Method and apparatus for transmitting the sync channel message in a multi-carrier communication system

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for transmitting and receiving broadcast information in a multi-carrier CDMA communication system.

II. Description of the Related Art

The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in <patcit id="pcit0001" dnum="US4901307A"><text>U.S. Patent No. 4,901,307</text></patcit>, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in <patcit id="pcit0002" dnum="US5103459A"><text>U.S. Patent No. 5,103,459</text></patcit>, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein. CDMA has been standardized by the Telecommunications Industry Association in Interim Standards IS-95A and IS-95B, entitled "Mobile Station-Base Station Compatibility Standard for Dual Mode Spread Spectrum Systems" (referred to hereafter collectively as IS-95).

In IS-95 communications systems channels of information transmitted from a common base station are distinguished from one another by orthogonal spreading codes. Each channel is spread by a unique orthogonal spreading sequence. The channels that are transmitted by an IS-95 base station include, a pilot channel, a sync channel, at least one paging channel and dedicated traffic channels. The pilot channels is used to provide a phase reference for coherent demodulation of the other channels by mobile stations within the coverage area of the base station. The sync channel carriers overhead information such as timing information, pilot PN offset information and other information that allows the reception of the other overhead channels. The paging channel notifies mobile stations of mobile terminated calls directed to the mobile station in the area. Dedicated traffic channels provide information directed to the user of a specific mobile station in the coverage area of the base station.

In IS-95, when the base station sends a <i>Sync Channel Message,</i> it shall uses the fixed-length message format of Table 1 below: <tables id="tabl0001" num="0001"><table frame="all"><title><b>TABLE 1</b></title><tgroup cols="2"><colspec colnum="1" colname="col1" colwidth="41mm" /><colspec colnum="2" colname="col2" colwidth="25mm" /><thead><row><entry valign="top"><b>Field</b></entry><entry valign="top"><b>Length (bits)</b></entry></row></thead><tbody><row><entry>MSG_TYPE ('00000001')</entry><entry>8</entry></row><row><entry>P_REV</entry><entry>8</entry></row><row><entry>MIN_P_REV</entry><entry>8</entry></row><row><entry>SID</entry><entry>15</entry></row><row><entry>NID</entry><entry>16</entry></row><row><entry>PILOT_PN</entry><entry>9</entry></row><row><entry>LC_STATE</entry><entry>42</entry></row><row><entry>SYS_TIME</entry><entry>36</entry></row><row><entry>LP_SEC</entry><entry>8</entry></row><row><entry>LTM_OFF</entry><entry>6</entry></row><row><entry>DAYLT</entry><entry>1</entry></row><row><entry>PRAT</entry><entry>2</entry></row><row><entry>CDMA_FREQ</entry><entry>11</entry></row></tbody></tgroup></table></tables><dl id="dl0001" compact="compact"><dt>MSG_TYPE</dt><dd>Message type.</dd><dt>P_REV</dt><dd>Protocol revision level.</dd><dt>MIN_P_REV</dt><dd>Minimum protocol revision level. The base station sets this field to prevent mobile stations which cannot be supported by the base station from accessing the system.</dd><dt>SID</dt><dd>System identification. The base station shall set this field to the system identification number for this system.</dd><dt>NID</dt><dd>Network identification. This field serves as a sub- identifier of a system as defined by the owner of the SID.</dd><dt>PILOT_PN</dt><dd>Pilot PN sequence offset index. The base station shall set this field to the pilot PN sequence offset for this base station, in units of 64 PN chips.</dd><dt>LC_STATE</dt><dd>Long code state. The base station shall set this field to the long code state at the time given by the SYS_TIME field of this message.</dd><dt>SYS_TIME</dt><dd>System time. The base station shall set this field to the System Time as of four Sync Channel super frames (320 ms) after the end of the last super frame containing any part of this <i>Sync Channel Message,</i> minus the pilot PN sequence offset, in units of 80 ms.</dd><dt>LP_SEC</dt><dd>The number of leap seconds that have occurred since the start of System Time.</dd><dt>LTM_OFF</dt><dd>Offset of local time from System Time. The base station shall set this field to the two's complement offset of local time from System Time, in units of 30 minutes.</dd><dt>DAYLT</dt><dd>Daylight savings time indicator. If daylight savings time is in effect, the base station shall set this field to '1'; otherwise, the base station shall set this field to '0'.</dd><dt>PRAT</dt><dd>Paging Channel data rate. The base station shall set this field to the PRAT field value shown in Table 2 corresponding to the data rate used by the Paging Channels in the system.</dd></dl><tables id="tabl0002" num="0002"><table frame="all"><title><b>TABLE 2. Paging Channel Data Rate</b></title><tgroup cols="2"><colspec colnum="1" colname="col1" colwidth="34mm" /><colspec colnum="2" colname="col2" colwidth="43mm" /><thead><row><entry align="center" valign="top"><b>PRAT Field (binary)</b></entry><entry align="center" valign="top"><b>Paging Channel data rate</b></entry></row></thead><tbody><row><entry align="center">00</entry><entry align="center">9600 bps</entry></row><row><entry align="center">01</entry><entry align="center">4800 bps</entry></row><row><entry align="center">10</entry><entry align="center">Reserved</entry></row><row><entry align="center">11</entry><entry align="center">Reserved</entry></row></tbody></tgroup></table></tables><dl id="dl0002" compact="compact"><dt>CDMA_FREQ</dt><dd>Frequency assignment. The base station shall set this field to the CDMA Channel number corresponding to the CDMA frequency assignment for the CDMA Channel containing a Primary Paging Channel.</dd></dl>

In IS-95 systems, each base station transmits a pilot channel that is covered only by a short PN sequence. In IS-95 systems, the short PN sequence repeats once every 26ms. The pilot signal transmissions from each base station are distinguished from one another by a phase offset with respect to one another. In particular, there each base station associated with a single base station controller differs in phase by at least 64 PN chips.

