FIELD OF THE INVENTION

[0001]
The present invention relates generally to fast cell search, and in particular to a method and apparatus for fast identification of a service cell or sector during initial or periodic access, or handover in a mobile communication system.
BACKGROUND OF THE INVENTION

[0002]
In a mobile cellular network, the geographical coverage area is divided into many cells, each of which is served by a base station (BS). Each cell can also be further divided into a number of sectors. When a mobile station (MS) is powered up, it needs to search for a BS to register with. Also, when the MS finds out that the signal from the current serving cell becomes weak, it should prepare for a handover to another cell/sector. Because of this, the MS is required to search for a good BS for communication. The ability to quickly identify a BS for initial registration or handover is important for reducing the processing complexity and thus lowering the power consumption.

[0003]
The cell search function is often performed based on a cellspecific reference signal (or preamble) transmitted periodically. A straightforward method is to perform an exhaustive search by trying to detect each reference signal and then determine the best BS. There are two important criteria when determining reference sequences for cells or sectors. First, the reference sequences should allow good channel estimation to all the users within its service area, which is often obtained through a correlation process with the reference of the desired cell. In addition, since a mobile will receive signals sent from other sectors or cells, a good cross correlation between reference signals is important to minimize the interference effect on channel estimation to the desired cell.

[0004]
Just like autocorrelation, the crosscorrelation between two sequences is a sequence itself corresponding to different relative shifts. Precisely, the crosscorrelation at shiftd is defined as the result of summing over all entries after an elementwise multiplication between a sequence and another sequence that is conjugated and shifted by d entries with respect to the first sequence. “Good” cross correlation means that the cross correlation values at all shifts are as even as possible so that after correlating with the desired reference sequence, the interference can be evenly distributed and thus the desired channel can be estimated more reliably. Minimization of the maximal crosscorrelation values at all shifts, which is reached when they are all equal, is refer to as “optimal” cross correlation.

[0005]
Priorart techniques, such as those described in US Patent Application Publication No. 2006/0039451 A1, (which is incorporated by reference herein) describe the use of reference sequences that are constructed from distinct “classes” of a Generalized ChirpLike (GCL) sequence. By assigning a base station a particular index of a GCL sequence, the identification of a sequence index will therefore provide the identification of the base station.

[0006]
While the abovedescribed technique allows for a desirable reference signal, multipath fading and intercellinterference may cause a detector to show an abnormally high correlation for GCL sequences having index values close to the index values being transmitted by base stations. Thus, for example, if a base station is transmitting a GCL sequence index of 25, a detector may have high correlation values to sequences 23 and 27, simply due to multipath fading and intercell interference. This will lead the detector to falsely believe that there exist base stations utilizing index values 23 and 27. Therefore, a need exists for a method and apparatus for a fast cell search technique that utilizes GCL reference sequences and reduces abnormally high correlations due to multipath fading and intercell interference.
BRIEF DESCRIPTION OF THE DRAWINGS

[0007]
FIG. 1 is a block diagram of a communication system.

[0008]
FIG. 2 illustrates reference signal transmission for the communication system of FIG. 1.

[0009]
FIG. 3 is a block diagram of a transmitter.

[0010]
FIG. 4 is a block diagram of a GCL sequence detector.

[0011]
FIG. 5 illustrates abnormally high GCL correlations.

[0012]
FIG. 6 illustrates a GCL deployment within a 3sectored communication system.

[0013]
FIG. 7 is a block diagram of a receiver designed to remove adjacent N−1 indices.

[0014]
FIG. 8 is a flow chart showing operation of masking circuitry.

[0015]
FIG. 9 illustrates input and output signals to and from the masking circuitry.
DETAILED DESCRIPTION OF THE DRAWINGS

[0016]
To address the abovementioned need, a method and apparatus for fast cell search is provided herein. During operation, a GCL sequence index of each cell/sector is assigned so that a difference of the indices between adjacent cells/sectors is more than N, where N≧1. During GCL index detection, the adjacent N−1 indices of an index with the largest magnitude can be deleted from handoff candidate cell indices. This greatly reduces abnormally high GCL correlations due to multipath fading and intercell interference.

[0017]
The present invention encompasses a method for fast cell search. The method comprises the steps of receiving an overtheair communication, determining powers of a plurality of GCL indices from the overtheair communication, and determining a GCL index having a maximum power. GCL indices surrounding the GCL index having the maximum power are eliminated and a handoff candidate cell indices are determined based on the powers of a plurality of GCL indices excluding the eliminated GCL indices.

