WO2008107825A1

WO2008107825A1 - Compensation of frequency-dependent delays

Info

Publication number: WO2008107825A1
Application number: PCT/IB2008/050722
Authority: WO
Inventors: Hugo Veenstra; Marc G. M. Notten
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-03-05
Filing date: 2008-02-28
Publication date: 2008-09-12

Abstract

The present invention relates to a transceiver, receiver module, computer program product and method for compensating a group delay variation in a transceiver, wherein an oscillator frequency used for transmitting a calibration signal is varied, and the calibration signal is transmitted via a transmission antenna (25) of the transceiver circuit and directly received via a receiving antenna (24) of the transceiver circuit. A DC component in 5 the received calibration signal is detected and stored in association with said transmission frequency, and a frequency dependent delay generated in the transceiver is compensated based on the stored DC component.

Description

Compensation of frequency-dependent delays

FIELD OF THE INVENTION

The present invention relates to a transceiver, receiver module, computer program product and method for compensating a group delay variation in a transceiver.

BACKGROUND OF THE INVENTION

Ultra- Wideband (UWB) is a Radio Frequency (RF) technology in which binary data is transmitted by using low energy and extremely short duration impulses or bursts (in the order of picoseconds) over a wide spectrum of frequencies. It delivers data over 15 to 100 meters and does not require a dedicated radio frequency, so is also known as carrier- free impulse or base-band radio. Technical standards and operational restrictions enable co-existence of UWB with existing radio technologies such as for example IEEE 802.11 or Wireless Fidelity (Wi-Fi), HomeRF, and High Performance Radio Local Area Network (HiperLAN).

A compelling application for UWB is radar in the automotive industry. It is suited for collision avoidance, parking aid, lane change assist, remote keyless entry, detecting the movement and location of objects near a vehicle, improving airbag activation and suspension settings. Studies prove conclusively that UWB will not interfere with the Global Positioning System (GPS), especially as the first cars to have collision avoidance will be the same premium models that also host GPS-based telematics systems. Fig. 1 shows a schematic diagram indicating the principle of radar detection of a distance and velocity to an object. The distance and velocity of an object is determined based on measured time delay between transmission and reception of a radar beam. The beam is amplified by a power amplifier (PA) and transmitted via a transmitting antenna towards the object. At least a portion of the beam is reflected at the object and received via a receiving antenna and a low noise amplifier (LNA). The distance can then be determined based on the total time delay

required for forward and backward transmission, e.g., by using the following equation:

where d designates the distance to the object, c designates the speed of light, tl designates the time required for forward transmission, and t2 designates the time required for backward transmission. Thus, when the total time delay (Δt) is known, the distance and relative velocity to an object can be determined. The relative velocity can be derived based on the Doppler effect.

Fig. 2 shows an example of a state of the art automotive radar receiver front end based on a UWB frequency spectrum. This system can be used to detect objects around a car and determine the relative speed and distance of these objects. The circuit of Fig. 2 comprises two input terminals InI and In2 where respective receiving (Rx) antennas can be connected. These two alternative inputs can be selected by a selection unit 13 in response to a selection control input sel. The signal received via the input terminal InI is amplified by a low noise amplifier (LNA) 11, while the signal received via the input terminal In2 is amplified by a LNA 12. Power supply is connected to a supply input VCC.

The selection output of the selection unit 13 is supplied to a bi-phase demodulator which consists of a mixer or multiplier 15 which multiplies the received signal with a delayed pseudo random bit stream (PRBS) to detect reflected pulses in the received signal. The delayed PRBS is delayed with respect to an original PRBS used for modulating the transmission signal and may be converted e.g. from CMOS (Complementary Metal Oxide Semiconductor) level to CML (Current Mode Logic), by a corresponding conversion unit 14. The demodulated received signal is then filtered by a filter 16 and supplied to a down- conversion unit 17 where it is mixed with an in-phase (I) and quadrature phase (Q) component obtained from a tunable oscillator 20, such as voltage controlled oscillator (VCO), of a clock generation unit 10. A divider 21 generates the I and Q components to be supplied to the down-conversion unit 17. A second divider 22 generates a reference signal for a phase locked loop (PLL) output. Additionally, the I or Q component is also supplied to a power amplifier (PA) output of the front end.

