MXPA99011637A - Mobile station synchronization within a spread spectrum communications system - Google Patents

Abstract

Each frame of a pilot channel transmission in a spread spectrum communications system is divided into a plurality of synchronization slots. Each of the synchronization slots includes a pilot code, and at least one of the synchronization slots further includes a framing synchronization code. To extract frame and slot synchornization information from the pilot channel transmission, pilot code timing is first identified by applying a matched filter or correlation to a received pilot signal, identifying peaks, and using the peaks to find a timing reference indicative of synchronization slot boundaries. Next, the set of known framing synchronization codes are correlated with the received signal over the included found synchronization slots. Given that the location within the frame of the known framing synchronization code(s) is known, once a correlation match is found at a certain slot location, the boundary of the frame (i.e., the frame synchronization) relative thereto is then also known.

Description

SYNCHRONIZATION OF MOBILE STATION WITHIN AN EXTENDED SPECTRUM COMMUNICATION SYSTEM BACKGROUND OF THE INVENTION Technical Field of the Invention The present invention relates to extended spectrum communication systems and, particularly, to the timing synchronization of a mobile station with a station of base in an extended spectrum communication system. Description of the Related Art The cell phone industry has advanced significantly in terms of commercial operations throughout the world. Growth in major metropolitan areas has far exceeded expectations and is exceeding the capacity of the system. If this trend continues, the effects of rapid growth will quickly reach even the smallest markets. The prevailing problem with regard to continued growth is that the customer base is expanding while the amount of electromagnetic spectrum allocated to cellular service providers for use to carry radio frequency communications remains fixed. Innovative solutions are required to meet these increased capacity requirements in the limited available spectrum as well as to maintain a high quality service and avoid a rise in prices.

Today, access to channels is primarily achieved by using multiple division frequency access (FDMA) and Time Division Multiple Access (TDMA) methods. In multiple frequency division access systems, a communication channel comprises a single radio frequency band in which the transmission energy of a signal is concentrated. In time division multiple access systems, a communication channel comprises a time segment in a periodic series of time slots on the same radio frequency. Even when satisfactory performance is obtained with the FDMA and TDMA communication systems, what is known as channel congestion occurs due to the growing demand from customers. Therefore, it is being proposed, considering and implementing alternative methods of accessing channels. The extended spectrum comprises a communication technique that is finding a commercial application as a new method of accessing channels in wireless communications. The spread spectrum systems have existed since the days of the Second World War. The initial applications were predominantly military (in relation to intentional interference and radar). However, there is now a growing interest in the use of extended spectrum systems in communication applications, including digital cellular radio, land mobile radio, and personal / indoor communication networks. The extended spectrum operates quite differently from conventional TDMA and FDMA communication systems. In a forward-sequence code division multiple-access extended spectrum (DS-CDMA) transmitter, for example, a stream of digital symbols at a basic symbol rate is extended at a speed of transmission symbols (or velocity of symbols). chips). This extension operation includes the application of a unique digital user code (the extension or signature sequence) to the stream of symbols that increases its symbol speed while adding redundancy. This application typically multiplies the stream of digital symbols by digital code. The resulting transmitted data sequences (chips) are modulated using an appropriate modulation scheme to generate an output signal. The output signal (known as a channel, such as for example traffic channel or pilot channel) is added to other output signals (channels) similarly processed (ie, extended) for transmission of multiple channels in a communication medium . The output signals of multiple users (channels) then profitably share a transmission communication frequency with the multiple signals appearing positioned one above the other both in the frequency domain and in the time domain. Since the digital codes applied are unique to the userhowever, each output signal transmitted on the shared communication frequency is likewise unique, and through the application of appropriate processing techniques at the receiver they can be distinguished from each other. In the DS-CDMA extended spectrum receiver, the received signals are demodulated and the appropriate digital code is applied to the user of interest (i.e., multiplied or reciprocated) to decode, or remove the