Under normal operation, the mobile station acquires the pilot signal first. The pilot signal carries no data and is simply the all ones sequence spread by the common short code, which also spreads all of the other channels transmitted by the base station. After acquiring the pilot channel, the mobile station receives the information described above from the sync channel. The frame and interleaver timing on the sync channel are aligned with the pilot PN sequence. The zero state of the short PN sequence marks the starting point of the sync channel frame and interleaver.

In the U.S. Personal Communication System (PCS) spectrum, the CDMA channel number N specifies the carrier frequencies of the forward and reverse link channels. In particular, channel number N corresponds to a reverse link carrier frequency of (1850 + 0.05N) MHz and a forward link carrier frequency of (1930 + 0.05N) MHz, where N ranges from 0 to 1199. The bandwidth of each CDMA channel is 1.25 MHz. Therefore, the channel numbers of adjacent CDMA channels are different by at least 25 (25 x 0.05 MHz = 1.25 MHz). To facilitate mobile stations' initial acquisition, certain carrier frequencies are designated as the preferred frequency assignments. Referring to <figref idref="f0001">FIG. 1</figref>, for an IS-95B CDMA system in the A block of PCS band, the channel numbers of the preferred frequency assignments are 25, 50, 75, 100, 125, 150, 175, 200, 225, 250, and 275. Mobile stations will search the preferred frequency assignments first upon powering up.

The International Telecommunications Union recently requested the submission of proposed methods for providing high rate data and high-quality speech services over wireless communication channels. A first of these proposals was issued by the Telecommunications Industry Association, entitled "The cdma2000 ITU-R RTT Candidate Submission" (hereafter cdma2000). cdma2000 proposes increasing the throughput of the forward link signals by transmitting portions of the information on three bands, each of 1.2288 MHz. This method is described as the "multi-carrier" approach.

A multi-carrier CDMA system, that uses three adjacent 1.25 MHz RF channels with channel spacing also equal to 1.25 MHz is defined in cdma2000. Referring to <figref idref="f0001">FIG. 2</figref>, a multi-carrier CDMA system in the A block of the PCS band can be deployed with the center channel on channel 50, 75, 100, 125,150, 175, 200, 225, or 250. Channels 50 and 250 are usually avoided to avoid interference to adjacent bands on the reverse link. In the proposed cdma2000 description, the reverse link may be direct-spread with a chip rate of 3.6864 Mcps, which makes the emission limit requirements more difficult to meet than for a chip rate of 1.2288 Mcps.

A mobile station, upon powering up, searches for pilot signal on a preferred frequency. If no pilot is found in the current channel, it changes its channel and searches again. Once a pilot is acquired, the mobile station demodulates the Sync Channel associated with that pilot to receive timing information, pilot PN offset, and other information enabling its reception of the other overhead channels.

In a multi-carrier approach, one method for providing Sync Channel data would be to tri-partition the Sync Channel message and place one third of the message on each of the three portions of the multi-carrier signal. If the Sync Channel of a multi-carrier system is spread over three channels, the mobile station will have to know the exact channels used by the system in order to demodulate the Sync Channel reliably. Since the exact channels are not known in advance, the mobile station has to try numerous combinations to receive the Sync Channel message. Given the number of preferred channels, the time spent on such trials could be excessive and therefore degrade the mobile station's initial acquisition time. Therefore, there is a need felt in the art for a method that minimizes the mobile station's search time.

SUMMARY OF THE INVENTION

The present invention is a novel and improved method and apparatus for transmitting broadcast information in a multi-carrier communication system. The proposed invention is to send the Sync Channel of the multi-carrier system in a 1.25 MHz channel bandwidth (i.e., over a single carrier), and to specify the preferred channels for the Sync Channel transmission instead of the preferred channels for the entire multi-carrier system. The Sync Channel Message will indicate the center frequency for a mutli-carrier system in the band if one exeists and the frequency of a single band system if one exists.

Considering the A block of the PCS band again, the preferred channels for Sync Channel transmission can be selected as channels 75, 150 and 225. This selection ensures that one of the preferred channels will always be used by any multi-carrier system regardless the location of its center channel. The mobile station, upon powering up, searches on the preferred channels for Sync Channel first. Once the pilot signal is acquired on any of these channels, the mobile station demodulates the Sync Channel on the channel as well. The mobile station learns from the Sync Channel Message the location of a multic arrier and a single carrier system in the band if either exists. It can be easily observed that the number of channels to search and the number of hypotheses to try are significantly reduced by using the proposal in this disclosure. As a result, it improves the mobile station's initial acquisition time.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> is an illustrative band diagram for a PCS system for 1x communications systems;</li><li><figref idref="f0001">FIG. 2</figref> is an illustrative band diagram for a PCS system for 3x communications systems;</li><li><figref idref="f0002">FIG. 3</figref> is a flowchart illustrating the acquisition method of the present invention;</li><li><figref idref="f0003">FIG. 4</figref> is a block diagram illustrating the major elements in a wireless communications system;</li><li><figref idref="f0004">FIG. 5</figref> is a simplified block diagram of a multi-carrier transmission system;</li><li><figref idref="f0005">FIG. 6</figref> is a block diagram of CDMA modulation system;</li><li><figref idref="f0006">FIG. 7</figref> is a simplified block diagram of a multi-carrier receiver system; and</li><li><figref idref="f0007">FIG. 8</figref> is a block diagram of a CDMA demodulation system.</li></ul>