The tunable oscillator 20 can be controlled by a tuning input Vtune where a tuning voltage can be applied in order to control the oscillation frequency. The obtained oscillator output can be monitored at a monitoring output mon. The down-converted I and Q components of the received signal are filtered by respective low pass filters 18, 19 and are supplied to respective outputs Outl and OutQ. However, in such a receiver front end, non-constant group delay will increase inaccuracy involved in the determination of distance and velocity to an object. Such a non- constant group delay can occur due to non- linear phase transfer of for example the PA, LNA and Rx/Tx antennas of the radar transceiver. UWB systems require wide-band antennas, baluns and wide-band circuits. For an accurate distance and velocity measurement it is required that all frequencies are delayed by an equal amount by antenna and electronics, which means that a constant group delay over frequency is required. The group delay can be expressed by an equation of τ_g=-δφ/δcϋ, wherein φ denotes a phase, and CO represents an angular frequency. That is, the group delay may represent a phase variance relative to a variance of the angular frequency. The group delay may be disadvantageous in that it may cause a signal distortion. Generally, a modulated input signal has respective sidebands at both ends of a carrier frequency. When the group delay occurs in such an input signal including at least two frequencies, time delays according to respective frequencies may cause different phase delays. In publications such as R. Michael Bueher, et al, "Characterization of the

Ultra-wideband channel" , 2003, and S. H. Choi, et al, "A new Ultra-wideband antenna for UWB applications' ', Microwave and optical technology letters, Vol. 40, No. 5, March 5 2004, it has been stated that wide-band antennas have a certain phase dispersion, which causes extra frequency dependent delays (i.e. group delays). Fig. 3 shows an example of a measured group-delay plot of a patch antenna, as described for example in the above latter publication, intended for the 3-12 GHz frequency range. A significant variation in group-delay over frequency can be seen, which causes inaccuracy in the distance and velocity detection.

Car radar systems operate at a different frequency than the 3-12 GHz used in the patch antenna measurements, but the group delay problems in a UWB system are similar. A further source of frequency-dependent group delay is the non- linear phase response of the LNA and PA circuits of radar transceivers.

Fig. 4 shows example of group delay (in s) vs. frequency (in Hz) of an LNA and a PA based on circuit simulations. This overall frequency-dependent delay in the UWB system also causes inaccuracy in the determination of the distance and velocity to a detected object.

Fig. 5 shows a diagram indicating distance (in m) vs. delay (in ns), from which it can be gathered that additional delay causes an offset in the detected distance. In the example of Fig. 5 the delay is constant and not frequency-dependent, which is an additional issue.

SUMMARY OF THE INVENTION An object of the present invention is to provide a receiver apparatus and calibration method, by means of which inaccuracies caused by group delay can be compensated.

This object is achieved by a transceiver as claimed in claim 1, by a receiver module as claimed in claim 8, and by a compensation method as claimed in claim 9. Accordingly, a compensation mechanism for frequency dependent delays is provided, wherein the DC component can be used as an indication of or measure for internal frequency dependent delays and can be used for compensation and thus correctly interpreting the signals from the receiving antenna.

The DC component may be stored in a look-up table, so that the compensation unit can be arranged to access the look-up table during the compensation. Thereby, the internal frequency dependent delay added by the transceiver is available for later compensation during actual operation of the transceiver.

According to a first aspect, a received signal can be post-processed after receipt in order to compensate for the group delay. The compensation is thus done after reception during evaluation of the content or information derived from the received signal. E.g., correction of a distance or velocity obtained from a reflected radar signal.

As an alternative or additional second aspect, a pre-equalizer or a pre- equalizing function may be provided in order to pre-equalize a transmission signal based on said DC component so as to compensate for a group delay obtained when a reflected component of the transmission signal is received. In this case, the frequency dependent delay or group delay is compensated prior to transmission, so that the received information is substantially free of internal frequency-dependent delays added during transmission and reception operation.

