Measuring angle of incidence in an ultrawideband communication system

Abstract

In an ultra-wideband (“UWB”) receiver, a received UWB signal is periodically digitized as a series of ternary samples. During a carrier acquisition mode of operation, the samples are continuously correlated with a predetermined preamble sequence to develop a correlation value. When the value exceeds a predetermined threshold, indicating that the preamble sequence is being received, estimates of the channel impulse response (“CIR”) are developed. When a start-of-frame delimiter (“SFD”) is detected, the best CIR estimate is provided to a channel matched filter (“CMF”). During a data recovery mode of operation, the CMF filters channel-injected noise from the sample stream. Both carrier phase errors and data timing errors are continuously detected and corrected during both the carrier acquisition and data recovery modes of operation. The phase of the carrier can be determined by accumulating the correlator output before it is rotated by the carrier correction. By comparing the carrier phases of two receivers separated by a known distance, d, the angle of incidence, θ, of the signal can be determined.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Stage of PCT Application No. PCT/EP2014/060722, filed 23 May 2014 (“Parent PCT Application”).

The

where:

• • I_tim=timing loop integrator;
  • I_car=carrier loop integrator;

$\mathrm{Scale}=\frac{F_s}{2F_c}$

• • F_s=sampling rate; and
  • F_c=carrier frequency.
    Since F_s and F_c are related this is simpler in practice:

	Fc (MHz)	Channel	Scale Factor
	3494.4	1	1/7
	3993.6	2, 4	1/8
	4492.8	3	1/9
	6489.6	5, 7	1/13

As shown in FIG. 14, we prefer to allow the carrier recovery loop to settle on a good quality estimate before seeding the timing recovery loop; we define the delay in terms of the carrier recovery loop gearing counter and prefer to make this threshold value programmable. Once this gearing counter threshold value is reached, the value held in the carrier recover loop integrator is scaled according to the table above (depending on the channel setting) to produce a timing seed value with the precision S[−1:−15]. Rounding does not need to be applied to this computation, just truncation, because the computation is a one-time event in the receiver 10′ and does not take part in a recursive loop—therefore the bias introduced by not rounding will not accumulate to cause significant inaccuracy.

In our preferred embodiment, we implement a register-based field programmable gear shifting mechanism. Ten gears may be configured; one is reserved for demodulation mode, allowing nine acquisition gears. Each gear is assigned: a count at which it is activated; a K_p value; and a K_i value. Writing a value of logic_0 as the count for a gear other than the first gear terminates the gear shifting table; whilst still switching to the demodulation gear when the acquisition phase is over. Note that two sets of demodulation coefficients must be specified, one for the 110 Kbps data rate case, and one for the 850K and 6.81 Mbps cases. The default values for each of the available programmable registers are given in the following table:

TABLE 4
Timing Estimator Default Programmable Gear Shifting Register Values
	Default
Register	Value	Count [9:0]	Ki [14:10]	Kp [19:15]
TR0	0XF8000	0	0	31
TR1	0x8141E	30	5	16
TR2	0X58428	40	1	11
TR3	0X00000	0	0	0
TR3	0X00000	0	0	0
TR5	0X00000	0	0	0
TR6	0X00000	0	0	0
TR7	0X00000	0	0	0
TR8	0X00000	0	0	0
TR9	0X5A161	N/A	⅛ (110 Kbps)	11
where:
Value comprises a 20-bit variable expressed in hexadecimal format;
Count comprises bits [9:0] of the Value;
Ki comprises bits [14:10] of the Value; and
Kp comprises bits [19:15] of the Value.

The K factors are coded as follows:

TABLE 5
Gear Shifting Register Value Decode
	Minimum	Maximum	Decode
	Count	1	1023	Sample count on which
				to apply gearing values
	Ki	0x00	0x1F	0x00 = 00
				0x01 = 1 × 2−7
				0x1F = 31 × 2−7
	Kp	0x00	0x1F	0x00 = 00
				0x01 = 1 × 2−7
				0x1F = 31 × 2−7

Computing Angle of Incidence:

In a practical coherent receiver, it is necessary to track the carrier of the transmitter. For example, in the system of FIG. 16, both receivers 70 will be doing this, but because of different noise input mixed with the received signal, they will not necessarily do it identically. In, for example, the clock tracking loop shown in FIG. 3, the correlator output is accumulated to identify the channel impulse response, but before it is accumulated it is rotated by a carrier correction. Because this rotation is likely to be different in each of the receivers 70, it must be undone in order to calculate the phase difference between the two carriers. For example, with reference to FIG. 3, during an angle of incidence calculation mode, the rotator 52 can be rendered inoperative (or, alternatively, LUT 50 can be configured to output a fixed rotation of 0□); otherwise, the logic 46 works as described above. The phase difference between the two carriers is therefore the difference between the angles of the first paths in the accumulators minus the individual phase corrections that have been applied at the time the accumulation of the channel impulse response stops and is measured. FIG. 21 shows the calculated angle of arrival when repeated, using this method, on 100 separate packets. In the test, the actual angle of arrival was −5□ and the antennas were separated by one wavelength. The carrier frequency used in this test was 4 GHz, and the standard deviation of the error was 2.1□.