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

<figref idref="f0001">FIG. 1</figref> illustrates a typical band diagram for a multi-band communications system. In currently contemplated wireless communications systems, mobile station that is attempting to begin service will tune to each possible frequency in a preferred set of frequencies and determine whether a system is available at that frequency. In <figref idref="f0001">FIG. 1</figref>, the channel numbers of the preferred frequency assignments are 25, 50, 75, 100, 125, 150, 175, 200, 225, 250, and 275, which corresponds to bands <b>200a, 200b, 200c, 200d, 200e, 200f, 200g, 200h, 200i, 200j</b> and <b>200k.</b> In the exemplary embodiment, each of these bands is 1.25Mhz wide and is used to carry an IS-95 CDMA transmissions.

<figref idref="f0001">FIG. 2</figref> illustrates the possible center bands of a three component multi-carrier communication system as is contemplated in cdma2000 (also referred to as IS-2000). In the multi-carrier communication system, the mobile station tunes to each possible grouping of three adjacent channels and attempts to receive the Sync Channel message. In the current designs for multi-carrier systems the Sync Channel message will be divided into three component parts with each component part transmitted separately and simultaneously on a different carrier of the multi-carrier band. The mobile station first attempts to receive the Sync Channel message on a multi carrier system comprising bands <b>300b, 300c, 300d.</b> If unsuccessful, the mobile station then attempts to acquire the Sync Channel on a multi-carrier system consisting of bands <b>300c, 300d,</b> and <b>300e.</b> This continues for each possible three band system until the mobile station checks for a multi-carrier system consisting of bands <b>300h, 300i</b> and <b>300j.</b> For reasons described above, in the exemplary embodiment, bands <b>300a</b> and <b>300k</b> will not be used in a multi carrier system.

This method of receiving the Sync Channel message is very inefficient and time consuming. If a mobile station is capable of operating in a multi-carrier or single carrier mode, the mobile station will potentially have to conduct eleven 1x searches or bands <b>200a-200k</b> and seven multi-carrier searches using center frequencies <b>300c-300i.</b> In a CDMA communication system, each band searched requires the mobile station to test a large number of PN offsets to detect the presence of the pilot signal. Thus, this method of acquisition requires a substantial time period.

The present invention provides a significantly more efficient method for acquiring the necessary system parameters in a potentially mixed bandwidth communication system. In the present invention, the Sync Channel is always transmitted in a 1x band. In the preferred embodiment, the preferred channels are channels 75, 150 and 225. Thus, a mobile station need only conduct at most three searches to receive the Synch Channel message in 1x bandwidth in order to acquire the necessary information for acquisition of its preferred system. The present invention greatly reduces acquisition time in a mixed bandwidth communication system. In addition, by providing the Sync Channel message only on the preferred channels reduces the capacity impact of providing that overhead messaging on many more channels.

The reason for the allocation of the preferred channels of the present invention is that it provides the greatest flexibility in the provision of a multi-carrier system. With the allocation of the preferred channels to channel numbers 75, 150 and 220 <b>(300c, 300f</b> and <b>300j),</b> a multi-carrier system provided anywhere in the band consisting of bands <b>300a-300k</b> will include one of the preferred channels. A multi-carrier system comprising the bands <b>300a, 300b</b> and <b>300c</b> will include the preferred channel <b>300c.</b> A multi-carrier system comprising the bands <b>300b, 300c</b> and <b>300d</b> will include the preferred channel of <b>300c.</b> A multi-carrier system comprising the bands of <b>300c, 300d</b> and <b>300e</b> will include the preferred channel of <b>300c.</b> A multi-carrier system comprising the bands of <b>300d, 300e</b> and <b>300f</b> will include the preferred channel of <b>300f.</b> Any combinations of three adjacent bands will include a preferred channel upon which the mobile station will be able to receive the necessary system parameters for operation.

In the present invention, the mobile station tunes to a preferred channel <b>(300c, 300f</b> or <b>300i)</b> and attempts to detect a pilot signal on that frequency band. If a pilot signal is detected the mobile station, receives demodulates and decodes the Sync Channel message. In the present invention, the Sync Channel message will provide information that identifies the center frequency of a multi-carrier system in the current set of frequency bands (if one exists) and the frequency of a 1x band in the current set of frequency bands (if one exists).

The mobile station in response to the information received in the Sync Channel selects the system that is appropriate for its needs or capabilities. If the mobile station prefers to use a multi-carrier system, the mobile station uses the center frequency of the multi-carrier system indicated in the Sync Channel message to tune to the multi-carrier system and receive the broadcast channel (BCH) message. The broadcast channel message will indicate to the mobile station the number of common control channels (CCCHs) that are employed by the current system. The mobile station takes the number of common control channels and using a predetermined hashing algorithm determines what code channel it will use to receive page messages from the transmitting base station.

