OUT OF CHANNEL CYCLIC REDUNDANCY CODE METHOD FOR A DISCRETE MULTITONE SPREAD SPECTRUM COMMUNICATIONS SYSTEM

Abstract: In a discrete multitone spread spectrum system, a base station distinguishes between normal collisions and noise bursts when receiving access request signals for remote units on a common access channel. The base station is then able to reply to the remote units with information about the quality of the common access channel and why their transmissions where not successful. The remote units then use this information to adapt their retry processes to the channel's quality, depending on whether there was a noise burst, a normal collision, or a successful transmission on the channel.

Description: TITLE OF THE INVENTION: OUT OF CHANNEL CYCLIC REDUNDANCY CODE METHOD FOR A DISCRETE MULTITONE SPREAD SPECTRUM COMMUNICATIONS SYSTEM 5 Cross-References to Related Applications: The invention disclosed herein is related to die copending US patent application by Brian Agee, et al. entitled "HIGHLY BANDWIDTH EFFICIENT COMMUNICATIONS", serial number , filed on the same day as the instant patent application, assigned to AT&T, and incorporated herein by reference. Thdthvention disclosed herein is related to the copending US patent application by Siavash Alamouti, Doug Stolarz, and Joel Becker, entitled " VERTICAL ADAPTIVE ANTENNA ARRAY FOR A DISCRETE MULTITONE SPREAD SPECTRUM COMMUNICATIONS SYSTEM", serial number , filed on the same day as the instant patent application, assigned to AT&T, and incorporated herein by reference. Theifivention disclosed herein is related to the copending US patent application by Alamouti, Michaelson, et al., entitled "PWAN Frequency Division Duplex Communications System" , serial number filed on the same day as the instant patent application, assigned to AT&T, adn incorporated herein by reference. 20 Background of the Invention Field of the Invention This invention involves improvements to communications systems and methods in a wireless discrete multitone spread spectrum communications system. 25 Description of Related Art Wireless communications systems, such as cellular and personal communications systems, operate over limited spectral bandwidths. They must make highly efficient use of the scarce bandwidth resource to provide good service to a large population of users. Code Division Multiple Access (CDMA) protocol has been used by wireless communications systems to efficiently make use 30 of limited bandwidths. The protocol uses a unique code to distinguish each user's data signal from other users' data signals. Knowledge of the unique code with which any specific information is transmitted, permits the separation and reconstruction of each user's message at the receiving end of the communication channel. Adaptive beamforming technology has become a promising technology for wireless service providers to offer large coverage, high capacity, and high quality service. Based on this technology, a wireless communication system can improve its coverage capability, system capacity, and performance significantly. The personal wireless access network (PWAN) system described in the referenced Agee, et al and Alamouti, et al. patent applications, uses adaptive beamforming combined with a form of the CDMA protocol known as discrete multitone spread spectrum ( DMT-SS ) to provide efficient communications between a base station and a plurality of remote units. (The Agee, et al. patent application uses the term "discrete multitone stacked carrier (DMT-SC) to refer to this protocol.) Every effort must be made to avoid loading normal, high priority traffic channels with system management information that has a lower priority. An example of system management information is the characterization of channel quality factors that are not immediately needed to control the realtime operation of the network. What is needed is a way to offload the communication of system management information from high priority traffic channels. Summary of the Invention The invention disclosed herein is a new method to make the most efficient use of the scarce spectral bandwidth in a wireless discrete multitone spread spectrum communications system. Each remote station and each base station in the network prepares an error detection code, such as a cyclic redundancy code (CRC), on each block of data to be transmitted over the traffic channels. The CRC value computed for a given block of data is a unique mapping of the data block that characterizes the data block. Any change in the data block will result in a different CRC value. Each data block to be sent is numbered with a block number to distinguish it from other data blocks being sent by a sending station. A data message is formed by concatenating the data block with the block number. A CRC value is computed for each data block. An error detection message is formed by concatenating the CRC value with the block number. The sending station prepares the data message by forming data vectors that will be spread using the discrete multitone spread spectrum ( DMT-SS ) protocol to distribute the data message over a plurality of discrete tone frequencies, forming a spread signal for the traffic channel. In accordance with the invention, the sending station prepares the error detection message for transmission over the link control channel of the network. The sending station prepares the error detection message by forming a link control channel vector that will be spread using the discrete multitone spread spectrum ( DMT-SS ) protocol to distribute the data message over a plurality of discrete tone frequencies, forming a spread signal for the link control channel. A link control channel is associated with communications session using the traffic channels. Normally, the link control channel carries control information needed by the sending and receiving stations during a session using the traffic channels. However, its capacity is under-utilized. In accordance with the invention, the instant of transmission of the error detection message is allowed to be different from the instant of transmission of the data message. This permits the error detection messages to be transmitted when capacity is available on the link control channel. The receiving station buffers the error detection messages it receives from the link control channel, so that they are accessible by their block numbers. When the receiving station receives a data message on the traffic channel, it performs a CRC calculation on the data block in the message to obtain a resulting new CRC value. The new CRC value is also buffered at the receiving station with the block number so that it is accessible by its block number. Then, when both the received error message and the new CRC value are both available at the receiving station, they are matched by their common block number. The received CRC value in the error detection message is compared with the new CRC computed from the received data block. If the comparison determines that there is a difference in the values, then an error signal is generated. The error signal can be processed and used in several ways. The error signal can initiate a negative acknowledgement signal to be send from the receiving station back to the sender requesting the sender to repeat the data block transmission. The error signal can initiate an update in the spreading and despreading weights at the receiving station in an effort to improve the signal and interference to noise ratio of the traffic channel. The error signal can initiate an alarm to be used for other realtime control. Or, the error signal can be logged for the compilation of a longer term report of the traffic channel quality. Currently, the invention has advantageous applications in the field of wireless communications, such as cellular communications or personal communications, where bandwidth is scarce compared to the number of the users and their needs. Such applications may be effected in mobile, fixed, or minimally mobile systems. However, the invention may be advantageously applied to other, non-wireless, communications systems as well. Brief Description of the Drawings In the drawings: FIGURE 1 is an architectural diagram of the PWAN system, including remote stations transmitting to a base station. FIGURE 2 is an architectural diagram of the remote station X as a sender. FIGURE 3 is an architectural diagram of the base station Z as a receiver. FIGURE 4 is a more detailed architectural diagram of the vector disassembly and CRC comparison logic at a receiving station. FIGURE 5 is an architectural diagram of the base station Z as a sender. FIGURE 6 is an architectural diagram of the remote station X as a receiver. FIGURE 7 is a flow diagram showing the remote station as the sender and the base station as the receiver. FIGURE 8 is a flow diagram showing the base station as the sender and the remote station as the receiver. Description of the Preferred Embodiment FIGURE 1 an architectural diagram of the personal wireless access network (PWAN) system described in the referenced Agee, et al and Alamouti, et al. patent applications. Two users, Alice and Bob, are located at the remote station X and wish to transmit their respective data messages to the base station Z. Station X is positioned to be equidistant from the antenna elements A, B, C, and D of the base station Z. Two other users, Chuck and Dave, are located at the remote station Y and also wish to transmit their respective data messages to the base station Z. Station Y is geographically remote from Station X and is not equidistant from the antenna elements A, B, C, and D of the base station Z. The remote stations X and Y and the base station Z use the form of the CDMA protocol known as discrete multitone spread spectrum ( DMT-SS ) to provide efficient communications between the base station and the plurality of remote station units. This protocol is designated in Figure 1 as multi-tone CDMA. In this protocol, the user's data signal is modulated by a set of weighted discrete frequencies or tones. The weights are spreading weights that distribute the data signal over many discrete tones covering a broad range of frequencies. The weights are complex numbers with the real component acting to modulate the amplitude of a tone while the complex component of the weight acts to modulate the phase of the same tone. Each tone in the weighted tone set bears the same data signal. Plural users at the transmitting station can use the same tone set to transmit their data, but each of the users sharing the tone set has a different set of spreading weights. The weighted tone set for a particular user is transmitted to the receiving station where it is processed with despreading weights related to the user's spreading weights, to recover the user's data signal. For each of the spatially separated antennas at the receiver, the received multitone signals are transformed from time domain signals to frequency domain signals. Despreading weights are assigned to each frequency component of the signals received by each antenna element. The values of the despreading weights are combined with the received signals to obtain an optimized approximation of individual transmitted signals characterized by a particular multitone set and transmitting location. The PWAN system has a total of 2560 discrete tones (carriers) equally spaced in 8 MHZ of available bandwidth in the range of 1850 to 1990 MHZ. The spacing between die tones is 3.125 kHz. The total set of tones are numbered consecutively form 0 to 2559 starting from the lowest frequency tone. The tones are used to carry traffic messages and overhead messages between the base station and the plurality of remote units. The traffic tones are divided into 32 traffic partitions, with each traffic channel requiring at least one traffic partition of 72 tones. In addition, the PWAN system uses overhead tones to establish synchronization and to pass control information between the base station and the remote units. A Common Link Channel (CLC) is used by the base to transmit control information to the Remote Units. A Common Access Channel (CAC) is used to transmit messages from the Remote Unit to the Base. There is one grouping of tones assigned to each channel. These overhead channels are used in common by all of the remote units when they are exchanging control messages with the base station. In the PWAN system, Time Division Duplexing (TDD) is used by the base station and the remote unit to transmit data and control information in both directions over the same multi-tone frequency channel. Transmission from the base station to d e remote unit is called forward transmission and transmission from the remote unit to the base station is called reverse transmission. The time between recurrent transmissions from either the remote unit or the base station is the TDD period. In every TDD period, there are four consecutive transmission bursts in each direction. Data is transmitted in each burst using multiple tones. The base station and each remote unit must synchronize and conform to the TDD timing structure and both the base station and the remote unit must synchronize to a framing structure. All remote units and base stations must be synchronized so that all remote units transmit at the same time and then all base stations transmit at me same time. When a remote unit initially powers up, it acquires synchronization from the base station so that it can exchange control and traffic messages within the prescribed TDD time format. The remote unit must also acquire frequency and phase synchronization for the DMT-SS signals so that the remote is operating at the same frequency and phase as the base station. Selected tones within each tone set are designated as pilots distributed throughout the frequency band. Pilot tones carry known data patterns mat enable an accurate channel estimation. The series of pilot tones, having known amplitudes and phases, have a known level and are spaced apart by approximately 30 KHz to provide an accurate representation of the channel response (i.e., the amplitude and phase distortion introduced by the communication channel characteristics) over the entire transmission band. In accordance with the invention, each remote station and each base station in the network prepares an error detection code, such as a cyclic redundancy code (CRC), on each block of data to be transmitted over me traffic channels. A variety of error detecting codes can be used, in accordance with the invention. Polynomial codes, also known as cyclic redundancy codes, are preferred for the invention. The sender and receiver must agree on a generator polynomial in advance of the communication. A checksum is computed for a data block based on me generator polynomial. The checksum is a unique mapping of me data block. Any changes to the bit pattern of the datablock will result in a different checksum. Examples of error detecting codes for the preferred embodiment of the invention are given in Tanenbaum, "Computer Networks", second edition, Prentice-Hall, 1989. The CRC value computed for a given block of data is a unique mapping of the data block that characterizes the data block. Any change in the data block will result in a different CRC value. Each data block to be sent is numbered witfi a block number to distinguish it from otiier data blocks being sent by a sending station. A data message is formed by concatenating die data block wim the block number. A CRC value is computed for each data block. An error detection message is formed by concatenating the CRC value with the block number. The sending station prepares the data message by forming data vectors that will be spread using the discrete multitone spread spectrum ( DMT-SS ) protocol to distribute the data message over a plurality of discrete tone frequencies, forming a spread signal for the traffic channel. In accordance witi the invention, the sending station prepares the error detection message for transmission over the link control channel of the network. The sending station prepares the error detection message by forming a link control channel vector that will be spread using the discrete multitone spread spectrum ( DMT-SS ) protocol to distribute the data message over a plurality of discrete tone frequencies, forming a spread signal for the link control channel. A link control channel is associated with communications session using the traffic channels. Normally, the link control channel carries control information needed by the sending and receiving stations during a session using the traffic channels. However, its capacity is under-utilized. In accordance with the invention, the instant of transmission of the error detection message is allowed to be different from the instant of transmission of the data message. This permits the error detection messages to be transmitted when capacity is available on the link control channel. The receiving station buffers the error detection messages it receives from the link control channel, so that ti ey are accessible by their block numbers. When the receiving station receives a data message on the traffic channel, it performs a CRC calculation on the data block in the message to obtain a resulting new CRC value. The new CRC value is also buffered at the receiving station with the block number so that it is accessible by its block number. Then, when boui the received error message and the new CRC value are both available at the receiving station, they are matched by their common block number. The received CRC value in the error detection message is compared with the new CRC computed from the received data block. If the comparison determines that mere is a difference in the values, then an error signal is generated. The error signal can be processed and used in several ways. The error signal can initiate a negative acknowledgement signal to be send from the receiving station back to the sender requesting the sender to repeat die data block transmission. The error signal can initiate an update in the spreading and despreading weights at die receiving station in an effort to improve die signal and interference to noise ratio of die traffic channel. The error signal can initiate an alarm to be used for otiier realtime control. Or, the error signal can be logged for the compilation of a longer term report of the traffic channel quality. FIGURE 2 is an architectural diagram of the remote station X as a sender. Alice and Bob each input data to remote station X. The data is sent to the vector formation buffer 202 and also to the cyclic redundancy code generator 204. Data vectors are output from buffer 202 to the trellis encoder 206. The data vectors are in the form of a data message formed by concatenating a 64K-bit data block witii its serially assigned block number. The LCC vectors output from the CRC generator 204 to the trellis encoder 206 are in the form of an error detection message formed by concatenating the CRC value with the block number. The trellis encoded data vectors and LCC vectors are then output to the spectral spreading processor 208. The resultant data tones and LCC tones are then output from processor 208 to the transmitter 210 for transmission to the base station. The first four steps in the flow diagram 700 of Figure 7 show the steps at remote station X when it is the sender. The steps in the metiiod of transmission from a remote station to a base station are first for the Remote Station in step 710 to generate a CRC value on the data block, assign a data block number to die CRC value, and concatenate the CRC value and die block number in a error message which is input as a vector to the link control channel (LCC). Then in step 720, the Remote Station performs trellis encoding of the CRC link control channel vector and die data block vectors. Then in Step 730, the Remote Station performs spectral spreading of die trellis encoded CRC link control channel vector and data block vectors. Then in Step 740, the Remote Station transmits die CRC link control channel tone and data block tones to the base station. The personal wireless access network (PWAN) system described in the referenced Agee, et al and Alamouti, et al. patent applications provides a more detailed description of a high capacity mode, where one traffic partition is used in one traffic channel. The Base transmits information to multiple Remote Units in its cell. The transmission formats are for a 64 kbits/sec traffic channel, together witii a 4 kbps Link Control Channel (LCC) between die Base and a Remote Unit. The binary source delivers data to the sender's transmitter at 64 kbits/sec. This translates to 48 bits in one transmission burst. The information bits are encrypted according to a triple data encryption standard (DES) algorithm. The encrypted bits are tiien randomized in a data randomization block. A bit to octal conversion block converts d e randomized binary sequence into a sequence of 3-bit symbols. The symbol sequence is converted into 16 symbol vectors. The term vector generally refers to a column vector which is generally complex. One symbol from the LCC is added to form a vector of 17 symbols. The 17-symbol vector is trellis encoded. The trellis encoding starts with the most significant symbol (first element of the vector) and is continued sequentially until the last element of the vector (die LCC symbol). This process employs convolutional encoding tiiat converts the input symbol (an integer between 0 and 7) to anodier symbol (between 0 and 15) and maps die encoded symbol to its corresponding 16QAM (or 16PSK) signal constellation point. The output of the trellis encoder is therefore a vector of 17 elements where each element is signal within d e set of 16 QAM (or 16PSK) constellation signals. (The term signal will generally refer to a signal constellation point.) A link maintenance pilot signal (LMP) is added to form an 18-signal vector, with die LMP as the first elements of the vector. The resulting (18 X 1) vector is pre-multiplied by a (18 x 18) forward smearing matrix to yield a (18 x 1) vector b. Vector b is element-wise multiplied by die (18 x 1) gain preemphasis vector to yield anodier (18 x 1) vector, c, where p denotes die traffic channel index and is an integer. Vector c is post-multiplied by a (1 x 32) forward spatial and spectral spreading vector to yield a (18 x 32) matrix R(p). The number 32 results from multiplying die spectral spreading factor 4 and spatial spreading factor 8. The 18 x 32 matrices corresponding to all traffic channels carried (on the same traffic partition) are then combined (added) to produce die resulting 18 x 32 matrix S. The matrix S is partitioned (by groups of four columns) into eight (18 x 4) submatrices (AQ to A 7 ). (The indices 0 to 7, corresponds to the antenna elements over which these symbols will eventually be transmitted.) Each submatrix is mapped to tones witiiin one traffic partition. A lower physical layer places the baseband signals in discrete Fourier transfer (DFT) frequency bins where the data is converted into me time domain and sent to its corresponding antenna elements (0 to 7) for transmission over the air. This process is repeated from die start for the next 48 bits of binary data to be transmitted in die next forward transmission burst. FIGURE 3 is an architectural diagram of the base station Z as a receiver. The data tones and LCC tones are received at the base station antennas A, B, C, and D. The receiver 310 passes the data tones and the LCC tones to the spectral and spatial despreading processor 312. The despread signals are then output from the processor 312 to the trellis decoder 314. The data vectors 400, 400', and 400" are then output to the vector disassembly buffer 316, shown in greater detail in Figure 4. The LCC vectors 402, 402', and 402" are output to the CRC comparison processor 320, shown in greater detail in Figure 4. Alice's data and Bob's data are output from the buffer 316 to the public switched telephone network (PSTN). Alice's data and Bob's data are also input to the CRC generator 318. CRC generator 318 computes a new CRC value for every 64 K-bit data block and outputs the new CRC value and the block number to the buffer 406 of the CRC comparison processor 320. FIGURE 4 is a more detailed architectural diagram of die vector disassembly and CRC comparison logic at a receiving station. The receiving station buffers in the CRC comparison processor 320 the error detection messages it receives from die link control channel, so that ey are accessible by their block numbers N, N+ l, N + 2, etc. When die receiving station receives a data message on die traffic channel, it performs a CRC calculation on die data block in die message witii CRC generator 318 to obtain a resulting new CRC value. The new CRC value is buffered in buffer 406 at die receiving station with the block number so that it is accessible by its block number. Then, when both die received error message and die new CRC value are both available at the receiving station, they are matched by selector 404 by tiieir common block number. The received CRC value in die error detection message 402 is compared with die new CRC computed from the received data block 400 by means of the comparator 408. If the comparison determines that there is a difference in the values, tiien an error signal is generated by generator 322. The error signal can be processed and used in several ways by the error processor 330. The error signal can initiate a negative acknowledgement signal to be sent from the receiving station back to the sender requesting the sender to repeat die data block transmission. The error signal can initiate an update in die spreading and despreading weights at die receiving station in an effort to improve the signal and interference to noise ratio of die traffic channel. The error signal can initiate an alarm to be used for other realtime control. Or, die error signal can be logged for d e compilation of a longer term report of die traffic channel quality. The last five steps in the flow diagram of Figure 7, show the base station as die receiver. In Step 750, die Base Station performs spectral and spatial despreading of die CRC link control channel tone and data block tones. Then, in Step 760, die Base Station performs trellis decoding of despread CRC link control channel tone and data block tones. Then in Step 770, the Base Station generates new a CRC value on the data block and uses die block number to select die corresponding CRC vector received from die link control channel. Then in Step 780, the Base Station compares the new CRC value computed on die received data block witii the CRC vector received from the link control channel. Then in Step 790, the Base Station generates an error signal if the new CRC does not compare with die received CRC. FIGURE 5 is an architectural diagram of the base station Z as a sender and FIGURE 6 is an architectural diagram of the remote station X as a receiver. FIGURE 8 is a flow diagram showing the base station as die sender and die remote station as the receiver. These three figures illustrate a communications direction opposite to that shown in Figures 2, 3, and 7. The same principle of the invention applies to figures 5, 6, and 8 as the principles discussed for Figures 2, 3, and 7. FIGURE 5 is an architectural diagram of the base station as a sender. The PSTN inputs data to base station Z. The data is sent to the vector formation buffer 502 and also to the cyclic redundancy code generator 504. Data vectors are output from buffer 502 to the trellis encoder 506. The data vectors are in the form of a data message formed by concatenating a 64 K-bit data block witii its serially assigned block number. The LCC vectors output from the CRC generator 504 to the trellis encoder 506 are in the form of an error detection message formed by concatenating die CRC value witii the block number. The trellis encoded data vectors and LCC vectors are then output to the spectral and spatial spreading processor 508. The resultant data tones and LCC tones are then output from processor 508 to the transmitter 210 for transmission to the remote station. The first four steps in the flow diagram 800 of Figure 8 show the steps at base station Z when it is the sender. The steps in die method of transmission from a base station to a remote station are first for the Base Station in step 810 to generate a CRC value on the data block, assign a data block number to die CRC value, and concatenate the CRC value and die block number in a error message which is input as a vector to die link control channel (LCC). Then in step 820, die Base Station performs trellis encoding of die CRC link control channel vector and die data block vectors. Then in Step 830, die Base Station performs spectral spreading of die trellis encoded CRC link control channel vector and data block vectors. Then in Step 840, die Base Station transmits the CRC link control channel tone and data block tones to die remote station. FIGURE 6 is an architectural diagram of the remote station X as a receiver. The data tones and LCC tones are received at the remote station antenna X. The receiver 610 passes the data tones and the LCC tones to the spectral despreading processor 612. The despread signals are then output from the processor 612 to the trellis decoder 614. The data vectors 400, 400', and 400" of Figure 4 are then output to the vector disassembly buffer 616, shown in greater detail in Figure 4. The LCC vectors 402, 402', and 402" are output to the CRC comparison processor 620, shown in greater detail in Figure 4. Data to Alice and data to Bob are output from the buffer 616 to Mice and to Bob. Data to Mice and Bob are also input to the CRC generator 618. CRC generator 618 computes a new CRC value for every 64 K-bit data block and outputs the new CRC value and the block number to the buffer 406 of the CRC comparison processor 620. FIGURE 4 is a more detailed architectural diagram of the vector disassembly and CRC comparison logic at a receiving station. The receiving station buffers in the CRC comparison processor 620 the error detection messages it receives from the link control channel, so that they are accessible by their block numbers N, N+ l, N+2, etc. When the receiving station receives a data message on the traffic channel, it performs a CRC calculation on die data block in die message witii CRC generator 618 to obtain a resulting new CRC value. The new CRC value is buffered in buffer 406 at the receiving station with the block number so that it is accessible by its block number. Then, when both the received error message and die new CRC value are both available at the receiving station, they are matched by selector 404 by tiieir common block number. The received CRC value in the error detection message 402 is compared witii the new CRC computed from the received data block 400 by means of the comparator 408. If the comparison determines mat there is a difference in the values, then an error signal is generated by generator 622. The error signal can be processed and used in several ways by die error processor 630. The error signal can initiate a negative acknowledgement signal to be sent from die receiving station back to die sender requesting the sender to repeat the data block transmission. The error signal can initiate an update in the spreading and despreading weights at the receiving station in an effort to improve the signal and interference to noise ratio of the traffic channel. The error signal can initiate an alarm to be used for other realtime control. Or, the error signal can be logged for the compilation of a longer term report of e traffic channel quality. The last five steps in the flow diagram of Figure 8, show the remote station as the receiver. In Step 850, the Remote Station performs spectral and spatial despreading of the CRC link control channel tone and data block tones. Then, in Step 860, die Remote Station performs trellis decoding of despread CRC link control channel tone and data block tones. Then in Step 870, the Remote Station generates new a CRC value on the data block and uses die block number to select the corresponding CRC vector received from the link control channel. Then in Step 880, the Remote Station compares the new CRC value computed on die received data block witii the CRC vector received from the link control channel. Then in Step 890, the Remote Station generates an error signal if the new CRC does not compare with the received CRC. Still anodier alternate embodiment applies the above described invention in d e PWAN Frequency Division Duplex Communications System described in the Alamouti, Michaelson et al. patent application cited above. Mthough the preferred embodiments of the invention have been described in detail above, it will be apparent to those of ordinary skill in the art that obvious modifications may be made to the invention without departing from its spirit or essence. Consequently, the preceding description should be taken as illustrative and not restrictive, and the scope of the invention should be determined in view of the following claims. CLAIMS What is claimed is: 1. A highly bandwidth-efficient communications method, comprising the steps of: receiving at a base station a spread signal comprising an incoming data traffic signal spread over a plurality of discrete traffic frequencies and an incoming error detection signal spread over a plurality of link control frequencies; adaptively despreading the signals received at die base station by using despreading weights; computing an error value for said data traffic signal; comparing the error value with said error detection signal; generating an error response signal at the base station in response to said error value not comparing witii said error detection signal. 