A rake receiver tracks a multi-path signal transmitted from a base station to a mobile station. The rake receiver comprises rake fingers each assigned to a multi-path component. Typically a rake finger performs an early late detection using early and late component of the energy of the component taken before and after a presumed occurrence of an optimum of the energy. An early-late signal is generated from a comparison between a product of a first integer and the early component and a product of another integer and the late component.

FIELD OF THE INVENTION

The invention relates to a method for tracking a resolved signal. The invention also relates to a rake receiver for tracking a resolved signal. The invention then concerns a computer readable medium for storing instructions for carrying out a method of the invention.

The invention is relevant to communications over a cellular network using Code-Division Multiple Access technology for tracking delays of multipath signals from a base station to a mobile station. The invention may also be applied in a rake receiver in a mobile communication device for resolving and tracking arriving delays of multi-path components of a transmitted signal.

BACKGROUND ART

Various multiple-access technologies may be used for cellular communications.

A first group of these technologies consists in narrowband channelized technologies such as the Frequency-Division Multiple Access (FDMA) technology and the Time-Division Multiple Access (TDMA) technology. In a FDMA communication system each user is assigned to a first specific frequency sub-band of the bandwidth reserved for up-link communications (from a mobile station to a base station) and to a second frequency sub-band of the bandwidth reserved for down-link communications (from a base station to a mobile station). In a TDMA system each user is assigned to a different time slot and accesses the entire reserved sub-bands.

A second group of multiple-access communication technologies consists in wideband channelized technologies. Among these, the Code-Division Multiple Access (CDMA) technology has been widely adopted as a standard. CDMA allows each user to use the entire bandwidth for the complete duration of a call.

CDMA is a spread spectrum technology which means that the information contained in the information signal is spread over a much greater bandwidth than that of the original signal. In the Direct Sequence Spread Spectrum (DS-SS) technology, the information signal of data rate Tb is multiplied in the transmitter by a pseudo-random binary sequence, the code sequence, of clock period T, so-called the chip period, where Tb>>T. This has the effect of increasing the bandwidth of the signal by the ratio Tb/T. The spread signal is then transmitted over the wider band with a reduced power spectral density relative to a corresponding de-spread signal. The code sequence is independent of the information signal and is known to the transmitter and the receiver.

At the receiver, the received wide-band spread spectrum signal must be de-spread in order for the information signal to be recovered. De-spreading is achieved by multiplying the spread signal by an exact replica of the code sequence used in the transmitter. The replica must be synchronized with the received spread signal. A local code sequence generator that generates the code sequence at the receiver must be aligned and synchronized within one chip of the received spread signal.

Code synchronization may be performed in two stages: a code acquisition followed by a fine code tracking. Acquisition reduces the alignment timing offset between the received spread signal and the locally generated code sequence to less than a chip period. Tracking aligns and maintains the two signals synchronized.

In a real communication environment such as urban and suburban areas, radio signals are reflected and scattered off various objects along the transmission path between the transmitter and the receiver. Therefore the spread signal, mentioned above, encounters multipath when transmitted from the base station to the mobile station. In addition, phase cancellation of signals following different paths may cause severe fading and may lower the received signal power. However CDMA provides robust operation in fading environments. CDMA takes advantage of multipath fading to enhance communication and voice quality. For this purpose, a rake receiver is present in each mobile station and allows selecting the strongest multipath signals incoming from the base station. Transmission delays are estimated for the strongest multi-paths and the estimated delays are assigned to specific “fingers” of the rake receiver. A finger is a processing element that correlates the received spread signal with the replica of the locally generated code sequence on the basis of the estimated time delay assigned to the finger. The fingers' outputs are then weighted and then coherently combined to produce an enhanced signal. Thus, the multi-path nature of the channel is used to create a diversity advantage in CDMA.

International application WO 99/35763 discloses a method for estimating multipath delays of a direct spread spectrum signal transmitted in fading environments. Delays are estimated by measuring the envelope of the signal.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a rake receiver comprising rake fingers performing low computational early-late detection to track multi-path components of a signal transmitted in a fading environment.

To this end, a method of the invention comprises:

determining a first value representative of an energy of the signal at a first instant before a presumed occurrence of a local optimum of the energy of the signal;

determining a second value representative of the energy at a second instant after the presumed occurrence of the local optimum;

calculating a first product of a first positive integer and the first value and calculating a second product of a second positive integer and the second value, with the first positive integer smaller than the second positive integer;

generating a first logical value from a comparison 25 between the first and the second products;

calculating a third product from a third positive integer and the first value and calculating a fourth product from a fourth positive integer and the second value, with the third positive integer smaller than the fourth positive integer;

generating a second logical value from a comparison between the third and the fourth products; and,

generating a detector output signal from a difference between the first logical value and the second logical value.