[0018]
The present invention additionally encompasses an apparatus comprising a GCL sequence index detector outputting a plurality of GCL indices and their power, and masking circuitry determining a GCL index with a maximum power, eliminating adjacent indices, and outputting a plurality of GCL indices and their associated power, excluding the eliminated adjacent indices.

[0019]
The present invention additionally encompasses a base station comprising a transmitter transmitting a first GCL reference signal in a first sector, and a second transmitter transmitting a second GCL reference signal in a second sector, wherein a GCL sequence index of the first GCL reference signal is more than N indices from a sequence index of the second GCL reference signal.

[0020]
Turning now to the drawings, where like numerals designate like components, FIG. 1 is a block diagram of communication system 100 that utilizes reference transmissions. Communication system utilizes an Orthogonal Frequency Division Multiplexing (OFDM) protocol; however in alternate embodiments communication system 100 may utilize other digital cellular communication system protocols such as a Code Division Multiple Access (CDMA) system protocol, a Frequency Division Multiple Access (FDMA) system protocol, a Spatial Division Multiple Access (SDMA) system protocol or a Time Division Multiple Access (TDMA) system protocol, or various combinations thereof.

[0021]
As shown, communication system 100 includes base unit 101 and 102, and remote unit 103. A base unit or a remote unit may also be referred to more generally as a communication unit. The remote units may also be referred to as mobile units. A base unit comprises a transmit and receive unit that serves a number of remote units within a sector. As known in the art, the entire physical area served by the communication network may be divided into cells, and each cell may comprise one or more sectors. When multiple antennas are used to serve each sector to provide various advanced communication modes (e.g., adaptive beamforming, transmit diversity, transmit SDMA, and multiple stream transmission, etc.), multiple base units can be deployed. These base units within a sector may be highly integrated and may share various hardware and software components. For example, all base units colocated together to serve a cell can constitute what is traditionally known as a base station. Base units 101 and 102 transmit downlink communication signals 104 and 105 to serving remote units on at least a portion of the same resources (time, frequency, or both). Remote unit 103 communicates with one or more base units 101 and 102 via uplink communication signal 106. A communication unit that is transmitting may be referred to as a source communication unit. A communication unit that is receiving may be referred to as a destination or target communication unit.

[0022]
It should be noted that while only two base units and a single remote unit are illustrated in FIG. 1, one of ordinary skill in the art will recognize that typical communication systems comprise many base units in simultaneous communication with many remote units. It should also be noted that while the present invention is described primarily for the case of downlink transmission from multiple base units to multiple remote units for simplicity, the invention is also applicable to uplink transmissions from multiple remote units to multiple base units. It is contemplated that network elements within communication system 100 are configured in well known manners with processors, memories, instruction sets, and the like, which function in any suitable manner to perform the function set forth herein.

[0023]
As discussed above, reference assisted modulation is commonly used to aid in many functions such as channel estimation and cell identification. With this in mind, base units 101 and 102 transmit reference sequences at known time intervals as part of their downlink transmissions. Remote unit 103, knowing the set of sequences that different cells can use and the time interval, utilizes this information in cell search and channel estimation. Such a reference transmission scheme is illustrated in FIG. 2. As shown, downlink transmissions 200 from base units 101 and 102 typically comprise reference sequence 201 followed by remaining transmission 202. The same or a different sequence can show up one or multiple times during the remaining transmission 202. Thus, each base unit within communication system 100 comprises reference channel circuitry 107 that transmits one or more reference sequences along with data channel circuitry 108 transmitting data. In a similar manner, each remote unit 103 within communication system 100 comprises GCL sequence detector 109.

[0024]
It should be noted that although FIG. 2 shows reference sequence 201 existing at the beginning of a transmission, in various embodiments of the present invention, the reference channel circuitry may include reference sequence 201 anywhere within downlink transmission 200, and additionally may be transmitted on a separate channel. Remaining transmission 202 typically comprises transmissions such as, but not limited to, sending information that the receiver needs to know before performing demodulation/decoding (so called control information) and actual information targeted to the user (user data).

[0025]
As discussed above, it is important for any reference sequence to have optimal crosscorrelation. With this in mind, communication system 100 utilizes reference sequences constructed from distinct “classes” of chirp sequences with optimal cyclic crosscorrelation. The construction of such reference sequences is described below. In a preferred embodiment of the invention, the method for fast cell search is based on such reference sequences.

