KR100210632B1 - Random access in mobile radio telephone system - Google Patents

Abstract

The present invention includes a system and method for minimizing crosstalk between two radio stations at the start of wireless communication, for example, between a mobile radiotelephone and a fixed base station. The mobile station (Figure 1) initiates random access at the lowest power level (118), increasing the transmit power level until the base station detects the access signal (116). If a signal is detected, the power level of the message remains at the detection level to avoid crosstalk of the signal. The present invention also provides a mechanism for synchronizing random access communication between a mobile station and a base station despite various remote distances between the mobile station and the base station.

Description

[Name of invention]

Random access in mobile phone system

Background of the Invention

The present invention relates to a system for minimizing crosstalk caused by a mobile radio station initiating and terminating communication with a fixed radio station.

In a cellular wireless telephone network, mobile subscribers are free to choose where and when to initiate telephone calls. This procedure is known as random access call set-up. The term random access also applies to the first transmission of a mobile station in response to a call initiated through the mobile station's fixed home base station. In both cases, there is considerable uncertainty in determining the transmit power level of the mobile station in approach.

By three basic broadcasts, a wireless telephone system can support multiple calls in progress within a given frequency band. Frequency division multiple access (FDMA) is a conventional method in which all call connections between a mobile station and a base station are assigned to a particular frequency channel that is occupied continuously until the call is terminated. In recent years, mobile telephone systems have been changing in a time-based manner of sharing communication resources from FDMA. In time division multiple access (TDMA) schemes, different radio transmitters are assigned to short time slots in periodic cycles, where the radio transmitters transmit consecutive information. In the third method, code division multiple access (CDMA), different voice / information signals are transmitted with different spread-spectrum codes so that the coded signals can be overlapped on time and at the same frequency. The received CDMA signals are decoded by correlation with a code associated with the required voice / information signal.

In all mobile telephone systems, the physical distance between the mobile station and the base station varies considerably. The signal propagation loss between the radio transmitter and receiver varies as a function of the square of their mutual distance [mutual distance ⁴ ]. As a result, the strengths of the signals received at the base stations from different mobile stations are largely different. Although conventional cellular radiotelephone systems use a variety of techniques to avoid crosstalk between different signals, nevertheless crosstalk when the differences between signal strengths received from various mobile stations increase This happens.

This crosstalk problem is particularly noticeable in CDMA systems where the mobile station signal occupies twice the system capacity, which is twice as strong as other mobile station signals. Unfortunately, it is not uncommon for powerful mobile stations to transmit signals thousands of times stronger than other mobile station transmissions. The loss of system capacity due to such a powerful mobile station is unacceptable, so power regulation is particularly important in CDMA systems. A US patent entitled Duplex Power Control filed April 10, 1992. In US 866,544, we describe a power regulation method and apparatus for a CDMA system. This application was used as a reference in the present invention.

United States Patent Application No. 07 / 739,446, entitled CDMA Subtractive Demodulation, issued September 29, 1992, entitled CDMA Subtractive Demodulation. In the call and other pending applications by the present inventors, a CDMA subtractive demodulation system is described in which a redundant coded signal is decoded from the strongest signal to the weakest signal. After each decoding, the decoded signal is removed or subtracted from the received composite signal before decoding the next strongest signal. Using this CDMA subtractive demodulation system, the difference in signal strength between mobile stations becomes less important and the capacity is increased. In other words, the signals with the greatest potential to cause crosstalk, the strongest signals, are first decoded or removed. In this way, there is a significant reduction in potential sources of crosstalk for weak signals.

However, even in such a CDMA subtractive demodulation system, there is still a problem that crosstalk occurs when the mobile station initiates random access call setup. Because of the difficulty in measuring the proper access power level, there is a risk of at least temporary interference with ongoing calls.

Another cause of potential confusion with ongoing calls during mobile random access is the misalignment of the time of mobile random access signals with respect to the frame timing of the base station. In order for mobile station signals to be received in the correct time slot (TDMA) or to be precisely time aligned to a particular correlation code (CDMA), the mobile station must adjust its approach signal transmission timing to compensate for the loud-trip propagation delay between the base station and the mobile station. do. Unfortunately, if no recent contact has been made with the base station, the mobile station will need a mechanism to set the correct time alignment for random access.

These problems hinder the efficient operation of current and future cellular systems. Given the frequency at which new cells are deployed by mobile telephone subscribers, particularly those in urban and other congested areas, it is required that the mobile station take random access without creating unnecessary confusion on the network. There is also a need to simply and effectively establish a call connection synchronized with the time-aligned structure of the base station from the mobile station to the base station.

Summary of the Invention

The present invention includes a method for minimizing crosstalk caused by wireless communications initiated between at least one of a plurality of first radio stations and a second radio station. The access message is sent from the first radio station at a relatively low power level. A determination is made as to whether an access message has been detected at the second radio station. If no message is detected, the access message is resent with increasing levels until the message is surely detected. If a message is detected, the power level is fixed at the detected level.

The first radio station may be a mobile radio telephone station, and the second station may be a base station. The access message itself contains an access code and an identification code identifying the first radio station. Each access message is transmitted using spread spectrum signal coding preferably including orthogonal block error correction codes. Furthermore, each access message is scrambled before being sent using a scrambling code selected from the reserved scrambling code group. If the base station has received the access message, the base station notifies the reception and instructs the mobile station to stop increasing the power level. This acknowledgment also includes time alignment information used by the mobile station in connection with the access message transmission power level to determine when the next mobile communication transmission should occur.

In one aspect of the invention, a mobile radio station for communicating with at least one other radio station comprises means for transmitting an access message to another radio station at a relatively low power level. The detecting means determines whether a response message has been received from another radio station. The mobile station has means for retransmitting the access message at an increased power level if no response message is detected. The retransmission means increases the power level of the random access transmission in accordance with the ramp function. The mobile station also includes means for selecting one scrambling code from the list of available scrambling codes broadcast from another radio station generating a random access message. The mobile station additionally has means for adjusting the transmission time of the access message based on the increased power level.

