Interference Suppression in Radio Receivers by using Frequency Retranslation T. Nesimoglu, Z. Charalampopoulos* and M. A. Beach* Middle East Technical University - Northern Cyprus Campus, Kalkanli, Guzelyurt, Mersin 10, Turkey. E-mail:
[email protected] * Centre for Communications Research, University of Bristol, University Walk, Bristol BS8 1TR, United Kingdom. E-mail:
[email protected].
[email protected].
Abstract In the presence of a strong received signal, non-linear radio frequency (RF) front-end components generate distortion products that add interference to a wanted signal. In this paper, an interference suppression technique using frequency retranslation is presented. When the wanted signal level is high, experimental results show a suppression ofintermodulation distortion (IMD) by 25 dB. When a small wanted signal is received along with high level interferers, the technique suppresses interferers by 14 dB and in-band interference by more than 18 dB. Index Terms: Interference suppression, frequency retranslation, intermodulation distortion, in-band interference.
1. Introduction Within a broadband receiver front-end [1]-[2], rejecting high level interferer signals and the linearity of the RF front-end is important for the performance of the radio. If the received wanted signal is high, the RF front-end components will operate in the non-linear region and will generate IMD products adding interference to the wanted signal. If the wanted signal is small but it is received along with a large interferer signal, IMD of the interferers may add in-band interference to the wanted channel. This makes it more difficult or, in the extreme case, impossible for the receiver to correctly detect the weak wanted signal. In radio receivers accommodating only one communication standard, the frequency of transmission and reception is fixed, and thus the out-of-band unwanted channels (blockers) can be rejected by filtering. In a reconfigurable radio application, the frequency of operation is variable since the receiver is required to
handle multiple standards. Therefore, rejecting strong nearby interferers necessitates a flexible filter [3]-[4] fully tunable over a broad bandwidth with high selectivity; such a filter is difficult to realize. In order to alleviate the inband interference and non-linearity problem, automatic gain control and digital attenuators are used to provide back-off. This reduces the dynamic range in which the receiver can be used. Alternatively linearization and interferer suppression techniques may be used. The first one improves the linearity of the RF front-end and the latter suppresses high level interferers to an acceptable level and prevents them from adding in-band interference to the wanted signal. Therefore, flexible filtering, linearization and interferer suppression are regarded as "enabling technologies" for software defined radio (SDR) applications [5]-[6]. This paper presents a technique, which is a potential solution to overcome non-linearity and interferer suppression problem not only for reconfigurable radio applications but also for a traditional receiver front-end.
2. Linearization by means of Frequency Retranslation In receivers, the received clean wanted signal and the output where the IMD products exist are at different frequencies due to the frequency translating mixer stage. Two linearization techniques based on feedforward were proposed for radio receiver applications, where the reference signal was frequency translated by a backed-off or a saturated secondary mixer. In [7] the secondary mixer is backed-off to operate in its linear region, as shown in Figure 1. This mixer downconverts the reference signal to the same IF as the output of the main mixer ideally undistorted. This signal when used as a reference is only an approximation to the
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required reference signal. The output of the main mixer, which includes IMD is coupled and added in anti-phase to the output of the secondary mixer, thus cancelling the wanted signals. This error signal is also an approximation to the requir ed error signal, which is then recombined at the output combiner to suppress the IMD at the IF output. According to our investigations the disadvantage of this architecture is that the signal-to-noise ratio (SNR) of the referen ce path is significantly reduced since it is operating at a much lower RF power. This adds noise to the main path when the error signal is combined at the output combiner to suppress the IMD, which would make the receiver less sensitive to the received signals, and also reduce the dynamic range of the receiver. The experimental results show 25 dB third-order IMD (1M3) reduction at 70 MHz of IF when the prototype was used as a downconverter with 500 MHz of RF input.
secondary mixer is driven with a much higher RF signal than the main mixer to provide a high level of IMD which is also an approximation to the required error signal, also providing a high SNR. This error signal is amplitude and phase adjusted before being added to the final IF for suppressing the IMD. Our investigations show that this technique offers a low dynamic range. High levels of 1M3 reduction was obtained, about 30 dB, but the performance was critically dependent on the amplitude matching of the IMD products and the mismatching characteristics of the two mixers.
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Figure 4: Frequency retranslation test-bed. The block diagram of frequency retranslation used for linearization is shown Figure 3 [9]-[10]. The distorted output of the downconverting mixer at IF is coupled , amplified, frequency retranslated back to RF by the upconverting mixer and then filtered to remove the unwanted image signals. The clean wanted reference signal at the receiver front-end is also coupled and added in anti-
phase to the frequency retranslated IF output (which is now at RF) with correct amplitude. This process cancels the wanted signals and produces an error signal ideally including only the IMD products. This error signal is then combined with the received RF input signal with correct amplitude and phase relation to predistort the saturated downconverting mixer. This provides suppression of the IMD without affecting the wanted signal level, if the signal cancellation is also correctly optimized.
2.1
Experimental Results of Linearization
The picture of the constructed prototype is shown in Figure 4, where two passive double-balanced mixers were used as the main non-linear components. The amplitude and phase adjustment of the different signal paths has been performed using voltage variable components .
