Abstract:
Rapid uplink synchronization is enabled by reducing a 2D search problem to two 1D search problems, which can generally be performed in less time. Advantage is taken of fact that a mobile device sends a ranging code on multiple sub-carriers. Using the assumption that adjacent sub-carriers will have approximately equivalent channel characteristics, phase ambiguity can be removed by differentially combining pairs of adjacent sub-carriers. Once the phase ambiguity is removed, the code, timing, and power level may be determined relatively quickly. In one embodiment, the values of correlations between received signals and possible codes are compared with a threshold.

Description:
RELATED APPLICATIONS 
     This application is related to and claims priority to Chinese Application No. 200610160842.9 filed Nov. 30, 2006 entitled “SYSTEMS AND METHODS FOR RAPID UPLINK AIR INTERFACE SYNCHRONIZATION”, the disclosure of which is hereby incorporated herein by reference. 
     TECHNICAL FIELD 
     This invention relates to air interface communication systems synchronization between base stations and mobile devices and more particularly to rapid uplink synchronization based on signals sent from the mobile devices. 
     BACKGROUND OF INVENTION 
     In wireless (air interface) communication systems, signals transferred from a plurality of mobile devices arrive at the base station with different propagation delays and different power. Large propagation delay and power difference often result in significant loss of signal at the base station. One method for the base station to control the propagation delays and power levels of the signal from mobile devices is to have each mobile device send a predetermined pseudo random code identifying itself on a defined ranging time slot or channel. These codes, or ranging signals are used by the base station (which could include any suitable distant end transmission point) to determine the time delay and transmission power level of the mobile device. 
     Since the base station does not know which code is being sent by the mobile device, the base station must isolate the sub-channel codes for each mobile device. One method of isolating the code from a mobile device is to match the incoming signal against a known signal in order to determine which code is being sent. However, because there are many possible codes and because they are not arriving at the base station with a known time (phase), the solution to the problem becomes a two-dimensional calculation, i.e., first the system must check to see if the signal contains a known code at a first time (first phase). If not, then the system must repeat the process for successive time slices (phases) to see if a particular code is being received. This is time consuming and requires high processor resources. Besides, the channel phase ambiguities acting on ranging channel will significantly deteriorate the measurement precise of propagation delays. 
     BRIEF SUMMARY OF THE INVENTION 
     A two-dimensional (2D) search problem is reduced to two one-dimensional (1D) search problems, which can generally be performed in less time. Advantage is taken of fact that each mobile device sends the randomly selected ranging code on multiple sub-channels. In Orthogonal Frequency Division Multiple Access (OFDMA) and Orthogonal Frequency Division Multiplexing (OFDM) systems, the ranging channel is often composed of a group of adjacent sub-carriers. An assumption can then be made that adjacent sub-carriers (because they are close in frequency and other characteristics) will have approximately (although not necessarily) same channel characteristics. By differentially multiplying pairs of adjacent received ranging sub-carriers, the channel phase ambiguity can be removed between those sub-carriers. Power levels for each ranging code can be calculated by correlating the differential received ranging sub-carriers with local predetermined differential ranging codes. All the ranging codes with power meeting a predetermined threshold are selected as the ranging codes transmitted from the mobile devices. Time delay measurement is then performed only for the selected ranging codes. Since in most cases the selected ranging codes belong to a subset of the total ranging codes, the computing complexity may be reduced. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
         FIG. 1  illustrates one embodiment of a flow chart for obtaining uplink synchronization for air interface communication between a base station and a mobile device; and 
         FIG. 2  shows a typical air interface system in which the concepts of the invention can be practiced. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates one embodiment of flow chart  10  for obtaining uplink synchronization for air interface communication ( 23   FIG. 2 ) between a base station, such as transmission point  201 , shown in  FIG. 2 , and a mobile device, such as device  21 - 1  shown in  FIG. 2 . The algorithm shown in  FIG. 1  can be run, for example, by processor  241  in conjunction with memory  242  operating in base station  24  which can be co-located with the actual point of transmission, or can be remote there from. 
     Process  101  extracts the ranging sub-carriers from the signal received from the mobile device from time to time. In effect, the ranging sub-carriers are separated from the other data, such as the payload data, etc. Each ranging channel is composed of multiple sub-carriers. For example, in an OFDMA system  144  ranging sub-carriers may be specified. The concepts discussed herein are particularly well-suited for OFDMA as well as OFDM systems. A mobile device selects a pseudo-random code and transmits that code on all of the ranging sub-carriers. The pseudo-random code identifies the mobile device, and the base station then determines, as will be discussed below, the average power level of each received random code in order to determine whether the mobile device which transmitted that code should adjust its transmission power. 
     Process  102  then differentially multiplies the adjacent ranging sub-carriers. An approximation may be made that channels of adjacent sub-carriers are coherent, in the sense that phase characteristics will be approximately same between adjacent sub-carriers. This approximation is useful if sub-carrier spacing is smaller than the channel coherent bandwidth. Channel phase rotation on each sub-carrier may then be removed by multiplying a specific sub-carrier with the conjugation of an adjacent sub-carrier. 
     Process  103  multiplies differential ranging sub-carriers with local replicas of the possible differential ranging codes, which may be pre-calculated and stored in memory  242  shown in  FIG. 2 , and then sums the multiplication results. This produces a correlation between the possible ranging codes and the ranging code transmitted by the mobile device. The correlation value of the ranging code transmitted by the mobile device will be highest value. In this manner, the correlation values can be used by base station  24 , as will be discussed below, to isolate ranging codes for each mobile device. 
     Process  104  calculates the power of the correlation values for the convenience of threshold comparison in a following process. Process  105  determines which ranging codes are transmitted by mobile devices. All of the ranging codes with power exceeding the predetermined threshold will be selected as the transmitted ranging codes. If no acceptable power level is found, the mobile device can be told to increase its power and transmit another ranging code in subsequence time frame. 
     At this point, the code has been identified for certain mobile device. Process  106  compares the power levels of the selected ranging codes with target power levels and thus determines the power adjustment value for that mobile device in subsequent transmissions. Process  107  calculates time delay using several methods, such as, for example, phase detection, inverse FFT or sine wave correlation. This time delay corresponds to the round trip delay between base station and mobile device, and the mobile device can use this value to adjust its transmission time in subsequent frames. 
     Transmitted ranging codes may be shown as:
 