If the mobile station selects to operate in a single band system, the mobile station uses the information received in the Sync Channel message to tune to the appropriate frequency of the single band system. The mobile station then receives the general paging channel message on the primary paging channel. The general paging channel message provides the number of paging channels used by the single band system. The mobile station uses a predetermine hashing function to determine the code channel upon which the mobile station will receive paging message for the transmitting base station.

The present invention is equally applicable to systems that may include 3X direct spread communications systems. In this embodiment, the Sync Channel message would include the addition information whether the 3X system is a direct spread system or a multi-carrier system. In addition, the Sync Channel message may also provide information respect whether the system uses a form of diversity transmission such as orthogonal transmit diversity (OTD). If methods of transmission diversity are possible in the multiple bandwidth communication system, specification of the means of transmit diversity greatly reduces the number of hypotheses that must be tested to acquire a system.

These changes to the Sync Channel messages can be accommodated without necessitating the extension of the Sync Channel message used in IS-95B and illustrated above. In the current Sync Channel message there a large number of reserved bits that can be used to provide the additional information.

<figref idref="f0002">FIG. 3</figref> is a flowchart illustrating the acquisition operation of the present invention. In block <b>2,</b> the mobile station tunes to a preferred channel (<b>300c, 300f</b> or <b>300i).</b> It will be understood that the present invention is disclosed in context of the PCS band of freqeuncies and can easily be extended to other bands such as the cellular band. Moreover, the selected set of preferred channels though preferred for three carrier, multi-carrier communications systems would be different for multi-carrier systems with a different number of carriers.

In block 6, the mobile station determines whether the search operation was successful. In the exemplary embodiment, the present invention is incorporated into a CDMA communications system, though the present invention would be equally applicable to other mixed bandwidth communications systems. In the exemplary embodiment, the mobile station tunes its RF receiver to a preferred channel <b>(300c, 300f,</b> or <b>300i</b>) and attempts to detect the presence of a pilot signal. In the exemplary embodiment of an IS-95 based CDMA communications system, each base station transmits its pilot signal using a unique pilot signal offset. Thus, the mobile station after tuning to a preferred channel <b>(300c, 300f,</b> or <b>300i),</b> searches the possible PN offset hypotheses.

The method and apparatus for detecting a pilot signal in a CDMA communication system are well known in the art and are described in detail in <patcit id="pcit0003" dnum="US5644591A"><text>U.S. Patent No. 5,644,591</text></patcit>, entitled "Method And Apparatus For Performing Search Acquisition In A CDMA Communication System", which is assigned to the assignee of the present invention and incorporated by reference herein. The mobile station tests each PN offset hypothesis by computing the correlation between the received signal within the preferred frequency band with the PN hypothesis being tested. If the correlation energy for all PN hypotheses is less than a threshold value, then the acquisition at the preferred frequency is not successful and the operation moves to block <b>4.</b> In block <b>4,</b> the next preferred channel <b>(300c, 300f,</b> or <b>300i)</b> to be tested by the mobile station is selected and the flow moves to block <b>2</b> and continues as described.

When the mobile station detects sufficient correlation energy between the received signal at the preferred channel frequency and a PN offset hypothesis, a successful acquisition is declared. At this point the operation moves to block <b>8.</b> In block <b>8,</b> the mobile station receives the Sync Channel message. In the present invention, the Sync Channel message is transmitted in a single 1x band (<b>300c, 300f,</b> or <b>300i</b>). In the exemplary embodiment of an IS-95 based CDMA communication system, the Sync Channel frame boundary and the interleaver boundary are aligned with the short PN sequence used to spread the pilot channel signal. Thus, upon successful acquisition of the pilot channel signal, the mobile station has enough information to de-interleave and decode the Sync Channel message.

After receiving the Sync Channel message, the operation moves to block 10. The mobile station determines from the Sync Channel message the center frequency of a multi-carrier system in the current set of bands (if one exists) and the location of a single carrier band in the current set of bands (if one exists). The mobile station decides whether to operate in a multi-carrier mode or a single channel mode based on its capabilities and needs.

The operation moves to control block <b>10.</b> If the mobile station capable of operating in a multi-carrier mode decides to operate in multi-carrier mode, then the flow moves to block <b>12.</b> In block <b>12,</b> the mobile station initiates its RF hardware for multi-carrier reception. The mobile station will know from the received Sync Channel message the center frequency of a multi-carrier system in the current set of frequency bands if one exists. The operation then moves to block <b>14,</b> where the mobile station receives the broadcast channel (BCH) signal and from that channel learn among other information the number of common control channels used by the communication system. The mobile station hashes the number of common control channels to determine the code channel that it should use to receive pages.

If, back in block <b>10,</b> the mobile station decides to use a single channel system because of limitations in its capability or other preference with respect to its service needs, the operation moves to block <b>16.</b> In block <b>16,</b> the mobile station initiates its RF hardware for single-carrier reception. The operation then moves to block <b>18,</b> where the mobile station receives the general page message on a predetermined code channel. The general page message will state the number of paging channels used by the system. The mobile station hashes the number of paging channels used to determine the code channel that it will use to receive directed pages from the serving base station.