In another aspect of the invention, there is provided a means for each mobile station to initially transmit an access message at a relatively low power level; Means for adjusting a power level of the transmitting means; And a communication system containing a plurality of mobile radiotelephone stations and at least one fixed base station having control means for controlling the adjusting means depending on whether the access message has been detected. The base station comprises means for receiving a combination of signals from the mobile station; Means for detecting mobile access messages; Means for decoding the detected access messages; And means for transmitting a response message to the mobile station corresponding to the detected access message.

The base station includes means for ordering received signals containing access messages according to signal strength; Means for selectively decoding the strongest signal; And means for removing the decoded signal from the received composite signal. The mobile station comprises means for encoding a scrambled access message using bi-orthogonal block codes and means for scrambled access messages using scrambling codes. The transmitting means of the base station broadcasts a list of reserved scrambling codes that are distinguished from the scrambling codes used for other wireless communications.

The mobile station includes means for adjusting the transmission time of the access message based on the adjusted power level and means for detecting time alignment information in the response message. The base station includes means for detecting a difference between the signal strength of the random access message detected at the base station and the predetermined signal strength, and means for determining a time difference between the time of the detected random access message and the predetermined time. . Finally, the base station detection means searches for a specific access message at staggered time intervals.

[Brief Description of Drawings]

The features and advantages of the present invention will be apparent from the detailed description described with reference to the accompanying drawings.

1 is a schematic diagram of the functional aspects of a mobile station transmitter in accordance with the present invention.

2 is a schematic diagram of the functional aspects of a transceiver portion of a base station according to the present invention.

3A illustrates an exemplary signal format for a random access message from a mobile station.

FIG. 3B is a diagram illustrating partial variation of the random access message start code sequence. FIG.

4a to 4e are diagrams showing the basic operation of the RAKE receiver.

5 is a schematic diagram of the functional aspects of a multi-stage power amplifier according to the present invention.

6 is a schematic diagram of the functional aspects of a balanced modulator in accordance with the present invention.

Detailed Description of the Preferred Embodiments

In order to facilitate understanding of the present invention, a specific example will be described in the background of a CDMA subtractive demodulation system as described in the above-mentioned US Patent No. 5,151,919. Those skilled in the art will appreciate that the present invention can be applied to any wireless communication system that requires minimizing crosstalk caused by random access call attempts between two wireless communication devices, including all cellular wireless telephone systems. know.

In brief, in a CDMA subtractive demodulation system, information between a plurality of mobile stations and a base station is transmitted in blocks of codewords, for example, in blocks every 42 codewords. A typical signal transmission format is a sequence of 128 bit codewords transmitted continuously on a wireless communication channel. The wireless receiver amplifies, filters, samples, and converts a complex received signal consisting of redundant communication signals into digital form for processing. The digitized composite signal is descrambled by a unique scrambling code corresponding to the information signal having the largest signal strength. The descrambled signal is independent of the spreading code known as the orthogonal (or bi-orthogonal) block code associated with the information signals. The 128 bit signal sample is decoded by a two-orthogonal block decoder to determine which block code has the best correlation with the sample signal, producing an eight bit information signal. The decoded information signal, i.e., 8 bits, identifies which of the 128 bit two-orthogonal codes was transmitted and subtracts this identification code from the composite signal before decoding the next strongest code information signal.

In the specific embodiment of the present invention shown in FIGS. 1 and 2, the mobile station transmitter 10 includes a radio frequency (RF) power amplifier 100 coupled to the duplex antenna 102. The frequency synthesizer 104 generates a transmit carrier waveform that is modulated by the quadrature modulator 106 with the information signal, e.g., voice. Quadrature modulator 106 selectively modulates the information bits on the in-phase (I) channel and the quadrature (Q) channel using waveforms generated by two low pass (LP) filters 108 and 110. A special technique can be implemented, such as an impulse-excited quadrature amplitude modulator (QAM). The complex modulator 112 calculates the shock response waveforms corresponding to the polarities of the received digital information signals and converts these waveforms into analog form. LP filters 108,110 basically remove the digital to analog conversion sampling frequency component. Optionally, the information signal may be initially mixed to a typical intermediate frequency and then converted to a higher carrier transmission frequency by a heterodyne that mixes the modulated intermediate frequency with an offset frequency.

Digital information signals received by the complex modulator 112 are generated by the block codeword generator and the scrambler 114 or the voice encoder 122. When the mobile station 10 is transmitting a random access message, i.e., before the start of the voice communication, the message is generated in the control processing unit 166 and input into the block codeword generator and the scrambler 114 at 8 bits simultaneously. However, once voice transmission is initiated, an 8 bit input to the block codeword generator and scrambler 114 comes from the voice digitizer and encoder 122. Voice encoder 122 receives a microphone signal from microphone 124 and outputs an 8 bit word. The switch 126 is controlled by the control processing unit 116. For input to the block codeword generator and scrambler 114, the control processing unit 116 selects itself for the transmission of the random access message or voice encoder 122 for the conversation transmission. Even after the random access procedure is completed, the control processing unit 116 may sometimes operate the switch 126 to select a message transmission and interrupt the voice transmission. This is done for high priority signaling message exchange between the base station 20 and the mobile station 10, such as, for example, a Fast Associated Control Channel (FACCH) message.

In the block codeword generator and scrambler 114, eight bits of information are spread, for example, into 128 bit codewords using the appropriate two orthogonal block codes. The 128 bit codeword is then scrambled by modularly adding the unique scrambling code to the codeword. Information bits and scrambling codes are generated from the control processing unit 116, which also selects the carrier frequency generated by the frequency synthesizer 104 and transmits a power level command signal to the power level controller 118.

Power level controller 118 includes a combination of attenuators and components to control the bias of power amplifier 100 to execute a commanded power level when transmitting each codeword. Combinations of attenuator and amplifier bias controllers are useful for achieving a suitable width of transmission power level control range, for example 60 dB, and a wide variety of combinations are known, among which the following techniques may be used as required by the present invention. have.

