Abstract:
A cellular communications system that utilizes OFDM in its radio interface is capable of utilizing either a first subcarrier spacing or a second subcarrier spacing. Which of these is presently in use is indicated by generating a first type of synchronization signal in response to the first subcarrier spacing presently being in use, and generating a second type of synchronization signal in response to the second subcarrier spacing presently being in use. Whichever of the first type of synchronization signal and the second type of synchronization signal was generated is transmitted. To distinguish between the first and second types of synchronization signals, a time domain representation of the second type of synchronization signal includes a plurality of instances of the first type of synchronization signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/891,265, filed Feb. 23, 2007, which is hereby incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    The invention relates to subcarrier spacing in mobile communication systems that employ Orthogonal Frequency Division Multiplexing (OFDM), and more particularly to techniques that enable a user equipment operating in such a system to detect which of a number of possible subcarrier spacings are presently being used for communications. 
         [0003]    In the forthcoming evolution of the mobile cellular standards like the Global System for Mobile Communication (GSM) and Wideband Code Division Multiple Access (WCDMA), new transmission techniques like OFDM are likely to occur. Furthermore, in order to have a smooth migration from the existing cellular systems to the new high capacity high data rate system in existing radio spectrum, the new system has to be able to operate in a dynamically changeable bandwidth. A proposal for such a new flexible cellular system is 3G Long Term Evolution (3G LTE, where “3G” stands for “Third Generation”) that can be seen as an evolution of the 3G WCDMA standard. OFDM will be used in this system in a technique called OFDM Multiple access (“OFDMA”) to enable multiple users to share access to the radio spectrum in the downlink. The system will be able to operate on bandwidths ranging from 1.25 MHz to 20 MHz. Furthermore, data rates up to 100 Mb/s will be supported on the largest bandwidth. 
         [0004]    Another important aspect of LTE is efficient support for broadcast and Multimedia Broadcast/Multicast Service (“MBMS”). In LTE, so called “Single Frequency Network” (SFN) operation is foreseen in which base stations are synchronized. Here, MBMS content is transmitted from several base stations using the same physical resources. The signals from these multiple transmissions are automatically “combined in the air”, so no additional receiver resources are required for this purpose. In order for this “over the air combining” to work, all participating base stations must be synchronized—both in the frequency domain and in the time-domain—down to the extent of a fraction of the length of a cyclic prefix. In order to ease time synchronization requirements a long cyclic prefix is advantageous. However, increasing the cyclic prefix without increasing the OFDM symbol duration increases overhead and is thus not attractive. One possible solution is to use a smaller subcarrier spacing (and corresponding bandwidth), thus increasing the OFDM symbol duration (the OFDM symbol duration is inversely proportional to the subcarrier spacing). For example, halving the subcarrier spacing results in OFDM symbols that are twice as long, thereby enabling a cyclic prefix that is twice as long. In this manner, the amount of overhead is maintained constant. Therefore, in addition to support for 15 kHz subcarrier spacing, LTE also supports the use of a 7.5 kHz subcarrier spacing for SFN operation. 
         [0005]    The physical layer of a 3G LTE system includes a generic radio frame having a duration of 10 ms.  FIG. 1  illustrates one such frame  100 . Each frame has 20 slots (numbered  0  through  19 ), each slot having a duration of 0.5 ms. A sub-frame is made up of two adjacent slots, and therefore has a duration of 1 ms. 
         [0006]    One important aspect of LTE is the mobility function. Hence, synchronization symbols and cell search procedures are of major importance in order for the User Equipment (UE) to detect and synchronize with other cells. To facilitate cell search and synchronization procedures, defined signals include Primary and Secondary Synchronization Signals (P-SyS and S-SyS, respectively), which are transmitted on a Primary Synchronization Channel (P-SCH) and a Secondary Synchronization Channel (S-SCH), respectively. The P-SySs and S-SySs are each broadcast twice per frame: once in sub-frame  0 , and again in sub-frame  5 , as shown in  FIG. 1 . 