<figref idref="f0003">FIG. 4</figref> introduces the elements and nomenclature of an very simplified wireless communications system. Base station <b>30</b> transmits forward link signal <b>32</b> to mobile station <b>40.</b> Mobile station <b>40</b> transmits reverse link signal <b>34</b> to base station <b>30.</b>

<figref idref="f0004">FIG. 5</figref> is simplified block diagram illustrating the exemplary embodiment of base <b>30</b> as multi-carrier CDMA transmission system with three forward link channels. Each of transmission subsystems <b>48</b> transmits a portion of forward link signal <b>32</b> on a different carrier frequency. Transmission subsystem <b>48a</b> transmits a portion of forward link signal <b>32</b> on frequency f₁, transmission subsystem <b>48b</b> transmits a portion of forward link signal <b>32</b> on frequency f₂, and transmission subsystem <b>48c</b> transmits a portion of forward link signal <b>32</b> on frequency f₃.

The data for transmission on forward link signal <b>32</b> is provided to de-multiplexer <b>50.</b> De-multiplexer <b>50</b> provides the data to one of the three transmission subsystems <b>48.</b> The present invention is described in term of a three carrier multi-carrier communication system because three IS-95 carriers each occupying 1.2288 MHz can fit into a 5MHz band. However, it will be understood by one skilled in the art that the teachings of the present invention can easily be extended to an arbitrary number of channels in a multi-carrier system.

The de-multiplexed data streams are provided to modulators <b>52.</b> In the exemplary embodiment, modulators <b>52</b> modulate the forward link data in accordance with a CDMA modulation format such as that described in the IS-95 standard and described in detail in the aforementioned <patcit id="pcit0004" dnum="US5103459A"><text>U.S. Patent No. 5,103,459</text></patcit>. The forward link data includes dedicated channel data that is for provision to a specific mobile station and broadcast channel data that is for provision to all mobile stations in the coverage area of base station <b>30</b> or a subset of mobile stations <b>30</b> in the coverage area of base station <b>30.</b> The Sync Channel message is an example of broadcast data that is transmitted to all mobile stations in the coverage area of base station <b>30.</b> In the present invention, the Synch Channel message is provided to a designated one of transmission subsystems <b>48</b> for transmission on a designated one of the three carriers.

The modulated forward link data from modulators <b>52</b> is provided to up converter <b>54.</b> Up converter <b>54</b> up coverts the modulated signal to a carrier frequency (f₁, f₂, or f₃) that is generated by a local oscillator (not shown). The up converted signals are then combined for transmission through antenna <b>56.</b>

<figref idref="f0005">FIG. 6</figref> illustrates the exemplary embodiment of modulators <b>52</b> which modulate a portion of the forward link signal for transmission on a single carrier of forward link signal <b>32.</b> In the exemplary embodiment, a pilot signal is transmitted to allow for coherent demodulation of the signal by the receivers, which improves the performance of the receiver by providing a phase reference for demodulation. A set of pilot symbols known to both base station <b>30</b> and mobile station <b>40</b> are provided to Walsh spreader <b>60.</b> Walsh spreader <b>60</b> spreads the pilot symbols by a Walsh sequence W_pilot. In the exemplary embodiment, Walsh sequences are used to distinguish the channels of the data transmitted on a single CDMA carrier. The Walsh function can be either of a fixed number of symbols as is described in the IS-95 specification or it can be an orthogonal function that varies in length in accordance with the rate of the data to be transmitted on the channel as is described in the cdma2000 proposal and in <patcit id="pcit0005" dnum="US5751761A"><text>U.S. Patent No. 5,751,761</text></patcit>, entitled "System and Method for Orthogonal Spread Spectrum Sequence Generation in Variable Data Rate Systems", which is assigned to the assignee of the present invention and incorporated by reference herein.

The Walsh spread pilot symbols are provided to complex PN spreader <b>62.</b> Complex PN spreader <b>62</b> spreads the Walsh spread pilot symbols in accordance with two separately generated pseudonoise (PN) sequences, PN_I and PN_Q. If the two inputs to complex PN spreader <b>62</b> are designated as I and Q, the result of the complex spreading operation are two channels I' and Q' given by the equations: <maths id="math0001" num="(1)"><math display="block"><mi mathvariant="normal">Iʹ</mi><mo mathvariant="normal">=</mo><msub><mi>PN</mi><mi mathvariant="normal">I</mi></msub><mo>⁢</mo><mi mathvariant="normal">I</mi><mo mathvariant="normal">-</mo><msub><mi>PN</mi><mi mathvariant="normal">Q</mi></msub><mo>⁢</mo><mi mathvariant="normal">Q</mi></math><img file="EP2207318A2_D0001.tif" /></maths><maths id="math0002" num="(2)"><math display="block"><mi mathvariant="normal">Qʹ</mi><mo mathvariant="normal">=</mo><msub><mi>PN</mi><mi mathvariant="normal">Q</mi></msub><mo>⁢</mo><mi mathvariant="normal">I</mi><mo>+</mo><msub><mi>PN</mi><mi mathvariant="normal">I</mi></msub><mo>⁢</mo><mi mathvariant="normal">Q</mi><mn>.</mn></math><img file="EP2207318A2_D0002.tif" /></maths> The purpose of the complex PN spreading is to more evenly distribute the loading on the in-phase and quadrature channels of the QPSK modulator, which results in a reduction of the peak to average ratio on the power amplifier (not shown) of base station <b>30</b> which in turn increases the capacity of base station <b>30.</b> Complex PN spreading is described in the cdma2000 RTT proposal and is described in detail in copending <patcit id="pcit0006" dnum="US886604A"><text>U.S. Patent Application Serial No. 08/886,604</text></patcit>, entitled "High Data Rate CDMA Wireless Communication System", which is assigned to the assignee of the present invention and is incorporated by reference herein. The complex PN spread pilot symbols are provided to transmitter (TMTR) <b>94,</b> which up converts, filters and amplifies the signal for transmission through antenna <b>56.</b>