coding of the desired transmitted signal and Go back to the basic symbol speed. Where this digital code is applied to other signals transmitted and received, however, there is no decoding since the signals remain with their chip speed. The decoding operation therefore effectively comprises a correlation process that compares the received signal with the appropriate digital code. Before a transfer of radio frequency communication or information between a base station and a mobile station of the spread spectrum communication system can occur, the mobile station must find and synchronize with the timing reference of this base station. In a forward-sequence code division multiple access extended spectrum communication system, for example, the mobile station must first find the downlink chip limits, symbol limits and frame boundaries of this timing reference clock . The most frequent solution implemented for this synchronization problem is that the base station transmits periodically (with a repetition period Tp) in a pilot channel, and the mobile station detects and processes a recognizable pilot code cp of length Np chips as shown in Figure 1 in a type of CDMA communication system, each base station employs a different known pilot code taken from a set of available pilot codes. In another type of CDMA communication system, each base station uses the same pilot code, with differences between base stations that are identified through the use of different bases for transmissions. In the extended spectrum receiver of the mobile station, the received signals are demodulated and applied to a filter corresponding to the pilot code (s). Obviously it is understood that alternative detection schemes, for example sliding correlation, can be used for the processing of the pilot code. The output of the corresponding filter reaches peaks at times corresponding to the reception times of the pilot code transmitted periodically. Due to the effects of multipath propagation, several types can be detected in relation to a single pilot code transmission. From the processing of these peaks received in known manner, a timing reference can be found in relation to the transmitting base station with an ambiguity equal to the repetition period Tp. If the repetition period is equal to the frame length, then this timing reference can be used to synchronize the mobile station and the base station communication operation in frame. While any length of Np in chips can be selected for the transmitted pilot code cp, the length of Np is practically limited by the complexity of the corresponding filter implemented in the mobile station receiver. At the same time, it is desirable to limit the instantaneous peak energy Pp of the pilot code signal / channel transmissions in order not to cause high instantaneous interference with other transmitted signals of spread spectrum / channels. In order to obtain a sufficient average power in relation to the pilot code transmissions provided at a certain chip length Np, it may be necessary in the CDMA communication system to employ a pilot code repetition period Tp that is shorter than a frame length Tf for the pilot channel as illustrated in figure 2. Another reason for transmitting multiple pilot codes cp within a single frame length Tf is to support an interfrequency downlink synchronization in the compressed mode known to the experts in the matter. With mode processing (compressed, a downlink synchronization on a given carrier frequency is carried out for only a part of a frame instead of the entire frame. It is then possible, with only a pilot code cp per frame, that a compressed mode processing may lose a significant period of time to fully detect the pilot code. By transmitting multiple cp pilots during each frame, multiple opportunities are provided per frame for compressed mode processing detection, and at least one pilot code transmission may be detected. However, there is a drawback in relation to the reception and synchronization that is observed with a multiple pilot code transmission cp within a single frame length Tf. Again, the received signals are demodulated and applied to a filter (or correlator) corresponding to the known pilot code. The corresponding filter output presents types in times corresponding to the reception times of the pilot code periodically transmitted. From the processing of these peaks, a timing reference for the transmitting base station can be found in relation to the pilot code repetition period Tp in a manner well known in the art. However, this timing reference is ambiguous in relation to the frame timing and therefore presents us with sufficient information to allow synchronization of the base station / mobile station frame with the timing reference. The ambiguous term "means here that the frame boundary (ie, its synchronization) can not be identified from the detected pilot code peaks alone." Thus, in relation to the transmission of multiple pilots cp within a length of Tf single box, there is a need for a procedure to determine the frame synchronization COMPENDIUM OF THE INVENTION Each frame of a base station transmission within an extended spectrum communication system in relation to a pilot channel is divided into a plurality of synchronization segments Each of