We propose two ways to solve the ambiguity in solutions that occurs at an antenna separation of more than ½ a wavelength. First, we measure the time of arrival of the packet at each antenna. The angle of incidence that is most consistent with the measured time of arrival differences is the one chosen. Take the example shown in FIG. 18. If a is measured as −125° then there are two possible values for θ either −20° or +40°. If the signal arrived at antenna A first, then −20° is the correct but if the signal arrives at antenna B first, then +40° is correct. Second, we resolve the ambiguity is by sending two packets but at different carrier frequencies. FIG. 20 shows an example of the relationship between α and θ for two different carrier frequencies. Because the curves are different, only one of the possible solutions occurs at both carrier frequencies. For example, if θ, the angle of incidence was −50° then a at 4 GHz would be measured as +90° which could correspond to an α of either −50° or of +15°. At 6.5 GHz would be measured at −90° which could correspond to an a of either −50°, 10° or +24°. Since −50° is the only solution in common, it must be the correct one. Of course in practice, noise in the system means that the common solution will not be exactly the same so it will be necessary to choose the solution which has the smallest difference in the two sets of possible solutions.

Even if the two receivers 70a and 70b are fed from the same clock, it may happen that the delay of this clock to one receiver is different to the delay to the other receiver. In this case there will be a fixed phase difference between the carriers. However, this phase difference can be calibrated, e.g., by measuring a at a known angle of arrival and subtracted from a, before applying the formula of Eq. 6.

Rather than supplying the two different PLLs, 76a and 76b, with the common crystal 72, there are other ways to synchronize the receivers 70, e.g, the two receivers 70a and 70b could be synchronized by supplying both with a clock from a single PLL, e.g., the PLL 76a.

Although we have described our invention in the context of particular embodiments, one of ordinary skill in this art will readily realize that many modifications may be made in such embodiments to adapt either to specific implementations. By way of example, it will take but little effort to adapt our invention for use with a different ADC scheme when it can be anticipated that the target application will not be subject to significant levels of in-channel CW interference. Further, the several elements described above may be implemented using any of the various known semiconductor manufacturing methodologies, and, in general, be adapted so as to be operable under either hardware or software control or some combination thereof, as is known in this art. Alternatively, the several methods of our invention as disclosed herein in the context of special purpose receiver apparatus may be embodied in computer readable code on a suitable computer readable medium such that when a general or special purpose computer processor executes the computer readable code, the processor executes the respective method.

Thus it is apparent that we have provided an improved method and apparatus for use in the receiver of a UWB communication system to determine angle of incidence. In particular, we submit that such a method and apparatus should provide performance generally comparable to the best prior art techniques but more efficiently than known implementations of such prior art techniques.

Claims

1. In a radio frequency (“RF”) system comprising an rf transmitter, a first rf receiver having a first antenna, and a second rf receiver having a second antenna, the first and second antennas being separated by a predetermined distance, d, a method comprising the steps of:

[1] in the first and second receivers receiver, using respective a first and second clock tracking loops loop to synchronize the carrier phase of said the first receiver to the transmitter and in the second receiver, using a second clock tracking loop to synchronize the carrier phase of the second receiver to the transmitter;

[2] in the transmitter, transmitting an ultra-wideband (“UWB”) signal having a predetermined carrier wavelength, λ;

[3] in the first receiver:

[3.1] receiving the transmitted UWB signal;

[3.2] developing a first phase value as a function of the complex baseband impulse response of said received UWB signal; and

[3.3] correcting the first phase value by subtracting the phase of the first clock tracking loop;

[4] in the second receiver:

[4.1] receiving the transmitted UWB signal;

[4.2] developing a second phase value as a function of the complex baseband impulse response of said received UWB signal; and

[4.3] correcting the second phase value by subtracting the phase of the second clock tracking loop;

[5] developing a phase difference value, α, as a function of difference between the corrected first and second phase values; and

[6] developing an angle of arrival, θ, of the transmitted UWB signal relative to the first receiver as a function of d, λ and α.

2. The method of claim 1 wherein, if d is at least λ/2, step [6] is further characterized as:

[6] developing a plurality of angles of arrival, θ, of the transmitted UWB signal relative to the first receiver according to the following:

3. The method of claim 1 wherein, if d is at least λ/2, step [6] is further characterized as:

[6] developing a plurality of angles of arrival, θ, of the transmitted signal relative to the first receiver according to the following:

4. A non-transitory computer readable medium including executable instructions which, when executed in a processing system, cause the processing system to perform all of the steps of a method according to any one of claims 1, 2 and 3.