         [0007]    The UE must detect, as soon as possible, whether it is connecting to a 7.5 kHz/subcarrier cell or a 15 kHz/subcarrier cell, since subsequent procedures may be different for the two cases. One possibility, of course, is to have two completely different synchronization signal designs, each uniquely associated with one of the subcarrier spacing sizes. However, here the UE would be required to have both synchronization signal designs implemented, in which case it would either have to run search algorithms for both synchronization signals in parallel—thus increasing complexity—or sequentially—thus increasing cell search time. 
         [0008]    It is therefore desired to have a technique that will enable a UE to detect what the subcarrier spacing is without having to have two different synchronization signal designs. 
       SUMMARY 
       [0009]    It should be emphasized that the terms “comprises” and “comprising”, when used in this specification, are taken to specify the presence of stated features, integers, steps or components; but the use of these terms does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. 
         [0010]    In accordance with one aspect of the present invention, the foregoing and other objects are achieved in apparatuses and methods that indicate which of a first subcarrier spacing (e.g., 15 kHz) and a second subcarrier spacing (e.g., 7.5 kHz) is presently in use in a cellular communication system. This includes generating a first type of synchronization signal in response to the first subcarrier spacing presently being in use, and generating a second type of synchronization signal in response to the second subcarrier spacing presently being in use. Whichever of the first type of synchronization signal and the second type of synchronization signal was generated is transmitted. A time domain representation of the second type of synchronization signal includes a plurality of instances of the first type of synchronization signal. 
         [0011]    In another aspect, a radio interface of the cellular communication system utilizes Orthogonal Frequency Division Multiplexing (OFDM); generating the first type of synchronization signal comprises mapping a set of synchronization symbols, S k , (0≦k≦N−1) onto a contiguous set of N subcarriers, each of the N subcarriers having the first subcarrier spacing; and generating the second type of synchronization signal comprises mapping the set of synchronization symbols, S k , onto a set of n·N subcarriers (n&gt;1) such that every n th  one of the n·N carries one of the N synchronization symbols, and remaining ones of the n·N subcarriers carry zero values, wherein each of the n·N subcarriers has the second subcarrier spacing. 
         [0012]    In yet another aspect, the time domain representation of the second type of synchronization signal consists of two instances of the first type of synchronization signal. 
         [0013]    In another aspect of embodiments consistent with the invention, apparatuses and methods detect which of a first subcarrier spacing (e.g., 15 kHz) and a second subcarrier spacing (e.g., 7.5 kHz) is presently in use in a cellular communication system. This includes processing a received signal to derive therefrom a synchronization signal. It is then detected whether a time domain representation of the synchronization signal comprises only one instance of a first type of synchronization signal. If the time domain representation of the synchronization signal comprises only one instance of the first type of synchronization signal, then the first subcarrier spacing is utilized in one or more subsequent communication operations. Otherwise, if the time domain representation of the synchronization signal comprises more than one instance of the first type of synchronization signal, then the second subcarrier spacing is utilized in one or more subsequent communication operations. 
         [0014]    In some embodiments, detecting whether the time domain representation of the synchronization signal comprises only one instance of the first type of synchronization signal comprises: generating correlation results by correlating the received signal against the first type of synchronization signal; and detecting whether the correlation results include distinct peaks separated in time by a predetermined amount. 
         [0015]    In alternative embodiments, detecting whether the time domain representation of the synchronization signal comprises only one instance of the first type of synchronization signal comprises generating correlation results by correlating the received signal with a delayed copy of the received signal, wherein an amount of delay applied to generate the delayed copy of the received signal corresponds to a time period of first type of synchronization signal; and detecting whether the correlation results include a distinct peak. 
         [0016]    In still other alternative embodiments, indicating which of a first subcarrier spacing and a second subcarrier spacing is presently in use in a cellular communication system involves generating a first type of synchronization signal in response to the first subcarrier spacing presently being in use, and generating a second type of synchronization signal in response to the second subcarrier spacing presently being in use. Whichever one of the first type of synchronization signal and the second type of synchronization signal was generated is then transmitted. In such embodiments, a time domain representation of the first type of synchronization signal includes a first plurality of instances of a basic waveform; a time domain representation of the second type of synchronization signal includes a second plurality of instances of the basic waveform; and the first plurality is not equal to the second plurality. 