Since the final stage of the power amplifier can only be controlled within a power range of 20 dB, a wide transmit power control range is difficult to achieve by controlling the bias of only one stage of the power amplifier. Thus, the two-stage power amplifier 100 controlling both stages produces a 40 dB control region and can obtain the required 60 dB region by giving a fixed 20 dB attenuator selectively switched to the output of the power amplifier. . Of course, if the bias control of a single amplifier stage is good, two independently controllable 20 dB fixed attenuators can be selectively switched in the output, resulting in the same 60 dB control area. An example of a multistage power amplifier is shown in FIG.

5 is a schematic diagram of the functional aspect of the multi-stage power amplifier 50 in which modulated in-phase (I) and quadrature (Q) signals are input to the quadrature modulator 106. Quadrature modulator 106 includes a level-switchable balanced modulator controlled by level control bits B ₁ -B ₄ to provide first 0-20 dB control. The first frequency F _1, which provides the carrier frequency, is also input from the transmitter frequency synthesizer 104 to the quadrature modulator 106. The output of the quadrature modulator 106 is input to an uplink frequency converter 504 equipped with a second frequency F ₂ from the transmitter frequency synthesizer 104. The heterodyne of the uplink frequency converter 504 mixes the modulated signal (at a low frequency for technical convenience) to a higher, fixed second frequency F ₂ to transition to a higher output frequency. Conversely, in order to convert the high frequency signal received at the antenna to a low fixed intermediate frequency so that amplification can be carried out more easily, a converter or super heterodyne mixer is usually used in the receiver at the down frequency. In either case, it is advantageous to modulate or demodulate the signal at a low fixed frequency and vary the frequency at the antenna by changing the oscillator driving the mixer.

The output of the converter 504 is input to the band pass filter 506 and supplied to the driver 507. The gain on driver 507 is controlled between 0-20 dB by power level controller 118. The output of the driver 507 is input to the power amplifier 508, the gain of which is controlled between 0-20 dB by gain control of the power level controller 118. The output of the last amplifier 508 is input to the antenna 102 for broadcasting. This circuit allows a total transmit power control range of 0-60 dB. It can be seen that a variable attenuator can also be used. Switchable and variable attenuators are commercially available from many of the same sources from Avantech, Santa Clara, California.

Suitable balance modulators are provided as shown in FIG. 6 for the inter-modulator 106 for controlling gain. The circuit of FIG. 6 includes pair balance modulators 602a and 602b. Each modulator 602 includes two pairs of twin transistors 603a, 603b; 605a, 605b with emitters coupled to each other within each pair. The bases of each pair of symmetrically opposing transistors 603a, 605b; 603b, 605a are coupled together to form a bridge, where a local oscillator (transmitter frequency synthesizer; 104) providing a carrier frequency F ₁ is a bridge circuit. Are connected in parallel. The collectors of transistors 603 and 605 are cross linked and form the output of quadrature modulator 602. The combined emitters of each balance modulator 602 are coupled to the collector of the multiple emitter transistor 604, and the bases of the multiple emitter transistors are controlled by individual modulation inputs. Each emitter of each multiple emitter transistor 604 is connected together by a separate series connected resistor pair R ₁ -R ₄ . Resistor pairs R ₁ -R ₄ are centrally tapped by switchable current generators 606 (I ₁ , I ₂ , I ₃ , I ₄ ), which are generated by control bits B ₁ -B. Controlled by _four . The values of the unbypass emitter resistors R ₁ -R ₄ in each balance modulator 602 selectively activate the tail current source 606 connected to the respective center tapped emitter resistors R ₁ -R ₄ . By means of which the balance modulator circuit 60 has a binary-programmed output level.

Controlling the transmit power level can also be accomplished by numerically scaling the digital values of I, Q before the digital I, Q values generated at the complex modulator 112 are converted to analog form for the quadrature modulator 106. Can be. The control range is somewhat limited, but the gain can be selected easily and accurately, for example in 0.1 dB steps.

2 illustrates an exemplary base station transceiver 20 for detecting mobile random access in a communication environment of overlapping ongoing wireless traffic signals. Antenna 200 receives a low noise, composite signal amplified by RF amplifier 202. The amplified signal is spectral shaped by filter 204, and dual analog-to-digital converter 206 carries the filtered analog signal with a real or in-phase part (I) and an imaginary part or right angle part (Q). To a stream of digitized signals with complex numbers. Optionally, an intermediate frequency mixing step will precede the amplifier 202 so that amplification and filtration occur at lower intermediate frequencies.

After the frequency demodulation process, the complex digitized composite signal is processed by the CDMA subtracted signal processor 208. Since the individual signals to be demodulated are each scrambled by a unique scrambling code generated by the control processing unit 116 of the mobile station, the CDMA signal processor 208 is combined by each scrambling code in order from the strongest signal to the weakest signal. Describing sequentially. The descrambled signal is decoded by correlation with all two orthogonal codes that could be used to encode to extract the eight bit information for each 128 bit two orthogonal code word. The correlation is described, for example, by the fast Walsh-Hadamard transform processor described by the inventor of the pending US patent application Ser. No. 07 / 735,805, filed July 25, 1991 (Fast Walsh-). By using a Hadamard transform processor). The decoded information bits are sent to the base station control processor 212 for further voice / data processing.

By selecting a scrambling code corresponding to the signal having the largest signal strength, the base station CDMA processor 208 demodulates the signals that are variously duplicated in order from the strongest to the weakest foreseen signal strength. Signal strength tracker and sorter 210 predict signal strengths from past observations and trim them. By recognizing the power level variation over time, the signal strength tracker and aligner 210 freely rearrange the signal decoding sequence to accommodate the corresponding power level variation. By extrapolating the next power level using the calculated power level variation, the expected signal strength level is predicted based on the history of the last power level.

Random access by the mobile station 10 using a low power level is detected only after a stronger signal is decoded. In addition to decoding the information or traffic signals, base station processor 208 searches for and decodes each coded mobile access signal according to the detected power level. Since the mobile access message is initially sent at the low power level, this message is not confused with the stronger signals that are decoded. As soon as the access message is detected, processor 208 subtracts the access message from the composite signal to minimize confusion with the next signal to be decoded in order of sorting by signal strength.