2. The highly bandwidth-efficient communications metiiod of claim 1, wherein said base station is part of a wireless discrete multitone spread spectrum communications system. 3. The highly bandwidtii-efficient communications method of claim 1, wherein said error detection signal is a checksum resulting from the operation of a polynomial generator on said data block. 4. The highly bandwidth-efficient communications metiiod of claim 1, wherein said error detection signal is a cyclic redundancy code. 5. The highly bandwidtii-efficient communications method of claim 1, which further comprises: said data traffic signal including a block number and said error detection signal including d e same block number; prior to said comparing step, buffering said error value for said data traffic signal and buffering said error detection signal; said comparing step further including the step of matching the block number of said error detection signal witii the block number of said data traffic signal. 6. The highly bandwidth-efficient communications metiiod of claim 1, which further comprises. initiating a negative acknowledgement signal to be sent from the base station to the sender requesting the sender to repeat the data block transmission, in response to said error response signal. 7. The highly bandwidtii-efficient communications method of claim 1, which further comprises: initiating an update in die spreading and despreading weights at die receiving station in an effort to improve die signal and interference to noise ratio of a traffic channel, in response to said error response signal. 8. The highly bandwidtii-efficient communications method of claim 1, which further comprises: initiating an alarm to be used for realtime control, in response to said error response signal. 9. The highly bandwidtii-efficient communications method of claim 1 , which further comprises: logging the error signal for compilation of a longer term report of a traffic channel quality, in response to said error response signal. 10. A highly bandwidtii-efficient communications method, comprising the steps of: receiving at a base station a first spread signal comprising an incoming data traffic signal having a data block portion and a block number portion spread over a plurality of discrete traffic frequencies; receiving at said base station a second spread signal comprising an incoming error detection signal having an error detection portion and said block number portion spread over a plurality of link control frequencies; adaptively despreading said first spread signal received at die base station by using despreading weights, recovering said data block portion and a block number portion; computing an error value for said data block portion at said base station; adaptively despreading said second spread signal received at die base station by using despreading weights, recovering said error detection portion and said block number portion; comparing the error value with said error detection portion at said base station; generating an error response signal at the base station in response to said error value not comparing with said error detection portion. 11. The highly bandwidtii-efficient communications method of claim 10, wherein said base station is part of a wireless discrete multitone spread spectrum communications system. 12. The highly bandwidth-efficient communications metiiod of claim 10, wherein said error detection signal is a checksum resulting from die operation of a polynomial generator on said data block. 13. The highly bandwidtii-efficient communications method of claim 10, wherein said error detection signal is a cyclic redundancy code. 14. The highly bandwidtii-efficient communications metiiod of claim 10, which further comprises: prior to said comparing step, buffering said error value for said data traffic signal and buffering said error detection signal; said comparing step further including die step of matching die block number of said error detection signal with the block number of said data traffic signal. 15. The highly bandwidtii-efficient communications method of claim 10, which further comprises: initiating a negative acknowledgement signal to be sent from the base station to the sender requesting the sender to repeat the data block transmission, in response to said error response signal. 16. The highly bandwidtii-efficient communications metiiod of claim 10, which further comprises: initiating an update in the spreading and despreading weights at the receiving station in an effort to improve the signal and interference to noise ratio of a traffic channel, in response to said error response signal. 17. The highly bandwidtii-efficient communications method of claim 10, which further comprises: initiating an alarm to be used for realtime control, in response to said error response signal. 18. The highly bandwidtii-efficient communications method of claim 10, which further comprises: logging the error signal for compilation of a longer term report of a traffic channel quality, in response to said error response signal. Abstract of the Disclosure A new method makes die most efficient use of die scarce spectral bandwidth in a wireless discrete multitone spread spectrum communications system. Each remote station and each base station in die network prepares an error detection field, such as a cyclic redundancy code (CRC), on each block of data to be transmitted over the traffic channels. The sending station prepares an error detection message for transmission over the link control channel of the network. The sending station prepares the error detection message by forming a link control channel vector that will be spread using die discrete multitone spread spectrum ( DMT-SS ) protocol to distribute the data message over a plurality of discrete tone frequencies, forming a spread signal for the link control channel. A link control channel is associated witii communications session using tiie traffic channels. The instant of transmission of the error detection message is allowed to be different from the instant of transmission of the data message. This permits the error detection messages to be transmitted when capacity is available on die link control channel. The receiving station buffers the error detection messages it receives from the link control channel, so that they are accessible by their block numbers. When the receiving station receives a data message on the traffic channel, it performs a CRC calculation on the data block in die message to obtain a resulting new CRC value. The new CRC value is also buffered at die receiving station with die block number so that it is accessible by its block number. Then, when both the received error message and die new CRC value are both available at the receiving station, they are matched by tiieir common block number. The received CRC value in the error detection message is compared witii the new CRC computed from the received data block. If the comparison determines tiiat tiiere is a difference in the values, tiien an error signal is generated. METHOD TO GAIN ACCESS TO A BASE STATION IN A DISCRETE MULTITONE SPREAD SPECTRUM COMMUNICATIONS SYSTEM REMOTE WIRELESS UNIT HAVING REDUCED POWER OPERATING MODE CROSS-REFERENCES TO RELATED APPLICATIONS: The invention disclosed herein is related to the co-pending U.S. patent application by Brian Agee et al. entitled "HIGHLY BANDWIDTH-EFFICIENT GIBBONS 1-1 COMMUNICATIONS", serial number , filed on the same day as the instant patent application, assigned to AT&T, and incorporated herein by reference. The invention disclosed herein is related to the co-pending U.S. patent application by Siavash Alamouti, Doug Stolarz, and Joel Becker, entitled " VERTICAL ADAPTIVE ANTENNA ARRAY FOR A DISCRETE MULTITONE SPREAD SPECTRUM COMMUNICATIONS SYSTEM", serial number , filed on the same day as the instant patent application, assigned to AT&T, and incorporated herein by reference. The invention disclosed herein is related to the copending US patent application by Alamouti, Michaelson, et al. , entitled "PWAN Frequency Division Duplex Communications System", serial number l filed on the same day as the instant patent application, assigned to AT&T, adn incorporated herein by reference. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvements to communications systems. More particularly, the present invention relates to wireless discrete multitone spread spectrum communications systems. 2. Description of the Related Art Wireless communications systems, such as cellular and personal communications systems, operate over limited spectral bandwidths and must make highly efficient use of the scarce bandwidth resource for providing good service to a large GIBBONS 1-1 population of users. A Code Division Multiple Access (CDMA) protocol has been used by wireless communications systems for efficiently making use of limited bandwidths and uses a unique code for distinguishing each user's data signal from data signals of other users. Knowledge of the unique code with which any specific information is transmitted permits separation and reconstruction of each user's message at the receiving end of the communication channel. Adaptive beamforming technology has become a promising technology for wireless service providers for offering large coverage, high capacity, and high quality service. Based on this technology, a wireless communication system can improve its coverage capability, system capacity, and performance significantly. A personal wireless access network (PWAN) system, described in the cross-referenced Agee et al. and Alamouti et al. patent applications, uses adaptive beamforming combined with a form of the CDMA protocol known as discrete multitone spread spectrum (DMT-SS) for providing efficient communications between a base station and a plurality of remote units (RUs). (The Agee, et al. patent application uses the term "discrete multitone stacked carrier (DMT-SC)" to refer to this protocol.) The remote units are powered primarily from AC power sources and include a battery for providing battery backup power when AC power fails. To conserve battery power, an RU has a sleep mode of operation with periodic power-up modes for checking whether any calls are attempting to be connected to the RU. When an RU is in a sleep mode, it expedient that the system operate in such a way so that appropriate actions are taken GIBBONS 1-1 for completing a call to a sleep mode RU. One approach for ensuring that calls are completed to a remote unit operating in a sleep mode is to maintain a database at a central location that stores the current operating mode of each remote in the system. When a remote unit enters a sleep mode of operation, the remote unit reports the change of operational status to the database. Similarly, the remote unit reports a change of status back to a standby operating mode. This approach has a drawback when a number of remote units recorded in the database experience frequent power outages. In such a situation, recording, managing and synchronizing power outage information in the database is particularly cumbersome when the database is large, perhaps holding status information for 3 to 4 thousand remote units. This drawback is further compounded when the database is duplicated multiple times throughout the system. When several thousand subscribers experience a power outage and AC power is restored before the database has completed recording the power outage, a database approach becomes unwieldy. Another complicated situation is when multiple remote units lose power at the same time. The affected remote units cannot all access the channel simultaneously for communicating their status to the database. A collision avoidance scheme must be implemented that spans a period of time and that is open for the possibility of power being restored before the database has been completely revised. This approach has another drawback in that a remote unit entering the sleep mode consumes system bandwidth in notifying the database. Figure 4 shows an exemplary flow of internal messaging that occurs between various layers of a remote unit when loss of GIBBONS 1-1 AC power is detected and a database is notified of the operational status change. Time is shown along the vertical axes of Figure 4, with advancing time being indicated toward the bottom of Figure 4. In_Figure 4, four layers of the remote unit operating system are shown: Health; OAM&P (Operations, Administration, Maintenance & Provisioning), MAC (Media Access Control) and physical. Only MAC layer of the base station is shown. At 40, AC power failure is detected by the Health layer. At 41, an EVENT message is sent from the Health layer to the OAM&P layer indicating that AC power has failed. The OAM&P layer sends an ACTION message to the MAC layer at 42. The MAC layer responds at 43 by sending an ACTION_RSP message to the OAM&P layer indicating that base station notification is pending. At 44, the MAC layer waits a random length period of time before sending an unsolicited CAC message at 45 to the MAC layer of the base station indicating the need for the remote unit to enter the sleep mode. At 46, the MAC layer of the base station sends an acknowledgment message to the MAC layer of the remote unit acknowledging receipt of the unsolicited CAC message. In response, the MAC layer of the remote unit sends an EVENT message at 47 to the OAM&P layer that the notification is done. The OAM&P layer first sends an EVENT message to the MAC layer indicating that the sleep mode has been entered at 48, and then sends a message at 49 to the physical layer to power down. What is needed is a way for a PWAN system to be aware that a remote unit is operating in a sleep mode so that appropriate actions can be taken by the system so that calls can be completed to a remote unit operating in a sleep mode. GIBBONS 1-1 SUMMARY OF THE INVENTION The present invention provides a method for reducing power consumption of a remote unit in a PWAN system. A remote unit is powered using a battery backup power supply when an AC power supply fails at the remote unit. A sleep mode of operation is entered at the remote unit that has a reduced power consumption for the battery backup power supply. The remote unit is synchronized to a TDD timing structure a predetermined period of time after entering the sleep mode of operation. A standby mode of operation is then entered at the remote unit in which a CONNECT message indicating an incoming call for the remote unit is scanned for by the receiver. When no CONNECT message is received, the remote unit reenters the sleep mode of operation. According to the invention, the predetermined period of time is a predetermined number of subframes after a boundary subframe of the TDD timing structure. Preferably, the predetermined