In the exemplary embodiment, the Sync Channel message is distinguished from other channels of information by being spread by a unique orthogonal spreading sequence W_sync. In the preferred embodiment of the present invention, the sync channel message is only transmitted by a selected one of modulators <b>48a, 48b</b> or <b>48c.</b> The selected modulator <b>48</b> transmits the Sync Channel message on a preferred channel. In the exemplary embodiment, the Sync Channel message indicates the center frequency of a multi-carrier system in the current band of frequencies, in one exists and the frequency of a single carrier system in the current set of frequency bands if one exists.

The Sync Channel message is provided to message formatter <b>64.</b> In the exemplary embodiment, message formatter <b>64</b> generates a set of cyclic redundancy check (CRC) bits and an optional set of tail bits and appends those bits to the Sync Channel message. IS-95 systems do not append tail bits to the Synch Channel message. cdma2000 (also referred to as IS-2000) systems append eight tail bits to the Sync Channel message. The Sync channel message with the CRC bits and tails bits appended are provided to encoder <b>66.</b> Encoder <b>66</b> encodes the sync channel message, CRC bits and tail bits in accordance with a predetermined forward error correction coding algorithm such as convolutional coding.

The encoded symbols are then provided to interleaver (INT) <b>68</b> which reorders the encoded symbols in accordance with a predetermined interleaving format. The interleaver is provided to provide time diversity in the transmitted stream of encoded symbols. Decoders have better error correction performance when the errors in the received stream are not burst errors.

The reordered symbols are provided to Walsh spreader <b>70</b> which spreads the reordered symbols in accordance with a predetermined code sequence W_sync. In the exemplary embodiment, W_sync is a code sequence that is orthogonal to all other code sequences used to channelized the channels of forward link signal <b>32.</b> The Walsh spread signal is then provided to complex PN spreader <b>62</b> and is spread as described above.

Common channel messages are transmitted to all subscriber stations or sets of subscriber stations within the coverage area of base station <b>30.</b> Examples of common channel messages include paging messages that alert mobile station of incoming calls and control channel messages that provide necessary control information to mobile stations in the coverage area of base station <b>30.</b> For illustration purposes a single control channel is shown. It will be understood by one skilled in the art that in practical implementations a plurality of control channels will be transmitted a base station <b>30.</b>

The Common Channel message is provided to message formatter <b>74.</b> In the exemplary embodiment, message formatter <b>74</b> generates a set of cyclic redundancy check (CRC) bits and a set of tail bits and appends those bits to the Common Channel message. The Common Channel message with the CRC bits and tails bits appended are provided to encoder <b>76.</b> Encoder <b>76</b> encodes the Common Channel message, CRC bits and tail bits in accordance with a predetermined forward error correction coding algorithm such as convolutional coding.

The encoded symbols are then provided to interleaver (INT) <b>78</b> which reorders the encoded symbols in accordance with a predetermined interleaving format. The interleaver is provided to provide time diversity in the transmitted stream of encoded symbols. Decoders have better error correction performance when the errors in the received stream are not burst errors.

The reordered symbols are provided to Walsh spreader <b>82</b> which spreads the reordered symbols in accordance with a predetermined code sequence W_cc. In the exemplary embodiment, W_cc is a code sequence that is orthogonal to all other code sequences used to channelized the channels of forward link signal <b>32.</b> The Walsh spread signal is then provided to complex PN spreader <b>62</b> and is spread as described above.

Dedicated Channel data is transmitted to a particular subscriber station within the coverage area of base station <b>30.</b> The Dedicated Channel data is provided to message formatter <b>84.</b> In the exemplary embodiment, message formatter <b>84</b> generates a set of cyclic redundancy check (CRC) bits and a set of tail bits and appends those bits to the frame of dedicated channel data. The frame of Dedicated Channel data with the CRC bits and tails bits appended is provided to encoder <b>86.</b> Encoder <b>86</b> encodes the frame of Dedicated Channel data, CRC bits and tail bits in accordance with a predetermined forward error correction coding algorithm such as turbo coding or convolutional coding.

The encoded symbols are then provided to interleaver (INT) <b>88</b> which reorders the encoded symbols in accordance with a predetermined interleaving format. The interleaver is provided to provide time diversity in the transmitted stream of encoded symbols. Decoders have better error correction performance when the errors in the received stream are not burst errors.

The reordered symbols are provided to Walsh spreader <b>90,</b> which spreads the reordered symbols in accordance with a predetermined code sequence W_T. In the exemplary embodiment, W_T is a code sequence that is orthogonal to all other code sequences used to channelized the channels of forward link signal <b>32</b>. The Walsh spread signal is then provided to complex PN spreader <b>62</b> and is spread as described above.