The power level of the initial random access message from the mobile station 10 is set by the power controller 118 to a low power level. The power controller 118 gradually increases the transmit power by a small intermediate, for example 0.1 dB, after each successive codeword is transmitted. These successive power increases continue while the total number of codewords making up the random access message are all transmitted. Preferably, the power level increases with a ramp function having a ramp slope that determines the magnitude of the power increment. The ram function is easily implemented and simplifies the signal strength prediction process of the base station signal tracker and aligner 210. Each successive transmission of the random access message by the mobile station 10 occurs at a higher power level than the preceding access message transmission until the base station 20 detects the access message and sends a response message to the mobile station 10. If the base station 20 indicates that a response message is received and the mobile station power level is within the required signal strength range, the mobile station 10 locks its transmit power to the level specified by the base station 20 in the response message.

It takes about 0.5 milliseconds to serially transmit a single 128-bit codeword. If the mobile station increases its transmit power level by only 0.1 dB per code, the rate of change of the power level is 200 dB per second, which is perceived as a high rate of change. Since the received power level range for a typical traffic or access signal is on the order of 60 dB, codewords of 600 or less need to be transmitted before the base station detects the access message. Thus, the worst case delay in access detection is only about 300 milliseconds. In this way, the present invention allows the mobile station to randomly access the cellular network without causing significant delay for the mobile subscriber and without any interference with other communication signals.

To simplify the task of the base station detecting the random access message, the first two codewords of every 42-codeword message are fixed to a first value A. Thus, the 42-codeword message starts with AA .... For simplicity, the message length was chosen to be 42 codewords, since this is also the length of the speech code frame in the system of the present invention. When transmitting voice traffic, the first two codewords also take the value AA .... to indicate when all the voice frames have been transmitted, but the remaining 40 codewords of the next speech island frame because the speaker is temporarily silent. Also takes BB .... as the second value when required to indicate that is not to be transmitted. This so-called discontinuous transmission or DTX is described in US Patent Application No. 866,555 entitled Discontinuous DCMA Reception, filed April 10, 1992.

Therefore, the base station receiver's ability to detect the occurrence of AA ..., BB ... is used in dual systems for identifying DTX voice frames as well as random access attempts in good systems.

The staggering of the relative occurrences of the codewords, AA, for a particular codeword, allows the receiver to predict when AA is generated, so that it is not necessary to search AA for every codeword every frame. It is related to word selection.

If the AA sequence is not detected immediately because the first mobile access attempt was made at the low power level, the power level is increased by 42 x 0.1, ie 4.2 dB, the next time the receiver of the base station searches for AA. In approach attempts, instead of increasing the power in a smoothed ramp-function fashion of 42 0.1 dB smaller steps between attempts, one step of 4.2 dB seems to be able to increase power as well, but Since the smooth increase in power is much simpler for the operation of the base station 20 after the detection of a random access attempt, the base station 20 decodes all codewords and small variations in power level between the codewords more easily. Can be traced

One object inherent in the present invention is that the base station 20 detects random access messages and raises their power to the target level immediately after this detection while the power level of the random access signal is too low to be confused with the ongoing traffic. Subtract the signals. If the random access was below the detectable level in the previous attempt, there is no risk of large crosstalk even if the signal level is increased by 4.2 dB before the base station detects the random access attempt again.

An exemplary random access message sequence showing the sequence of 42 codewords generated by the control processing unit 116 is shown in FIG. 3A. The first two 128-bit codewords A, A have a fixed bit pattern recognized by the base station control processor 212 as a random access message, and the remaining 40 codewords (3, 4, 5, 6 ..., 42). ) Includes the identification code / telephone number and communication information of the mobile station in the form of an error correction code.

Codeword A is one of the 128 possible Walsh-Hadamard codewords used to indicate random access and is scrambled with one of a number of reserved scrambling codes not used for traffic. Base station 20 descrambles the received signal with a reserved scrambling code and then correlates A to determine whether random access has been initiated. Base station 20 must detect one of the A codewords to detect random access initiation, and if A is detected, base station 20 demodulates the remaining 40 codewords. Codewords A and A are also used to synchronize communication between the mobile station 10 and the base station 20 as described below.

In order to prevent overloading of the base station 20 caused by requiring all the scrambling codes of the 42-codeword sequence in the same 128-bit block to find all the AA sequences, the position of the AA sequence is determined for three scrambling codes. It changes according to the scrambling code used as shown in FIG. 3b. Similar positional changes are provided in the rest of the scrambling code. In this method, the base station 20 searches for the codewords A and A at different times for each scrambling code, thereby avoiding the processing load that occurs when AA sequences are simultaneously detected in all scrambling codes.

In a preferred embodiment of the present invention, the random access message transmission of each mobile station uses a scrambling code selected from several available scrambling codes that are not soon assigned to current traffic communication. These are indicated in the broadcast message sent by the base station 20 over the designated CDMA channel on the same frequency. The mobile station 10 attempting to access the network scans the broadcast list of available scrambling codes by tuning to this frequency. The mobile station 10 selects an available code to send a scrambled random access message. Currently unused scrambling code on the _first predetermined frequency Fd is identified during the broadcast transmission that uses a signal of the CDMA signals overlapping on the _first frequency Fd.

Preferably this is the strongest of the downlink signals on the first frequency Fd _{1 and} therefore can be decoded by any mobile station 10 as well. This effectively acts as a calling channel where the signal acts as a pilot signal to assist the mobile demodulator. The purpose of this is that each channel maintains itself as long as broadcast information is relevant. If an access is detected and recognized as an access code, the base station control processor 212 removes this code from the broadcast list and decodes the access message as described above. Optionally, the access code currently in use can be broadcast as a list such that the mobile station 10 can select the rest on the list stored therein.

