Abstract:
Disclosed is a method of, and apparatus for, processing a Random Access Channel (RACH) burst comprising a processed preamble group having at least two processed preambles. The processed preambles may be derived from short preambles, i.e., preamble comprising less than 887 bits, such that the RACH burst may be detected over shorter coherent accumulation time intervals relative to prior art coherent accumulation time intervals, thereby improving RACH burst detection. A transmitter generates a RACH burst comprising two or more processed preambles and transmits the RACH burst to a receiver. The receiver processes the RACH burst by correlating the two or more processed preambles to a plurality of reference signals in a frequency domain to produce a set of two or more frequency domain correlated outputs for each of the plurality of reference signals. The RACH burst is then detected based on energy associated with at least one frequency domain correlated output in the set of two or more frequency domain correlated outputs to a threshold energy value.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to wireless communication systems and, in particular, to random access channels. 
       BACKGROUND OF THE INVENTION 
       [0002]    To increase system capacity, Universal Mobile Telecommunication System (UMTS) based wireless communication systems will change from a Code Division Multiple Access (CDMA) air interface to an Orthogonal Frequency Division Multiple Access (OFDMA) air interface. The OFDMA air interface will comprise a plurality of orthogonal sub-carrier frequencies. A subset of the plurality of orthogonal sub-carrier frequencies will support a non-synchronized Random Access Channel (RACH) for User Equipments (UE) to use when initially accessing the wireless communication system and performing uplink synchronization, among other things. 
         [0003]    Several possible structures for the non-synchronized RACH have been proposed. In one proposal, the non-synchronized RACH comprises an access slot occupying, in terms of time and frequency space, a transmission time interval (TTI) of 1.0 ms and a 1.25 MHz bandwidth, respectively. The access slot comprises a first gap period, a preamble period and a second gap period. The first gap period corresponds to a time interval T DS1  associated with a maximum delay spread or multi-path delay. The preamble period corresponds to a preamble transmission time interval T P1 . The second gap period corresponds to the time interval T DS1  plus a time interval T GP1  associated with a maximum round trip propagation delay between a Node B and a UE (which is within a cell associated with the Node B). 
         [0004]    Bursts are transmitted over the non-synchronized RACH (referred to herein as “RACH bursts”) by UEs attempting, for example, to initially access the system. Each RACH burst comprises a first gap sequence, a processed preamble and a second gap sequence. The first gap sequence comprises N DS1  zero samples, wherein N DS1 ≧1. The first gap sequence is transmitted over the first gap period. The second gap sequence comprises N DS1  plus N GP1  zero samples, wherein N GP1 ≧1. The second gap sequence is transmitted over the second gap period. 
         [0005]    The processed preamble is derived from a preamble comprising approximately 887 bits. Specifically, the processed preamble is obtained by processing the preamble in accordance with at least a discrete Fourier Transform (DFT) operation and an Inverse Fast Fourier Transformer (IFFT) operation. The processed preamble comprises N P1  samples and is transmitted over the preamble period. 
         [0006]    The RACH burst is detected at the Node B using periodic correlation during which the RACH burst is coherently integrated over a coherent accumulation time interval equal to the length of the preamble. Increasing the coherent accumulation time interval enhances processing gain, thereby improving RACH burst detection. Thus, it is desirable to use preambles with long lengths, e.g., 877 bits, to derive the RACH bursts because it increases the coherent accumulation time interval. 
         [0007]    However, under fading conditions, phase offsets are introduced into the RACH burst detection process. Such phase offsets can cause degradation in RACH burst detection. The amount of phase offset introduced will depend, in part, on the length of the coherent accumulation time interval. As the coherent accumulation time interval increases, so does the phase offset. And as the phase offset increases, so does degradation of detection performance. 
       SUMMARY OF THE INVENTION 
       [0008]    An embodiment of the present invention is a method of, and apparatus for, processing a Random Access Channel (RACH) burst comprising a processed preamble group having at least two processed preambles. The processed preambles may be derived from short preambles, i.e., preamble comprising less than 887 bits, such that the RACH burst may be detected over shorter coherent accumulation time intervals relative to prior art coherent accumulation time intervals, thereby improving RACH burst detection under fading conditions. In one embodiment, a transmitter generates a RACH burst comprising two or more processed preambles and transmits the RACH burst to a receiver. The receiver processes the RACH burst by correlating the two or more processed preambles to a plurality of reference signals in a frequency domain to produce a set of two or more frequency domain correlated outputs for each of the plurality of reference signals. The RACH burst is then detected based on energy associated with at least one of the frequency domain correlated outputs to a threshold energy value. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