the synchronization segments includes a pilot code cp transmitted with a predetermined timing offset relative to the segment limit At least one of the synchronization segments further includes a frame synchronization code cs transmitted with a predetermined timing offset relative to another one mite segment or its associated pilot code cp. The pilot code cp and the frame synchronization code cs preferably are not spliced. In cases in which multiple frame synchronization codes cs are transmitted (eg, one per synchronization segment), the frame synchronization codes are unique for each segment in a frame, but are repeated in each frame. In addition, the multiple frame synchronization codes c "a are preferably mutually orthogonal and preferably orthogonal in relation to the pilot code cp. To obtain synchronization information, a mobile station first identifies a pilot code timing by applying a filter adapted to cp to a received signal and identification peaks From these peaks, a timing reference can be found in relation to the synchronization segment limits by using the known timing offset between the pilot code and the segment limit While this timing reference is ambiguous in terms of frame timing, knowledge of the synchronization segment limits indirectly indicates the location of the frame synchronization code cs in the synchronization segment. of codes framing synchronization known with the signal received at the location of a framing synchronization code. Since both the timing offset of each frame synchronization code location cs in relation to the segment limit and the position of the segment limit in relation to the frame boundary are known, once a corresponding correlation is found in the location, the frame boundary is also known in relation to this (and consequently frame synchronization). BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the method and apparatus of the present invention can be had with reference to the following Detailed Description when taken in combination with the accompanying drawings where: Figure 1 previously described, is a diagram illustrating a Prior art pilot channel signal transmission format in a direct sequence code division multiple access communication system (DS-CDMA); Figure 2, previously described, is a diagram illustrating an alternate prior art pilot channel signal transmission format in a direct sequence code division multiple access communication system; Figure 3 is a diagram illustrating the pilot channel signal transmission format of the present invention in a direct sequence code division multiple access communication system; Fig. 4 is a flow chart illustrating a process performed by a mobile station to obtain a base station timing reference from the processing of a pilot channel signal transmission in the format of Fig. 3; Fig. 5 is a flow diagram illustrating a process performed by a mobile station to find a frame (limit) timing within the process of Fig. 4; Fig. 6 is a diagram illustrating a pilot channel signal transmission format of an alternative mode in a direct sequence code division multiple access communication system; Figure 7 is a flow chart illustrating a process performed by a mobile station to obtain a base station timing reference from the processing of a pilot channel signal transmission in the alternative format of Figure 6; and Figure 8 is a simplified block diagram of an extended spectrum communication system receiver. DETAILED DESCRIPTION OF THE DRAWINGS With reference to Figure 3, a diagram illustrating a pilot channel signal transmission format of the present invention in an extended spectrum communication system (such as a multiple access communication system) is shown. by division of direct sequence code). Each frame having a length Tf of a pilot channel transmission is divided into a plurality (M in number) in synchronization segments sO, si,, sM-1. The length of each synchronization segment s is equal to a pilot code repetition period Tp. Each of the synchronization segments includes a pilot code cp and a frame synchronization code cs. The pilot code is the same in each synchronization segment and in the repetitive frames. The pilot code cp and the frame synchronization code cs preferably are not spliced. With the transmission of several frame synchronization codes (M in number) cs, 0, cs,!,, Cs, M --_ (one per synchronization segment sO, si,, sM-l), synchronization codes Framing patterns are unique for each segment in a frame, and are repeated in each frame. In addition, the multiple frame synchronization codes cs, 0, cs, l,, cs, M-l are preferably mutually orthogonal and are preferably orthogonal to the pilot code. The pilot code cp has a known timing offset tl in relation to a limit 30 of the synchronization segment. The frame synchronization code cs has a known timing offset t2 in relation to its associated pilot code cp and a known timing offset t3 in relation to the limit 30 of the synchronization segment. In addition, the synchronization segments have a known position in relation to a frame boundary 34. Reference is now made to FIG. 4 where a flowchart is shown illustrating a process performed with a mobile station to obtain a timing reference. base station from the processing of a received signal having the pilot