number of subframes is based on an identification number of the remote unit. The present invention also provides a remote unit for a personal wireless area network that includes a receiver, an AC power supply, a battery-backup power supply and a controller. The battery-backup becomes operative when the AC power supply fails and supplies power to the receiver. The controller detects when the AC power supply fails and controls the receiver and the battery-backup power supply by invoking a sleep mode of operation. The sleep mode of operation is periodically interrupted by the controller controlling the receiver and the battery-backup power supply to enter a standby mode of operation in which the receiver scans a CONNECT message indicating an incoming call. GIBBONS 1-1 The controller coordinates the sleep mode and the standby mode of operations based on a frame count that is generated from an identification number of the remote unit. In accordance with another aspect of the invention, a highly bandwidth- efficient communications method is disclosed for the base station to enable it to communicate with a remote unit that is in the sleep mode. The remote unit has a unique identification value that is different from the identification value of other remote units that may be communicating with the base station. The base station begins by establishing a periodic reference instant at the base station and at the remote station. Then the base station determines a delay interval following the periodic reference instant at the base station, the delay interval being derived from the unique identification value of the remote unit. The base station receives spread signals from the remote units with which it communicates, each comprising an incoming data traffic signal spread over a plurality of discrete traffic frequencies. The base station adaptively despreads the signals received it receives by using despreading weights. The base station attempts to initiate a communication with the remote unit that is currently in the sleep mode. If the attempting step fails to initiate communications with the remote unit, the base station concludes that the remote unit is in the sleep mode. In response to this, the base station waits for the delay interval following the periodic reference instant at the base station before transmitting to the remote unit. The base station then transmits to the remote unit a spread signal comprising an outgoing data traffic signal spread over a plurality of discrete traffic frequencies. The remote unit has simultaneously changed from the sleep mode to the standby mode and is able to receive and GIBBONS 1-1 respond to the spread signal transmitted from the base station. In accordance additional aspects of the invention, the base station is part of a wireless discrete multitone spread spectrum communications system. Further, the periodic reference instant is established by a beginning subframe count instant that is incremented by a packet count value at the base station and at the remote unit. In addition, the delay interval is determined by a value N of a quantity of M least significant bits of the unique identification value of the remote unit, the delay interval being an interval required for the occurrence of a plurality of N of the beginning subframe count instants. The resulting invention enables the base station to be aware that a remote unit is operating in a sleep mode so that appropriate actions can be taken by the base station to assure that calls can be completed to the remote unit. BRIEF DESCRIPTION OF THE DRAWINGS The present invention is illustrated by way of example and not limitation in the accompanying figures in which like reference numerals indicate similar elements and in which: Figure 1 is an architectural diagram of the PWAN system, including remote stations transmitting to a base station; Figure 2 is an architectural diagram of the remote station X as a sender; Figure 3 is an architectural diagram of the remote station X as a receiver; Figure 4 shows an exemplary messaging flow occurring between various layers GIBBONS 1-1 of an exemplary remote unit and through an airlink to a base station when a loss of AC power at the remote unit is detected; Figure 5 shows a message flow sequence for a terminating call for the situation when a target remote unit is operating in the sleep mode; Figure 6 shows a sequence of events with respect 6 ms subframe structure of the present invention; Figure 7 is an exemplary graph showing Battery Operating Time, measured in hours, for Sleep Mode Duty Cycle (: 1); and Figure 8 is an architectural diagram of the base station Z. DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 1 shows an architectural diagram of the personal wireless access network (PWAN) system described in the referenced Agee et al. and Alamouti et al. patent applications and which is the environment of the present invention. Two users, Alice and Bob, are located at a remote station unit, or remote unit (RU), X and wish to transmit their respective data messages to a base station Z. Remote unit X is positioned to be equidistant from each of antenna elements A, B, C, and D at base station Z. Two other users, Chuck and Dave, are located at a remote station unit Y and also wish to transmit their respective data messages to base station Z. Remote unit Y is geographically different from remote unit X and is not equidistant from each of antenna elements A, B, C, and D of base station Z. Remote units X and Y, and base station Z use a form of the CDMA protocol known as GIBBONS 1-1 discrete multitone spread spectrum (DMT-SS) which is used for providing efficient communications between base stations and remote units. The DMT-SS protocol is indicated in Figure 1 as a multi-tone CDMA. In the DMT-SS protocol, a user data signal is modulated by a set of weighted discrete frequencies or tones. The weights are spreading weights that distribute the data signal over many discrete tones covering a broad range of frequencies. The weights are complex numbers having a real component that is used for modulating the amplitude of a tone and a complex component that is used for modulating the phase of the same tone. Each tone in the weighted-tone set bears the same data signal. Plural users at a transmitting station can use the same tone set for transmitting their data, but each of the users sharing the tone set has a different set of spreading weights. The weighted-tone set for a particular user is transmitted to the receiving station where it is processed with despreading weights that are related to the user's spreading weights for recovering the user's data signal. For each of a plurality of spatially separated antennas at the receiver, the received multitone signals are transformed from time-domain signals to frequency-domain signals. Despreading weights are assigned to each frequency component of the signals that are received by each antenna element. The values of the despreading weights are combined with the received signals for obtaining an optimized approximation of individual transmitted signals characterized by a particular multitone set and transmitting location. The PWAN system has a total of 2560 discrete tones (carriers) that are equally spaced in 8 MHZ of available bandwidth in the frequency range of 1850 to 1990 MHZ, with GIBBONS 1-1 a spacing between the tones of 3.125 KHz. The tones are used for carrying traffic messages and overhead messages between the base station and the plurality of remote units. The total set of tones are numbered consecutively from 0 to 2559, starting from the lowest frequency tone. The tones used for traffic messages are divided into 32 traffic partitions, with each traffic channel requiring at least one traffic partition of 72 tones. The overhead message tones are used for establishing synchronization and for passing control information between base stations and remote units. A Common Link Channel (CLC) is used by a base station for transmitting control information to remote units. A Common Access Channel (CAC) is used by a remote unit for transmitting messages to the base station. There is one grouping of tones assigned to each channel. The overhead channels are used in common by all remote units when control messages are exchanged with a base station. Transmission from a base station to a remote unit is called "forward transmission" and transmission from a remote unit to a base station is called "reverse transmission". Time Division Duplexing (TDD) is used by base stations and remote units for transmitting data and control information in both directions over the same multi-tone frequency channel. The time between recurrent transmissions in either direction is called a TDD period which, is equal to 3 ms. For every TDD period, there are four consecutive transmission bursts in each direction. Data is transmitted during each burst using multiple tones. The base station and each remote unit synchronize and conform to a TDD timing structure and framing structure that has 1 frame equal to 8 subframes and 1 subframe equal GIBBONS 1-1 to 2 TDD periods. A superframe is 256 subframes, or 1536 ms. All remote units and base stations are synchronized such that all remote units transmit simultaneously and then all base stations transmit simultaneously. When a remote unit initially powers up, it acquires synchronization from a base station so that control and traffic messages can be exchanged within the prescribed TDD time format. A remote unit must also acquire frequency and phase synchronization for the DMT-SS signals so that the remote unit is operating at the same frequency and phase as an associated base station. Selected tones within each tone set are designated as pilot tones that are distributed throughout the frequency band and carry known data patterns for enabling an accurate channel estimation. A series of pilot tones, having known amplitudes and phases, are spaced apart in frequency by approximately 30 KHz for providing an accurate representation of a channel response over the entire transmission band, that is, the amplitude and phase distortion introduced by the communication channel characteristics over the transmission band. FIGURE 2 shows an architectural diagram of remote station X operating as a sender station. Alice and Bob each input data to remote station X. The data is sent to a vector formation buffer 202 and also to a cyclic redundancy code generator 204. Data vectors are output from buffer 202 to a trellis encoder 206. The data vectors are in the form of a data message formed by concatenating a 64K-bit data block with a serially assigned block number. CRC generator 204 generates LCC vectors that are output to trellis encoder 206. The LCC vectors are in the form of an error detection message formed by GIBBONS 1-1 concatenating a CRC value with the serially assigned block number of the data block. The trellis encoded data vectors and LCC vectors are then output to a spectral spreading processor 208. The resultant data tones and LCC tones are then output from processor 208 to a transmitter 210 for transmission to the base station. The personal wireless access network (PWAN) system described in the cross- referenced Agee et al. and Alamouti et al. patent applications provides a more detailed description of a high-capacity mode, where one traffic partition is used in one traffic channel. A base station transmits information to multiple remote units that are located in the base station's cell. The transmission formats are for a 64 Kbps traffic channel, together with a 4 Kbps Link Control Channel (LCC) between the base station and a remote unit. A binary source, for example, Alice or Bob, delivers data, or information bits, to a sender transmitter at 64 Kbits/sec. This translates to 48 bits in one transmission burst. The information bits are encrypted according to a triple data encryption standard (DES) algorithm. The encrypted bits are then randomized in a data randomization block. A bit-to-octal conversion block converts the randomized binary sequence into a sequence of 3-bit symbols. The symbol sequence is converted into 16 symbol vectors. The term vector generally refers to a column vector, which is generally complex. One symbol from the LCC is added to form a vector of 17 symbols. The 17-symbol vector is trellis encoded starting with the most significant symbol (first element of the vector) and is continued sequentially until the last element of the vector (the LCC symbol). This process employs convolutional encoding for converting the GIBBONS 1-1 input symbol (an integer between 0 and 7) to another symbol (between 0 and 15) and maps the encoded symbol to its corresponding 16 QAM (or 16 PSK) signal constellation point. The output of the trellis -encoder is therefore a vector of 17 elements where each element is a signal within a set of 16 QAM (or 16 PSK) constellation signals. (The term signal will generally refer to a signal constellation point.) A link maintenance pilot signal (LMP) is added to form an 18-signal vector, with the LMP as the first element of the vector. The resulting (18 x 1) vector is pre- multiplied by a (18 x 18) forward smearing matrix yielding an (18 x 1) vector b. Vector b is element- wise multiplied by an (18 x 1) gain preemphasis vector yielding another (18 x 1) vector c. Vector c is post-multiplied by a (1 x 32) forward spatial and spectral spreading vector yielding a (18 x 32) matrix R(p), where p denotes the traffic channel index and is an integer. The 32 columns of matrix R results from multiplying the spectral spreading factor 4 and spatial spreading factor 8. The (18 x 32) matrices corresponding to all traffic channels carried (on the same traffic partition) are then combined (added) for producing a resulting 18 x 32 matrix S. Matrix S is partitioned by groups of four columns into eight (18 x 4) submatrices A 0 to A 7 . The indices 0 to 7 of sub matrices A 0 to A 7 correspond to the antenna elements over which these symbols will eventually be transmitted. Each submatrix is mapped to tones within one traffic partition. A lower physical layer places the baseband signals in discrete Fourier transfer (DFT) frequency bins where the data is converted into the time-domain and sent to its corresponding antenna elements (0 to 7) for transmission. This GIBBONS 1-1 process is repeated from the start for the next 48 bits of binary data to be transmitted in the next forward transmission burst. Figure 3 js an architectural block diagram of remote station X operating as a receiving station. Data tones and LCC tones are received by remote station antenna X and a receiver 610. Receiver 610 passes the data tones and the LCC tones to a spectral despreading processor 612 which despreads the data tones and LCC tones. The despread signals are then output from processor 612 to a trellis decoder 614. Trellis decoder 614 generates data vectors from the despread signals. The data vectors are then output to a vector disassembly buffer 616. Data for Alice and data to Bob are output from buffer 616 to Alice and Bob, respectively. Data for Alice and Bob are also input to a CRC generator 618. CRC generator 618 computes a new CRC value for every 64 K-bit data block and outputs the new CRC value with the block number to a buffer within a CRC comparison processor 620. The receiving station buffers error detection messages that are received from the link control channel in CRC comparison processor 620 so that the error detection messages are accessible by their block numbers N, N+ l, N+2, etc. When the receiving station receives a data message on the traffic channel, it performs a CRC calculation on the data block in the message with CRC generator 618 for obtaining a resulting new CRC value. If the comparison determines that there is a difference in the values, then an error signal is generated by an error signal generator 622. The error signal can be processed and used in several ways by an error processor 630. For example, the error signal can initiate a negative acknowledgment signal that is to be sent from the receiving station back to the sender station GIBBONS 1-1 requesting that the sender repeat transmission of the data block. The error signal can also