           [0010]      FIG. 1  depicts a wireless communication system used in accordance with the present invention; 
           [0011]      FIG. 2  depicts a non-synchronized RACH used in accordance with an embodiment of the present invention; 
           [0012]      FIG. 3  depicts a transmitter used in accordance with one embodiment of the present invention; and 
           [0013]      FIG. 4  depicts a receiver used in accordance with one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    For purposes of illustration, the present invention will be described herein with reference to  FIG. 1 , which depicts a wireless communication system  100  used in accordance with the present invention. Wireless communication system  100  comprises a Node B  110  and a User Equipment (UE)  120 . An Orthogonal Frequency Division Multiple Access (OFDMA) air interface is used for communications between Node B  110  and UE  120 , for example, as described in the well-known Universal Mobile Telecommunication System (UMTS) standard specification. 
         [0015]    The OFDMA air interface comprises N SYS  orthogonal sub-carrier frequencies, N SYS &gt;1. A subset of N RACH  orthogonal sub-carrier frequencies is used to support a non-synchronized Random Access Channel (RACH), where N SYS &gt;N RACH . A burst is transmitted by UE  120  to Node B  110  over the non-synchronized RACH when UE  120  attempts, for example, to initially access wireless communication system  100 . Such burst is referred to herein as a “RACH burst.” 
         [0016]      FIG. 2  depicts a non-synchronized RACH  200  used in accordance with another embodiment of the present invention. The non-synchronized RACH  200  comprises an access slot  210  for transmitting a RACH burst  220 . Access slot  210 , in terms of time and frequency space, occupies a time interval T AS  and a bandwidth f BW , which includes N SYS  orthogonal sub-carriers. In one embodiment, time interval T AS  is 1.0 ms and the bandwidth f BW  is an integer multiple of 1.25 MHz. 
         [0017]    Access slot  210  comprises a cyclic prefix (CP) period  260 , a preamble period  270  and a gap period  280 . Preamble period  270  corresponds to at least a time interval T P . CP period  260  and gap period  280  both correspond to at least a time interval T DS  plus a time interval T GP , i.e., CP period=gap period=T DS +T GP , wherein time interval T DS  corresponds to a maximum delay spread and time interval T GP  corresponds to a maximum round trip propagation delay between Node B  110  and UE  120  (which is within a cell associated with Node B  110 ). In one embodiment, time interval T DS  is based on a typical urban environment, such as the maximum delay spread used for GSM TU power-delay profile. 
         [0018]    RACH burst  220  comprises N AS  samples, which include a CP  230 , a processed preamble group  240  and a gap sequence  250 . CP  230  comprises N DS  plus N GP  samples from processed preamble group  240 , wherein N DS , N GP ≧1. In one embodiment, the N DS  plus N GP  samples are taken from the end of processed preamble group  240 . Gap sequence  250  comprises N DS  plus N GP  zero samples. CP  230  and gap sequence  250  are transmitted over CP period  260  and gap period  280 , respectively. 
         [0019]    Note that in another embodiment, CP  230  comprises N DS  plus N GP  zero samples. In yet another embodiment, CP  230  comprises N DS  zero samples or N DS  samples from processed preamble group  240 , and CP period  260  corresponds to at least time interval T DS . 
         [0020]    Processed preamble group  240  comprises z processed preambles, where z is a repetition factor and is greater than or equal to 2. Each processed preamble comprises N P  samples and is derived from a short preamble (or other sequence with good auto and cross correlation properties) of length L, where N P  ≧1. For purposes of this application, the term “short preamble” should be construed to include preambles comprising less than 887 bits, i.e., L&lt;887. In one embodiment, the number of bits comprising the short preamble is a prime number less than 887, such as 449 and 223. The short preamble can be a CAZAC sequence, such as a Generalized Chirp Like (GCL) sequence or a Zadoff-Chu with zero cross correlation zone (ZCZ) sequence. 
         [0021]    Each processed preamble is derived by processing the short preamble in accordance with at least a DFT operation and an IFFT operation. In one embodiment, processed preamble group  240  comprises at least two processed preambles derived from a same short preamble. The two processed preambles may be exactly identical or inverse versions of each other. 
         [0022]    Parameters N DS , N P  and N GP  are dependent upon a variety of factors including, for example, bandwidth, sampling rate and access slot, among others. Table 1 depicts transmission parameters for the non-synchronized RACH for various bandwidths and sampling rates in accordance with one embodiment of the present invention. In this embodiment, non-synchronized RACH  210  comprises an access slot of 1.0 ms duration, RACH burst  220  is derived from a short preamble comprising 449 bits, and processed preamble group  240  includes two processed preambles, i.e., z=2. A maximum cell radius of 13.4 km and a typical urban environment in accordance with a GSM TU profile are assumed for calculating T GP  and T DS , respectively. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Sampling 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Bandwidth 
                 Rate 
                 T P   
                 T DS   
                 T GP   
               