A two-tone-test was applied at 920 MHz to provide a downconverted signal at an IF of 160 MHz where the LO signal was set to 760 MHz. The IF output of the receiver with and without the technique applied is given in Figure 5, indicating about 25 dB suppression of 1M3. The signal cancellation provides about 40 dB suppression of wanted signals (see Figure 6). Noise power measurements indicate only 0.2 dB increase in the noise figure when the Iinearisation is applied, which is negligible. The frequency response of the signal cancellation process across 10 MHz of bandwidth centered at 920 MHz is shown in Figure 7. The notch of the signal cancellation in linearization application was tuned to 920 MHz in order to suppress the wanted signals and obtain an error signal that consists of unwanted IMD products (see Figure 6). CHI S2 1/k M
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3. Interferer Suppression by means of Frequency Retranslation
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Figure 6: Error signal obtained in signal cancellation.
The frequency retranslation receiver architecture used for interferer suppression is shown in Figure 8. Here, the wanted signal is received along with high level interferers. The notch of the signal cancellation loop (see Figure 7) was tuned to the frequency of the wanted signal so that the wanted signal is cancelled. This produces an error signal containing only the unwanted interferers and the IMD products. The error signal is amplitude and phase adjusted and then combined with the input received signal such that the interferers and thus the IMD products are suppressed at the receiver output.
down-converted wanted signal at 165.5 MHz. The 1M3 at 170 MHz is the highest IMD product of the interferers. If it masks the wanted signal, the detection of the wanted signal will be impossible under these operating conditions. As shown in Figure 10, the application of the frequency retranslation technique has reduced the high level interferers by 14 dB and the 1M3 by more than 18 dB with no chang e in wanted signal level. - 10
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Figure 8: The frequency retranslation applied for interferer suppression in a receiver.
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Experimental Results of Interferer Suppression
The interferer and in-band interference suppr ession performance of the system was measured by applying a three-tone-test where the high level interferers are the twotones with equal magnitudes at 940 and 950 MHz, and the small wanted signal is the single-tone at 925.5 MHz (see Figure 9). The interferers were set to -5 dBm which was sufficient to generate non-linear distortion whereas the small wanted signal was at -45 dBm. The frequency separation between the tones was intentionally set unequal so that the in-band interference, i.e. the IMD products and the wanted signal would not overlap and they could be measured independently. Figure 9 shows the error signal produced by the signal cancellation process; the wanted signal was suppressed by more than 14 dB with a minimal effect on the other nearby signals. The LO frequency was set to 760 MHz and as shown in Figure 10, this provides a
4. Conclusion In this paper, frequency retranslation has been presented as a potential solution to minimise the degrading effects of high level interferers and RF front-end non-linearity in radio receiver applications. Experimental results indicate 25 dB suppression of 1M3 products without degrading the SNR of the receiver. Also, more than 18 dB suppr ession of in-band interference was demonstrated whereas the interferers were suppressed by about 14 dB. In a practical application, received signal characteristics change as a function of the environment and an automatic control system is advised for this scheme to maintain the system performance,
REFERENCES [1] J. Mitola, The Software Radio Architecture, IEEE Communications Magaz ine, May 95, Pages: 26-38. [2] J. R. MacLeod, M. A. Beach, P. A. WaITand T. Nesimoglu, A Software Defined Radio Receiver Test-Bed, IEEE Vehicular Technology Conference, VTC'OI-Fall, Atlantic City, Vol: 3, Pages: 1565-1569.
[3] B. Carey-Smith, P. A. WaIT, M. A. Beach and T. Nesimoglu, Tunable lumped-distributed capacitivelycoupled transmission-line filter, lEE Electronics Letters ,Vol: 40, Issue: 7, April 2004, Pages:434 - 436. [4] B. E. Carey-Smith, P. A. WaIT, M. A. Beach and T. Nesimoglu, Wide Tuning-Range Planar Filters Using Lumped-Distributed Coupled Resonators, IEEE Transactions on Microwave Theory and Techniques, Volume: 53, Issue: 2, Feb. 2005, Pages: 777-785. [5] W. Tuttlebee, Software Defined Radio: Enabling Technologies, June 2002, John Wiley & Sons, ISBN: 0470843187. [6] 1. R. MacLeod, T. Nesimoglu, M. A. Beach and P. A. WaIT, Enabling Technologies for Software Defined Radio Transceivers, Military Communications ConferenceMILCOM, October 2002, California, USA, Pages: 354-358.
[7] T. 1. Ellis., Modified feed-forward technique for mixer linearization, IEEE MTT-S International Microwave Symposium Digest, v 3, 1998, Pages: 1423-1426. [8] M. Chongcheawchamnan and I. D. Robertson, Linearized microwave mixer using simplified feedforward technique, Electronics Letters, v 35, n 9, 1999, Page(s): 724-725. [9] T. Nesimoglu, M. A. Beach, P. A. WaIT and 1. R. MacLeod, Linearised Mixer using Frequency Retranslation, lEE Electronics Letters, December 2001, Vol. 37, No. 25, Pages: 1493-1494. [10] T. Nesimoglu, M. A. Beach, 1. R. MacLeod and P. A. WaIT, Mixer Linearisation for Software Defined Radio Applications, IEEE Vehicular Technology Conference, September 2002, Canada, Pages: 134-138.