initiate an update in spreading and despreading weights at the receiving station for improving the signal-to-interference _and noise ratio of the traffic channel. Another use of the error signal is for initiating an alarm used for other real time control. Yet another use of the error signal is as part of a logging signal for compilation of a long term report relating to traffic channel quality. According to the invention, a remote unit includes a standby mode of operation and a sleep mode of operation. Normally, the standby mode is the mode in which a remote unit scans the CLC channel for a CONNECT message for the remote unit. The sleep mode of operation provides a reduced power consumption operating mode for extending remote unit battery runtime during an AC power outage condition. During the sleep mode of operation, the remote unit periodically switches between the standby mode and sleep mode, with the overall effect being a reduction in the average power required by the remote unit. Delivery of a CONNECT message to a remote unit operating in the sleep mode is scheduled so that the remote unit is in the standby portion of the sleep mode. That is, the remote unit is synchronized and ready for receiving data from the CLC when the base station begins transmitting on the CLC. In order to achieve synchronization, a system wide Packet Count (PKT_CNT) is used. The basic unit of measure for synchronization is a mod[8] PKT_CNT, which is called a subframe count (SUBFRM CNT). The SUBFRM_CNT is incremented every 256 PKT_CNTs, or every 6 ms. GIBBONS 1-1 The base station and the remote unit both preferably use the least significant 8 bits of the remote unit ID for determining the particular SUBFRM CNT at which the CLC CONNECT message should be sent to the remote unit and, simultaneously, the appropriate time at which the remote unit should be in the standby portion of the sleep mode for receiving the CONNECT message. When the least significant 8 bits of the remote unit ID are used, the remote unit enters the standby mode once every 256 subframes and is ready for receiving an incoming call. The particular subframe that a remote unit will be ready for receiving an incoming call is called the N Usten for the remote unit. To avoid using a remote unit power status database that is maintained at a central location, the sleep mode features of the present invention are preferably implemented as part of a standard terminating call retry mechanism. That is, when a terminating call request is received at the base station MAC Layer, the MAC Layer Access Manager proceeds normally through a terminating call setup procedure by transmitting a CONNECT message on the CLC to the target remote unit. In the situation when the target remote unit is operating in the sleep mode at the time of the CONNECT message transmission, the remote unit will generally be unable to process the message. The base station MAC Layer Access Manager will time-out and retry transmission of the CONNECT message. Preferably, a retry timer T r is nominally set to 72 ms. The base station MAC Layer Access Manager retries the CONNECT message for a predetermined number of tries that is set by a system manager. Preferably, the retry count is 2. When the number of retries equals the retry count, the base station MAC GIBBONS 1-1 Layer Access Manager determines that the remote unit is in the sleep mode and, consequently, attempts to deliver the CONNECT message at a scheduled time that is based on the target remote unit ID. The scheduled time is a subframe occurring N Usten subframes after the boundary subframe for the TDD timing structure. The base station MAC Layer Access Manager also reserves the CLC slot(s) required for completing the CLC CONNECT message transmission at the time the N Usten subframe number is derived. That is, when the base station MAC Layer Access Manager has reached its retry count for a CONNECT message and has determined the N listeo subframe, CLC slot availability is examined for reserving the appropriate CLC slot(s) for use. As an alternative, a remote unit can scan up to 3 CLC slots for a CONNECT message when in the sleep mode so that a base station can select from 3 CLC slots in case a specific slot is unavailable. Figure 5 shows a message flow sequence for a terminating call for the situation when a target remote unit is operating in the sleep mode. The MAC Layer of the base station receives a terminating call request at 50. At 51 , the MAC Layer of the base station sends a CLC CONNECT message to the target remote unit. Since the remote unit is in the sleep mode, it does not receive the CLC CONNECT message and, therefore, does not respond. Since there is no response from the target remote unit during the T,^ period 52, the MAC Layer of the base station sends a second CLC CONNECT message to the target remote unit at 53. The remote unit does not respond during T rctry 54, so the MAC Layer of the base station determines the N Usten subframe for the remote unit using the least significant 8 GIBBONS 1-1 bits of the remote unit ID and waits for the particular N^^ subframe at 55. At N bsten for the remote unit, the MAC Layer of the base station sends a CLC CONNECT message at 56. At Ni l ,,, * ,, the remote unit is in the standby mode and ready to receive the CLC CONNECT message at 57. In response, the remote unit MAC Layer sends a CAC_ACK message to the base station at 58. The following definitions are used for describing the sleep mode of operation of the present invention: T !leep = the time that a remote unit is in a low-power mode (i.e. , sleeping). T, ync = the time required by a remote unit for re-acquiring synchronization when exiting the sleep mode. T ec , ck = the time that a remote unit is operating in a standby mode scanning the CLC for a CONNECT message. T 8Undby = the total time a remote unit is running (i.e., T sync + T κan clc ) D sleep = T skcp + T^^/T^ by , that is, the definition of the duty cycle of the sleep mode duty cycle. Since the base station transmits the CONNECT message at the N rιsten subframe so that the call can be completed, and the remote unit therefore must be ready for receiving the messages on the CLC channel at the N Ustco subframe. The remote unit MAC Layer Access Manager is capable of deriving the N, Urt 8ync subframe number and insures that all hardware required for the remote unit synchronization and CLC scanning efforts are released from sleep mode at that time. This is done, for example, by using a programmable hardware GIBBONS 1-1 counter 640 that is clocked in synchronism with the TDD subframe of the system, as shown in Figure 3. Prior to entering the sleep mode, or at the time the sleep mode is entered, CPU 650 preferably uses the least significant 8 bits of the remote unit ID for determining the N ϋsten subframe for the remote unit. CPU 650 loads counter 640 with a value related to N^^ and synchronizes counter 640 using a Start Sync signal. Counter 640 provides an interrupt to CPU 650 once every 256 subframes, initiating a re-synchronization process. CPU 650 responds by controlling power supply 660 to provide power 661 to the various components of remote unit used for receiving a CLC CONNECT message. CPU 650 also outputs an enabling signal to the spectral despreading processor 612 to enable the remote unit to receive messages from the base station. The remote unit begins its re-synchronization effort at a subframe N s rt sync that occurs some determined period of time prior to the occurrence of the N llsten subframe. Simulations of the remote unit synchronization algorithms indicate that a remote unit acquires synchronization with a base station when exiting a period of sleep in a minimum time of 122 ms and a maximum time of 200 ms. The actual time additionally depends on hardware component tolerances, the ambient temperature and numerous other factors. For the purposes of this disclosure, a worst case synchronization acquisition time T sync of 200 ms is used. This equates to approximately 34 subframes. Therefore, N start _, ync = N llsteo - 34 subframes. Figure 6 is a timing diagram showing the sequence of events for a remote unit operating in the sleep mode. Each vertical line in Figure 6 represents a subframe boundary. GIBBONS 1-1 The time between each subframe boundary is 6 ms. A remote unit is shown as being in a sleep mode. At N atart sync , counter 640 sends an interrupt request to CPU 650 (Figure 3). CPU 650 responds by controlling power supply 660 to provide power to the various components of the remote unit needed for receiving a CLC CONNECT message. In Figure 6, the remote unit is in the sleep mode at 60. At 61, N 9taIt sync occurs and the remote unit resynchronizes for a number of subframes. Preferably, about 34 subframes are required for a remote unit to reacquire synchronization. At N lιsten , the remote unit scans the CLC channel for any CLC CONNECT messages for the remote unit. The remote unit scans for 2 subframes, as shown in Figure 6 at 62. The remote unit can also be set to scan for a CLC CONNECT message over a different number of subframes other than 2 subframes depending upon system requirements. If no CLC CONNECT message is received at N lιsten , the remote unit returns to the sleep mode at 63. If a CLC CONNECT message is received, the call is established in a normal manner. As a first illustrative example of the timing aspects of the sleep mode of the present invention, the least significant 8 bits of a remote unit ID are used so that the N Usten cycle time is 1536 ms (256 x 6 ms). The remote unit synchronization acquisition time N 9ync is estimated to be 34 subframes (204 ms), and a CLC scan time for 2 CLC subframes is chosen. It follows that, T slcep = 220 subframe times = 220 x 6 ms = 1.320 s T sync = 204 ms = 34 subframes x 6 ms Tj c - n c i = 12 ms = 2 subframes x 6 ms GIBBONS lrl τ s-m by = 212 ms Therefore, the total sleep mode/standby mode cycle time is 1536 ms, and the total remote unit power-on time is 212 ms. The overall duty cycle is 7.25: 1. For this example, the maximum delay for delivery of a CONNECT message is 1.530 seconds (1536 ms - 6 ms). The nominal CONNECT message delay delivery time is about 0.766 seconds. Using a longer delay in CONNECT message delivery time permits the remote unit to be in the sleep mode for a greater period of time. As another example, the N Ugten subframe is determined by using the least significant 9-bits of a remote unit ID. Thus, the Nii Meα interval is 512 subframes. In this example, even though the sleep time is longer, the maximum synchronization acquisition time T sync remains the same. This is based on the fact that any temperature change of the remote unit is not sufficient for requiring a coarse TDD synchronization to be performed. It follows that, Tji eep = 476 subframe times = 476 x 6 ms = 2.856 s T,^,. = 204 ms = 34 subframes x 6 ms T^ clc = 12 ms = 2 subframes x 6 ms T standby = 212 ms The total sleep mode/standby mode cycle time is 3072 ms (512 x 6 ms), and the total remote unit power-on time is 212 ms. The overall duty cycle is 14.5: 1. For this example, the maximum delay of delivery of a CONNECT message is 3.066 seconds (3072 ms - 6 ms). The nominal time for delivery of a CONNECT message is about 1.536 s. Table I below summarizes various scenarios: GIBBONS 1-1 TABLE I The situation of a call originating from a remote unit that is operating in the sleep mode is straight forward compared to the situation when a call terminates at a sleeping remote unit. That is, the remote unit exits the sleep mode in response to a user command. The originating call delivery time, i.e., the time taken for delivering an ACCESS message on the CAC, is delayed by approximately 200 ms since the remote unit must re-acquire synchronization before the ACCESS message may be transmitted. In normal system operation, a base station polls remote units at a periodic rate for determining status of each remote unit. Each remote unit responds to the Poll Request message with a Poll response message using the CAC channel. When a remote unit is in a sleep mode of operation, the Poll Request message will not be received and, consequently, the remote unit will not respond with a Poll Response message. The present invention provides two alternatives for handling such a situation from the system point of view. The GIBBONS 1-1 first approach is to always schedule a Poll Request message to arrive at a remote unit during the N Ustca subframe for the remote unit whether the remote is in the standby or the sleep mode. The remote unit will receive the Poll Request message regardless of AC power status. A disadvantage associated with this approach is that the CAC channel is used by the remote unit for a Poll Response message, causing the remote unit transmitter to be used, effectively wasting battery power when in the sleep mode. The alternative approach is for a remote unit to ignore the Poll message from the base station during AC power outage situations and allow an OAM&P Layer at the base station to recognize that a non-responsive remote unit may possibly be in the sleep mode and, consequently, be aware of the power status of the remote unit in questions power. Figure 7 is an exemplary graph showing Battery Operating Time, measured in hours, for Sleep Mode Duty Cycle (: 1). From Figure 7, it is apparent that the length of time that a remote unit is sleeping has a significant impact on the run time of the battery. Also, from Figure 7, it is also apparent that the battery run time begins to flatten with duty cycle after about a 10: 1 ratio. Lab results for simulated sleep mode operation with a new, 7.2 amp-hour battery installed in a prototype uninterruptable power supply have yielded runtimes between 12 hours, 12 minutes to 12 hours, 32 minutes under the conditions that the remote unit is at room temperature, the sleep mode period is set for 3 seconds, and the sleep mode duty cycle is 10: 1 (0.3 s standby state and a 2.7 s sleep state). A remote unit operating in the sleep mode preferably provides the following characteristics: GIBBONS 1-1 Sleep time = 2856 ms RU Synchronization Time = 200 ms Call delivery delay = 1428 ms nominally RU CLC Scan time = 36 ms (i.e., three slots for flexibility at Base MAC Layer) Total Cycle Time = 3092 ms Standby Time = 236 ms Duty Cycle = 13:1 (approx.) Battery Operating Time = 12.5 hours (approx.) FIGURE 8 is an architectural diagram of the base station as a sender. The PSTN inputs data to base station Z. The data is sent to the vector formation buffer 502 and also to the cyclic redundancy code generator 504. Data vectors are output from buffer 502 to the trellis encoder 506. The data vectors are in the form of a data message formed by concatenating a 64 K-bit data block with its serially assigned block number. The LCC vectors output from the CRC generator 504 to the trellis encoder 506 are in the form of an error detection message formed by concatenating the CRC value with the block number. The trellis encoded data vectors and LCC vectors are then output to the spectral and spatial spreading processor 508. The resultant data tones and LCC tones are then output from processor 508 to the transmitter 210 for transmission to the remote station. The base station transmits the CONNECT message at the Nu^ subframe so that the call can be completed to the remote unit. The base station knows to send the GIBBONS 1-1 messages on the CLC channel at the N Ustal subframe. The base station's MAC Layer Access Manager is capable of deriving the N sUrt sync subframe number. This is done, for example, by using a programmable hardware counter 540 that is clocked in synchronism with the TDD subframe of the system, as shown in Figure 8. When the base station wants to send a message to the remote unit, the CPU 550 preferably uses the least significant 8 bits of the remote unit ID for determining the Nn atcn subframe for the remote unit. CPU 550 loads counter 540 with a value related to N listen and synchronizes counter 540 using a Start Sync signal. Counter 540 provides an interrupt to CPU 550 once every 256 subframes, initiating a re-synchronization process. CPU 550 responds by outputting an enabling signal to the spectral and spatial spreading processor 508 to enable the base station to transmit messages to the remote unit when the remote unit is in its standby mode. Still another alternate embodiment applies the above described invention in the PWAN Frequency Division Duplex Communications System described in the Alamouti, Michaelson et al. patent application cited above. Although the preferred embodiments of the invention have been described in detail above, it will be apparent to those of ordinary skill in the art that obvious modifications may be made to the invention without departing from its spirit or essence. Consequently, the preceding description should be taken as illustrative and not restrictive, and the scope of the invention should be determined in view of the following claims. GIBBONS 1-1 CLAIMS What is claimed is: 1. In a wireless communications network, a method in a base station to communicate with a remote unit that is in a sleep mode, the remote unit having a unique identification value, comprising the steps of: establishing a periodic reference instant at the base station and at the remote station; determining a delay interval following said periodic reference instant at the base station, said delay interval being derived from said unique identification value of said remote unit; and transmitting a message from the base station to the remote unit at a second instant following said delay interval, said remote unit having changed from said sleep mode to a standby mode after said delay interval. 