[0026]
In one embodiment, the time domain reference signal is an Orthogonal Frequency Division Multiplexing (OFDM) symbol that is based on Npoint FFT. A set of lengthN_{p }sequences are assigned to base units in communication system 100 as the frequencydomain reference sequence (i.e., the entries of the sequence will be assigned onto a set of N_{p }(N_{p}<=N) reference subcarriers in the frequency domain). The spacing of these reference subcarriers is preferably equal (e.g., 0, 1, 2, etc. in subcarrier(s)). The final reference sequences transmitted in time domain can be cyclically extended where the cyclic extension is typically longer than the expected maximum delay spread of the channel (L_{D}). In this case, the final sequence sent has a length equal to the sum of N and the cyclic extension length L_{CP}. The cyclic extension can comprise a prefix, postfix, or a combination of a prefix and a postfix. The cyclic extension is an inherent part of the OFDM communication system. The inserted cyclic prefix makes the ordinary auto or crosscorrelation appear as a cyclic correlation at any shift that ranges from 0 to L_{CP}. If no cyclic prefix is inserted, the ordinary correlation is approximately equal to the cyclic correlation if the shift is much smaller than the reference sequence length.

[0027]
The construction of the frequency domain reference sequences depends on at least two factors, namely, a desired number of reference sequences needed in a network (K) and a desired reference length (N
_{p}). In fact, the number of reference sequences available that has the optimal cyclic crosscorrelation is P−1 where P is the smallest prime factor of N
_{p }other than “1” after factoring N
_{p }into the product of two or more prime numbers including “1”. For example, the maximum value that P can be is N
_{p}−1 when N
_{p }is a prime number. But when N
_{p }is not a prime number, the number of reference sequences often will be smaller than the desired number K. In order to obtain a maximum number of sequences, the reference sequence will be constructed by starting with a sequence whose length N
_{G }is a prime number and then performing modifications. In the preferred embodiment, one of the following two modifications is used:

 1. Choose N_{G }to be the smallest prime number that is greater than N_{p }and generate the sequence set. Truncate the sequences in the set to N_{p}; or
 2. Choose N_{G }to be the largest prime number that is smaller than N_{p }and generate the sequence set. Repeat the beginning elements of each sequence in the set to append at the end to reach the desired length N_{p}.

[0030]
The above design of requiring N_{G }to be a prime number will give a set of N_{G}−1 sequences that has ideal auto correlation and optimal cross correlation. However, if only a smaller number of sequences are needed, N_{G }does not need to be a prime number as long as the smallest prime factor of N_{G }excluding “1” is larger than K.

[0031]
When a modification such as truncating or inserting is used, the crosscorrelation will not be precisely optimal anymore. However, the auto and crosscorrelation properties are still acceptable. Further modifications to the truncated/extended sequences may also be applied, such as applying a unitary transform to them.

[0032]
It should also be noted that while only sequence truncation and cyclic extension were described above, in alternate embodiments of the present invention there exist other ways to modify the GCL sequences to obtain the final sequences of the desired length. Such modifications include, but are not limited to extending with arbitrary symbols, shortening by puncturing, etc. Again, further modifications to the extended/punctured sequences may also be applied, such as applying a unitary transform to them.

[0033]
As discussed above, in the preferred embodiment of the present invention Generalized ChirpLike (GCL) sequences are utilized for constructing reference sequences. There are a number of “classes” of GCL sequences and if the classes are chosen carefully (see GCL property below); sequences with those chosen classes will have optimal crosscorrelation and ideal autocorrelation. Classu GCL sequence (S) of length N
_{G }are defined as:
S _{u}=(
a _{u}(0)
b, a _{u}(1)
b, . . . , a _{u}(
N _{G}−1)
b), (1)
where b can be any complex scalar of unit amplitude and
$\begin{array}{cc}{a}_{u}\left(k\right)=\mathrm{exp}\left(j\text{\hspace{1em}}2\pi \text{\hspace{1em}}u\text{\hspace{1em}}\frac{k\left(k+1\right)/2+q\text{\hspace{1em}}k}{{N}_{G}}\right),& \left(2\right)\end{array}$
where,
 u=1, . . . N_{G}−1 is known as the “class” of the GCL sequence,
 k=0, 1, . . . N_{G}−1 are the indices of the entries in a sequence,
 q=any integer.

[0037]
Each class of GCL sequence can have infinite number of sequences depending on the particular choice of q and b, but only one sequence out of each class is used to construct one reference sequence. Notice that each class index “u” produces a different phase ramp characteristic over the elements of the sequence (i.e., over the “k” values).