A method of the invention allows tracking a signal using the first and second values of the energy without necessarily determining a metric involving a ratio of these two values that may be complicated to calculate and costly in terms of hardware. Such a method of tracking the signal does not require normalization of the first and second values of the energy of the signal. An advantage of one or more embodiments of the invention is to provide simple and efficient tracking.

Elements within the drawing having similar or corresponding features are identified by like reference numerals.

Preferred Embodiment

FIG. 1is a communication system100of the invention comprising a first transceiver200communicating with at least a second transceiver300. The transceiver200may be a base station and the transceiver300may be a mobile station such as a handset or a cell phone in a CDMA cellular communication system. The transceivers200and300comprise respective transmitters T200, T300for transmitting information signals and comprise respective receivers R200, R300for receiving information signals. The transmitter T200transmits via an antenna210an information signal S spread by correlation with a pseudo-random noise code sequence. The signal S was also previously modulated by correlation with a carrier signal of carrier frequency fc. The spread signal S is received by an antenna310of the transceiver300.

While transmitted from the transmitter T200to the receiver R300, the signal S encounters multipath. In this embodiment, the signal S is reflected and scattered off a mountain110and a building120. The spread signal S is the superposition of at least two multipath signals S1and S2. The multipath signals S1and S2have different transmission paths and different transmission delays. The path attenuation and phase shift to which the signals S1and S2are subjected are assumed to be random-like and mutually independent. As a result the signal S can be thought as the superposition of a number of randomly attenuated and phase rotated signals containing among others the signals S1and S2.

FIG. 2is a conventional block diagram of the receiver R300. The signal S transmitted from the base station200is received by the antenna310and inputted to a demodulation circuit305of the receiver R300. The signal S is passed through a RF receiver320and thereafter processed by a divider330for division into two radio signals I1and Q1. The radio signal I1is mixed in a mixer340with the oscillator output fc of an oscillator360resulting in an in-phase demodulated base-band signal I2. The radio signal Q1is mixed in a mixer350with the oscillator output fc shifted to π/2 in a phase shifter370resulting in a quadrature demodulated base-band signal Q2. The base-band signals I2and Q2are then respectively passed through low pass filters380and390for providing channel selectivity. Both filtered signals I and Q are then provided to a rake receiver400for multi-path components resolving and diversity combining into a signal R.

FIG. 3is another block diagram of the receiver R300. The receiver R300comprises the demodulation circuit305for extracting the in—phase and quadrature components I and Q further transmitted as a complex signal S* to the rake receiver400. The rake receiver400comprises three rake fingers410,412, and414. Each finger410,412, and414is assigned a multi-path component S1, S2and S3, respectively, of the received signal S for acquisition and tracking. The rake receiver400also comprises a maximal ratio combiner420for combining the multi-path components S1-S3resolved by the fingers410,412, and414to provide diversity. The resulting signal is the signal R.

FIG. 4is a circuit block diagram representing one example of a structure of the rake finger410,412, and414of the invention. Initially the rake finger410,412, and414adopts an acquisition mode. Acquisition is performed in the acquisition unit422for synchronizing a replica of the code sequence, originally used to spread the information signal, with the multi-path component S1-S3assigned to the finger410,412, and414. The replica of the code sequence is generated in a pseudo-noise generator424.

Thereafter, in a tracking mode, the rake finger410,412, and414maintains the generated code sequence aligned to the assigned multi-path component S1-S3. The base-band signals I and Q are provided as the complex input signal S* to the rake finger410,412, and414. The signal S* is then branched in two branches for determining early and late components E and L representing respective early and late reception of the assigned signal component S1-S3with respect to a presumed reception of the signal S1-S3which is represented by a peak of the energy of the assigned signal S1-S3.

In a first branch, the early component E is determined by correlating in a mixer428the signal S* with the code sequence taken at a first instant tE (seeFIG. 6) before the presumed peak of the energy of the signal S1-S3. The early component E is then determined by processing the output signal of the mixer428in a low-pass filter432and by complex magnitude squaring in a squared arrangement436. As shown inFIG. 6, a presumed peak of the energy of the assigned signal S1-S3, represented in dashed line, is expected to occur at instant t0. The component E is representative of the energy of the signal S1-S3taken at the first instant tE before the presumed occurrence at t0of the optimum of the energy of the signal S1-S3.