2. The method of claim 1, wherein said base station is part of a wireless discrete multitone spread spectrum communications system. GIBBONS 1-1 3. The method of claim 1, wherein said periodic reference instant is established by a beginning subframe count instant that is incremented by a packet count value at the base station and at the remote unit. 4. The method of claim 3, wherein said delay interval is determined by a value N of a quantity of M least significant bits of said unique identification value of said remote unit, the delay interval being an interval required for the occurrence of a plurality of N of said beginning subframe count instants. 5. The method of claim 4, wherein said remote unit changes from said sleep mode to a standby mode after said delay interval. GIBBONS 1-1 6. In a wireless communications network, a method in a base station to communicate with a remote unit that is in a sleep mode, the remote unit having a unique identification value, comprising the steps of: establishing a periodic reference instant at the base station and at the remote station; determining a delay interval following said periodic reference instant at the base station, said delay interval being derived from said unique identification value of said remote unit; attempting to initiate a communication from said base station to said remote unit; concluding at the base station that the remote unit is in a sleep mode if said attempting step fails to initiate communications with the remote unit; waiting for said delay interval following said periodic reference instant at the base station; and transmitting a message from the base station to the remote unit at a second instant following said delay interval, said remote unit having changed from said sleep mode to a GIBBONS 1-1 standby mode after said delay interval. 7. The method of claim 6, wherein said base station is part of a wireless discrete multitone spread spectrum communications system. 8. The method of claim 6, wherein said periodic reference instant is established by a beginning subframe count instant that is incremented by a packet count value at the base station and at the remote unit. 9. The method of claim 8, wherein said delay interval is determined by a value N of a quantity of M least significant bits of said unique identification value of said remote unit, the delay interval being an interval required for the occurrence of a plurality of N of said beginning subframe count instants. 10. The method of claim 9, wherein said remote unit changes from said sleep mode to a standby mode after said delay interval. GIBBONS 1-1 11. A highly bandwidth-efficient communications method in a base station to communicate with a remote unit that is in a sleep mode, the remote unit having a unique identification value, comprising the steps of: establishing a periodic reference instant at the base station and at the remote station; determining a delay interval following said periodic reference instant at the base station, said delay interval being derived from said unique identification value of said remote unit; receiving at a base station a spread signal comprising an incoming data traffic signal spread over a plurality of discrete traffic frequencies; adaptively despreading the signals received at the base station by using despreading weights; attempting to initiate a communication from said base station to said remote unit; concluding at the base station that the remote unit is in a sleep mode if said GIBBONS 1-1 attempting step fails to initiate communications with the remote unit; waiting for said delay interval following said periodic reference instant at the base station; and transmitting at the base station to the remote unit a spread signal comprising an outgoing data traffic signal spread over a plurality of discrete traffic frequencies. 12. The method of claim 11, wherein said base station is part of a wireless discrete multitone spread spectrum communications system. 13. The method of claim 11, wherein said periodic reference instant is established by a beginning subframe count instant that is incremented by a packet count value at the base station and at the remote unit. 14. The method of claim 13, wherein said delay interval is determined by a value N of a quantity of M least significant bits of said unique identification value of said remote unit, the delay interval being an interval required for the occurrence of a plurality of N of said beginning subframe count instants. GIBBONS 1-1 15. The method of claim 14, wherein said remote unit changes from said sleep mode to a standby mode after said delay interval. 16. A remote unit for a personal wireless area network comprising: a receiver; an AC power supply coupled to the receiver and supplying power to the receiver; a battery-backup power supply coupled to the receiver, the battery-backup power supply becoming operative to supply power to the receiver when the AC power supply fails; and a controller coupled to the receiver, the AC power supply and the battery- backup power supply, the controller detecting when the AC power supply fails and in response controls the receiver and the battery-backup power supply by invoking a sleep mode of operation, the sleep mode operation being periodically interrupted by the controller controlling the receiver and the battery-backup power supply to enter a standby mode of operation in which the receiver scans for a CONNECT message indicating an incoming call, the controller controlling the sleep mode and the standby mode of operations based on a frame count that is generated from an identification number of the remote unit. 17. The remote unit according to claim 16, wherein the receiver scans for a GIBBONS 1-1 connect message for a predetermined number of subframes of a TDD timing structure. 18. The remote unit according to claim 17, wherein the predetermined number of subframes equals 3. 19. The remote unit according to claim 17, wherein when the remote unit enters the standby mode, the remote unit reacquires synchronization to the TDD timing structure. 20. The remote unit according to claim 19, wherein the remote unit reacquires synchronization to the TDD timing structure in about 34 subframes. 21. The remote unit according to claim 19, wherein the remote unit scans for a CONNECT message at a subframe that is related to an identification number of the remote unit. 22. A method for reducing power consumption of a remote unit in a PWAN system, comprising the steps of: powering a remote unit using a battery backup power supply when an AC power supply fails at the remote unit; entering a sleep mode of operation at the remote unit, the sleep mode having a reduced power consumption for the battery backup power supply; GIBBONS 1-1 entering a standby mode of operation at the remote unit a predetermined period of time after entering the sleep mode of operation scanning for a CONNECT message indicating an incoming call for the remote unit; and reentering the sleep mode of operation when no CONNECT message is received. 23. The method according to claim 22, further comprising the step of synchronizing the remote unit to a TDD timing structure before the step of entering the standby mode of operation. 24. The method according to claim 23, wherein the predetermined period of time is a predetermined number of subframes after a boundary subframe of the TDD timing structure. 25. The method according to claim 24, wherein the predetermined number of subframes is based on an identification number of the remote unit. GIBBONS 1-1 ABSTRACT OF THE DISCLOSURE A remote unit for a personal wireless area network includes a receiver, an AC power supply, a battery-backup-power supply and a controller. The battery-backup becomes operative when the AC power supply fails and supplied power to the receiver. The controller detects when the AC power supply fails and controls the receiver and the battery- backup power supply by invoking a sleep mode of operation. The sleep mode of operation is periodically interrupted by the controller controlling the receiver and the battery-backup power supply to enter a standby mode of operation in which the receiver scans for a CONNECT message from a base station indicating an incoming call. The controller coordinates the sleep mode and the standby mode of operations based on a frame count that is generated from an identification number of the remote unit. A highly bandwidth-efficient communications method is employed in the base station to enable it to coordinate communication with the remote unit when it changes from the sleep mode to the standby mode. Cross-References to Related Applications: The invention disclosed herein is related to the copending US patent application by Brian Agee, et al. entitled "HIGHLY BANDWIDTH EFFICIENT COMMUNICATIONS", serial number , filed on the same day as the instant patent application, assigned to AT&T, and incorporated herein by reference. GIBBONS 1-1 The invention disclosed herein is related to the copending US patent application by Siavash Alamouti, Doug Stolarz, and Joel Becker, entitled " VERTICAL ADAPTIVE ANTENNA ARRAY FOR A DISCRETE MULTITONE SPREAD SPECTRUM COMMUNICATIONS SYSTEM", serial number , filed on the same day as the instant patent application, assigned to AT&T, and incorporated herein by reference. The invention disclosed herein is related to the copending US patent application by Alamouti, Michaelson, et al. , entitled "PWAN Frequency Division Duplex Communications System", serial number l filed on the same day as the instant patent application, assigned to AT&T, adn incorporated herein by reference. Background of die Invention Field of the Invention This invention involves communications methods that a wireless remote station uses to gain access to a base station in a discrete multitone spread spectrum communications system. Description of Related Art Wireless communications systems, such as cellular and personal communications systems, operate over limited spectral bandwidths. They must make highly efficient use of die scarce bandwidth resource to provide good service to a large population of users. Code Division Multiple Access (CDMA) protocol has been used by wireless communications systems to efficiently make use of limited bandwidths. The protocol uses a unique code to distinguish each user's data signal from GIBBONS 1-1 other users' data signals. Knowledge of die unique code with which any specific information is transmitted, permits the separation and reconstruction of each user's message at the receiving end of die communication channel. The personal wireless access network (PWAN) system described in the referenced Alamouti, et al. patent application, incorporated herein by reference, uses a form of the CDMA protocol known as discrete multitone spread spectrum ( DMT-SS ) to provide efficient communications between a base station and a plurality of remote units. (The Agee, et al. patent application uses the term "discrete multitone stacked carrier (DMT-SC) to refer to this protocol.) In this protocol, the user's data signal is modulated by a set of weighted discrete frequencies or tones. The weights are spreading codes that distribute the data signal over many discrete tones covering a broad range of frequencies. The weights are complex numbers with the real component acting to modulate the amplitude of a tone while the complex component of the weight acts to modulate the phase of the same tone. Each tone in the weighted tone set bears the same data signal. Plural users at the transmitting station can use the same tone set to transmit their data, but each of die users sharing the tone set has a different set of spreading codes. The weighted tone set for a particular user is transmitted to the receiving station where it is processed witii despreading codes related to die user's spreading codes, to recover the user's data signal. For each of the spatially separated antennas at the receiver, the received multitone signals are transformed from time domain signals to frequency domain signals. Despreading weights are assigned to each frequency component of the signals received by each antenna element. The values of die despreading weights are combined witii the received signals to obtain an optimized approximation of individual transmitted signals characterized by a particular multitone set and transmitting location. The PWAN system has a total of 2560 discrete tones (carriers) equally spaced in 8 MHz of available bandwidth in the range of 1850 to 1990 MHz. The GIBBONS 1-1 spacing between the tones is 3.125 kHz. The total set of tones are numbered consecutively from 0 to 2559 starting from die lowest frequency tone. The tones are used to carry traffic messages and overhead messages between die base station and die plurality of remote units. The traffic tones are divided into 32 traffic partitions, witii each traffic channel requiring at least one traffic partition of 72 tones. In addition, the PWAN system uses overhead tones to establish synchronization and to pass control information between the base station and die remote units. A Common Link Channel (CLC) is used by die base to transmit control information to the Remote Units. A Common Access Channel (CAC) is used to transmit messages from d e Remote Unit to the Base. There is one grouping of tones assigned to each channel. These overhead channels are used in common by all of the remote units when they are exchanging control messages with die base station. In the PWAN system, Time Division Duplexing (TDD) is used by the base station and the remote unit to transmit data and control information in both directions over the same multi-tone frequency channel. Transmission from the base station to the remote unit is called forward transmission and transmission from the remote unit to tiie base station is called reverse transmission. The time between recurrent transmissions from either the remote unit or the base station is the TDD period. In every TDD period, there are four consecutive transmission bursts in each direction. Data is transmitted in each burst using multiple tones. The base station and each remote unit must synchronize and conform to the TDD timing structure and boti the base station and die remote unit must synchronize to a framing structure. All remote units and base stations must be synchronized so that all remote units transmit at the same time and then all base stations transmit at the same time. When a remote unit initially powers up, it acquires synchronization from the base station so that it can exchange control and traffic messages within the prescribed TDD time format. The remote unit GIBBONS 1-1 must also acquire frequency and phase synchronization for the DMT-SS signals so that the remote is