               
                 (MHz) 
                 (MHz) 
                 (μs) 
                 (μs) 
                 (μs) 
                 N AS   
                 L 
                 N P   
                 N DS   
                 N GP   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1.25 
                 1.92 
                 800 
                 5.2 
                 94.8 
                 1920 
                 449 
                 768 
                 10 
                 182 
               
               
                 2.5 
                 3.84 
                 800 
                 5.2 
                 94.8 
                 3840 
                 449 
                 1536 
                 20 
                 364 
               
               
                 5 
                 7.68 
                 800 
                 5.2 
                 94.8 
                 7680 
                 449 
                 3072 
                 40 
                 728 
               
               
                 10 
                 15.36 
                 800 
                 5.2 
                 94.8 
                 15360 
                 449 
                 6144 
                 80 
                 1456 
               
               
                 20 
                 30.72 
                 800 
                 5.2 
                 94.8 
                 30720 
                 449 
                 12288 
                 160 
                 2912 
               
               
                   
               
             
          
         
       
     
         [0023]    UE  120  includes a transmitter for transmitting RACH burst  220  of  FIG. 2 .  FIG. 3  depicts a transmitter  300  used in accordance with one embodiment. Transmitter  300  comprises a preamble generator  310 , a block repeater  320 , a serial to parallel (S/P) converter  330 , a Discrete Fourier Transform (DFT) precoder  340 , a RACH mapper  350 , an Inverse Fast Fourier Transformer (IFFT)  360 , a parallel to serial (P/S) converter  370 , and a CP and gap inserter  380 . These elements may be implemented, for example, through dedicated or shared hardware including, but not limited to hardware capable of executing software. 
         [0024]    Preamble generator  310  is a device for generating a short preamble  315 . In one embodiment, the preamble generator  310  selects short preamble  315  from a set comprising S short preambles, wherein S&gt;1 and each of the short preambles in the set has a bit length L. Short preamble  315  is provided as input to block repeater  320 . Short preamble  315  is repeated z times by block repeater  320  to produce block repeater output  325  comprising z short preambles. In one embodiment, all z short preambles in block repeater output  325  are identical. In another embodiment, at least one of the short preambles in block repeater output  325  is identical to inputted short preamble  315  and at least one of the short preambles in block repeater output  325  is an inverse version of inputted short preamble  315 . 
         [0025]    S/P converter  330  converts block repeater output  325  from a serial stream to a S/P output  335  comprising z sets of N DFT  parallel streams, where N DFT =N RACH . DFT precoder  340  performs a discrete Fourier transform to convert S/P output  335  from the time domain into the frequency domain and produce a DFT precoder output  345  comprising z sets of N DFT  parallel streams of frequency domain signals. RACH mapper  350  maps each set of N DFT  parallel streams to the subset of N RACH  orthogonal sub-carrier frequencies which support the non-synchronized RACH. RACH mapper  350  produces a RACH mapper output  355  comprising z sets of N IFFT  parallel streams of mapped frequency domain signals, where N IFFT =N SYS . The z sets of N IFFT  parallel streams include z sets of N DFT  occupied orthogonal sub-carriers and z sets of N IFFT -N DFT  unoccupied orthogonal sub-carriers which have been mapped to zero samples. 
         [0026]    IFFT  360  performs an inverse fast Fourier transform to convert RACH mapper output  355  from the frequency domain to the time domain. IFFT  360  produces an IFFT output  365  comprising z sets of N IFFT  parallel streams of samples. P/S converter  370  converts IFFT output  365  into a P/S output  375  comprising a serial stream of z sets of N P  samples or processed preambles, i.e., processed preamble group  240 , where N P =N SYS . CP and gap inserter  380  appends CP  230  and gap sequence  250  to P/S output  375  to produce RACH burst  220 , which is subsequently transmitted by a radio transmitter interface at transmitter  300 , not shown. In one embodiment, CP  230  is added to the beginning of P/S output  375 , and gap sequence  250  is added to the end of P/S output  375 . 
         [0027]    The transmitted RACH burst  220  of  FIG. 2  is received by a receiver at Node B  110 .  FIG. 4  depicts a receiver  400  used in accordance with one embodiment. Receiver  400  comprises a preprocessor  410 , a block partitioner  420 , a frequency domain correlator  430  and an energy detector  440 . These elements may be implemented, for example, through dedicated or shared hardware including, but not limited to hardware capable of executing software. 
         [0028]    RACH burst  220  is received by a radio receiver interface, not shown, at receiver  400 . Preprocessor  410  removes CP  230  from received RACH burst  220  to produce preprocessor output  415 . Specifically, preprocessor  410  removes from the received RACH burst  220  a fixed number of samples corresponding to the CP samples. 
