Abstract:
An OFDM-CDMA transmission apparatus may include a first spreader that carries out spreading processing on a plurality of transmission signals using different spreading codes, respectively. A second spreader carries out spreading processing on at least one known signal using a spreading code different from the spreading codes employed by the first spreader. A frequency division multiplexer breaks down the transmission signals spread by the first spreader and the known signal spread by the second spreader into individual chips and subjects these chips to frequency division multiplexing, thereby assigning one chip data signal string per subcarrier. The frequency division multiplexer operates such that information from each of the plurality of transmission signals and the known signal is multiplexed into every chip assigned to a different subcarrier. After the chips are assigned to the subcarriers, they are transmitted by a transmitter.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a communication apparatus, and more particularly, to a communication apparatus that carries out radio communications combining a CDMA (Code Division Multiple Access) system and OFDM (Orthogonal Frequency Division Multiplexing) system in mobile communications. 
   2. Description of the Related Art 
   An error rate characteristic in a communication based on a CDMA system deteriorates in a multi-path environment because of interference between spreading codes. On the other hand, a well-known communication system resistant to interference between codes is an OFDM communication that uses a guard interval. Thus, a radio communication based on an OFDM-CDMA system that implements a CDMA-based communication with multiple carriers and performs transmission with subcarriers assigned to their respective chips then subjected to frequency division multiplexing is now a focus of attention as a next-generation radio communication system. 
   In an OFDM-CDMA-based communication, a plurality of signals is spread using mutually not correlated spreading codes by assigning one spread signal to one subcarrier. If these spreading codes are completely orthogonal to each other, signals other than the necessary ones are completely removed through despreading processing at the time of reception regardless of the degree of signal multiplexing. 
   Hereinafter, a conventional OFDM-CDMA-based communication apparatus will be explained using  FIG. 1 .  FIG. 1  is a block diagram showing a configuration of a conventional OFDM-CDMA-based communication apparatus. 
   In the transmission system shown in  FIG. 1 , each spreading section  11  carries out spreading processing by multiplying transmission signals  1  to n by their respective spreading codes  1  to n. Here, suppose their spreading factor is k. 
   Addition section  12  adds up the transmission signals subjected to spreading processing. Serial/parallel (hereinafter referred to as “S/P”) converter  13  converts a serial signal to a plurality of parallel signals. This S/P converter  13  divides the transmission signals thus spread and added up by spread signal or breaks down spread transmission signals  1  to n by spread signal (chip), that is, A 1st to kth chip. 
   IFFT processing section  14  carries out inverse Fourier transform processing on a plurality of parallel signals. This IFFT processing section  14  assigns one subcarrier to one chip data signal string and carries out frequency division multiplexing. 
   That is, the number of subcarriers corresponds to the spreading factor and it is “k” in this case. Suppose the 1st chip of transmission signals  1  to n is placed in subcarrier  1  and the kth chip of transmission signals  1  to n is placed in subcarrier k. That is, a chip data string is subjected to frequency division multiplexing.  FIG. 2  shows this mode. Antenna  15  transmits/receives a radio signal. 
   In the reception system, quasi-coherent detection section  16  carries out quasi-coherent detection processing on the reception signal from antenna  15 . That is, quasi-coherent detection section  16  carries out quasi-coherent detection processing under the control of a local signal subjected to frequency offset correction from frequency offset correction section  17 , which will be described later. In this way, frequency offset correction is performed. 
   Frequency offset correction section  17  detects a frequency offset using the signal after quasi-coherent detection processing and creates a local signal based on this frequency offset. That is, frequency offset correction section  17  outputs the local signal subjected to frequency offset correction to quasi-coherent detection section  16 . 
   FFT processing section  18  carries out Fourier transform processing on the reception signal subjected to quasi-coherent detection processing and extracts each subcarrier signal (chip data signal string). Transmission path compensation sections  19  are provided in one-to-one correspondence with subcarriers and carry out compensation processing such as phase compensation on their respective subcarrier reception signals. 
   Parallel/serial (hereinafter referred to as “P/S”) converter  20  converts a plurality of parallel signals into a single serial signal. This P/S converter  20  rearranges the subcarrier signals from one chip to another and outputs the 1st chip of a signal on which spread transmission signals  1  to n are multiplexed at time t 1 , the 2nd chip of a signal on which spread transmission signals  1  to n are multiplexed at time t 2 , . . . up to the kth chip of a signal on which spread transmission signals  1  to n are multiplexed at time t k . 
   Despreading sections  21  carry out despreading processing by multiplying the reception signal which has been converted to a single serial signal by their respective spreading codes  1  to n and extracting only the signals spread using those codes. 
   However, the above OFDM-CDMA-based communication apparatus has problems as shown below. That is, if the frequency offset detected by frequency offset correction section  17  above contains a detection error, the reception signal after FFT processing contains a residual phase error. 
   This results in the reception signal after FFT processing involving phase rotation. For example, as shown in  FIG. 3 , if the frequency offset contains a detection error of Δ f, the 1st chip to kth chip corresponding to 2nd transmission signals  1  to n contain a residual phase error with 2π Δ fT. The 1st chip to kth chip corresponding to 3rd transmission signals  1  to n contain a residual phase error with 2π Δ f2T. Here, T is signal transmission speed before spreading processing. 