As an alternative or additional third aspect, an equalizer or an equalizing function may be provided in order to equalize based on the DC component a reflected component of a transmission signal transmitted by the transceiver. Here, contrary to the second aspect, the frequency dependent delay or group delay is compensated during reception, so that the received information is substantially free of internal frequency- dependent delays added during transmission and reception operation The DC component may be detected and stored during an initial calibration procedure performed prior to an actual use of the transceiver. The compensation information is thus gathered and stored prior to actual use of the transceiver, so that calibration delay(s) during operation can be prevented. Further advantageous embodiments are defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present invention will be described in greater detail based on embodiments with reference to the accompanying drawings in which: Fig. 1 shows a schematic diagram indicating a radar detection principle;

Fig. 2 shows a schematic block diagram of a conventional receiver front end; Fig. 3 shows a diagram indicating group delay of a patch antenna; Fig. 4 shows a diagram indicating typical LNA and PA group delay; Fig. 5 shows a diagram indicating distance vs. delay with and without additional delay;

Fig. 6 shows a schematic block diagram of a receiver front end according to a first embodiment;

Fig. 7 shows a schematic block diagram of a receiver front end according to a second embodiment; Fig. 8 shows a schematic block diagram of a receiver front end according to a third embodiment;

Fig. 9 shows a flow diagram of a calibration method according to a fourth embodiment; and

Fig. 10 shows a schematic block diagram of a software-based implementation according to a fifth embodiment.

DESCRIPTION OF THE EMBODIMENT

The embodiments of the present invention will now be described in greater detail based on modifications of the UWB receiver front end circuit or module shown in Fig. 1. It is however noted that the present invention can be applied to any transceiver type to thereby reduce group delay inaccuracies.

According to various embodiments, the frequency dependent (group-)delay is compensated to prevent inaccuracy in the determination of the distance and velocity to an object. Possible implementation options of the compensation process include look up table(s), pre-equalizing in the transmission (Tx) path or equalizing in the receiving (Rx) path. These measures serve to remove or at least alleviate the frequency dependent group-delay caused by the circuitry and antennas in an exemplary UWB system similar to Fig. 2.

Fig. 6 shows a schematic block diagram of a transceiver with receiver front end according to a first embodiment. Contrary to Fig. 2, a transmitter 23 and a digital baseband processing unit 30 are also shown in addition the receiver module 100 which may be implemented as an integrated circuit (IC) or chip, or as an electronic circuit on a circuit board. The transmitter 23 comprises a modulator or multiplier 26 connected via an amplifier to a transmission antenna 25. One of the multiplier inputs is supplied with a component of the oscillation signal of the tunable oscillator 20, and the other multiplier input is connected to an input for supplying a PRBS signal. However, during the proposed calibration procedure, the binary PRBS signal is kept at a fixed value, such as "-1" or "1", so that a calibration signal with fixed amplitude is generated at the output of the transmitter 23. The control and supply of the PRBS signal as well as the control and supply of the tuning control signal Vtune to the oscillator which generates the transmission frequency of the calibration signal may be achieved by a digital baseband unit 30 arranged for processing and evaluating reflected and received radar signals to determine at least one of distance and relative velocity.

The operation of the transceiver circuit of Fig. 6 will now be explained with reference to signals A, B, and C indicated in Fig. 6. Signal A can be expressed by the following equation (2) with COo designating a carrier frequency and A_LO designating the amplitude of the oscillator signal:

A =A_LOcos(ω_ot) (2)

The filtered or matched RF signal B has an amplitude A_RF and a certain phase difference φ; according to the group delay behavior of the circuits and is expressed by the following equation (3):

Finally, equation (4) illustrates the IF signal C at the mixer output:

C = cos((ύot+§i)cos((ύ_ot) C= l/2cos(2ω_ot+$i) + l/2cos($0 (4)

where the first term is the sum frequency which is suppressed by the low pass filter (LPF) 18 (e.g. 4GHz LPF) and the second term indicates a DC component which indicates the phase delay φ; caused by the circuits and Tx/Rx antennas 24, 25.

To compensate for this frequency dependent group delay a calibration procedure is proposed. This calibration procedure could be implemented intermittently during system operation or as an initial or preparatory procedure before the system is active and actually used. In the latter case, when the system is switched on, the calibration can be done as follows. A calibration signal is generated, transmitted by the Tx antenna 25 and directly received by the Rx antenna 24. The oscillator frequency is swept (or otherwise varied) in a predetermined operation range (e.g., from 22 to 29 GHz) by applying a ramp signal (or a signal with another shape) to the Vtune input. The calibration procedure may be controlled by the baseband unit 30 or another separate control or processing unit. Due to the frequency dependent group delay in the system, a DC component of signal C can be detected or retrieved by the baseband unit 30 and stored in the digital domain in association with the transmission frequency, e.g., in a lookup table 37. The value of this DC component represents the frequency-dependent group delay at the respective transmission frequency. Thus, the lookup table 37 is now available and can be used to post- process the IF signal by a compensation unit 33 during actual operation or use of the system.