operating at the same frequency and phase as die base station. When a caller at a remote unit goes off-hook, an access request message is sent by the remote unit over the Common Link Channel (CLC) to the base station during d e reverse TDD interval when all of the remotes are allowed to transmit. If more than one remote unit sends a message over the CAC channel during the same reverse TDD interval, there is a collision of the signal tones. If the base station receives the combined signal from die collided tones, the signal will not be intelligible. In that case the base station will reply with a negative acknowledgement signal. Alternately, if die base station never receives the collided signals, the absence of an acknowledgement signal from die base station will be inferred by botii remote units as a collision. In either case, die remote units in the present PWAN system will delay repeating their transmissions by a random interval. This collision detection multiple access technique is generally known as the aloha protocol. Each remote unit will delay retransmission by a random interval, known as a back-off interval, that is usually different for the two units. The remote unit whose random interval is the first to expire, will be the first to retransmit its message. A problem arises when the collision is not between the transmissions from two remote units, but instead is between a transmission from one remote unit and a noise burst. Noise bursts are typically of a longer duration than the typical back-off interval of the standard aloha protocol. If the remote unit infers from the base station's negative acknowledgement signal or from the lack of an acknowledgement signal that there has been a collision with a transmission from another remote station, the remote unit will not delay long enough to avoid a second collision with the noise burst when it retransmits its signal. However, the possible solution of merely lengthening the aloha backoff intervals for all detected collisions would unnecessarily delay most retransmissions after normal GIBBONS 1-1 collisions with other remotes. GIBBONS 1-1 Summary of the Invention The invention solves this problem by providing the base station with the ability to distinguish between normal collisions and noise bursts on the Common Access Channel (CAC). The base station is then able to reply to the remote units with information about the quality of the CAC channel and why their CAC channel transmissions were not successful. The remote units can then use this information to adapt their retry processes to the channel's quality, depending on whether tiiere was a noise burst, a normal collision, or a successful transmission on the CAC channel. The CAC channel transmissions are discrete tones received by the base station from one or more remote units during each reverse interval of a TDD period. The tones have been modulated with data such as an access request by the remote unit, using a 16 QAM modulation scheme. The received tones are sampled, digitized, passed through a fast Fourier transform (FFT) processor, and stored in FFT incremental frequency bins as complex numbers. These numbers represent points in a 16 QAM modulation constellation and are related to the average amount of energy of the received tone in die increment of frequency represented by die FFT bin. In accordance with the invention, the base station uses tiie information about the signals received, as represented by die FFT bins, to prepare notices of CAC channel conditions to be sent back to die remote units. In a first example, with no noise on the CAC channel, the average energy of the received tone represented by die numbers in the FFT frequency bins is equal to unity. In a second example, with no noise on the CAC channel, when a collision occurs between two tones that are received by die base station at die same time from two different remote units, the average energy represented by die numbers in the FFT frequency bins is greater than unity. In a third example, witii noise on the CAC channel but no GIBBONS 1-1 transmitted tones, the average energy of the received noise will be measurable but very small. In a fourth example, with noise on the CAC channel and a tone transmitted from a remote unit, the average energy measured will be less than expected for a tone received over a quiet CAC channel. This is due to the randomness with which the noise adds to and subtracts from the tone signal . The combination of noise and a received tone on the CAC channel will be represented by die numbers in die FFT frequency bins being less than unity. In accordance witii the invention, if no noise is detected by the base station on the CAC channel, then it responds with a normal reply when a tone is received on die CAC channel from a remote unit. If noise is detected by die base station on die CAC channel when a tone is received from a remote unit, then the base station responds on die Common Link Channel (CLC) with a negative acknowledgement (NACK) signal specifying that there is noise on the CAC channel. If a collision is detected by the base station on the CAC channel when two tones have been transmitted by two remote units, then the base station responds on die CLC channel with a negative acknowledgement (NACK) signal specifying that there has been a collision on the CAC channel. In an alternate embodiment of the invention, whenever noise is detected by the base station on the CAC channel, the strength and duration and specific frequency range of the noise is measured and recorded. Only die most recent noise measurement data is retained at die base station. Then when a tone is received from a remote unit accompanied by die noise, tiien the base station responds on die Common Link Channel (CLC) with a negative acknowledgement (NACK) signal specifying the strength and duration and specific frequency range of die noise on the CAC channel. Alternately, the base station can periodically broadcast updates to all remote stations about the strength and duration and specific frequency range of noise bursts recently measured on the CAC channel. In accordance witii the invention, the remote unit waits for a reply on the CLC channel after GIBBONS 1-1 it transmits an access request to the base station on the CAC channel. If the remote unit does not receive some form of response from the base unit before a timeout interval, then d e remote unit infers that there is noise interference on the CAC channel and its access request did not get to die base station. Since noise bursts are typically of a longer duration than the typical back-off interval of die standard random back-off and retry protocol, die remote unit will add an extra delay period before beginning the random back-off and retry process. In an alternate embodiment of the invention, the duration of the extra delay period can vary in response to information received from the base station specifying the strength and duration and specific frequency range of the noise recently measured on the CAC channel. If a remote unit receives a reply to its access request on the CAC channel, it analyzes the reply message to determine if it is normal reply, or if it is a NACK message indicating noise or a collision. If the base station has sent a normal reply, then the remote unit completes the access process in the normal manner. If the base station has sent a negative acknowledgement message indicating tiiat there is noise on the CAC channel, then the remote unit will add an extra delay period before beginning the random back-off and retry process, as described above. In an alternate embodiment of the invention, the duration of the extra delay period can vary in response to information received from die base station specifying the strength and duration and specific frequency range of the noise measured on the CAC channel. If the base station has sent a negative acknowledgement message indicating that there has been a collision with the tone from another remote unit on the CAC channel, then both remote units will receive the NACK message and both will begin the random back-off and retry process. In this manner, access requests from remote units are processed in die minimum amount of time in the face of varying traffic congestion and noise burst interference on the CAC channel. GIBBONS 1-1 In an alternate embodiment of die invention, the remote unit and die base station can exchange their respective request and response messages over the same frequency channel, such as a common broadcast channel. If either a collision or a noise burst is detected by the base station on the common chancel, the base station can respond on die same channel to the remote station with information on the conditions on die channel. The remote unit will respond as described above, witii a selected type of back-off and retry operation, depending on the information in the response from the base station. If the base station's response back to the remote unit in not received by die remote unit, then after a timeout interval, the remote unit will infer that there is a noise condition on d e common channel. Currently, the invention has advantageous applications in the field of wireless communications, such as cellular communications or personal communications, where bandwidth is scarce compared to die number of the users and their needs. Such applications may be effected in mobile, fixed, or minimally mobile systems. However, the invention may be advantageously applied to other, non-wireless, communications systems as well. Brief Description of the Drawings In the drawings: FIGURE 1A is a diagram illustrating a collision on die CAC channel by two remote units attempting to gain access to d e base station at the same time. FIGURE IB is a diagram illustrating a NACK message response on the CLC channel from the base station informing the remote units tiiat there has been a collision. GIBBONS 1-1 FIGURE IC is a diagram illustrating noise on the CAC channel while one remote unit attempts to gain access to tiie base station. FIGURE ID is a diagram illustrating a NACK message response on die CLC channel from the base station informing the remote unit that there has been noise on the CAC channel. FIGURE 2A is a diagram illustrating 16 QAM modulated signals for a normal transmission, a collision, and a noise event on the CAC channel. FIGURE 2B is a diagram illustrating the composite signal amplitude for a collision on the CAC channel. FIGURE 2C is a diagram illustrating the composite signal amplitude for a noise event on the CAC channel. FIGURE 2D is a diagram illustrating back-off and retry by a remote unit after a collision on the CAC channel. FIGURE 2E is a diagram illustrating back-off and retry by a remote unit after a noise event on die CAC channel. FIGURE 2F is a flow diagram of the process at the base station for notifying the remote units of either a collision or a noise event on the CAC channel. FIGURE 2G is a flow diagram of the process at the remote unit for acting on notification from the base station of either a collision or a noise event on the CAC channel. GIBBONS 1-1 Detailed Description of the Preferred Embodiment FIGURE 1A is a diagram illustrating a collision on the CAC channel by tones from two remote units attempting to gain access to the base station at the same time. Remote unit X receives an access request signal shown as a white data signal from a first sender. In accordance witii one aspect of the personal wireless access network (PWAN) system described in die referenced Alamouti, et al. patent application, incorporated herein by reference, the encoder uses a discrete multitone spread spectrum protocol to encode the white data signal onto multiple discrete frequencies or tones, here represented by one common access channel (CAC) tone. The white data signal is copied onto each of the CAC tones. FIGURE 1A also shows remote unit Y receiving an access request signal shown as a black data signal from a second sender. In accordance with one aspect of the PWAN system, the encoder at station Y uses a discrete multitone spread spectrum protocol to encode die black data signal onto the same multiple discrete frequencies or tones, here represented by one common access channel (CAC) tone. The black data signal is copied onto each of die CAC tones. FIGURE 1A shows the transmitters at remote units X and Y being positioned close to one anodier, so tiiat the transmitted signals from them are not significantly different in their spacial characteristics. The transmitted signals from the two remote units X and Y also have the same CAC discrete frequencies or tones. Since the CAC channel is commonly used by all remote units to make access requests to die base station, when two remote units transmit during the same interval, their tones collide, as is shown in the figure. FIGURE 1A shows the base station Z receiving the discrete multitone signals on its antenna A from the remote units X and Y. The signals are processed by a signal processor computer and stored in a memory. The memory at die receiving station Z is organized into sections called bins. Each bin GIBBONS 1-1 is associated with one antenna at the receiving station and witii one tone of the multitone set. The antenna A has separate bins in the memory for one each different tone frequency. Each bin is further divided into four sub-bins for each of the four possible phases, Tl, T2, T3, and T4. FIGURE 1A shows how the bins and sub-bins in the memory of station Z store the patterns of the white data received from remote unit X and die black data received from remote unit Y. The signal processor at station Z uses the process of spectral despreading, in accordance witii one aspect of the PWAN system, to distinguish the white data from the black data. The base station is shown detecting a collision of die CAC tones from remote units X and Y. The CAC channel transmissions are discrete tones received by d e base station from one or more remote units during each reverse interval of a TDD period. The tones have been modulated witii data such as an access request by the remote unit, using a 16 QAM modulation scheme. FIGURE 2A is a diagram illustrating 16 QAM modulated signals for a normal transmission, a collision, and a noise event on die CAC channel. The received tones are sampled, digitized, passed through a fast Fourier transform (FFT) processor, and stored in FFT incremental frequency bins as complex numbers. These numbers represent points in a 16 QAM modulation constellation and are related to the average amount of energy of the received tone in the increment of frequency represented by the FFT bin. In accordance with the invention, the base station uses the information about the signals received, as represented by the FFT bins, to prepare notices of CAC channel conditions to be sent back to the remote units. In a first example, with no noise on the CAC channel, the average energy of the received tone represented by the numbers in the FFT frequency bins is equal to unity. In a second example, witii no noise on the CAC channel, when a collision occurs between two tones that are received by the base station at die same time from two different remote units, die average energy represented by the numbers in the FFT frequency bins is greater than unity (greater than a threshold T). FIGURE 2B is a diagram illustrating die GIBBONS 1-1 composite signal amplitude for a collision on the CAC channel. In a third example, with noise on the CAC channel but no transmitted tones, die average energy of the received noise will be measurable but very small. In a fourth example, with noise on the CAC channel and a tone transmitted from a remote unit, die average energy measured will be less than expected for a tone received over a quiet CAC channel. This is due to the randomness with which the noise adds to and subtracts from the tone signal. The combination of noise and a received tone on die CAC channel will be represented by die numbers in the FFT frequency bins being less than unity (less tiian a threshold t). FIGURE 2C is a diagram illustrating the composite signal amplitude for a noise event on the CAC channel. FIGURE 2F is a flow diagram of die process at the base station for notifying