         [0029]    Block partitioner  420  partitions preprocessor output  415  into a set of z blocks  425 , wherein each block  425  comprises N P  samples of preprocessor output  415 . The set of z blocks  425  is provided as input to frequency domain correlator  430 , which is a block-wise processor for correlating the set of z blocks  425  to a plurality of reference signals in the frequency domain, thereby producing frequency correlated outputs. 
         [0030]    Frequency domain correlator  430  comprises a Fast Fourier Transformer (FFT)  450 , a RACH selector  460 , a multiplier  470 , a plurality of reference signal generators  480 , and an Inverse Discrete Fourier Transformer (IDFT)  490 . FFT  450  converts each block  425  from the time domain into the frequency domain to produce a set of z FFT outputs  455 , wherein each FFT output  455  comprises N FFT  parallel streams of frequency domain signals and N FFT =N SYS . RACH selector  460  selects, from each FFT output  455 , the N RACH  streams corresponding to the orthogonal sub-carrier frequencies which support the non-synchronized RACH. RACH selector  460  outputs a set of z RACH selector outputs  465 , wherein each RACH selector output  485  comprises a set of N RACH  streams. 
         [0031]    Multiplier  470  multiplies each RACH selector output  465  with a reference signal  485  provided by one of the plurality of reference signal generators  480  to produce a set of z multiplier outputs  475  for each reference signal  485 , wherein each multiplier output  475  comprises parallel streams of multiplied signals, i.e., frequency domain signals multiplied with a reference signal. Note that the same reference signal  485  will be used to process, i.e., multiply, the entire set of z RACH selector outputs  465 . After the entire set of z RACH selector outputs  465  have been processed with that reference signal  485 , then another reference signal  485  will be used to multiple the same set of z RACH selector outputs  465 . Such iterative processing may continue until the set of z RACH selector outputs have processed with each reference signal  485 . 
         [0032]    In one embodiment, the number of reference signal generators  480  is equal to S, i.e., number of short preambles in the set of short preambles. Each of the plurality of reference signal generators  480  comprises a FFT  540  and a conjugate module  550 . A different short preamble (from the set of S short preambles) is used by each of the reference signal generators  480  to generate a different reference signal  485 . In each reference signal generator  480 , a short preamble is transformed by FFT  540  from the time domain into the frequency domain to produce a FFT output  545 , i.e., frequency domain representation of the short preamble. Conjugate module  550  converts FFT output  545  into reference signal  485 , which is a complex conjugate representation of FFT output  545 . Alternately, reference signals  485  may be pre-computed and stored in some buffer to reduce the amount of real-time computation. 
         [0033]    For each set of z multiplier outputs  475  associated with a same reference signal  485 , IDFT  490  converts each multiplier output  475  in that set from the frequency domain to the code domain to produce a set of z IDFT outputs  495 , wherein each IDFT output  495  comprises correlation values corresponding to the delay spread. Such set of z IDFT outputs  495  corresponds to a set of frequency domain correlated outputs for a particular reference signal  485 . 
         [0034]    The set of z IDFT outputs  495  (associated with a same reference signal  485 ) is provided as input to energy detector  440  for determining whether a RACH burst has been received. Energy detector  440  comprises a search window limiter  500 , an energy module  510 , a summer  520  and a threshold module  530 . Search window limiter  500  limits each IDFT output  495 , in terms of time, to a search window size corresponding to time interval T GP  plus time interval T DS  to produce a set of z limited outputs  505 . Alternately, the search window size corresponds to time interval T GP  or time interval T DS . 
         [0035]    Energy module  510  determines an amount of energy associated with each limited output  505 , for example, by squaring a magnitude or gain value associated with that particular limited output  505 . A set of z energy outputs  515  is produced by energy module  510  for the set of z limited outputs  505 . In summer  520 , two or more energy outputs  515  in the same set of z energy outputs  515  (associated with a same reference signal  485 ) are summed together to produced a summer output  525 . Threshold module  530  determines whether a RACH burst is present by comparing summer output  525  to a threshold energy value. If summer output  525  is greater than the threshold energy value, then a RACH burst is deemed detected. If summer output  525  is not greater than the threshold energy value, then energy detector  440  checks the next set of z IDFT outputs  495  (i.e., IDFT outputs  495  associated with another reference signal  485 ) to determine whether a RACH burst has been received. 
         [0036]    Note that, in an alternate embodiment, energy detector  440  does not include summer  520 . In such an embodiment, individual energy outputs  515  are compared to the threshold energy value to determine whether a RACH burst has been received. 
         [0037]    Although the present invention has been described in considerable detail with reference to certain embodiments, other versions are possible. Therefore, the spirit and scope of the present invention should not be limited to the description of the embodiments contained herein.