Thus, for every transmission frequency generated during the calibration process, the phase difference between signals A and B results in a DC component at signal C. The DC component is determined by the group delay of the circuits and antennas, because the transmitted signal is received via the directly coupled Tx and Rx antennas 25 and 24 and not via any object.

When the system is active and an object has been detected this stored DC value can be used in the calculations for the velocity and distance to an object. By means of this the frequency dependent delay caused by the system is compensated so that accuracy of the detection process can be increased. Of course, other possibilities exist to implement frequency dependent group delay compensation.

Fig. 7 shows a schematic block diagram of a receiver front end according to a second embodiment. Here, compensation of the frequency-dependent (group) delay is achieved by means of an additional pre-equalizer 32 provided in the transmitter 23, which is controlled by the baseband unit 30 (or another control or processing unit) based on the stored DC components derived from the calibration procedure. The transmission signal is thus pre- processed by the pre-equalizer 32 before actual transmission, in order to compensate for the group delay introduced in the electronics and Rx/Tx antennas 24, 25. This can be done purely in the analog domain or as shown in Fig. 7 under control of the digital baseband unit 30. The pre-equalizer may introduce a frequency-dependent phase shift or delay inverse to the frequency dependency or characteristic of the unwanted group delay.

Fig. 8 shows a schematic block diagram of a receiver front end according to a third embodiment. Here, another possible solution is presented, where an additional equalizer 35 is provided in the receive path, which can be controlled by the digital baseband unit 30 based on a control signal EQin. The functionality of the equalizer 35 is similar to the pre- equalizer 32 of Fig. 7 with the difference that the inverse characteristic or dependency is applied after receipt of the reflected component.

Fig. 9 shows a general flow diagram of a compensation method with initial calibration according to a fourth embodiment.

In an initial calibration phase, a calibration signal is transmitted and directly coupled and received by a receiver of the same system (step SlOl). Therefore, only internal frequency-dependent delay introduced by the system itself is measured. In step S 102, the transmission frequency of the calibration signal is swept in or through a predetermined frequency range to cover the delay characteristic of the whole operation range of the system. DC components obtained after frequency conversion of the received RF signal are retrieved and stored in step S 103. Finally, the stored DC components are used in step S 104 during a subsequent operation phase to compensate for detected internal frequency-dependent delay(s) introduced by the system itself. Thereby, system accuracy can be improved and/or phase distortions reduced.

Fig. 10 shows a schematic block diagram of an alternative software-based fifth embodiment of the proposed compensation with initial calibration for achieving group delay compensation. The required functionalities can be implemented in the digital baseband unit 30 or any other control functionality or system which comprises a processing unit 210, such as a processor or computer device with a control unit which performs control based on software routines of a control program stored in a memory 212. Program code instructions are fetched from the memory 212 and are loaded to the control unit of the processing unit 210 in order to perform the processing steps of the above functionalities described the flow diagram of Fig. 9. These processing steps may be performed on the basis of input data DI and may generate output data DO, wherein the input data DI may correspond to the Dc components obtained via the reference channel (e.g. pilot channel), and the output data DO may correspond to the control signals or words applied to the compensation functionalities, e.g., the compensation unit 33 in Fig. 6, the pre-equalizer 32 in Fig. 7 and/or the equalizer 35 in Fig. 8.

In summary, a transceiver, receiver module, computer program product and method for compensating a frequency dependent delay or group delay variation in a transceiver have been described, wherein an oscillator frequency used for transmitting a calibration signal is varied, and the calibration signal is transmitted via a transmission antenna of the transceiver circuit and directly received via a receiving antenna of the transceiver circuit. A DC component in the received calibration signal is detected and stored in association with said transmission frequency, and the frequency dependent delay generated in the transceiver is compensated based on the stored DC component.