channel signal transmission format of Figure 3. In step 10, the mobile station receives a signal. Then, in step 12, the mobile station processes the received signal s to find the pilot code timing cp that is, the location of the synchronization segments. This process occurs in accordance with the adapted filter or correlation method mentioned above and is a process well known in the art. From the pilot code timing cp found, the mobile station then identifies in step 14 the location (s) of the framing synchronization code (s) included (in step 14). This frame synchronization code location identification c • follows naturally using the known timing offset t2, as shown in Figure 3., once the location of the synchronization segments has been found (pilot codes cp). The mobile station then processes the framing synchronization code (s) ca (step 16) within the received signal s at the synchronization segment locations to find the frame timing (i.e., frame boundary) using the timing offset t2 and / or the timing offset t3 as well as the known position of the synchronization segment limit 30 in relation to the frame boundary 34. Referring now to FIG. 5, a flow chart is shown illustrating a process performed by a mobile station to find a frame timing (limit) within the process of Figure 4 (step 16). The frame synchronization code location is already known from step 14 of FIG. 4. A portion of the received signal s is an identified frame synchronization code location (found using the timing offset t2) is then correlated in step 20 with the set of allowed framing synchronization codes possible cs, 0, cs, l, is, Ml. This operation step can be represented mathematically by the following: R? = l < s, c.fl > ! (1) where 0 (< i < Ml (ie, over the M synchronization segments s); and (a, b) indicates the correlation operation, then, in step 22, the process determines the position within the the portion of the received signal s in the location identified in relation to both the segment limit 30 and the frame limit 34. In accordance with this operation, if Ri has a maximum when i = n, then the portion of the received signal s in the identified location is positioned within the n-th synchronization segment s of the frame in timing offset t 3. Then, in step 24, the frame timing (limit) is found as the position of the the received signal s identified as positioned within the n-th synchronization segment s, the limit 30 is known in relation to the frame boundary 34. The correlation operation of step 20 can be carried out in several code intervals. synchronization of consecutive synchronization segments. This step can be represented mathematically by the following: R 1. = f and TQ i < s J, c. m tmcdMJ) (2) where: 0 < i < M - l (that is, in the M synchronization segments s); sj are portions of the received signal in identified frame synchronization code locations of consecutive synchronization segments; L is the number of framing synchronization code intervals; and < a, b > indicates the correlation operation between the received signal s within the j-th range of framing synchronization code and the (l + j) -avo frame synchronization code cs (modM). In step 22, the process determines the position within the frame of the portion of the received signal s at the identified location. In accordance with this operation, Ri has a maximum when i = n, then it is considered that the portion of the received signal s at the location identified in the first interval (j = 0) is placed within the n-th synchronization segment. of the frame in the timing offset t3. Then in step 24, the frame timing (limit) is found as the position of the portion of the received signal s identified as being placed within the n-th synchronization segment s, the limit 30 is known in relation to the limit of Table 34. A fuller understanding of the process implemented in Figures 4 and 5 can be obtained with reference to a specific example. Accordingly, reference is now made to Figure 3. The operation of step 12 applies a filter adapted for cp or correlation with the received signal. The peaks found from this filtering identify the limits of synchronization segment 30 using the known time shift tl. Once those limits of segments 30 are known, and given the knowledge of the pilot channel formatting implemented, the location 32 of the included frame synchronization code (s) cs (step 14) is also known by the use of the known timing displacement t2. Then, the correlation operation of step 20 is carried out using either Equation (1) or Equation (2) to adapt a portion of the received signal to a certain location of identified frame code locations 32 ' with one of the set of allowed framing synchronization codes possible cs, 0, cs, l,, cs, Ml. From a correspondence, a corresponding particular segment limit 30 'is identified (step 22) employing the known timing offset t3. Once this particular segment limit 30 J is known and given the knowledge of the corresponding frame synchronization code location in a given segment within the frame, the frame boundary 34 is identified (step 24). With reference to Figure 6, there is shown a diagram illustrating a pilot channel signal transmission format of an alternative mode in a direct sequence code division multiple access communication system.

