1. Field of the Invention
Systems, apparatuses, and methods consistent with the present invention generally relate to a communication system using a wide band, and more specifically, to an apparatus and a method for generating a radio signal used in a wide-band communication.
2. Description of the Related Art
In general, a communication system transmits data to a reception side using a carrier frequency, which requires the generation of the carrier frequency. A local oscillator (LO) is utilized to generate a frequency for data transmission. The data is transmitted using a signal generated by the LO, or, a predetermined procedure is firstly performed when the signal generated by the LO is not a desired signal. The desired signal is generated by performing the predetermined procedure with respect to the signal generated by the LO using a specific signal. Hereinafter, a LO signal denotes the signal generated by the LO.
In a wireless system applying quadrature phase shifting keying (QPSK), a mixer mixes the LO signal and intermediate frequency (IF) signals having a phase difference of 90° with respect to the LO signal so as to generate two signals having 90° phase difference at the same time. In general, a single mixer mixes the LO signal and the IF signal to simultaneously output two signals. Specifically, based on the frequency of the LO signal, one signal having a frequency higher than that of the IF signal and the other signal having a frequency lower than that of the IF signal are output A filter is used to select one of the two output signals. An undesired frequency is removed generally using a band-pass filter to pass only the desired frequency.
However, as the frequency becomes higher, it is harder to implement a narrow-band filter. It is preferred not to use the narrow-band filter since the implementation of the narrow-band filter is hard on an integrated circuit. To overcome these shortcomings, a single sideband (SSB) system is used to selectively generate one desired signal alone. The SSB system is described in detail below.
By Fourier-transforming and frequency-analyzing a signal of which amplitude is modulated, the signal is shifted to frequencies above and below the carrier frequency (IF), to thus generate an upper sideband (USB) and a lower sideband (LSB) carrying the same information. A double sideband (DSB) transmission transmits both of the USB and the LSB, and the SSB transmission transmits only one sideband by removing unnecessary sidebands. Since audio or music does not have signal components in low frequency band, the SSB transmission alone is enough to communicate. Since the signal is transmitted to the reception side in one of the USB and the LSB, the scale of the reception side is reduced and the power consumption is lowered.
FIG. 1 illustrates a conventional SSB generator, which is described below. The SSB generator includes two mixers 102 and 106 and an operator 108.
The respective mixers 102 and 106 mix the input IF signal and LO signal. The mixed signals are furnished to the operator 108. The operator 108 performs an addition operation or a subtraction operation on the signals. The SSB generator outputs only one signal based on the following Equations.
Let the IF signal input to the mixer 102 be cos(wIFt) and let the LO signal be cos(wLOt). The mixer 102 outputs cos(wIFt)cos(wLOt) by mixing the received signals based on Equation 1. Let the IF signal input to the mixer 106 be sin(wIFt) and let the LO signal be sin(wLOt). The mixer 106 outputs sin(wIFt) sin(wLOt) by mixing the received signals based on Equation 2.
                                          cos            ⁡                          (                                                w                  LO                                ⁢                t                            )                                ⁢                      cos            ⁡                          (                                                w                  IF                                ⁢                t                            )                                      =                              1            2                    ⁡                      [                                          cos                ⁡                                  (                                                            (                                                                        w                          LO                                                +                                                  w                          IF                                                                    )                                        ⁢                    t                                    )                                            +                              cos                ⁡                                  (                                                            (                                                                        w                          LO                                                -                                                  w                          IF                                                                    )                                        ⁢                    t                                    )                                                      ]                                              [                  Equation          ⁢                                          ⁢          1                ]                                                      sin            ⁡                          (                                                w                  LO                                ⁢                t                            )                                ⁢                      sin            ⁡                          (                                                w                  IF                                ⁢                t                            )                                      =                              1            2                    ⁡                      [                                          -                                  cos                  ⁡                                      (                                                                  (                                                                              w                            LO                                                    +                                                      w                            IF                                                                          )                                            ⁢                      t                                        )                                                              +                              cos                ⁡                                  (                                                            (                                                                        w                          LO                                                -                                                  w                          IF                                                                    )                                        ⁢                    t                                    )                                                      ]                                              [                  Equation          ⁢                                          ⁢          2                ]            
Equation 1 represents the operations of the mixer 102, and Equation 2 represents the operations of the mixer 106.
The operator 108 performs the addition operation with respect to the signals received from the mixers 102 and 106. The following Equation 3 represents operations of the operator 108.