         [0017]    In still other embodiments, detecting which of a first subcarrier spacing and a second subcarrier spacing is presently in use in a cellular communication system involves processing a received signal to derive therefrom a synchronization signal, and detecting how many instances of a basic waveform are included in a time domain representation of the synchronization signal. If the time domain representation of the synchronization signal comprises a first plurality of instances of the basic waveform, then the first subcarrier spacing is utilized in one or more subsequent communication operations. If the time domain representation of the synchronization signal comprises a second plurality of instances of the basic waveform, then the second subcarrier spacing is utilized in one or more subsequent communication operations, wherein the first plurality is not equal to the second plurality. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The objects and advantages of the invention will be understood by reading the following detailed description in conjunction with the drawings in which: 
           [0019]      FIG. 1  is a timing diagram of a generic radio frame utilized in an LTE system. 
           [0020]      FIG. 2A  is a diagram illustrating a first type of synchronization signal, which is used when a subcarrier spacing is a larger of two possible sizes. 
           [0021]      FIG. 2B  is a diagram illustrating a second type of synchronization signal, which is used when a subcarrier spacing is a smaller of two possible sizes. 
           [0022]      FIG. 3A  is a time-frequency diagram showing the mapping of the P-SYS MODE1  signal, in which all N of the modulation symbols, S k , are mapped to contiguous subcarriers. 
           [0023]      FIG. 3B  is a time-frequency diagram showing the exemplary mapping of the P-SYS MODE2  signal, in which the same N modulation symbols, S k , are mapped to every other one of a group of 2N subcarriers, with remaining subcarriers set to zero, in accordance with an aspect of the invention. 
           [0024]      FIG. 4A  illustrates an exemplary correlation results waveform when the received signal includes a P-SYS MODE1 . 
           [0025]      FIG. 4B  illustrates an exemplary correlation results waveform when the received signal includes a P-SYS MODE2 , in accordance with an aspect of the invention. 
           [0026]      FIG. 5  is a block diagram of relevant components in a transmitter for generating a P-SyS signal in accordance with aspects of the invention. 
           [0027]      FIG. 6  is a block diagram of relevant components of an exemplary synchronization channel receiver in a UE that is capable of detecting the indication of subcarrier spacing in accordance with another aspect of embodiments consistent with the invention. 
           [0028]      FIG. 7  is a flow chart of exemplary steps that can be carried out by circuitry in a UE that detects the indication of subcarrier spacing in accordance with another aspect of embodiments consistent with the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    The various features of the invention will now be described with reference to the figures, in which like parts are identified with the same reference characters. 
         [0030]    The various aspects of the invention will now be described in greater detail in connection with a number of exemplary embodiments. To facilitate an understanding of the invention, many aspects of the invention are described in terms of sequences of actions to be performed by elements of a computer system or other hardware capable of executing programmed instructions. It will be recognized that in each of the embodiments, the various actions could be performed by specialized circuits (e.g., discrete logic gates interconnected to perform a specialized function), by program instructions being executed by one or more processors, or by a combination of both. Moreover, the invention can additionally be considered to be embodied entirely within any form of computer readable carrier, such as solid-state memory, magnetic disk, optical disk containing an appropriate set of computer instructions that would cause a processor to carry out the techniques described herein. Thus, the various aspects of the invention may be embodied in many different forms, and all such forms are contemplated to be within the scope of the invention. For each of the various aspects of the invention, any such form of embodiments may be referred to herein as “logic configured to” perform a described action, or alternatively as “logic that” performs a described action. 
         [0031]    In an aspect of the embodiments consistent with the invention, a Primary SyS (“P-SyS”) is provided and utilized in a communication system, wherein a first type of synchronization signal, herein denoted P-SYS MODE1 , is utilized when the subcarrier spacing is the larger of two possible spacings (e.g., 15 kHz per subcarrier) and a second type of synchronization signal, herein denoted P-SYS MODE2 , is used when the subcarrier spacing is the smaller of the two possible spacings (e.g., 7.5 kHz per subcarrier), wherein the second type of synchronization signal is formed as a function of the first type of synchronization signal. In a more specific embodiment, the second type of synchronization signal comprises two or more instances of the first type of synchronization signal in immediate succession. 