Random access transmission starts at a specific boundary (i.e., the beginning of the 42-codeword message block AA...) Dividing the mobile access message into a certain number of codewords, e. The scrambled start code AA of each two words is staggered in time to possibly spread the mobile station's task of searching for a random access message, as described above. As a result, the control processor 212 only needs to search for a portion of the random access scrambling code possible for each block decoding operation for one unit of time interval. The CDMA signal processor 208 also searches for random access using the scrambling code of the mobile station 20 in neighboring cells that are received with a signal strength strong enough to cause crosstalk.

The time-alignment information is provided by the base station transmitter 214 from the processing unit 116 to the transmission timing controller 120. In response to detecting the random access message, the base station transmitter 214 transmits response information such as a timing difference between the time received by the mobile access transmitting base station 20 and the preset target timing value to the control processing unit 116. do. The other information includes the difference between the signal strength of the detected random access message and the predetermined signal strength.

Even before random access, the mobile station 10 receives the calling channel (ie, the strongest of the overlapping CDMA on the same frequency). This is locked to this signal and thus establishes timing synchronization with the base station 20 at the 42-codeword frame level as well as the spreading code chip level.

The mobile station 10 has been receiving the call channel broadcast from the base station 20 from the start. Also, the signal structure on the channel being called is AA…. Consists of a 42-codeword message. Therefore, the mobile station knows the time of occurrence of AA on the downlink calling channel and from this time the time- staggered of all other codewords is determined. For example, if the calling channel uses the symbol scrambling code C _o and the traffic channel uses the scrambling code C1, C2, etc., the occurrence time of AA in the signal scrambled with code C1 occurs on the calling channel After 2 codewords, and similarly, the occurrence time of AA in the signal scrambled with codeword C2 is after 4 codewords after AA on the calling channel.

First, it is necessary to obtain synchronization with finer resolution than simply determining the 0.5 mS slot in which A occurs. In fact, each codeword like A consists of 128 chips. A chip in CDMA terminology is actually identical to a bit, meaning that CDMA actually transmits a number of such chips to carry each bit of information.

In order to accurately descramble the codeword and perform 128-bit correlation, it is necessary to align the descrambling code to the received signal with one chip of precision. This means that synchronization is achieved at the frame structure level as well as the chip level. Synchronization to the frame structure refers to the search for AA sequencing at any of the 42 possible codeword positions in the frame structure, and synchronization to the chip resolution means that the descrambling code is decoded to 128 possible chip positions within this codeword period. This means deciding where to align.

This is done by actually testing several locations and analyzing how many correlations were found at each location.

This refers to the operation of the mobile receiver, but the timing so found is such as block code generation and scrambler 114 from the mobile receiver to generate frames, codewords and chip alignments related to those received from the base station 20. Is transferred to the mobile transmitter parts. That is, the base station 20 serves as a main timing reference for all mobile stations 10 to synchronize and transmit signals in accordance with a predetermined time relationship.

However, because the distances from the different mobile stations 10 to the same base station 20 are not the same, even if the mobile station 10 transmits signals simultaneously aligned in time, the signals are not received in the base station 20 in alignment. Will not. Therefore, the transmission code generator 114 is commanded by the control processing unit 116 to generate a slightly faster transmission signal to compensate for the loop propagation delay from the base station to the mobile station 10 and vice versa. The exact amount of timing progression required may be determined by the known receiver, which measures the time alignment of the received mobile signal and determines how fast or slow the signal is relative to this predetermined location, and then the mobile station 10 A message is sent to the mobile station 20 providing the amount of time adjustment (e.g., + or-chip number) to adjust.

When the mobile station 10 makes its first transmission to the base station 20 during random access, the mobile station associates the transmission frame, word and chip timing with the timing it is receiving. Therefore, by comparing the timing of the received mobile signal with the timing on the ongoing call channel, the base station 20 can determine a round trip delay. The base station 20 determines how fast or slow the timing of movement is in relation to the desired time alignment required for all the movement signals to be received. The base station then transmits the information to the mobile station 10 so that the mobile station 10 can adjust the transmission timing to the correct position in response to confirming receipt of the random access transmission. This information may be an absolute timing progression used by the mobile station 10 (in this case, the base station 20 needs to know how much timing progression is already in use by the mobile station 10), and the base station 20 may use the mobile station 10 as an example. May be the amount of adjustment desired to be applied in this case (the base station 20 does not need to know the timing progression that the mobile station 10 is already using).

The choice between them is not critical to the present invention and is a matter of system principles. The current preferred solution is to include details of the timing progression and the power level that the mobile station 10 used for transmission as well as the mobile ID of the mobile station in random access. The base station 20 then additionally adjusts this signal and sends this absolute value back to the mobile station 10. The mobile station 10 then ramps the power and timing continuously at the same relaxed rate until the target value most recently received from the base station 20 is reached.

As an example of a mobile random access message, a 40 codeword and a ratio 1/2 error correction code of data present in a 42-word frame generates 20 bytes of information after decoding. These bytes are expanded into a random access message as shown below.

The response from the base station to the mobile station upon detection of the first transmission may be in a similar form.

There are 42 codewords in one frame, which is the coarsest unit in timing resolution. Each codeword contains 128 chips, which are more granular units of timing resolution, but in addition, in a good system, a 4 μS long chip will have a very high frequency clock (i.e. a much shorter pulse than 4 μS). Generated by dividing) by a predetermined integer. For example, in one embodiment the 12.8 MHz clock is divided by 48 to generate a pulse of one chip duration. Since each pulse of 12.8 MHz is 1/48 of the duration of one chip, the system of the present invention can fine-tune any timing adjustment with a fraction of one chip resolution if necessary.

Indeed, in the present invention, it is preferable to gently ramp between one value and the next to a smaller step than to shift the timing to a larger step for the same reason that changing the power level to a larger step is undesirable. It was done together. However, it is only necessary to specify the target or target value with an accuracy or resolution of 1/4 of a chip. For example, if it is detected by the base station receiver that a particular moving signal arrives between 0 and 1 chip, i.e. on average 0.71 chip faster than the other signal, this can be rounded up to 3/4 of one chip, resulting in timing The progress byte value is changed to 3 and included in the timing adjustment message and delivered to the mobile station. On the next few codewords, the mobile station ramps the timing in 1/48 chip steps per codeword and adjusts the 3/4 chip. It takes a total of 36 codewords or 18 mS. This is fast enough for practical use.