   Thus, the reception signals obtained from those signals containing residual phase errors have a deteriorated error rate characteristic. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an OFDM-CDMA-based communication apparatus capable of compensating residual phase errors. 
   This object is achieved by a transmission system and reception system carrying out processing as shown below. That is, first, the transmission system carries out spreading processing on a known signal provided apart from each transmission signal using a spreading code assigned to this known signal and inserts each spread transmission signal and the spread known signal into each subcarrier. Then, the reception system detects a residual phase error using the received known signal obtained by despreading processing using the spreading code above and the known signal above and carries out compensation processing on each reception signal using the detected residual phase error. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the invention will appear more fully hereinafter from a consideration of the following description taken in connection with the accompanying drawing wherein one example is illustrated by way of example, in which; 
       FIG. 1  is a block diagram showing a configuration of a conventional OFDM-CDMA-based communication apparatus; 
       FIG. 2  is a schematic diagram showing an example of subcarrier placement of the conventional OFDM-CDMA-based communication apparatus; 
       FIG. 3  is a schematic diagram showing an amount of phase rotation contained in a reception signal of the conventional OFDM-CDMA-based communication apparatus; 
       FIG. 4  is a block diagram showing a configuration of an OFDM-CDMA-based communication apparatus according to Embodiment 1 of the present invention; 
       FIG. 5  is a schematic diagram showing an example of subcarrier placement of the OFDM-CDMA-based communication apparatus according to Embodiment 1 above; 
       FIG. 6  is a block diagram showing a configuration of a residual phase error detection section of the OFDM-CDMA-based communication apparatus according to Embodiment 1 above; 
       FIG. 7  is a block diagram showing a configuration of a phase compensation section of the OFDM-CDMA-based communication apparatus according to Embodiment 1 above; and 
       FIG. 8  is a block diagram showing a configuration of an OFDM-CDMA-based communication apparatus according to Embodiment 2 of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference now to the attached drawings, embodiments of the present invention will be explained in detail below. 
   Embodiment 1 
     FIG. 4  is a block diagram showing a configuration of an OFDM-CDMA-based communication apparatus according to Embodiment 1 of the present invention. In the transmission system shown in  FIG. 4 , each spreading section  101  carries out spreading processing by multiplying transmission signals  1  to n by their respective spreading codes  1  to n. Spreading section  102  carries out spreading processing by multiplying a known signal by a spreading code for the known signal. Here, suppose their spreading factor is k. 
   Addition section  103  multiplexes the transmission signals subjected to spreading processing by each spreading section and the known signal. S/P converter  104  divides the multiplexed and spread transmission signals and known signal by spread signal and breaks down spread transmission signals  1  to n and known signal by spread signal. That is, S/P converter  104  breaks down spread transmission signals  1  to n and known signal into a 1st chip to kth chip. 
   IFFT processing section  105  carries out inverse Fourier transform processing on a plurality of parallel signals. Here, IFFT processing section  105  assigns one subcarrier (carrier) to one chip data signal string and carries out frequency division multiplexing. That is, the number of subcarriers corresponds to the spreading factor and it is k in this case. Suppose the 1st chip of transmission signals  1  to n is placed in subcarrier  1  and the kth chip of transmission signals  1  to n is placed in subcarrier k. In other words, IFFT processing section  105  subjects a chip data string to frequency division multiplexing.  FIG. 5  shows this mode. Antenna  106  transmits/receives a radio signal. 
   In the reception system, FFT processing section  107  carries out Fourier transform processing on the reception signal from antenna  106  and extracts each subcarrier signal (chip data signal string). Here, the reception signal sent to FFT processing section  107  can also be the one subjected to frequency offset correction according to the above conventional system. 
   Each transmission path compensation section  108  is provided in one-to-one correspondence with subcarriers. Each transmission path compensation section  108  carries out compensation processing such as phase compensation on their respective subcarrier reception signals. 
   P/S converter  109  converts a plurality of parallel signals into a single serial signal. This P/S converter  109  rearranges subcarrier signals from one chip to another and outputs the first chip of a signal on which spread transmission signals  1  to n and the known signal are multiplexed at time t 1 , the second chip of a signal on which spread transmission signals  1  to n and the known signal are multiplexed at time t 2 , . . . up to the kth chip of a signal on which spread transmission signals  1  to n and the known signal are multiplexed at time t k . 
   Each despreading section  110  carries out despreading processing by multiplying the reception signal which has been converted to a single serial signal by their respective spreading codes  1  to n and extracting only the signals spread by those codes. Despreading section  111  carries out despreading processing by multiplying the reception signal which has been converted to a single serial signal by a known signal spreading code and extracting only the known signal spread by this code. 
   Residual phase error detection section  113  detects a residual phase error using the known signal, that is, the same known signal used in the transmission system and the despread signal (received known signal) from the despreading section  111 . Here, the method of detecting a residual phase error by residual phase error detection section  113  will be explained using  FIG. 6 .  FIG. 6  is a block diagram showing a configuration of the residual phase error detection section of the OFDM-CDMA-based communication apparatus according to Embodiment 1 of the present invention. 
   Here, suppose residual phase error θ (nT) exists in the despread signal. In this case, despread signal RX(nT) is expressed in the following expression:
 