Symmetrically, the late component L is derived by first correlating in a mixer426the signal S* with the code sequence taken at a second instant tL after the presumed occurrence of the maximum. The late component L is then determined by processing the output signal of the mixer426in the low-pass filter430and by complex magnitude squaring in a squaring arrangement434. The component L is representative of the energy of the signal S1-S3taken at the second instant tL after the presumed occurrence at t0of the optimum of the energy of the signal S1-S3.

The early and late components E and L are then inputted to a delay detector500that processes the two components E and L and determines the early-late state of the reception of the assigned multi-path component S1-S3indicated by an early-late signal V. In this embodiment the delay detector500is a digital signal processing (DSP) unit. The signal V is then provided to a loop filter438where an appropriate correction signal CORR is derived and transmitted to the pseudo-noise generator424. The correction signal CORR allows monitoring the phase of the code sequence so that the code sequence is kept synchronized with the assigned component S1-S3.

An information signal is then obtained from the multi-path signal S1-S3at the output of a mixer420. The mixer420allows de-spreading the assigned component S1-S3of the signal S* by correlating the signal S* with the code sequence aligned to the multi-path signal S1-S3.

A detailed embodiment of the detector500is given in FIG.5. The detector500comprises a calculating unit510, a calculating unit530and a logical comparator550for providing a logical signal V1. The signal V1is obtained from a comparison of a first product K1*E of the early value E and a positive integer K1derived in the unit510with a second product K2*L of the late value L and a positive integer K2derived in the unit530. In this embodiment K1is smaller than K2(K2/K1>1). V1is 1 when K2*L>K1*E is true as shown in Table 1, V1is 0 otherwise. V1=0 indicates that E/L>K2/K1>1, i.e. the tracking of the assigned component S1-S3is early.

The detector500further comprises a calculating unit520, a calculating unit540and a logical comparator560for providing a logical signal V2. The signal V2is obtained from a comparison of a first product K3*E of the early value E and a positive integer K3derived in the unit520with a second product K4*L of the late value L and a positive integer K4derived in the unit540. In this embodiment K3is smaller than K4(K3/K4<1). V2is 1 when K4*L>K3*E is true as shown in Table 1, V2is 0otherwise. V2=0 indicates that E/L<K3/K4<1, i.e. the tracking of the assigned component S1-S3is late.

The units510,520,530, and540may be implemented as logic gates in the form of ICs or alternatively the functions carried out by the units510,520,530, and540may be fulfilled by means of instructions in a software component.

The signals V1and V2are combined in an adder570. The adder570allows calculating the value of the signal V of the expression V1-V2. The value of the signal V is given in Table 1 depending on the position of E/L with respect to the values K3/K4and K2/K1.

TABLE 1E/L < K3/K4 < 1K3/K4 < E/L < K2/K1E/L > K2/K1 > 1V1110V2011V10−1V = 1 indicates that the tracking of the assigned component S1-S3 is late.V = 0 indicates that tracking of the assigned component is quite on-time.V = −1 indicates that tracking of the assigned component is early or in advance.

V=1 indicates that the tracking of the assigned component S1-S3is late. V=0 indicates that tracking of the assigned component is quite on-time. V=˜1 indicates that tracking of the assigned component is early or in advance.

K1, K2, K3and K4may be arbitrarily chosen with K1<K2and K3<K4. K1and K3may be chosen such that K1=K3and K2and K4may be chosen such that K2=K4. For example, K1=K3=2 and K2=K4=3 allow a simple implementation of the units510,520,530, and540using a limited number of logic gates. The positive integers K1, K2, K3and K4may be fixed to different constants depending on the sensitivity required for the early-late detection. The values for K1=K3and K2=K4may be chosen greater to reduce the on-time zone and to permit sensitive phase control of the code sequence generator424.

It is to be noted that, with respect to the described method and communication system, modifications or improvements may be proposed without departing from the scope of the invention. For instance, it is clear that this method may be implemented in several manners, such as by means of wired electronic circuits or, alternatively, by means of a set of instructions stored in a computer-readable medium, said instructions replacing at least a part of said circuits and being executable under the control of a computer or a digital processor in order to carry out the same functions as fulfilled in said replaced circuits.

In view of the foregoing it will be evident to a person skilled in the art that various modifications may be made within the spirit and the scope of the invention as hereinafter defined by the appended claims and that the invention is thus not limited to the examples provided. The word “comprising” does not exclude the presence of other elements or steps than those listed in a claim.