The complex PN spread data is provided to transmitter (TMTR) <b>94</b> and up converted, filtered and amplified for transmission through antenna <b>56.</b>

Turning to <figref idref="f0006">FIG. 7</figref>, a simplified multi-carrier receiver which in the exemplary embodiment is provided in mobile station <b>40</b> is illustrated. In the exemplary embodiment, mobile station <b>40</b> is capable of simultaneously receiving forward link signals <b>32</b> transmitted on up to three carrier channels. It will be understood by one skilled in the art that the present invention can be extended to multi-carrier reception of an arbitrary number of channels. The received signal is provided to each of receive subsystems <b>105</b> which down convert and demodulate a different component of forward link signal <b>32</b> in accordance with a unique carrier frequency.

The forward link signal <b>32</b> is received at antenna <b>100</b> and provided to receivers <b>102.</b> Each of receivers <b>102a, 102b</b> and <b>102c</b> down convert, filter and amplify the received signal in accordance with a different frequency f₁, f₂, or f₃, respectively. The down converted signals are provided to demodulators <b>104.</b> In the exemplary embodiment, demodulators <b>104</b> demodulated each of the down converted signals in accordance with a code division multiple access (CDMA) modulation format. The implementation of demodulators <b>104</b> are described in detail in the aforementioned <patcit id="pcit0007" dnum="US5103459A"><text>U.S. Patent No. 5,103,459</text></patcit>. The demodulated components of forward link signal <b>32</b> are provided to multiplexer (MUX) <b>106</b> which re-assembles the transmitted data stream.

In the present invention, mobile station <b>40</b> initially only uses a single receiver <b>102</b> and demodulator <b>104.</b> Mobile station <b>40</b> tunes a selected receivers <b>102</b> to a preferred channel <b>(300c, 300f</b> or <b>300i)</b> and attempts to acquire the pilot signal at the preferred channel frequency using a corresponding one of demodulators <b>104.</b> If sufficient correlation energy is detected, the acquisition is declared to be successful. The mobile station then still only down converting at the single frequency, demodulates, de-interleaves and decodes the Sync Channel message. From the Sync Channel message, mobile station <b>40</b> determines the center frequency of a multi-carrier system in the current frequency bands, if one exists, and the frequency of a single carrier system in the current frequency bands, if one exists.

Mobile station <b>40</b> decides whether, it will operate in a multi-carrier mode or a single carrier mode. If mobile station <b>40</b> decides to operate in a multi-carrier mode, then mobile station <b>40</b> activates the RF circuitry additional receivers <b>102,</b> tuning to the appropriate set of frequencies indicated in the Sync Channel message and begins to receive the forward link signal on a plurality of carrier frequencies. If mobile station <b>40</b> decides to operate in a single-carrier mode, then mobile station <b>40</b> tunes to the appropriate frequency indicated in the Sync Channel message and begins to receive the forward link signal on a single carrier frequency band.

<figref idref="f0007">FIG. 8</figref> illustrates the apparatus to receive forward link signal <b>32</b> in the exemplary embodiment of a CDMA communication system. Initially, mobile station 40 must tune to a preferred channel and attempt to acquire the pilot signal of that channel in the following manner.

The forward link signals <b>32</b> are received at antenna <b>100</b> and provided to receiver <b>102.</b> Receiver <b>102</b> tunes to a preferred channel frequency and down converts, filters and amplifies the received signal. In the exemplary embodiment, receiver <b>102</b> is a quaternary phase shift keyed receiver and outputs an in-phase (I) and Quadrature (Q) component of the received signal.

The two components of the received signal are provided to complex PN despreader <b>112.</b> Complex PN despreader <b>112</b> despreads the received signal in accordance with two pseudonoise sequences PN_I and PN_Q. In the exemplary embodiment, the PN despreading is a complex PN despreading as is described in detail in the aforementioned copending <patcit id="pcit0008" dnum="US886604A"><text>U.S. Patent Application Serial No. 08/886,604</text></patcit>. In the exemplary embodiment, the PN sequences used to spread forward link signals <b>32</b> are generated using a generator polynomial that is common to all base stations <b>30.</b> The spreading from base stations is distinguished from one another by the offset of the sequence.

Control processor <b>128</b> provides offset hypotheses to complex PN despreader <b>112.</b> Complex PN despreader <b>112</b> despreads the received signals in accordance with the PN offset hypotheses provided by control processor <b>128</b> as well as W_pilot. The received signal is despread in accordance with the PN offset hypothesis and the resultant signal is provided to pilot filter <b>114.</b> Pilot filter <b>114</b> despreads the signal from complex PN despreader <b>112</b> in accordance with an orthogonal sequence W_pilot and low pass filters the result of complex despreader <b>112</b> In the exemplary embodiment, the Walsh sequence comprising of all 1's is used to channel the pilot signal.