 RX ( nT )= TX ( nT )exp( j θ( nT ))  {circle around (1)}
 
   where TX(nT) is transmission signal n (n=1, 2, 3, . . . ). 
   Furthermore, if residual phase error θ (nT) exists, despread known signal RXPi(nT), that is, the signal from despreading section  111  is expressed in the following expression.
 
 RXPi ( nT )=A( nT ) Pi ( nT )exp( j θ( nT ))  {circle around (2)}
 
   where A(nT) is reception amplitude information of the known signal and Pi(nT) is the known signal. 
   In  FIG. 6 , multiplication section  301  multiplies despread known signal RXPi(nT) shown in expression {circle around (2)} by known signal Pi(nT). In this way, the signal output by multiplication section  301  is expressed in the following expression. Here, suppose |RXPi(nT)|=1.
 
A( nT ) Pi ( nT )exp( j θ( nT )) Pi ( nT )=A( nT ) RXPi ( nT ) 2 exp( j θ( nT ))=A( nT )exp( j θ( nT ))  {circle around (3)}
 
   Then, division section  302  normalizes the signal from multiplication section  301 , that is, the signal shown in expression {circle around (3)} using the reception amplitude information A(nT) from envelope generation section  303 . In this way, from division section  302  a residual phase error expressed in the following expression is detected.
 
A( nT )exp( j θ( nT ))/A( nT )=exp( j θ( nT ))  {circle around (4)}
 
   Furthermore, conjugate generation section  304  generates a conjugate complex number of the signal from division section  302 , that is, the signal shown in expression {circle around (4)}. In this way, conjugate complex number of the residual phase error exp(−jθ(nT)) is created. This is how the residual phase error detection section  113  detects a residual phase error. 
   In  FIG. 4 , residual phase error detection section  113  outputs the conjugate complex number of the detected residual phase error to each of phase compensation sections  112 . Each phase compensation section  112  compensates the residual phase error for the despread reception signals from despreading sections  110  using the above conjugate complex numbers of the above residual phase errors. Here, the method of compensating a residual phase error by phase compensation sections  112  will be explained using  FIG. 7 .  FIG. 7  is a block diagram showing a configuration of a phase compensation section of the OFDM-CDMA-based communication apparatus according to Embodiment 1 of the present invention. 
   As shown in  FIG. 7 , multiplication section  401  multiplies reception signal RX(nT) subjected to despreading processing by the conjugate complex number of residual phase error exp(−jθ(nT)). This allows multiplication section  401  to produce a reception signal with its residual phase error compensated as shown in the following expression:
 
