The present invention relates generally to digital communication receivers, and more particularly, to techniques for compensating for Doppler frequency shifts in such digital communication receivers.
Doppler induced carrier frequency offset is a common impairment in mobile wireless communication systems. Doppler frequency shift results in a drift in the carrier frequency and the symbol frequency of a mobile digital receiver system, which in some operating environments causes a significant degradation in receiver performance. Currently, such Doppler frequency shift is mitigated using a carrier recovery algorithm that compensates for the drift in carrier frequency, followed by a timing recovery algorithm that compensates for symbol timing offset and a phase recovery algorithm that compensates for a constant yet unknown phase shift due to the distance between the transmitter and the receiver.
While the carrier recovery algorithm effectively compensates for carrier frequency drift, the complexity of the carrier recovery algorithm unnecessarily increases the cost and complexity of TDMA digital receivers. A need therefore exists for an improved and less computationally intensive method and apparatus that compensate for Doppler induced carrier frequency offset in TDMA digital mobile receivers.
Generally, methods and apparatus are provided for compensating for Doppler induced carrier frequency offset in a digital receiver. According to one aspect of the invention, a received signal is digitized and a differential detection algorithm is applied to the digitized received signal to compensate for the Doppler induced carrier frequency offset. A symbol timing recovery algorithm can also be applied to the digitized received signal to compensate for symbol timing offset.
A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings.
The present invention applies a differential detection technique as a pre-processing step that mitigates the effects of Doppler induced carrier frequency offset, so that only the symbol timing recovery algorithm is required for Doppler offset compensation. According to one aspect of the invention, the differential detection pre-processing reduces the phase drift caused by carrier offset to a constant value consisting of the phase offset of a single symbol period, thereby achieving the carrier frequency offset compensation objective. Thus, the Doppler compensation task is reduced to only compensating for symbol timing offset.
The present invention recognizes that in differential Phase Shift Keying (PSK) TDMA mobile phone systems, such as PHS (Personal Handy Phone System) and Interim Standard 136 (IS-136; also referred to as “Digital AMPS”), the Doppler shift is generally relatively small with respect to the system transmission data rate (ƒd<<ƒsymb). In this manner, a differential detection technique can pre-process the input signals to reduce the Doppler carrier frequency shift to a constant phase shift corresponding to the shift in a single symbol period (which can normally be ignored).
r(t)=I(t)·cos {2π(ƒc+ƒd)t−φ}−Q(t)·sin {2π(ƒc+ƒd)t−φ}. (1)
Here, fc is the carrier frequency, fd is the Doppler carrier frequency shift due to the motion between the transmitter and the receiver, φ is a constant phase shift due to the distance between the transmitter and the receiver, and r(t) is normalized, i.e., I2(t)+Q2(t)=1.
After demodulation at stage 110, the baseband signals for in-phase and quadrature-phase can be expressed as:
i(t)=I(t)·cos (2πƒdt−φ) (2)
q(t)=Q(t)·sin (2πƒdt−φ) (3)
Thereafter, the i(t) and q(t) signals are each sampled by an Analog to Digital Converter (A/D) 120 with a sampling rate of N times the symbol rate. Following this is the carrier frequency recovery circuit 130 to remove the Doppler carrier frequency shift and then the recovered samples are further processed by a symbol timing recovery algorithm 140 and a phase recovery algorithm 150, as well as additional post-processing 160, such as equalization, demapping, descrambling, and decoding to get the final output. For a more detailed discussion of the conventional TDMA digital mobile receiver system 100, see, for example, Theodore Rappaport, Wireless Communications: Principles and Practice (2001), incorporated by reference herein.
As shown in
i(k)=I(tk)·cos (2πƒdtk−φ) (4)
q(k)=Q(tk)·sin (2πƒdtk−φ) (5)
According to the present invention, the differential detection approach is then applied at stage 230 to pre-process i(k) and q(k) with symbol time interval spacing as follows:
z(k)=i(k)·i(k−N)+q(k)·q(k−N) =cos {2πƒd/ƒsymb+Δθ(k)} (6)
w(k)=i(k−N)·q(k)−i(k)·q(k−N) =sin {2πƒd/ƒsymb+Δθ(k)} (7)
where fsymb is the symbol rate, N is the number of samples per baud, and Δθ(k) is the phase transition between the sample in the current symbol and the corresponding sample in the immediately preceding symbol, which contains the transmission bit information for a differential PSK system. For a more detailed discussion of the differential detection approach applied during stage 230, see, for example, Theodore Rappaport, Wireless Communications: Principles and Practice, Ch. 6 (2001), incorporated by reference herein.
Since ƒd/ƒsymb<<1 for TDMA mobile phone systems, equations (6) and (7) simplify to:
z(k)=cos {Δθ(k)} (8)
w(k)=sin {Δθ(k)} (9)
Equations (6) and (7) show that the unknown phase is eliminated and the Doppler shift fd is reduced to a constant phase offset. For Doppler shifts that are small relative to the symbol frequency, equations (8) and (9) show that the Doppler shift fd and unknown phase φ are both compensated.
As shown in
In an environment with only minor intersymbol interference (ISI), and for a data frame short enough that the amount of symbol timing offset that would accumulate in one frame can be ignored, the transmitted bit information can be recovered directly from the outputs z, w of equations (8) and (9), respectively. The only post processing 250 needed is the demapping, descrambling and differential decoding. Thus, in such a system the differential detection module 230 is actually used as the main module of the receiver.
The present invention recognizes that the differential detector can be used as a pre-processor in those situations where the differential detector alone will not function as a receiver. If there is sufficient ISI or accumulated symbol timing offset to require the use of an equalizer or a timing recovery algorithm, then the differential detector by itself will fail and a coherent detector as shown in
The TDMA digital mobile receiver system 200 of the present invention replaces the frequency and phase recovery circuits 130, 150 which are required in a conventional TDMA mobile phone system 100 with a differential detection operation 230 resulting in lower complexity and cost. Among other benefits, the differential detection pre-processing of the present invention reduces the real time signal processing requirements and hence increases the battery life for the receiver.
It is to be understood that the embodiments and variations shown and described herein are merely illustrative of the principles of this invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.