INTERFACE BETWEEN RADIO RECEIVER AND BASEBAND RECEIVER AND A METHOD FOR CONVERTING RF-SIGNALS TO BB-SIGNALS

Abstract

The invention relates to an analog-to-digital converter (ADC). The objective of the invention to have an analog-to-digital converter with the capability of non-equidistant sample time spacing and minimizing energy consumption will be solved by an apparatus comprising a sigma-delta modulator and a sample-time-counter, both controlled by a sample clock, a next-sample-time-computation unit configured to compute a sample-time-counter value when a next digital output sample is requested, a sample-computation-trigger unit connected to the next-sample-time-computation unit configured to compare an actual sample-time-counter value with the sample-time-counter value when the next digital output sample is requested and to trigger a computation unit for calculating a next digital sample when requested and by powering off the sigma-delta modulator in intervals where its delivered samples are not used for any computed decimator output sample. The objective is also solved by a method using the aforementioned analog-to-digital converter.

Claims

1. An interface between a radio receiver on a RF-side and a baseband receiver on a BB-side, wherein the radio receiver comprises means for receiving radio frequency signals and an analogue-to-digital converter for converting received analogue signals to digital signals, wherein the digital signals are further processed in the baseband receiver by a digital signal processing unit, and wherein the analogue-to-digital converter is a sigma-delta converter comprising a sigma-delta modulator on the RF-side and a decimation filter on the BB-side.

2. An interface between a radio receiver on the RF-side and a baseband receiver on a BB-side according to claim 1, wherein the decimation filter has a sampling rate and a FFT-length and the decimation filter is operable for selecting its sampling rate and its FFT-length according to a signal-to-noise ratio of the radio receiver.

3. An interface between a radio receiver on the RF-side and a baseband receiver on a BB-side according to claim 1, wherein the sigma-delta modulator is a multi-bit analog sigma-delta modulator.

4. An interface between a radio receiver on the RF-side and a baseband receiver on a BB-side according to claim 1, wherein the sigma-delta modulator comprises single-bit I and Q streams output lines connected to the baseband receiver.

5. An interface between a radio receiver on the RF-side and a baseband receiver on a BB-side according to claim 1, wherein the decimation filter is operable for selecting various output sampling frequencies.

6. An interface between a radio receiver on the RF-side and a baseband receiver on a BB-side according to claim 1, wherein the baseband receiver includes a signal-to-noise estimator.

7. An interface between a radio receiver on the RF-side and a baseband receiver on a BB-side according to claim 6, wherein the signal-to-noise estimator is connected to switches for selectable switching between different decimation filters and an output switch for outputting the digital signal.

8. A method for converting a radio-frequency signal received by a radio receiver on a radio-frequency side to a baseband signal in a baseband receiver on a baseband side wherein a signal-to-noise ratio of the radio receiver is estimated and according to the signal-to-noise ratio a decimation filter on the baseband side is selected.

9. The method for converting a radio-frequency signal received by a radio receiver on a radio-frequency side to a baseband signal in a baseband receiver on a baseband side according to claim 8, wherein a sampling rate and a FFT-length of the decimation filter is selected.

10. The method for converting a radio-frequency signal received by a radio receiver on a radio-frequency side to a baseband signal in a baseband receiver on a baseband side according to claim 8, wherein at a low signal-to-noise ratio the sampling rate is decimate to a smallest sampling frequency of the decimation filter and do decimation before the FFT.

11. The method for converting a radio-frequency signal received by a radio receiver on a radio-frequency side to a baseband signal in a baseband receiver on a baseband side according to claim 8, wherein at a high signal-to-noise ratio the sampling rate is decimate to a higher sampling frequency of the decimation filter and do part of a decimation inside the FFT.

12. An interface between a radio receiver on the RF-side and a baseband receiver on a BB-side according to claim 2, wherein the sigma-delta modulator is a multi-bit analog sigma-delta modulator.

13. An interface between a radio receiver on the RF-side and a baseband receiver on a BB-side according to claim 12, wherein the sigma-delta modulator comprises single-bit I and Q streams output lines connected to the baseband receiver.

14. An interface between a radio receiver on the RF-side and a baseband receiver on a BB-side according to claim 13, wherein the decimation filter is operable for selecting various output sampling frequencies.

15. An interface between a radio receiver on the RF-side and a baseband receiver on a BB-side according to claim 14, wherein the baseband receiver includes a signal-to-noise estimator.

16. The method for converting a radio-frequency signal received by a radio receiver on a radio-frequency side to a baseband signal in a baseband receiver on a baseband side according to claim 9, wherein at a low signal-to-noise ratio the sampling rate is decimate to a smallest sampling frequency of the decimation filter and do decimation before the FFT.