         [0032]    In another aspect of embodiments consistent with the invention, in use, if the detected synchronization signal contains two or more of the first type of synchronization signal in immediate succession, then the detected cell is ascertained to have a subcarrier spacing associated with the smaller of the two spacings (e.g., 7.5 kHz), whereas if no repetitions are detected, the detected cell is ascertained to have a subcarrier spacing associated with the larger of the two spacings (e.g., 15 kHz). 
         [0033]    These and other aspects are described in greater detail in the following discussion. 
         [0034]    The following description is set in the context of an LTE system. However, this is done merely to facilitate the reader&#39;s understanding of the various aspects (e.g., by utilizing terminology that will be readily understood by those having ordinary skill in the art). The use of LTE terminology, methods and apparatuses to illustrate the various aspects of the invention should not be construed to limit the scope of the invention, or to in any way suggest that the invention is limited only to LTE embodiments. To the contrary the various aspects of embodiments consistent with the invention are equally applicable to many other systems that share relevant characteristics with LTE systems (e.g., reliance on a synchronization signal to, for example, make an initial determination of things such as signal timing), but are different in other ways. 
         [0035]    The P-SyS in LTE is constructed by mapping a sequence S k , k=0, 1, . . . , N−1, onto the subcarrier used for the synchronization signal. After Inverse Fast Fourier Transform (IFFT) processing, this results in the time domain signal 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
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         [0000]    where n is a sample number, and N is the number of subcarriers. 
         [0036]    Some of the S k  values can actually be set to zero in order to confine the time domain signal, s(n), to its bandwidth. The inclusion of zeros may also be necessary to avoid DC distortion. 
         [0037]    In the exemplary embodiment, two modes of operation are presented, each having its own subcarrier spacing: a first mode which does not involve SFN (e.g., unicast transmissions), and a second mode which does involve SFN. In an aspect of the invention, the P-SyS for the second type of subcarrier spacing (P-SYS MODE2 ) is created such that—in the time domain—it is a replica of the P-SyS used in the first mode of operation (e.g., non-SFN use) (P-SYS MODE1 ) plus one or more immediate repetitions of that signal.  FIGS. 2A and 2B  are time domain signal diagrams that illustrate this point. Specifically, each of  FIGS. 2A and 2B  is a graph of the amplitude of transmitted signals plotted as a function of time. The shapes of the various illustrated signals are intended to merely represent the presence of the signals, and are not intended to represent actual waveforms. 
         [0038]      FIG. 2A  illustrates a first type of synchronization signal  201 , which is used when a subcarrier spacing is a larger of two possible sizes (herein illustrated as P-SYS MODE1 ). The duration of the first type of synchronization signal  201  is herein denoted t MODE1 . The first type of synchronization signal  201  is generated in accordance with Equation (1), with the symbols, S k , being mapped onto N contiguous subcarriers. In embodiments such as LTE, one of the subcarriers is DC, and for this reason is punctured. Accordingly, in such embodiments SDC should be set equal to zero. 
         [0039]      FIG. 2B  is a diagram illustrating a second type of synchronization signal  203 , which is used when a subcarrier spacing is a smaller of two possible sizes (herein illustrated as P-SYS MODE2 ). The second type of synchronization signal  203  comprises two (or in other embodiments, more than two) components (in this example denoted first component  203   a  and second component  203   b ) transmitted in immediate succession during two (or in other embodiments, more than two) contiguous time periods for a total duration of t MODE2 . In the time domain, the waveform of each of the first and second components  203   a,    203   b  of the second type of synchronization signal  203  is identical to that of the first type of synchronization signal  201 . Consequently, each of the first and second components  203   a,    203   b  has a duration equal to t MODE1  and can be considered an instance of the first type of synchronization signal  201 . 