[0038]
It should also be noted that if an N_{G}point DFT (Discrete Fourier Transform) or IDFT (inverse DFT) is taken on each GCL sequence, the member sequences of the new set also have optimal cyclic crosscorrelation and ideal autocorrelation, regardless of whether or not the new set can be represented in the form of (1) and (2). In fact, sequences formed by applying a matrix transformation on the GCL sequences also have optimal cyclic crosscorrelation and ideal autocorrelation as long as the matrix transformation is unitary. For example, the N_{G}point DFT/IDFT operation is equivalent to a sizeN_{G }matrix transformation where the matrix is an N_{G }by N_{G }unitary matrix. As a result, sequences formed based on unitary transformations performed on the GCL sequences still fall within the scope of the invention, because the final sequences are still constructed from GCL sequences. That is, the final sequences are substantially based on (but are not necessarily equal to) the GCL sequences.

[0039]
If N_{G }is a prime number, the crosscorrelation between any two sequences of distinct “class” is optimal and there will be N_{G}−1 sequences (“classes”) in the set. When a modification such as truncating or inserting is used, the modified reference sequence can be referred to as nearlyoptimal reference sequences that are constructed from GCL sequences.

[0040]
The integer “u” is the GCL sequence index. This sequence index is assigned to each cell. N_{G }in the equation is the length of the GCL sequence. A total of N_{G}−1 different sequences are available for use in different cells. N_{G }is a prime number equal or near the needed sequence length. If the needed sequence length is not a prime number, the nextlargest prime number can be used for NG and the resulting GCL sequence can be truncated to the desired length N_{p}.

[0041]
FIG. 3 is a block diagram of transmitter 107 used to transmit a reference (e.g., a GCL) sequence. The cell index enters GCL sequence generator 301 and a GCL sequence with the particular index is output to subcarrier mapper 303. Subcarrier mapper 303 maps the GCL sequence with unique cell index to subcarriers and outputs the subcarriers to OFDM modulator 305. Modulator 305 then transmits the timedomain reference signal as illustrated in FIG. 2.

[0042]
FIG. 4 is a block diagram of GCL sequence detector 109. During operation, OFDM receiver 401 transforms the received signal to frequency domain and extracts subcarrier symbols as the received GCL sequence. The subcarrier symbols are sent to the GCL sequence index detector 109 where they enter differential encoder 403.

[0043]
The following equation shows the differential encoder output in the case where a GCL sequence of a cell is received. Y(m) in the equation is the GCL sequence index detector input on the mth subcarrier. H(m) in the equation is the channel response of the mth subcarrier. This equation assumes the channel response of adjacent subcarriers is almost the same. The differential encoder output is a complex exponential with a frequency that corresponds to the original sequence index.
$Z\left(m\right)=Y\left(m\right)*{Y}^{*}\left(m+1\right)\approx {\uf603H\left(m\right)\uf604}^{2}{S}_{u}\left(m\right){{S}_{u}}^{*}\left(m+1\right))={\uf603H\left(m\right)\uf604}^{2}\mathrm{exp}\left\{j\text{\hspace{1em}}2\text{\hspace{1em}}\pi \text{\hspace{1em}}u\text{\hspace{1em}}\frac{m+1}{{N}_{G}}\right\},m=1,\cdots \text{\hspace{1em}},{N}_{p}1$
Each of differential encoder output is optionally averaged by frame averaging circuitry 405 over multiple frames to enhance robustness. IFFT 407 retransforms the averaged differential encoder outputs to time domain, where T is the IFFT size.
{z(n)}=IFFT _{T}({Z(m)}), m=1, . . . , N _{p}−1, n=1, . . . , T

[0044]
Power calculator 409 calculates the power of each possible index value, z(n)^{2 }(n=1, . . . ,T), and outputs this information to peak position detector 411. The indices with larger magnitude in the GCL sequence index detector output are detected and are converted into cell indices by detector 411.
$u=\frac{n}{T}\xb7{N}_{G}$
These detected cell indices could be stored as handoff cell indices including campon cell.

[0045]
As described above, while the abovedescribed technique allows for a desirable reference signal, multipath fading and intercellinterference may cause a detector to show an abnormally high correlation for GCL sequences having index values close to the index values being transmitted by base stations. Thus, for example, if a base station is transmitting a GCL sequence index of 25, a detector may have high correlation values to sequences 23 and 27, simply due to multipath fading and intercell interference. This will lead the detector to falsely believe that there exist base stations utilizing index values 23 and 27. This is illustrated in FIG. 5.