17. The method for converting a radio-frequency signal received by a radio receiver on a radio-frequency side to a baseband signal in a baseband receiver on a baseband side according to claim 16, wherein at a high signal-to-noise ratio the sampling rate is decimate to a higher sampling frequency of the decimation filter and do part of a decimation inside the FFT.

Description

[0031] The appended drawings show

[0032] FIG. 1 A radio receiver, an interface boundary between RF and baseband is at the analog-to-digital converter (ADC) output according to the state-of-the-art;

[0033] FIG. 2 A sigma-delta modulator as a 1st order single-bit continuous-time baseband sampling sigma-delta ADC, according to the state-of-the-art;

[0034] FIG. 3 A transmit spectrum of a narrowband LTE signal without special means for out-of-band power reduction (no pulse shaping);

[0035] FIG. 4 An anti-aliasing filter impulse response for different sampling rates;

[0036] FIG. 5 Signal-to-noise ratio of a receiver according to the sampling rate;

[0037] FIG. 6 Signal-to-noise ratio degradation of a receiver caused by added distortion;

[0038] FIG. 7 Architectural mapping of the inventive interface between radio receiver and baseband receiver;

[0039] FIG. 8 Detailed view of the inventive baseband receiver.

[0040] FIG. 7 shows the architectural mapping of the inventive interface between radio receiver 2 and baseband receiver 3. The sampling rate and the associated FFT length are selected depending on the receiver signal-to-noise ratio, in order to minimize sensitivity degradation caused by inter-symbol interference resulting from the anti-aliasing filter impulse response. At any sampling rate and FFT length the FFT will be part of the decimation filtering 17, thus relaxing requirements and simplifying the dedicated decimation filter 17 before the FFT 13. That cannot be achieved with state-of-the-art decimation filtering included in a sigma-delta ADC. Typical time domain FIR or IIR decimation filters have limited filter selectivity, with a wide transition band between passband and stopband. In contrast, FFT-based filtering is very sharp, with the filter bandwidth being the number of used FFT bins times the bin bandwidth. Therefore, the sigma-delta ADC 14 is split to keep only the sigma-delta modulator 22 on the RF side while moving the decimation filter 17 to the baseband side. Single-bit I and Q streams 1 pass the sigma-delta-modulator 22 output signals from the RF to the baseband IC, at a fixed rate. The decimation filters 171, 172 are selectable for various output sampling frequencies. Thus, the baseband side uses the FFT 13 as part of the decimation and can select: At low SNR, decimate to smallest sampling frequency and do basically all decimation before FFT 13; at high SNR, decimate to a higher sampling frequency and do part of the decimation inside FFT 13. FIG. 7 shows the signal flow. The FIR or IIR decimation filters 17 have a fairly wide transition band, and the FFT 13 performs sharp FFT bin filtering.

[0041] FIG. 8 shows the invention in more detail. The baseband receiver 3 includes a signal-to-noise estimator 19. The signal-to-noise estimator 19 estimates the signal-to-noise ratio of the receiver 2. If the estimated SNR falls below a certain threshold, a first sampling rate is selected, using a first decimation factor along with a first cyclic prefix removal unit 121 and a first FFT size 131. If the estimated SNR exceeds the threshold, a second sampling rate that is larger than the first sampling rate is selected, using a second decimation factor along with a second cyclic prefix removal unit 122 and a second FFT size 132.

REFERENCE SIGNS

[0042] 1 bit stream [0043] 2 radio receiver on RF-side [0044] 3 baseband receiver on BB-side [0045] 4 antenna [0046] 5 low-noise amplifier [0047] 6 mixer [0048] 7 low pass filter [0049] 8 voltage gain amplifier [0050] 9 analog-to-digital converter [0051] 10 PCM multiplexer [0052] 11 PCM demultiplexer [0053] 12 removing cyclic prefix [0054] 13 fast-fourier transformation means [0055] 14 sigma delta analog to digital converter [0056] 15 integrator [0057] 16 comparator [0058] 17 digital filter [0059] 18 digital signal [0060] 19 signal-to-noise estimator [0061] 20 input signal [0062] 21 internal quantizier [0063] 22 sigma-delta modulator [0064] 23 switch [0065] 24 output switch [0066] 121 first removing cyclic prefix means [0067] 122 second removing cyclic prefix means [0068] 131 first fast-fourier transformation means [0069] 132 second fast-fourier transformation means [0070] 171 first digital filter [0071] 172 second digital filter