                                                        1              2                        ⁡                          [                                                cos                  ⁡                                      (                                                                  (                                                                              w                            LO                                                    +                                                      w                            IF                                                                          )                                            ⁢                      t                                        )                                                  +                                  cos                  ⁡                                      (                                                                  (                                                                              w                            LO                                                    -                                                      w                            IF                                                                          )                                            ⁢                      t                                        )                                                              ]                                +                                    1              2                        ⁡                          [                                                -                                      cos                    ⁡                                          (                                                                        (                                                                                    w                              LO                                                        +                                                          w                              IF                                                                                )                                                ⁢                        t                                            )                                                                      +                                  cos                  ⁡                                      (                                                                  (                                                                              w                            LO                                                    -                                                      w                            IF                                                                          )                                            ⁢                      t                                        )                                                              ]                                      =                  cos          ⁡                      (                                          (                                                      w                    LO                                    -                                      w                    IF                                                  )                            ⁢              t                        )                                              [                  Equation          ⁢                                          ⁢          3                ]            
In Equation 3, the operator 108 generates the LSB signal.
Let the IF signal input to the mixer 102 be sin(wIFt) and let the LO signal be cos(wLOt). The mixer 102 outputs sin(wIFt)cos(wLOt) by mixing the received signals based on Equation 4. Let the IF signal input to the mixer 106 be cos(wIFt) and let the LO signal be sin(wLOt). The mixer 106 outputs cos(wIFt) sin(wLOt) by mixing the received signals based on Equation 5.
                                          cos            ⁡                          (                                                w                  LO                                ⁢                t                            )                                ⁢                      sin            ⁡                          (                                                w                  IF                                ⁢                t                            )                                      =                              1            2                    ⁡                      [                                          sin                ⁡                                  (                                                            (                                                                        w                          LO                                                +                                                  w                          IF                                                                    )                                        ⁢                    t                                    )                                            -                              sin                ⁡                                  (                                                            (                                                                        w                          LO                                                -                                                  w                          IF                                                                    )                                        ⁢                    t                                    )                                                      ]                                              [                  Equation          ⁢                                          ⁢          4                ]                                                      sin            ⁡                          (                                                w                  LO                                ⁢                t                            )                                ⁢                      cos            ⁡                          (                                                w                  IF                                ⁢                t                            )                                      =                              1            2                    ⁡                      [                                          sin                ⁡                                  (                                                            (                                                                        w                          LO                                                +                                                  w                          IF                                                                    )                                        ⁢                    t                                    )                                            +                              sin                ⁡                                  (                                                            (                                                                        w                          LO                                                -                                                  w                          IF                                                                    )                                        ⁢                    t                                    )                                                      ]                                              [                  Equation          ⁢                                          ⁢          5                ]            
Equation 4 represents the operations of the mixer 102, and Equation 5 represents the operations of the mixer 106.
The operator 108 performs the addition operation with respect to the signal received from the mixers 102 and 106. The following Equation 6 represents the operations of the operator 108.
                                                        1              2                        ⁡                          [                                                sin                  ⁡                                      (                                                                  (                                                                              w                            LO                                                    +                                                      w                            IF                                                                          )                                            ⁢                      t                                        )                                                  -                                  sin                  ⁡                                      (                                                                  (                                                                              w                            LO                                                    -                                                      w                            IF                                                                          )                                            ⁢                      t                                        )                                                              ]                                +                                    1              2                        ⁡                          [                                                sin                  ⁡                                      (                                                                  (                                                                              w                            LO                                                    +                                                      w                            IF                                                                          )                                            ⁢                      t                                        )                                                  +                                  sin                  ⁡                                      (                                                                  (                                                                              w                            LO                                                    -                                                      w                            IF                                                                          )                                            ⁢                      t                                        )                                                              ]                                      =                  sin          ⁡                      (                                          (                                                      w                    LO                                    +                                      w                    IF                                                  )                            ⁢              t                        )                                              [                  Equation          ⁢                                          ⁢          6                ]            
In Equation 6, the operator 108 generates the USB signal.
The wide-band communication system uses a plurality of carriers to increase the data transmission amount FIG. 2 illustrates a frequency generator in the communication system which transmits data using the plurality of carriers.
Referring to FIG. 2, one LO signal and a plurality of the IF signals are combined to generate the desired number of carriers. The number of the IF signals is n-ary.
The LO signal generated by the LO is provided to the SSB generator 202. The switch 200 receives the n-ary IF signals, selects one of the n-ary IF signals, and provides the selected signal to the SSB generator 202. The SSB generator 202 performs the same operations as in FIG. 1, to thus output a single signal.
As shown in FIG. 2, the signal provided to the SSB generator 202 is not preset, but is selected from the n-ary signals. The SSB generator 202 performs the phase shift on the received signals by a certain unit. Referring back to FIG. 1, the SSB generator 202 shifts the phase of the received signal to 90°. If the number of the received signals is plural, it is hard to perform the phase shift on each signal.
In general, the SSB generator performs accurately the phase shift on one signal. Accordingly, upon receiving other signals, the SSB generator cannot accurately perform the phase shift. If the SSB generator is constructed to shift the phase with respect to ƒ1 alone, it is infeasible to accurately shift the phase with respect to input ƒn. The inaccurate phase shift results in error components in the signal output from the SSB generator.