Each frame having a length Tf of a pilot channel transmission is divided into several synchronization segments (M) sO, si,, sM-l. The length of each synchronization segment s is equal to a pilot code repetition period Tp. Each of the synchronization segments includes a pilot code cp. The pilot code is the same in each synchronization segment and through the repetitive frames. One of the synchronization segments s, for example, the first segment sO as illustrated, in the table is additionally designated to include a frame synchronization code cs. The pilot code cp and the frame synchronization code cs preferably are not spliced. The pilot code cp has a known timing offset tl relative to a limit 30 of the synchronization segment. The frame synchronization code cs has a synchronization displacement t2 known in relation to the pilot code cp and a known timing displacement T3 in relation to the limit 30 of the synchronization segment. In addition, the synchronization segments and particularly the designated synchronization segment have a known position in relation to a frame boundary 34. Reference is now made to FIG. 7 where a flow diagram illustrating a process performed by the station is shown. mobile to obtain a base station timing reference for processing a received signal having the pilot channel signal transmission format of figure 6. In step 40, the mobile station receives the pilot channel signal. Then in step 42, the mobile station processes the received signal to find the pilot code timing cp (i.e., the location of the synchronization segments). This process occurs in accordance with the procedures described above and well known in the art and employs knowledge of the timing offset tl. From the pilot code timing cp found, the mobile station knows the repetition period of the pilot code Tp. Then, in step 44, M intervals of the received signal s ", which correspond to the possible framing synchronization code locations identified within the synchronization segments s found from the known pilot repetition period Tp, are then correlated with the authorized framing synchronization code cs This operation step can be represented mathematically as follows: R? = I < s ?, cs > (3) where: 0 (i (M-1 (ie, over the M intervals); and <a, b> indicates the correlation operation.

Next, in step 46, the frame timing (limit) is found. In accordance with this operation, if Ri has a maximum when i = n, then it is considered that the n-ava portion of the received signal s is positioned in the synchronization segment s in the frame designated for the frame synchronization code ( that is, the first segment sO in the illustrated mode). The frame timing (limit) is then found as the relative position is known relative to the frame limit of the designated synchronization segment. A more complete understanding of the process implemented in Figure 7 can be obtained with reference to a specific example. Accordingly, reference will now be made again to FIG. 6. The operation of step 42 applies a filter adapted for cp to the received signal. The peaks found from this filtering identify synchronization segment limits 30 that employ the known timing offset tl. Once these segment boundaries 30 are known, and given the knowledge of the type of pilot channel formatting implemented (i.e., timing offset t2 and frame code position), the frame synchronization code cs included is found by the use of the correlation operation of step 44 and Equation (3) which maps consecutive portions of the signal s into the M candidate locations 32 for frame synchronization codes within the segments identified with the authorized frame synchronization code cs. Once the framing synchronization code location 32 is known, and given the knowledge of the corresponding location within the frame (for example, timing offset t3 in the first synchronization segment, as illustrated), the frame boundary 34 is identified (step 46). Reference is now made to Figure 8, where a simplified block diagram of an extended spectrum communication system receiver 50 is shown. A receiving antenna 52 collects the signal energy of a transmitted modulator extended data sequence and passes this. energy to a radio receiver 54. The receiver 54 amplifies, filters, mixes and converts from analog to digital, as necessary, to convert the received radio signal into a baseband signal. The baseband signal is usually sampled at least once per chip period, and may or may not be stored in a buffer (not shown). The baseband signals pass to a plurality of traffic channel correlators 56 (by employing a RAKE receiver configuration). The operational function of the correlators 56 is sometimes known as decoding since the correlation coherently combines the multiple extended data values back to a unique information value when a given decoding sequence is correctly aligned in time with the sample sequence received. The output correlations are provided to one or several detectors 58 that reproduce the original information data stream. The shape of the detector used depends on the characteristics of the radio channel and complexity limitations. It may include a channel estimate and a coherent RAKE combination, or differential detection and combination, as necessary. In the context of the present invention, the baseband signals are passed to a pilot code search engine 60 specifically designated to process pilot channels. The pilot code search engine 60 processes the baseband signal to find the pilot code timing cp using the known actions of applying a filter adapted for cp, identifying peaks and locating a timing reference in relation to the base station transmissions and then identify the location (s) (s) of the frame synchronization code (s) included within the cs pilot code location cs.