         [0040]    A P-SYS MODE2  having this property is easily constructed by using the sequence S k  associated with the P-SYS MODE1  (i.e., the P-SyS of the larger of the two possible sizes) and mapping it to every n-th one of n·N subcarriers (where n&gt;1). The remaining subcarriers within the bandwidth allocated for the P-SyS are then populated with zeros. (Subcarriers outside the P-SyS bandwidth should also be treated as zeros, but this can be achieved by filtering on the receiver side, so actual population with zeros is not essential in practice.) The resulting time domain waveform of the P-SYS MODE2  will have a waveform comprising n contiguous instances of the P-SYS MODE1  waveform. For the case in which the spacing for SFN operation is half the original subcarrier spacing, the original P-SyS sequence is preferably mapped to every second subcarrier (i.e., n=2), resulting in two successive (and contiguous) instances of the P-SYS MODE1  waveform in the time domain. This is illustrated in  FIGS. 3A and 3B , wherein  FIG. 3A  is a time-frequency diagram showing the mapping of the P-SYS MODE1  signal (all N of the modulation symbols, S k , are mapped to contiguous subcarriers), and  FIG. 3B  is a time-frequency diagram showing the exemplary mapping of the P-SYS MODE2  signal (the same N modulation symbols, S k , are in this case mapped to every other one of a group of 2N subcarriers, with remaining subcarriers set to zero). 
         [0041]    More generally, mapping should be in accordance with the ratio of the “original” subcarrier spacing divided by the new (smaller) subcarrier spacing. For example, if the subcarrier spacing were only one-third the size of the “original” subcarrier spacing, the N modulation symbols, S k , would be mapped to every third one of a group of 3N subcarriers, with remaining subcarriers set to zero. 
         [0042]    With mapping to every other one of a group of 2N subcarriers as described above, the size of the IFFT needed to convert the P-SYS MODE2  is twice as large (i.e., 2N instead of N), so the number of generated time-domain samples is also twice as large: 2N. Assume that the sequence representing P-SYS MODE2  is given by: 
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         [0000]    where S k/2  is the sequence for P-SYS MODE1 . Then the time domain signal obtained from the exemplary P-SYS MODE2  after IFFT processing is 
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         [0000]    Now, substituting k→2k′ and letting k′ run from 0 to N−1 (k then runs from 0 to 2N−1 populating only even values), we obtain: 
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         [0000]    which is exactly two instances of the original signal (i.e., P-SYS MODE1 ) generated in contiguous succession. 
         [0043]    With this P-SyS structure the presence or absence of multiple instances of the P-SyS waveform indicates which of the subcarrier spacings is being used: only one instance means that the subcarrier spacing is 15 kHz, two (or in other embodiments, more) instances means that the subcarrier spacing is 7.5 kHz. Since the P-SyS waveform for the 7.5 kHz case is the same as the P-SyS waveform for the 15 kHz case but transmitted multiple times (preferably two times), the same synchronization circuitry in the UE is capable of detecting the P-SyS for both subcarrier spacings: In the case of a 15 kHz subcarrier spacing, the synchronization metric has one peak and in the case of a 7.5 kHz subcarrier spacing, the synchronization metric poses multiple peaks. 
         [0044]    In practice, the synchronization metric peaks are typically made up of a number of clustered peaks due to the radio channel delay spread (i.e., the clustered peaks correspond to the entire power delay profile—“PDP”). However, in a typical LTE system, the delay spread is on the order of a few microseconds (e.g., worst case up to about 15-20 microseconds), so the P-SYS MODE1  waveform can be detected. An exemplary correlation results waveform  401  obtained when a received signal containing a P-SYS MODE1  is correlated against the known P-SYS MODE1  is illustrated in  FIG. 4A . By contrast, the multiple peaks that occur when a received signal containing a P-SYS MODE2  is correlated against the known P-SYS MODE1  (i.e., the correlation results obtained in the 7.5 kHz subcarrier spacing case) are separated by a time distance that is on the order of 65-70 microseconds (corresponding to one OFDM symbol, in the 15 kHz subcarrier spacing case). The correlation results  403  obtained for the 7.5 kHz subcarrier spacing case are illustrated in  FIG. 4B . Because of the relatively large separation between the multiple peaks associated with the 7.5 kHz subcarrier spacing, they are easily distinguishable from multiple peaks associated with typical delay spread. 