In this format, the spare byte is used by the base station 20 to instruct the mobile station 10 to switch to a less heavily loaded frequency channel.

One example of the use of the message format assumes that mobile station 10 initiates random access transmission at the lowest power level. If the signal at this level has a chance to establish communication, zero timing progression is used since the mobile station 10 is not too far from the base station 20. Thus, the first message from the mobile station 10

Power byte = 0;

Timing Progress = 0

It includes.

The mobile station 10 continuously increases the power by about 0.1 dB per codeword so that the first repetitive message starts at about 4.1 dB higher than the first message. By quantizing the power byte to 0.5 dB, the value included in the message is 9. Timing progress does not need to be changed until the power level reaches a very high value without notification from the base station 20, indicating that the mobile station 10 is located at a significant distance. The maximum predicted distance in portable communications is typically 30 Km, with a relative timing progression of 200 μS required to compensate for the round-trip delay. Therefore, at full power, the timing byte contained in the random access repetition message indicates progress in the 200 μS range. Since the mobile station 10 may need higher power for reasons other than distance, for example, for example, local shadowing, it is not about to use the maximum time progression, but about as little as about 100 μS. This 100 μS leaves the delayed signal to be coped by the capacity of the base station to accommodate up to a predetermined maximum delay, for example, one chip length of 32 chips per cycle. In this exemplary figure, a quarter chip is 1 microsecond in length, and the timing byte value represents the timing progression of microseconds currently used by the mobile station 20.

When the mobile station signal is first detected by the base station 20 in the seventh attempt at which half the maximum power (in dB) is reached (i.e., the power byte value reaches 60), the mobile station 10 is configured as an example. For example, a timing progress byte of 40 is used. The base station 20 has detected the mobile station 10 even if the signal is very weak, and it is desirable for the mobile station 10 to continuously increase the power to, for example, 16 dB to bring it into the target range. Moreover, the signal of the mobile station is detected, for example 20 μs later than the good timing window of the mobile station. Therefore, the notification message of the mobile station 20 includes the following values.

Power Bytes Used = 60 + 16 / 0.5 = 92

Timing byte to use = 40 + 20 = 60

Upon receipt of this message, the mobile station 10 ramps the power and timing continuously until it reaches the commanded value.

Due to a processing delay at the base station 20 and the mobile station 10 or a failure in detecting the first notification message due to temporary radio noise, the mobile station 10 attempts at least one attempt before the notification message is received (e.g., Power = 68, timing = 42). It is also detected by the base station 20 at a higher power level which is within the target range of 12 dB and within the desired optimum timing of 18 μS. Therefore, the base station 20 sends a message with another notification having the following values.

Power Bytes Used = 68 + 12 / 0.5 = 92

Timing byte to use = 42 + 18 = 60

Thus, the base station 20 provides a consistent command to the mobile station 10 so that the mobile station 10 adopts the required parameters.

Based on this information, the processing unit 116 timing the transmission of the mobile access message to the timing controller 120 by the timing progression element to compensate for the propagation delay caused by the distance between the mobile station 10 and the base station 20. Instruct to proceed.

The access transmission timing element may be zero and then increase to affect the transmission time faster by the access message transmission power level. The larger the distance between the mobile station 10 and the base station 20, the greater the power level needed, and the earlier the access message should be transmitted earlier to properly time align. As described above, the base station 214 broadcasts information that allows the mobile station control unit 116 to correctly apply the timing controller 120 in accordance with the time progression / power level increase relation to the transfer law belonging to the cell of the base station. Good.

This information includes the effective emission power of the base station transmitter and the radiated propagation loss profile in different directions.

One problem that is solved by a message between the mobile station 10 and the base station 20 is that the mobile station 10 can receive base station transmissions using traffic scrambling codes rather than the channel code being called, and the channel capacity being called. To save, the base station 20 confirms this so that the base station 20 can stop transmitting on the channel being called.

Moreover, other mobile stations 10 in the cell may be disturbed by the sudden output of new base station transmission at any power level. Therefore, the same random access problem that prevents confusion by the sudden appearance of new code is solved in the reverse direction.

Preferred means for causing the base station 20 to begin transmission with an existing unused code initiates at a lower power level than any signal currently in progress, which is nearly equal to the mobile station 10 at a rate of 0.1 dB per 0.5 mS codeword. Similarly, ramp slowly. At the same time, this code is removed from the list of unassigned codes or otherwise added to the list of used code broadcasts from the base station 20.

The format for the list of busy codes broadcast from base station 20 is as follows:

Therefore, within a 20-byte data message, this byte can be assigned to the broadcast message as follows:

This message can be read by the mobile station 10 other than the mobile station that is attempting random access to warn the mobile station of the onset of urgent transmission by an existing idle code. It is also readable by the idle mobile station 10 and can be used to determine traffic loading on each frequency channel that the mobile station scans, thus allowing the mobile station to select random access attempts on the most freely loaded frequency channel. . Since it is not easy for the base station 20 to begin transmitting the power level to be 35 dB below the strongest signal, this increases from the weakest position to the strongest position at a rate of 0.1 dB per codeword, only 360 codewords (180 Hz). Only takes During this time, when the mobile station 10 originates a call, the base station 20 can hand over the already called number to the terrestrial network, so that the delay in the terrestrial network connecting with the called subscriber is not sufficient for completion of random access. It happens in parallel, not in series.

In other words, a land-line connection occurs simultaneously with the completion of the random access and not sequentially.

During this period, the mobile station 10 not only continuously demodulates the common call channel, but also listens for the base station signal which appears using its own scrambling code. The base station 20 transmits the same data in both the calling channel and the code of the mobile station, thereby facilitating the subtraction of the signal by the other mobile station 10 as well as the signal detection by the intended mobile station 10. When the mobile station 10 has successfully detected a signal using its own scrambling code, it notifies the base station 20 of this fact by using an uplink notification format as follows.

