all modifications, alterations and changes coming within the spirit and scope of the invention as defined in the following claims are herein meant to be included.