         [0045]      FIG. 5  is a block diagram of relevant components  500  in a transmitter for generating a P-SyS signal in accordance with aspects of the invention. In this exemplary embodiment, it is assumed that n=2 (i.e., to indicate the smaller (e.g., 7.5 kHz) subcarrier spacing, the synchronization signal sequence elements are mapped to every other subcarrier, with zeros mapped in-between. A synchronization signal generator  501  generates the sequence, S k , and supplies these to the input of a switch. The switch  503  is controlled by a signal  505  that indicates whether the subcarrier spacing is the smaller (e.g., 7.5 kHz) or the larger (e.g., 15 kHz) of the two possible values. When the smaller of the two possible spacings is indicated by the signal  505 , the switch  503  routes the sequence, S k , to logic  507  that inserts zeros in-between the S k  symbols. In this example, only one zero is inserted in-between two S k  symbols, but in other embodiments, the logic  507  that inserts zeros in-between the S k  symbols can insert more than one zero between S k  symbols, depending upon whether it is desired to generate more than two instances of the basic P-SyS waveform. The output of the logic  507  that inserts zeros in-between S k  symbols is supplied to those inputs of an Inverse Fast Fourier Transform (IFFT)  509  associated with the P-SyS. 
         [0046]    If the signal  505  indicates that the larger of the two possible subcarrier spacings is in effect, then the switch  503  causes the S k  symbols to be routed directly to the corresponding inputs of the IFFT  509 —the logic  507  that inserts zeros in-between S k  symbols is, in this instance, bypassed. 
         [0047]    A data unit  511  generates other data that is supplied to other inputs of the IFFT  509 . The signal  505  is supplied to a control input of each of the data unit  511  and the IFFT  509  to control, in the case of the data unit  511 , how many symbols are generated, and, in the case of the IFFT  509 , how many input symbols will be processed and the effective size of the IFFT  509 . 
         [0048]    The output of the IFFT  509  is the OFDM modulated signal that will be transmitted during the OFDM symbol time associated with the P-SyS. 
         [0049]      FIG. 6  is a block diagram of relevant components of an exemplary synchronization channel receiver  600  in a UE that is capable of detecting the indication of subcarrier spacing in accordance with another aspect of embodiments consistent with the invention. It is assumed that the supplied signal, Y t , is supplied by a filter that substantially eliminates frequency components lying outside the allocated spacing of the P-SyS. The synchronization channel receiver  600  includes a matched filter (correlator)  601  that is matched to one period of the P-SyS. The output (D t ) of the matched filter  601  is supplied to a control unit  603  that analyzes the matched filter output signal and generates therefrom the 5 ms timing information and a signal that indicates whether the detected cell is using a 7.5 kHz or 15 kHz subcarrier spacing. As described earlier, the control unit  603  in this example bases this decision on whether only a single P-SyS response was detected (i.e., P-SYS MODE1 ), or whether multiple (e.g., two) P-SyS power delay profiles (i.e., P-SYS MODE2 ) were detected. 
         [0050]      FIG. 7  is a flow chart of exemplary steps that can be carried out by circuitry (e.g., the control unit  603 ) in a UE that detects the indication of subcarrier spacing in accordance with another aspect of embodiments consistent with the invention. A signal is received (step  701 ) and processed by a matched filter, which correlates the received signal to a known P-SYS MODE1  (step  703 ). The results from this processing are then analyzed by, for example, a control unit that tests whether the correlation result includes distinct peaks separated by a distance greater than an expected delay spread associated with the radio channel (PDP max ) (decision block  705 ). A value of PDP max  can, for example, be 20 microseconds. If distinct peaks are not detected (“NO” path out of decision block  705 ), then 15 kHz subcarrier spacings have been detected, and processing proceeds accordingly (step  707 ). Otherwise (“YES” path out of decision block  705 ), 7.5 kHz subcarrier spacings have been detected, and processing proceeds accordingly (step  709 ). 
         [0051]    Alternative subcarrier spacing detection algorithms can also be implemented for use in a UE. In one such example, a differential correlator correlates the received signal with a delayed copy of itself. In this case, the delay corresponds to the time period of the P-SyS for the larger subcarrier spacing (i.e., P-SYS MODE1 ) (e.g., 1/15 kHz=66 μsec), and the integration time can be any duration larger than the longest cyclic prefix and up to the time period of the original P-SyS—e.g., 1/15 kHz. When the time domain representation of the synchronization signal comprises more than one instance of the P-SYS MODE1 , the correlator results will include a distinct peak. Such a detector is similar to a blind cyclic prefix estimator and therefore the smallest integration time should be larger than the longest cyclic prefix, otherwise a cyclic prefix might be detected rather than a repetition. 
