1. Field of the Invention
This invention relates to Automatic Frequency Control during communications between a base station and a mobile unit in a 3rd Generation Partnership Project wireless communications network. More specifically, a device and method to compensate for a Doppler shift induced frequency offset between the base station and the mobile unit is disclosed.
2. Description of the Prior Art
A mobile unit in a wireless communications network functions in a difficult environment. Structures and terrain scatter and reflect a signal transmitted from a base station to the mobile unit. As a result, the signal picked up by a receiving antenna is a sum of all the scattered and reflected, or multipath, signals. In general, the quality of this received multipath signal is affected by two major factors.
The first factor is called slow fading or lognormal fading. Slow fading results from absorption of the signal by terrain between the base station and the mobile unit. A good example of slow fading is a mobile unit moving through a tunnel, possibly resulting in loss of signal strength.
The second factor is called fast fading, multipath fading, or Rayleigh fading. Rayleigh fading results when the multipath signals arrive at the mobile unit and combine destructively, possibly causing a loss of the entire bandwidth. Another form of Rayleigh fading is a Doppler shift in frequency due to motion of the mobile unit relative to the base station.
The frequency shift between the transmitter and the receiver interferes with many functions in a Wideband Code Division Multiple Access (WCDMA) Universal Mobile Telecommunications System Terrestrial Radio Access Network (UTRAN). For example, the bandwidth of Channel Estimation (CE) in the WCDMA must be designed for the Doppler spread. This is easily illustrated. FIG. 1 shows a spectrum of channel complex gain 15 of a received signal neatly centered within an allotted bandwidth 10 when no frequency offset exists. FIG. 2 shows how signals are distorted when a received signals spectrum of channel complex gain 25 falls outside of the allotted bandwidth 10 due to a frequency offset.
An Automatic Frequency Control (AFC) is an efficient solution to compensate for a frequency offset. As shown in FIG. 3, a Phase-Locked Loop (PPL) 40 is a common structure for an AFC. The PPL 40 comprises a Phase Detector (PD) 42, a Loop Filter (LP) 44, and a Voltage Controlled Oscillator (VCO) 46. In FIG. 3, u1 (t) is the input signal and u2 (t) is the output of the VCO 46. The error (the phase difference in this case) is detected by the PD 42 and the output ud(t) of the PD 42 is proportional to the error. The detected output ud(t) is further filtered by the LP 44 and the output uf(t) of the LP 44 is sent to the VCO 46 where the control signal of the VCO 46 is generated. The phase error between the output u2 (t) of the VCO 46 and the input signal u1 (t) is detected again by the PD 42. This negative feedback reduces the phase error between u1 (t) and u2 (t).
To implement the PPL 40 in baseband, the VCO 46 is replaced with a variable complex tone generator, and a multiplier and a frequency-offset detector replace the PD 42 to put the AFC into practice. A conventional offset detector 50 is shown in FIG. 4. The offset detector 50 takes the derivative of the input phase. In a digital baseband, after multiplying the input signal u1 (n) with the compensating signal u2 (n), this is achieved by multiplying the conjugate of the previous sample with the current sample. Stated mathematically, ud(n)=u1 (n)u2 (n)[u1 (n-1)u2 (n-1)]*. This kind of detector is easily interfered with by Doppler spread and therefore the phase noise increases.