However, in general, the present invention is not restricted to the above embodiments or application examples and can be implemented in any discrete circuit arrangement or integrated architecture. The proposed alternative compensation approaches of the first to third embodiments may as well be combined to increase overall efficiency. I.e., at least two of the post-processing compensation unit 33, pre-equalizer 32, and equalizer 35 may be provide in a single transceiver system. The proposed calibration-based compensation procedure can be applied in any transceiver system (architecture) which functionality depends on retrieving the correct delay between the transmitted and received signal. More specifically, the proposed compensation can be applied to any kind of transceiver where internal frequency dependent delays are critical and should be compensated. It can be implemented in the analog or digital domain and applies for all general purpose and special commercial products (like integrated circuits used in consumer electronics, mobile phones, radar devices, etc.). The above embodiments may thus vary within the scope of the attached claims.

Finally, it is noted that the term "comprises" or "comprising" when used in the specification including the claims is intended to specify the presence of stated features, means, steps or components, but does not exclude the presence or addition of one or more other features, means, steps, components or group thereof. Further, the word "a" or "an" preceding an element in a claim does not exclude the presence of a plurality of such elements. Moreover, any reference sign does not limit the scope of the claims.

Claims

CLAIMS:

1. A transceiver comprising: a transmitter (23) for transmitting a calibration signal via a transmission antenna (25); and - a receiver (100) for directly receiving said transmitted calibration signal via at least one receiving antenna (24); a calibration unit (30) for varying a transmission frequency of said calibration signal, and for detecting and storing a DC component in said received calibration signal in association with said transmission frequency; and - a compensation unit (32; 33; 35) for compensating a frequency dependent delay generated in said transceiver based on said stored DC component.

2. The transceiver according to claim 1, wherein said calibration unit (30) is arranged to store said DC component in a look-up table (37), and wherein said compensation unit (35) is arranged to access said look-up table during said compensation.

3. The transceiver according to claim 1 or 2, wherein said compensation unit (33) is arranged to post-process a signal received by said receiver unit (100) in order to compensate for said frequency dependent delay.

4. The transceiver according to claim 1 or 2, wherein said compensation unit comprises a pre-equalizer (32) provided in said transmitter (23) and arranged to pre-equalize a transmission signal based on said DC component in order to compensate for a frequency dependent delay obtained when a reflected component of said transmission signal is received by said receiver (100).

5. The transceiver according to claim 1 or 2, wherein said compensation unit comprises an equalizer (35) provided in said receiver (100) and arranged to equalize based on said DC component a reflected component of a transmission signal transmitted by said transmitter (23).

6. The transceiver according to claim 4 or 5, wherein said calibration unit (30) is arranged to detect and store said DC component during an initial calibration procedure performed prior to an actual use of said transceiver.

7. An automotive radar front end comprising a transceiver according to any one of claims 1 to 6.

8. A receiver module comprising: a receiver (100) for receiving a calibration signal via at least one receiving antenna (24); a calibration unit (30) for generating a control output to be used for controlling a transmission frequency of said calibration signal, and for detecting and storing a DC component in said received calibration signal in association with said transmission frequency; and a compensation unit (32; 33; 35) for compensating based on said stored DC component a frequency dependent delay generated in said receiver.

9. A method of compensating a group delay in a transceiver, said method comprising: varying a transmission frequency of a calibration signal; transmitting said calibration signal via a transmission antenna (25) of said transceiver circuit; directly receiving said transmitted calibration signal via at least one receiving antenna (24) of said transceiver circuit; detecting and storing a DC component in said received calibration signal in association with said transmission frequency; and - compensating a frequency dependent delay generated in said transceiver based on said stored DC component.

10. The method according to claim 9, further comprising storing said DC component in a look-up table (37), and accessing said look-up table during compensation.

11. The method according to claim 9 or 10, further comprising post-processing a signal received by said transceiver in order to compensate for said frequency dependent delay.

12. The method according to claim 9 or 10, further comprising pre-equalizing a transmission signal based on said DC component in order to compensate for a frequency dependent delay obtained when a reflected component of said transmission signal is received.

13. The method according to claim 9 or 10, further comprising equalizing based on said DC component a reflected component of a transmission signal transmitted by said transceiver.

14. The method according to claim 12 or 13, further comprising detecting and storing said DC component during an initial calibration procedure performed prior to an actual use of said transceiver.

15. A computer program product comprising code means for producing the steps of method claim 9 when run on a processing device.