         [0052]    The proposed P-SyS structure enables a very simple structure and technique for detecting subcarrier spacing. No additional synchronization channel detection circuitry is needed for the smaller (e.g., 7.5 kHz) subcarrier spacing because the synchronization channel detection circuitry matched to the larger (e.g., 15 kHz) subcarrier spacing automatically also detects the P-SyS when 7.5 kHz subcarrier spacings are used. 
         [0053]    The invention has been described with reference to particular embodiments. However, it will be readily apparent to those skilled in the art that it is possible to embody the invention in specific forms other than those of the embodiment described above. 
         [0054]    For example, the exemplary embodiments described above assumed that only two different subcarrier spacings needed to be indicated and detected. However, alternative embodiments could be designed, using the principles described above, in which more than two different subcarrier spacings are indicated and detected. In such cases, each of the possible subcarrier spacings can be uniquely associated with a different number of instances of the “original” P-SyS waveform, for example one instance of the waveform to indicate use of a first subcarrier spacing, two instances of the waveform to indicate use of a second subcarrier spacing, three instances of the waveform to indicate use of a third subcarrier spacing, and so on. 
         [0055]    Also, the exemplary embodiments described above assumed that one of the subcarrier spacings (e.g., 15 kHz) was associated with the occurrence of only one instance of the “original” P-SyS waveform. However, this is not essential to the invention. To the contrary, the invention can more generally be defined as involving a number of different subcarrier spacings wherein each is uniquely associated with one of a like number of possible P-SyS waveforms, each of the possible P-SyS waveforms having a different number of instances of a “basic” waveform, thereby making them distinguishable from one another. For example, in an embodiment involving two possible subcarrier spacings (e.g., 15 kHz and 7.5 kHz), a first of these subcarrier spacings can be indicated by two occurrences of a “basic” waveform, and a second of these subcarrier spacings can be indicated by four occurrences of the “basic” waveform. This can be achieved by, for example, mapping the S k  symbols onto every other subcarrier (with zeros inserted in-between) for the first of the subcarrier spacings, and mapping the S k  symbols onto every fourth subcarrier (with zeros inserted in-between) for the second of the subcarrier spacings. 
         [0056]    In yet another exemplary embodiment involving two possible subcarrier spacings (e.g., 15 kHz and 10 kHz), a first of these subcarrier spacings can be indicated by two occurrences of a “basic” waveform and a second of these subcarrier spacings can be indicated by three occurrences of the “basic” waveform. To implement this, the S k  symbols are mapped onto every second subcarrier (with zeros inserted in-between) for the case of 15 kHz subcarrier spacing, and are mapped onto every third subcarrier (with zeros inserted in-between) for the case of 10 kHz. This works out because in the 15 kHz subcarrier spacing mode, the OFDM symbol length is 1/15 kHz=66.67 μs. When the S k  symbols are placed on every second subcarrier, the “effective” subcarrier spacing is 2×15=30 kHz, and the duration of the “basic” P-SyS waveform is 1/30 kHz=33.33 μs. Therefore, in one OFDM symbol (=66.67 μs) two basic waveforms are contained. 
         [0057]    For the 10 kHz subcarrier spacing mode, the OFDM symbol length is 1/10 kHz=100 μs. When the S k  symbols are placed on every third subcarrier (with zeros inserted in-between), the “effective” subcarrier spacing is 3×10=30 kHz, and the duration of the “basic” P-SyS waveform is 1/30 kHz=33.33 μs. Therefore, in one OFDM symbol (=100 μs) three basic waveforms are contained. That three instances of the basic waveform will be produced by placing the S k  symbols onto every third subcarrier can be shown by equations analogous to equations 2 through 5. 
         [0058]    Accordingly, the described embodiments are merely illustrative and should not be considered restrictive in any way. The scope of the invention is given by the appended claims, rather than the preceding description, and all variations and equivalents which fall within the range of the claims are intended to be embraced therein.