 X   t ( k,l )ε{−1,1}
 
where k is ranging sub-carrier index (k=1, . . . , K) and l is ranging sequence index (l=1, . . . , L).
 
     Received ranging codes in frequency domain is: 
                 X   r     ⁡     (     k   ,   l     )       =         X   t     ⁡     (     k   ,   l     )       ·     H   ⁡     (     k   ,   l     )       ·     ⅇ       -   j     ⁢       2   ⁢   π   ⁢           ⁢   k   ⁢           ⁢     τ   ⁡     (   l   )         N                 
where H(k,l) is the complex channel transfer function of the k-th sub-carrier of the l-th transmitted ranging sequence, τ(l) is propagation delay corresponding to the l-th ranging sequence and N is the sub-carrier number.
 
     
       
         
           
             
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                   X   r     ⁡     (       k   +   1     ,   l     )       ·       X   r   *     ⁡     (     k   ,   l     )         =         X   t     ⁡     (       k   +   1     ,   l     )       ·       X   t   *     ⁡     (     k   ,   l     )       ·     H   ⁡     (       k   +   1     ,   l     )       ·       H   *     ⁡     (     k   ,   l     )       ·     ⅇ       -   j     ⁢       2   ⁢   π   ⁢           ⁢   k   ⁢           ⁢     τ   ⁡     (   l   )         N                 
Assuming adjacent channels are coherent, we have:
 
                   X   r     ⁡     (       k   +   1     ,   l     )       ·       X   r   *     ⁡     (     k   ,   l     )         =         X   t     ⁡     (       k   +   1     ,   l     )       ·       X   t   *     ⁡     (     k   ,   l     )       ·            H   ⁡     (     k   ,   l     )            2     ·     ⅇ       -   j     ⁢       2   ⁢   π   ⁢           ⁢   k   ⁢           ⁢     τ   ⁡     (   l   )         N                 
Multiplying received differential ranging codes with local differential ranging codes and summing the results yields:
 
                     R   ⁡     (     l   ,     l   ′       )       =       ∑   k     ⁢     [           X   r     ⁡     (       k   +   1     ,   l     )       ·     X   r   *       ⁢       (     k   ,   l     )     ·       X   t     ⁡     (       k   +   1     ,     l   ′       )       ·       X   t   *     ⁡     (     k   ,     l   ′       )           ]                   l   ′     =   1     ,   2   ,   …   ⁢           ,   L               
Power is then:
 
 P ( l,l ′)=| R ( l,l ′)| 2  
 
Maximum P(l,l′) can be obtained when l′=l, i.e.:
 
     
       
         
           
             
               
                 
                   
                     
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     Therefore, selection of ranging codes sequences with power P exceeding the predetermined threshold P th  may be shown as:
 
 L   s   ={l′:P ( l,l ′)&gt; P   th }
 
One way to calculate the transmit time is the Inverse Fast Fourier Transform (IFFT) method. The channel impulse response of a ranging channel is calculated by:
 
                     h   ⁡     (     l   ,     t   -   τ       )       =       IFFT   k     ⁡     (         X   r     ⁡     (     k   ,   l     )       ·       X   t     ⁡     (     k   ,   l     )         )                     =       IFFT   k     (       H   ⁡     (     k   ,   l     )       ·     ⅇ       -   j     ⁢       2   ⁢   π   ⁢           ⁢   k   ⁢           ⁢     r   ⁡     (   l   )         N           )       ⁢                             l   ∈     L   s           
Transmit time delay τ may be obtained based on the first path of the channel impulse response. However, this method requires an IFFT operation, which may be time consuming and resource intensive. Another way to estimate the transmit time delay is the phase detection method, which calculates phase rotation θ of a differential correlation value and obtains transmit time delay τ from:
 
     
       
         
           
             
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     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.