This information is then passed to a synchronization code finder 62 which implements the specific pilot channel processing of the present invention and hence determines the frame timing (i.e., the frame boundary). The operation of the pilot code finder 60 and synchronization code finder 62 are defined through the flow diagrams of Figures 4,5 and 7, as well as through Equations (1), (2) and (3) ). The pilot channel box and the timing / segment synchronization information generated by the pilot code finder 60 and the synchronization code finder 62 is then used by the traffic channel correlators 56 and detectors 58 to reproduce and process the current original information data. Even though modalities of the method and apparatus of the present invention were illustrated in the accompanying drawings and even though such embodiments were described in the above Detailed Description, it will be understood that the invention is not limited to the embodiments presented, but can be carried out with numerous rearrangements, modifications and substitutions without departing from the spirit of the invention in accordance with what has been presented and defined in the appended claims.

Claims (1)

1. CLAIMS A format for a code division multiple access pilot channel transmission, comprising: a repeating frame comprising a plurality of synchronization segments; a repeated cp pilot code in each repetitive frame synchronization segment; and a frame synchronization code cs in at least one of the synchronization segments of the repetitive frame. The format of claim 1, wherein the pilot code cp and the framing synchronization code "cs are not spliced." The format according to claim 1, wherein the framing synchronization code cs, if it is in more than one of the synchronization segments are unique by synchronization segments and are repeated in each frame The format according to claim 3, wherein the pilot code cp and the individual codes of the frame synchronization codes are not spliced. with claim 3, wherein the plurality of framing synchronization code are mutually orthogonal, n method for processing a signal including a pilot channel to obtain timing synchronization information, wherein the signal includes a repeating frame divided into a plurality of synchronization segment, each segment includes a pilot code cp and at least one segment that includes a framing synchronization code cs, comprising the steps of: correlating a received signal with the pilot code cp in order to find synchronization segment locations; and correlating the received signal at a location within the synchronization segment found with a frame synchronization code set cs with the object of finding frame synchronization timing information. The method according to claim 6, wherein the second correlation step comprises the step of matching the framing synchronization code cs to a location within each of the several synchronization segment locations found consecutively in the repeating frame. The method according to claim 6, wherein the frame synchronization code cs, if it is in more than one of the synchronization segments, is unique by synchronization segment, and, where the second correlation step comprises the step of matching the plurality of framing synchronization code with the location within a synchronization segment. 9. The method according to claim 8, wherein the various frame synchronization codes are mutually orthogonal. The method according to claim 6, wherein the second correlation step further comprises the step of identifying the frame synchronization timing information from a relative position known in the repetitive frame relative to the frame synchronization code. cs. The method according to claim 6, wherein the pilot code cp and the frame synchronization code cs are not spliced. 12. An apparatus for processing a code division multiple access signal including a pilot channel for obtaining timing synchronization information, comprising: a receiver for receiving a signal including a repetitive frame divided into several segments of synchronization, each segment includes a pilot code cp and at least one segment that includes a frame synchronization code c "s, and a pilot channel finder connected to the receiver to receive a portion of the pilot channel of the signal, the search engine operates to correlating a received pilot channel portion with a pilot code cp for the purpose of finding synchronization segment locations, and correlating the pilot channel portion received at a location within the synchronization segment found with the synchronization code fs cs to the object to find box synchronization timing information 13. The device in accordance with claim 11, where the searcher operation also matches the frame synchronization code cs with a location within each of the several consecutive synchronization segments found in the repetitive frame. The apparatus according to claim 12, wherein the frame synchronization code cs, if it is in more than one of the synchronization segments, is unique by synchronization segments, and where the second correlation step comprises the step of matching the various framing synchronization codes with the location within each synchronization segment. 5. The apparatus according to claim 14, wherein the various frame synchronization codes are mutually orthogonal. The apparatus according to claim 12, wherein the searcher operation further identifies the frame synchronization timing information from a relative position known in the repetitive frame relative to the frame synchronization code cs. The apparatus according to claim 12, wherein the pilot code cp and the framing synchronization code cs are not spliced.