 RX ( nT )= TX ( nT )exp( j θ( nT ))exp(− j θ( nT )= TX ( nT )  {circle around (5)}
 
   That is, phase compensation sections  112  output signals quasi-equivalent to the transmission signals in the transmission system as reception signals with a residual phase error compensated. This is how compensation sections  112  compensate a residual phase error. 
   As shown above, according to this embodiment, the transmission system carries out spreading processing on a known signal provided apart from each transmission signal using a spreading code assigned to this known signal and inserts the despread known signal and each despread transmission signal into each subcarrier, while the reception system detects a residual phase error using the above known signal and received known signal obtained through the despreading processing using the above spreading code and carries out compensation processing using the detected residual phase error on the reception signal obtained through despreading processing using each spreading code, thus allowing a reception signal with an optimal error rate characteristic to be extracted. Thus, this embodiment can provide an OFDM-CDMA-based communication apparatus capable of compensating a residual phase error. 
   This embodiment describes the case where the transmission system uses one known reference signal, but the present invention is not limited to this and is also applicable to cases where the transmission system uses two or more known reference signals. In such cases, the reception system averages detected residual phase errors using each known reference signal, thus further improving the accuracy in detecting residual phase errors. 
   Embodiment 2 
   Embodiment 2 is an improved version of Embodiment 1 with the transmission system having a known signal whose signal level is higher than the levels of other transmission signals and the reception system with an improved signal-to-noise ratio when receiving the above known signal, thus improving the accuracy in detecting phase errors and preventing deterioration of the error rate characteristic of each reception signal. The OFDM-CDMA-based communication apparatus according to this embodiment will be explained using  FIG. 8 . 
     FIG. 8  is a block diagram showing a configuration of the OFDM-CDMA-based communication apparatus according to Embodiment 2 of the present invention. The parts with the same configuration as that in Embodiment 1 ( FIG. 4 ) are assigned the same reference numerals and their explanations are omitted. 
   In  FIG. 8 , multiplication section  501  receives gain-related information and a known signal as inputs and outputs a signal obtained by multiplying this known signal by a factor indicating the above gain to spreading section  102 . This allows the reception system to have an improved signal-to-noise ratio when receiving the above known signal, which improves the accuracy in detecting phase errors in residual phase error detection section  113 . This makes it possible to further suppress deterioration of the error rate characteristic of each reception signal compared to Embodiment 1. 
   Thus, this embodiment can prevent the error rate characteristic of each reception signal from deteriorating in the reception system by having a known signal whose signal level is higher than the levels of other transmission signals in the transmission system. 
   When two or more known reference signals are used, it goes without saying that the transmission system raises the signal level of each known reference signal as shown above. This allows the accuracy in detecting residual phase errors to be further improved, making it possible to prevent the error rate characteristic of each reception signal from deteriorating in the reception system. 
   Multiplexing a spread known signal with data is also applicable to a direct spreading CDMA system, but when the reception signal level falls or when interference between codes is large, deterioration of the residual phase error detection characteristic is also large. 
   On the other hand, when multiplexing a spread known signal with data is applied to an OFDM-CDMA system, it is possible to obtain a high accuracy residual phase error detection characteristic even when the reception signal level falls or when interference between codes is large for the following reasons: 
   {circle around (1)} Even if the reception level of a certain subcarrier falls, there are still other subcarriers whose reception level has not fallen, which allows a frequency diversity effect to be obtained. 
   {circle around (2)} A guard interval prevents influences of interference between codes. 
   As described above, according to the present invention, the transmission system carries out spreading processing on a known signal provided apart from each transmission signal using a spreading code assigned to this known signal and inserts each transmission signal subjected to spreading processing and the known signal subjected to spreading processing into each subcarrier, and the reception system detects a residual phase error using the received known signal obtained through despreading processing using the above spreading code and the above known signal and carries out compensation processing on each reception signal using the detected residual phase error, thus providing an OFDM-CDMA-based communication apparatus capable of compensating residual phase errors. 
   The present invention is not limited to the above described embodiments, and various variations and modifications may be possible without departing from the scope of the present invention. 
   This application is based on the Japanese Patent Application No. HEI 11-198943 filed on Jul. 13, 1999, entire content of which is expressly incorporated by reference herein.