[0046]
As shown in FIG. 5, base stations are currently transmitting with GCL index values of 25, 76, and 127. Peak detector 411 will assume that the indices, which should be detected as handoff cell indices, are 25, 76 and 127. However, because the GCL sequence index detector output of the index 127 is smaller than that of the undesired index 27, the desired index 127 is not detected as a handoff cell index. The cause of this error is multipath fading and intercellinterference. In order to detect index 127, more multiframe averaging would be required. Therefore, it takes long periods of time for priorart detection of the desired handoff candidate cell indices.

[0047]
In order to address this issue, the GCL sequence index of each cell is assigned so that the difference of the indices between adjacent cells/sectors is more than N, where N≧1. In other words, N is a minimum number of indices between any two sectors/cells. This proposed method assures that the difference of the indices between handoff candidate cells is more than N.

[0048]
FIG. 6 shows an example of the code index assignment. More particularly, FIG. 6 shows a plurality of base stations 601 (only one labeled), each having three sectors 602 (only one sector labeled). Because each base station 601 will have three sectors 602, each base station 601 comprises at least three base units 101 (at least one for each sector). Thus, each base station 601 will comprise at least a transmitter transmitting a first GCL reference signal in a first sector, a second transmitter transmitting a second GCL reference signal in a second sector, and a third transmitter transmitting a third GCL reference signal an a third sector. As described, the first, second, ant third GCL reference signal indices are more than N indices from any adjacent sector.

[0049]
Each sector in FIG. 6 is labeled with its particular GCL sequence index. FIG. 6 assumes 3 sectors, and hence 3 base units per cell site (base station) and 57 (19×3) indices are reused by 19 base stations 601. The index of each sector is represented as ID(k,l)=k+l*M, where k is cell number, I is sector number and M is number of cells reusing available indices. Note, the index 56 and the index 18 are swapped in FIG. 6. Therefore, the minimum number of indices between adjacent cells N becomes 5, which in this case equals the number of indices between index 13 and 18. Because of this, the adjacent N−1 indices of the index with the largest magnitude in the GCL sequence index detector output can be deleted from handoff candidate cell indices.

[0050]
FIG. 7 is a block diagram of a receiver designed to remove adjacent N−1 indices surrounding an index. As is evident, the receiver of FIG. 7 is identical to the receiver of FIG. 4 except for the addition of masking circuitry 701. Masking circuitry 701 utilizes the code assignment characteristic that the difference of the indices between handoff candidate cells is more than N. Circuitry 701 receives a value for N and a number of candidate handoff cells. Circuitry 701 then deletes N−1 adjacent indices of the detected cell index from each candidate cell. FIG. 8 is a flow chart showing operation of circuitry 701 assuming the number of handoff candidate cell indices is three. Additionally, it is assumed that an overtheair communication has been received containing GCL reference signals from potential handover candidates.

[0051]
The logic flow begins at step 801 where circuitry 701 receives a power of a plurality of GCL indices from the overtheair communication and detects the index with a largest power. As described above, the power values of each GCL index is output by GCL sequence index detector 109. At step 803, circuitry 701 eliminates the N−1 adjacent indices surrounding the index with the largest power. The logic flow continues to step 805 where circuitry 701 detects an index with a second largest power and eliminates N−1 adjacent indices surrounding the index with the second largest power (step 807). At step 809 circuitry 701 detects the index with the third largest power and eliminates the N−1 adjacent indices surrounding the index with the third largest power (step 811). Finally, at step 813 the indices and their normalized powers (excluding the eliminated indices) are output to peak detector 411. The detected three peak indices are output as handoff candidate cell indices.

[0052]
As is evident, potential handoff candidate cells are determined based on the powers of the plurality of GCL indices excluding the eliminated GCL indices. More particularly, the peak indices output by peak detector 411 are based on the powers of a plurality of GCL indices excluding the eliminated GCL indices.

[0053]
FIG. 9 shows an example of inputs and outputs of the index mask function. As is evident, the input to circuitry 701 shows high correlations surrounding index values 25 and 76. The output of circuitry 701 remove all power for the N−1 indices surrounding indices 25, 76, and 127. By adopting this proposed method, the false detection because of multipath fading and intercellinterface is reduced without multiframe averaging. That is to say, handoff candidate search time is reduced by this proposed method.

[0054]
While the invention has been particularly shown and described with reference to a particular embodiment, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.