The mobile station 10 repeats this message, incrementing the number of messages until the base station 20 stops transmitting power and time alignment message types and transmits new message types or traffic. Therefore, the process of base station / mobile station switching achieves the purpose of the present invention, i.e., duplex base station-mobile station link setup on a particular channel.

If the power level and time-alignment of the base station 20 are satisfied, the base station 20 transmits a traffic channel assignment message to the mobile station 10. This traffic channel assignment message indicates to the mobile station 10 that random access has been achieved and that the mobile station 10 can proceed to the next call setup step. In the call setup initiated by the mobile station, the mobile station 10 transmits the telephone number of the part to be called. When a traffic channel assignment message is sent to the mobile station 10, the mobile station 10 switches to the code / frequency parameter indicated in the message. Therefore, closed loop control of timing and power levels is established between the mobile station 10 and the base station 20. Preferably this power level control loop functions according to the system described in U.S. Patent Application No. 866,544, filed Apr. 10, 1992 and pending dual power control and used as a reference. In particular, however, the traffic channel assignment message includes instructions for changing the CDMA code from a random access code, which may be common with all mobile stations attempting random access, to a code assigned in traffic communication. Incidentally, this traffic channel assignment message contains an instruction to change to another frequency. Procedures beyond this point may be named completion of call setup and may include other message exchanges for the purpose of verifying or vice versa in the called network. Incidentally, the transmit power level applied by the mobile station 10 and the base station 20 beyond the stop of power and time alignment may be controlled using a technique called dual power control described in the above-mentioned US patent application 866,554.

To compensate for the time-of-arrival of the random access message as well as the time-spread and echo on the wireless channel, the CDMA signal processor 208 processes several different shifts of the received and digitized signal samples. Combine the decoded result of each shift in a conventional RAKE detector receiver.

The RAKE receiver allows this signal to flow through the channel as long as it adds a delayed echo.

Figure 4a shows that the desired components / shifts of the original transmitted signal appear in alignment at discrete points of amplitude C, time T. On the other hand, when an echo is added on the transmission path so that a desired signal component appears at T1 having an amplitude C1, this signal is shown as shown in Fig. 4B. In general, the RAKE receiver allows multiple echoes to be received in relation to amplitudes C, C1, C2, C3, etc. at T, T + dT, T + 2dT, T + 3dT, as shown in FIG. 4C.

The predicted position and amplitude are predicted from the prior art and a number of RAKE taps are placed at the predicted position in the delay register to collect these echoes and then added with the appropriate weight.

Therefore, the RAKE receiver can search for the signal at the earliest possible time of arrival (T) by placing one RAKE tap on T, which also delays the time (T + dT) by placing a single RAKE tap on T + dT. You can search for a signal arriving at. The total possible spread of taps on the delay register determines the total amount of uncertainty in arrival time that can be searched. In a preferred embodiment, up to 32 chip time delays corresponding to up to 128 μS time delays may be searched.

When a tap containing significant energy has been identified, the signal is detected by setting several RAKE taps to gather energy from this time shift, and add taps at appropriate weights to minimize signal to noise and crosstalk ratios. In a preferred embodiment, the total spread of tabs combined in this manner is 8 chips. That is, taps containing significant energy can be delayed up to 32 chips, but all of them added for detection are chosen to lie within an 8-chip window within this 32 chip range. This proved to be acceptable without undue loss of performance in practical simplicity rather than in terms of principle. In order to show how to manipulate the RAKE tap to allow the receiver to receive a + or-(more or less) delayed signal, FIG. 3 shows zero weights. This arrangement combines the received energy starting at the first time position T4 and decodes it. In FIG. 4E, the tabs 4, 5, 6 and 7 are zero while the tabs 0, 1, 2 and 3 have the values that the tabs 4, 5, 6 and 7 had in FIG. 4d. . Therefore, the RAKE receiver can precisely receive the same signal as in the first case, i.e., T, which is an earlier arrival time instead of T4. Therefore, by adjusting the relative values of the position and tap weights, the RAKE receiver can receive signals with a + or-(more of less) propagation delay.

Similar problems with crosstalk prevention exist when the call ends. The risk of crosstalk due to the sudden disappearance of a signal is less than due to the sudden appearance of a new signal, but even this small risk can be eliminated by applying a gentle termination procedure as shown below.

After the base station 20 or mobile station 10 transmits the final traffic frame, it proceeds to the discontinuous transmission mode described in detail in US Patent Application No. 866,555, entitled Discontinuous CDMA Reception, filed April 10, 1992. This removes 40 codewords out of the 42 codewords transmitted per frame, leaving only two codewords named DTX FLAGS out of the 42 codewords, so that other receivers are transmitting. They transmit while gradually decreasing the power supply level to about -4 dB per 42-word frame. After 10 frames, this signal level can be terminated by a 40 dB decrease of 1, which only takes about 200 mS. The base station 20 then removes the used access code from the broadcast list of codes in use, so that other mobile stations 10 can freely access the code.

Although specific embodiments of the invention have been described and practiced, the invention is not limited to these specific embodiments, as variations thereof can be made by those skilled in the art, and only by the scope of the appended claims. It is limited.