the remote units of either a collision or a noise event on the CAC channel. Step Bl receives a signal on the CAC channel. Step B2 samples, digitizes, and performs an FFT on the CAC channel signal, step B3 compares the 16 QAM constellation of the signal with a normal signal, noise interference, and collision interference. In step B4, in accordance with the invention, if no noise is detected by the base station on die CAC channel, then it responds witii a normal reply when a tone is received on the CAC channel from a remote unit. In step B5, if noise is detected by die base station on die CAC channel when a tone is received from a remote unit, then the base station responds on die Common Link Channel (CLC) witii a negative acknowledgement (NACK) signal specifying that there is noise on the CAC channel. FIGURE IC is a diagram illustrating noise on the CAC channel while one remote unit attempts to gain access to tiie base station. FIGURE ID is a diagram illustrating a NACK message response on the CLC channel from the base station informing the remote unit tiiat there has been noise on the CAC channel. In step B6 of FIGURE 2F, if a collision is detected by die base station on die CAC channel when two tones have been transmitted by two remote units, as shown in FIGURE 1A, men the base station responds on the CLC channel with a negative acknowledgement (NACK) signal specifying that there has GIBBONS 1-1 been a collision on the CAC channel. FIGURE IB is a diagram illustrating a NACK message response on the CLC channel from the base station informing the remote units that there has been a collision. In an alternate embodiment of the invention, whenever noise is detected by the base station on the CAC channel, the strength and duration and specific frequency range of the noise is measured and recorded. Only the most recent noise measurement data is retained at die base station. Then when a tone is received from a remote unit accompanied by die noise, tiien the base station responds on die Common Link Channel (CLC) witii a negative acknowledgement (NACK) signal specifying the strength and duration and specific frequency range of the noise on the CAC channel. Alternately, the base station can periodically broadcast updates to all remote stations about the strength and duration and specific frequency range of noise bursts recently measured on the CAC channel. FIGURE 2G is a flow diagram of the process at die remote unit for acting on notification from die base station of either a collision or a noise event on the CAC channel. In step Rl, the remote unit sends an access request on the CAC channel to the base station. In step R2, in accordance witii die invention, die remote unit waits for a reply on die CLC channel after it transmits an access request to the base station on the CAC channel. If the remote unit does not receive some form of response from the base unit before a timeout interval, then the remote unit infers that there is noise interference on the CAC channel and its access request did not get to the base station. Since noise bursts are typically of a longer duration than the typical back-off interval of the standard random back-off and retry protocol, the remote unit will add an extra delay period before beginning the random back-off and retry process. FIGURE 2E is a diagram illustrating back-off and retry by a remote unit after a noise event on the CAC channel. In an alternate embodiment of the invention, the duration of the extra delay period can vary in response to information received from the base station specifying the strength and duration and specific frequency range of die noise recently measured on the CAC channel. GIBBONS 1-1 In step R3 of FIGURE 2G , if a remote unit receives a reply to its access request on the CAC channel, it analyzes the reply message to determine if it is normal reply, or if it is a NACK message indicating noise or a collision. In step R4, if the base station has sent a normal reply, then the remote unit completes the access process in the normal manner. In step R5, if the base station has sent a negative acknowledgement message indicating that there is noise on the CAC channel, then the remote unit will add an extra delay period before beginning the random back-off and retry process, as described above. FIGURE 2E is a diagram illustrating back-off and retry by a remote unit after a noise event on the CAC channel. In an alternate embodiment of die invention, die duration of the extra delay period can vary in response to information received from the base station specifying the strength and duration and specific frequency range of the noise measured on the CAC channel. In step R6 of FIGURE 2G , if the base station has sent a negative acknowledgement message indicating that there has been a collision with the tone from another remote unit on the CAC channel, then both remote units will receive the NACK message and botii will begin the random back-off and retry process. The following provides an additional explanation of how the base station determines the difference between congestion and noise. When die remote unit (RU) submits a packet to the CAC, it waits for a subsequent response from the base station, which will vary depending on die particular protocol procedure. In general, a waiting period is timed and when expiration takes place, d e packet is resubmitted. However, the resubmission takes place only after a delay of some number of time periods. The number is chosen at random between 1 and some maximum number M. In the PWAN system, no information is used concerning die underlying reason for the unsuccessful transmission. In general it could be due to congestion on the channel or due to fading. In accordance witii die invention, a better GIBBONS 1-1 decision can be made if die base station distinguishes between die two cases. This enables d e remote unit (RU) to use different number m not equal to M or perhaps a different random distribution altogether. Take as a base case for comparison, the case where one RU uses the channel to transmit to the base station (Base) and no abnormal noise is present on the channel. In the Base, samples are continuously being taken via the analog to digital converter, and placed into FFT bins as complex numbers. These numbers represent points in the constellation that is determined by die chosen modulation scheme, e.g., 16 QAM for the PWAN system. With no noise, and no packet transmitted, the average energy will be 0, or With no noise, when a packet is received, die average energy represented by die contents of die FFT bins is equal to 1. With no noise, when two packets are received at die same time, (i.e., a collision has occurred), die average energy will be higher still, say higher than a threshold value T, 1 " 1 T- 2 2 . —, ■ ∑ X J + J > τ as in FIGURE 2A. With noise present, the ambient energy measured in die FFT bins while no packet is being received will be higher than with no noise but still very small. Finally, with noise present and a packet being received, die average energy measured will be less than expected by die reception of a packet over die quiet case, say less than some threshold value t. I f 2 > t N- y j GIBBONS 1-1 Hence the Base, in principle, has the ability to distinguish die problem of congestion from the problem of noise. With this in mind, the operating principle of the invention is as follows. Assume the CAC channel is quiet. Two RU's send packets which collide at die Base. The Base detects this, (average energy > T) and sends an indication over die CLC channel tiiat there is congestion. The RU's respond accordingly. Next, in die presence of noise, an RU transmits a packet to the Base, which is recognized as having die characteristics associated witii noise, (average energy is < t). The Base sends out a message over the CLC indicating noise on die CAC. If die CLC message is received intact, d e RU's note this fact and use it in their next attempt to access the CAC. What if the CAC message is not received intact? For the particular RU which has transmitted a packet over die CAC and is awaiting a response from the Base, and has timed out, if d e noise indication message is not received on die CLC, nor any congestion message received on die CLC, tiien that RU assumes tiiat die CLC message has been corrupted and responds accordingly. In this manner, access requests from remote units are processed in the minimum amount of time in tiie face of varying traffic congestion and noise burst interference on the CAC channel. In an alternate embodiment of die invention, die remote unit and die base station can exchange their respective request and response messages over the same frequency channel, such as a common broadcast channel. If either a collision or a noise burst is detected by d e base station on die common chancel, die base station can respond on die same channel to die remote station with information on the conditions on die channel. The remote unit will respond as described above, witii a selected type of backoff and retry operation, depending on die information in the response from the base station. If the base station's response back to the remote unit in not received by die remote unit, tiien after a timeout interval, GIBBONS 1-1 the remote unit will infer that there is a noise condition on die common channel. Still anodier alternate embodiment applies die above described invention in die PWAN Frequency Division Duplex Communications System described in die Alamouti, Michaelson et al. patent application cited above. The invention disclosed herein is suitable for wide application in die telecommunications field. The invention finds particular application in die personal wireless access network (PWAN) system which is described in die referenced Alamouti, et al. patent application, incorporated herein by reference. Although the preferred embodiments of the invention have been described in detail above, it will be apparent to those of ordinary skill in the art that obvious modifications may be made to the invention witiiout departing from its spirit or essence. Consequently, the preceding description should be taken as illustrative and not restrictive, and the scope of the invention should be determined in view of die following claims.

Claims: GIBBONS 1-1 CLAIMS What is claimed is: 1. A highly bandwidtii-efficient communications method, comprising: receiving at a base station during a first time period a first spread signal from a first remote unit comprising a first common access channel data signal spread over a first plurality of discrete tones in accordance witii a first spreading code assigned to at least the first and a second remote units; receiving at the base station during die first time period a second spread signal from die second remote unit comprising a second common access channel data signal spread over the first plurality of discrete tones in accordance with the first spreading code; adaptively despreading die first spread signal and die second spread signal received at die base station by using first despreading codes tiiat are based on die characteristics of the received signals; determining at the base station that a collision has occurred between die first and second spread signals; and transmitting from the base station a notice of the collision to the first and second remote units. 2. The highly bandwidtii-efficient communications network of claim 1 , which further comprises: GIBBONS 1-1 said first and second spread signals are discrete tones received by die base station from the first and second remote units during a reverse interval of a time division duplex period. 3. The highly bandwidth-efficient communications network of claim 2, which further comprises: said discrete tones are modulated witii an access request by die first and second remote units using a 16 QAM modulation scheme. 4. The highly bandwidtii-efficient communications network of claim 3, which further comprises: said discrete tones are sampled, digitized, passed through a fast Fourier transform (FFT) processor, and stored in FFT incremental frequency bins as complex numbers which represent points in a 16 QAM modulation constellation which are related to an average amount of energy of the discrete tones. 5. The highly bandwidtii-efficient communications network of claim 4, which further comprises: said base station using said complex numbers to prepare said notice of the collision. 6. The highly bandwidth-efficient communications network of claim 5, which further comprises: said base station distinguishing die collision from a noise burst by using said complex numbers. 7. The highly bandwidtii-efficient communications network of claim 1, which further comprises: GIBBONS 1-1 said first remote unit and said second remote unit receiving said notice and in response thereto, beginning a first type back-off and retry process to avoid a second collision. 8. The highly bandwidtii-efficient communications network of claim 7, which further comprises: said first remote unit and said second remote unit failing to receive said notice and in response thereto, beginning a second type back-off and retry process to minimize effects of noise bursts. 9. A highly bandwidtii-efficient communications method, comprising: receiving at a base station during a first time period a first spread signal from a first remote unit comprising a first common access channel data signal spread over a first plurality of discrete tones in accordance witii a first spreading code assigned to at least die first and a second remote units; receiving at die base station during the first time period a noise burst signal comprising that at least partially interferes with the first plurality of discrete tones; adaptively despreading tiie first spread signal and processing the noise burst signal received at die base station by using first despreading codes tiiat are based on the characteristics of the first spread signal; determining at die base station tiiat a noise burst has occurred while receiving the first spread signal; and transmitting from the base station a notice of the noise burst to the first remote unit. GIBBONS 1-1 10. The highly bandwidtii-efficient communications network of claim 9, which further comprises: said first spread signal is discrete tones received by the base station from die first remote unit during a reverse interval of a time division duplex period. 11. The highly bandwidtii-efficient communications network of claim 10, which further comprises: said discrete tones are modulated witii an access request by the first remote unit using a 16 QAM modulation scheme. 12. The highly bandwidtii-efficient communications network of claim 11, which further comprises: said discrete tones are sampled, digitized, passed through a fast Fourier transform (FFT) processor, and stored in FFT incremental frequency bins as complex numbers which represent points in a 16 QAM modulation constellation which are related to an average amount of energy of the discrete tones. 13. The highly bandwidtii-efficient communications network of claim 12, which further comprises: said base station using said complex numbers to prepare said notice of the noise burst. 14. The highly bandwidth-efficient communications network of claim 13, which further comprises: said base station distinguishing die noise burst from collisions with spread signals from otiier remote units GIBBONS 1-1 by using said complex numbers. 15. The highly bandwidtii-efficient communications network of claim 9, which further comprises: said first remote unit receiving said notice and in response thereto, beginning a first type back-off and retry process to minimize effects of the noise burst. 16. The highly bandwidtii-efficient communications network of claim 15, which further comprises: said first remote unit failing to receive said notice and in response thereto, beginning said first type backoff and retry process to minimize effects of noise bursts.

Inventors: GIBBONS DAVID, MAXWELL ROBERT LEE, RYAN DAVID JAMES, GIBBONS, DAVID, MAXWELL, ROBERT, LEE, RYAN, DAVID, JAMES
Applicants: AT & T WIRELESS SERVICES INC, AT & T WIRELESS SERVICES, INC.
Application year: 1998
Main classification: H04B72
IPC codes: H04B1, H04L1