The resultant signals from pilot filter <b>114</b> are provided to energy detector <b>118</b> which sums the squares of the resultant samples from pilot filter <b>114</b> to provide a received pilot energy value. The received pilot energy value is provided to control processor <b>128,</b> where it is compared to a predetermined threshold value. If the computed energy exceeds the threshold acquisition is declared to be successful and the mobile station begins to receive the Sync Channel message. If the energy falls below the threshold value, then the acquisition is declared unsuccessful and the next PN hypothesis is provided by control processor <b>128</b> to complex PN despreader <b>112.</b> The method and apparatus for searching PN offsets in a CDMA communication system is described in detail in <patcit id="pcit0009" dnum="US5644591A"><text>U.S. Patent No. 5,644,591</text></patcit>, entitled "Method And Apparatus For Performing Search Acquisition In A CDMA Communication System", which is assigned to the assignee of the present invention and incorporated by reference herein. If exhausting the possible PN offset hypotheses, the received pilot energy fails to exceed the threshold value, then control processor sends message to receiver <b>102</b> to begin down converting the received signal at a different preferred frequency channel.

Upon successful acquisition of a pilot channel on a preferred channel frequency, mobile station <b>40</b> demodulates and decodes the Sync Channel message. The received signal from receiver <b>102</b> is despread using the PN offset determined in the pilot search algorithm. The pilot signal is processed by pilot filter <b>114</b> as described above.

The PN despread signal is also provided to Walsh despreader <b>116.</b> Walsh despreader <b>116</b> despreads the received signal in accordance with a Walsh code sequence W_chan. When demodulating the Sync Channel, the W_chan is Walsh sequence allocated for the transmission of the Sync Channel message. Walsh despreader <b>116</b> despreads the signal components in accordance with the orthogonal sequence W_chan and provides the result to dot product circuit <b>120.</b>

As forward link signal <b>32</b> traverse the propagation path to mobile station <b>40,</b> an unknown phase component is introduced into the received signal. Dot product circuit <b>120</b> computes the projection of the received signal onto the received pilot signal to provides a scalar result without the phase errors. The implementation of dot product circuits for coherent demodulation are well known in the art and a method and apparatus for performing the dot product procedure is described in detail in <patcit id="pcit0010" dnum="US5506865A"><text>U.S. Patent No. 5,506,865</text></patcit>, entitled "Pilot Carrier Dot Product Circuit" which is assigned to the assignee of the present invention and is incorporated by reference herein.

The scalar outputs from dot product circuit <b>120</b> are provided to multiplexer <b>122</b> which combines the two received streams into a signal data stream. The data stream from multiplexer <b>122</b> is provided to de-interleaver <b>124</b> which reorders the received symbols in accordance with a predetermined de-interleaving format. The reordered symbols are provided to decoder <b>126</b> which decodes the symbols of the Sync Channel message to provide the received Sync Channel message.

The received Sync Channel message is provided to control processor <b>128</b>. In the present invention, control processor <b>128</b> determines from the Sync Channel message the channel frequency for the primary overhead channel of the single carrier system or the center frequency of the multi-carrier system. In response to the Sync Channel message, control processor <b>128</b> initiates the correct number of receive subsystems <b>105</b> and tunes them to the appropriate channels to receive forward link signal <b>32.</b>

The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. The various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. <ol id="ol0001" compact="compact" ol-style=""><li>1. A multi-carrier base station operating within a predetermined set of frequencies wherein data components of forward link data are transmitted simultaneously on a plurality of frequency bands, said base station comprising: a first transmission subsystem for transmitting a sync channel message on a single carrier frequency of a set of frequencies of said multi carrier system; and at least one additional transmission subsystem for transmitting remaining components of said forward link data.</li><li>2. The base station of embodiment 1 wherein said sync channel message indicates the center frequency of at least one multi carrier system in said predetermined set of frequencies.</li><li>3. The base station of embodiment 1 wherein said sync channel message indicates the frequency of a single carrier system in said predetermined set of frequencies.</li><li>4. The base station of embodiment 2 wherein said sync channel message indicates the frequency of a single carrier system in said predetermined set of frequencies.</li><li>5. The base station of embodiment 2 wherein said sync channel message is transmitted on one of a set of a preferred frequency channels wherein the number of frequencies in said set of preferred frequency channels is less number of frequencies in said predetermined set of frequencies.</li><li>6. The base station of embodiment 5 wherein said set of predetermined frequencies are the set of frequency bands in a personal communications system block of frequencies.</li><li>7. The base station of embodiment 6 wherein the channel numbers of the set of preferred frequency channels are 75,150 and 225.</li><li>8. A multi-carrier mobile station comprising: a control processor for controlling the operation of a plurality of receiver subsystems in accordance with information indicated in a received sync carrier message; a first receiver subsystem for receiving said sync channel message on single carrier frequency and for providing said sync carrier message to said control processor and for receiving a first portion of a multi-carrier signal; at least one additional receiver subsystem for receiving additional portions of said multi-carrier signal.</li><li>9. The mobile station of embodiment 8 wherein said control processor is further for deciding whether to operate in a single band mode or a multicarrier mode and for directing said first receiver system to tune to a frequency indicated in said sync channel message for the reception of a single band system when said mobile station decides to operate in a single band mode and for directing said first receiver subsystem to tune to a first frequency and for directing said at least one additional receiver subsystem to tune to at lest one additional frequency when said mobile station decides to operate in a multi-carrier mode.</li><li>10. The mobile station of embodiment 8 wherein said control processor directs said first receiver subsystem to tune to one of a predetermined set of preferred frequencies.</li><li>11. The mobile station of embodiment 8 wherein said mobile station is operating within a personal communication system (PCS) set of frequencies and wherein said predetermined set of preferred frequencies consist of the frequency channel numbers 75,150 and 225.</li></ol>