Claims (43)

CLAIMS 1. A method for minimizing crosstalk caused by wireless communications initiated between one or more first and second wireless stations of a plurality of first wireless stations, the method comprising: transmitting an access message from the at least one first wireless station at a relatively low power level; Transmitting, determining whether the access message was detected by the second radio station, retransmitting the access message at an increased power level until the access message is detected, and detecting the access message And transmitting power setting information for the first radio station from the second radio station. 2. The method of claim 1 wherein the first radio station is a mobile radiotelephone and the second radio station is a base station. 2. The method of claim 1, wherein said access message comprises an access code and an identification code of said first radio station. 10. The method of claim 1, wherein the access message is transmitted using spread spectrum signal coding. 5. The method of claim 4, wherein the access message includes an orthogonal block error correction code. 5. The method of claim 4, wherein said transmitting step comprises scrambling said access message using a scrambling code selected from a reserved group of scrambling codes. 7. The method of claim 6, wherein the scrambling code can be identified with an information broadcast on a common call channel. 2. The method of claim 1, further comprising notifying the reception of the access message at the second radio station. 9. The method of claim 8, wherein the notifying step includes instructing the first radio station to stop increasing the power level after a particular delay. The method of claim 1, wherein the access message transmission time is based on the power level. 10. The method of claim 8, wherein said notifying includes transmitting time alignment information. 9. The method of claim 8, wherein said notifying includes transmitting power adjustment information. 2. The communication initiation of claim 1 including the step of gradually reducing the power level and terminating the wireless communication including terminating the communication if the power level is reduced by a predetermined amount. A method for minimizing crosstalk that occurs. 14. The method of claim 13, wherein said terminating step comprises entering a discontinuous transmission mode. A mobile radio station for communicating with one or more other radio stations, comprising: means for transmitting a random access message from the mobile radio station to the other radio station at a relatively low power level, means for detecting a response message from the other radio station; And means for instructing the transmitting means to retransmit the random access message at an increased power level if the response message containing a notification providing power setting information is not detected. 16. A mobile station as claimed in claim 15, wherein said retransmission means increases said power level in accordance with a ramp function. 16. A mobile station as claimed in claim 15, wherein said means for transmitting comprises means for selecting a scrambling code from a list of scrambling codes broadcast from said other radio station to generate said random access message. 16. The mobile station of claim 15, further comprising means for adjusting the transmission time of the access message based on the increased power level. Means for initially transmitting an access message at a relatively low power level, means for adjusting the power level of the transmitting means, and control means for controlling the adjusting means depending on whether the access message has been detected or not; It includes a number of mobile radiotelephone stations; And means for receiving overlapping composite signals transmitted from the mobile stations on the same frequency channel, means for detecting the access message, and means for transmitting a response message to the mobile station corresponding to the detected access message. A communication system comprising at least one base station nested. 20. The apparatus of claim 19, wherein the base station comprises means for trimming the received signals containing the access message according to signal strength, means for selectively decoding a strongest signal, and decoded the decoded signal in the received composite signal. And means for removing. 21. The apparatus of claim 20, wherein each mobile station means for encoding an access message using an orthogonal or two-orthogonal code to generate a coded access message, and scrambling the coded access messages using a unique scrambling code. And means for performing the communication system. 22. The communications system of claim 21 wherein the scrambling codes are reserved for random access messages. 23. A communication system as claimed in claim 22, wherein the base station transmitting means broadcasts the list of reserved scrambling codes to enable a mobile station to determine an available access code. 23. A communication system according to claim 22, wherein said base station transmitting means broadcasts said list of reserved scrambling codes distinguished from scrambling codes used for other wireless communications. 23. A communication system according to claim 22, wherein said base station transmitting means broadcasts said reserved list of scrambling codes used for other wireless communications. 20. The communication system of claim 19, wherein said access message contains a mobile station identification code and a random access code. 20. The apparatus of claim 19, wherein each mobile station comprises means for adjusting the transmission time of the access message based on the adjusted power level, and means for detecting time alignment information in the response message. Communication system. 28. The communication system of claim 27, wherein the base station comprises means for determining a difference between a signal strength of the random access message detected at the base station and a predetermined signal strength. 29. A communication system according to claim 28, wherein said determining means determines a time difference between a time at which said random access is detected and a predetermined time. 20. A communication system according to claim 19, wherein said receiving means comprises means for correlating with a predetermined signal spreading code associated with said access message. 31. A communication system as claimed in claim 30, wherein said detecting means is a RAKE detector. 20. A communication system according to claim 19, wherein said base station detecting means searches for a specific access message at staggered time intervals. A method for establishing wireless communication between one or more second radio stations and one or more first ones of a plurality of first stations, wherein the load of the one or more second stations is caused by the one or more first stations. Selecting less jammed channels; And selecting, by the first radio station, a scrambling code identified from the list of codes broadcast by the second radio station, initiating a call by the first radio station to contain the selected code at a relatively low power level. Transmitting a message, and gradually increasing the power level of the transmitted message until a notification message is received by the first radio station from the second radio station. Communication system. 34. The method of claim 33, wherein the channel selection step includes receiving and decoding signals on a channel from one or more of the second radio stations at various radio frequencies, determining a load on each received channel, and in a clean reception state. And selecting a channel with a low load according to the present invention. 34. The wireless communication of claim 33, wherein the random access procedure further comprises initiating the random access procedure upon receipt of a call initiation signal sent by the first and second wireless stations. How to open. 36. The method of claim 35, wherein the random access procedure determines, by the second radio station, whether a code from the code list is being transmitted by a first radio station, by the second radio station using the selected code. Transmitting a notification message upon reception and decoding of the transmission message, transmitting, by the first radio station, a second message to the second radio station upon detection of the notification message, and receiving and Stopping transmission of the notification message by the second radio station upon decoding. 34. The method of claim 33, wherein the notification message is sent on a common call channel and includes timing adjustment information by the second radio station. 37. The method of claim 36, wherein the second radio station initially transmits the notification message at a low power level and gradually increases the power level until receipt of the second message. 37. The method of claim 36, wherein said second message includes signal strength information. 36. The method of claim 35, wherein the first radio station receives the notification message from both a common call channel and the selected channel. 34. The method of claim 33, further comprising the step of terminating the communication including gradually decreasing the power level of the transmitted message and terminating the communication if the power level is reduced by a predetermined amount. Wireless communication establishment method characterized by the above-mentioned. CLAIMS 1. A method for minimizing crosstalk caused by termination of wireless communication between a first radio station and a second radio station, the method comprising: entering a call termination mode after a last communication segment, gradually decreasing a transmit power level, and the power Terminating transmission if the level is reduced by a predetermined amount. 40. The method of claim 39, wherein the end mode is a discontinuous transmission mode in which frames of data are reduced to two or less flags.