This invention relates to a low Intermediate Frequency (‘IF’) radio receiver and especially, but not exclusively, to Very Low IF (‘VLIF’) receivers. The expression “low IF” refers to intermediate frequencies that are comparable with the bandwidth of the resulting baseband signal and VLIF to intermediate frequencies that are much smaller than the bandwidth of the resulting baseband signal.
Very Low IF receivers are widely used for slot-based radio communications and, in particular Time Division Multiple Access (‘TDMA’) protocols such as the Global System for Mobile communications (‘GSM’), Digital Enhanced Cordless Telecommunications (‘DECT’) and Enhanced Data for GSM Evolution (‘EDGE’), in the General Packet Radio Service (GPRS), an extension to the GSM standard that provides higher speed access. These receivers need to reject interferer signals that fall as image frequencies on or very close to the wanted signal when the wanted signal is translated to the desired intermediate frequency. This is achieved through image cancelling mixers or poly-phase filters at low frequency. The success of these image cancelling techniques depends upon the balance achieved in terms of phase and gain of the I (in phase) and Q (quadrature phase) paths in the receiver. Patent specifications U.S. Pat. No. 6,597,748 and EP 1 058 378 describe receivers of this kind.
The balance of I and Q in the receiver may be achieved by precision analogue design and by compensation in the form of a digital equaliser. This adds cost in terms of yield, software overhead, manufacturing overhead and extra hardware. Moreover, the image rejection varies with temperature and frequency band.
The image channel is often one of the adjacent frequency channels or alternate adjacent channels to the wanted one. The other adjacent or alternate adjacent channel is not an image frequency and can be rejected adequately without the need for such accurate quadrature balance. Instead it is easily rejected by standard filter topologies. Selectivity is especially a problem for alternate channels on the ‘low side’ IF, that is to say where the local oscillator (‘LO’) frequency used to convert the carrier frequency down to IF is smaller than the carrier frequency, so that the IF is positive, regarding interferers at twice channel spacing (400 kHz in the example shown in
Patent specification US2002/0183030 describes radio receiver and transmitter apparatus that includes Local Oscillator (LO) frequencies LowLO and HighLO selectable based on the frequency band of operation of the mobile communication device in the transmitter, whereas in the receiver the LO inputs LowLO and HighLO are connected to different receiver paths via a sub-harmonic mixer.
The specifications for low IF radio receivers include stringent requirements regarding the rejection of interferers and there is a need to meet or surpass these specifications at minimal cost.
The present invention provides a radio receiver as described in the accompanying claims.
The receiver shown in
In operation, the wanted signal presents a bandwidth 14, shown in
As described in patent specification U.S. Pat. No. 6,597,748, the image rejection depends on the gain and phase imbalances of the I- and Q-channels. Digital gain and phase correction as described in patent specification EP 1 058 378, for example, enables significant compensation of these errors to be achieved and gives a high degree of image rejection. However, as described above, it is desirable to improve image rejection since the receiver treats them like an image and part of the interferer spectrum falls in the wanted band. In particular, in certain applications, selectivity is desired to be at detected carrier to interference (C/I) levels better than 10 dB for a received interferer (I) at 41 dB greater than the carrier (C) where carrier is the wanted channel, even at carrier frequencies greater than 2 GHz.
In the embodiment of the invention shown in the drawings, the local oscillator means includes frequency alternation means for causing the local oscillator frequency to alternate in successive time-slots between first and second values one of which is greater and the other smaller than the desired carrier frequency of the input signal. More specifically, in the embodiment shown in
In order to maintain the polarities of the VLIF I- and Q-channel mixer stage 11, this mixer stage comprises first and second I-channel mixers 21 and 22 and first and second Q-channel mixers 23 and 24. A VLIF two-position switch 25 synchronised with the LO two-position switch 20 provides phase alternation and alternately applies the VLIF LO signal from the VLIF local oscillator 19 to the first I- and Q-mixers 21 and 23 in one time-slot and the phase-shifted VLIF LO signal from the VLIF phase shifter 13 to the second I- and Q-mixers 22 and 24 in the next time-slot. The I-mixers 21 and 22 mix these signals with the signal from the I-channel ADC 9 and the Q-mixers 23 and 24 mix these signals with the signal from the Q-channel ADC 10. The signals from the first I-mixer 21 and the second Q-mixer 24 are supplied to an adder 26, which adds the mixed signals to produce a Q-output signal at a Q-output 27 and the signals from the second I-mixer 22 and the first Q-mixer 23 are supplied to a subtractor 28, which subtracts the mixed signals to produce an I-output signal at a I-output 29.
The signal amplitudes and phases can be expressed as follows:
In the case where the Local RF oscillator is higher than the Wanted channel frequency, the High Side Low IF, the low IF receiver I,Q vector output can be expressed as:
I
out(t)+j·Qout(t)=Aw(t)/2·e−j·φw(t)**Hwanted+Ai(t)/2·e−j·(φi(t)+2·n·finterferer·t)**Hwanted (term a)+Aw(t)/2·e+j·(φw(t)−4·n·fIF·t)**Himage+Ai(t)/2·e+j·(φi(t)+2·n·finterferer·t−4·n·fIF·t)**Himage (term b)
where Aw(t) is the wanted channel amplitude signal,
where φw(t) is the wanted channel phase signal,
where fIF is the intermediate IF frequency,
where Ai(t) is the interferer channel amplitude signal,
where φi(t) is the interferer channel phase signal,
where Finterferer is the offset of the interferer frequency versus the wanted channel frequency (e.g in GSM the +400 khz alternate channel interferer case is considered),
where Hwanted is the wanted channel filter impulse response expressed as Hwanted=(HI+HQ)/2 where HI is the I path channel impulse response and HQ is the Q path channel impulse response, and
where Himage is the Image channel filter impulse response expressed as Himage=(HI-HQ)/2 where HI is the I path channel impulse response and HQ is the Q path channel impulse response.
Ideally if HI=HQ, ie perfect quadrature over frequency, then Himage is nullified.
The term b represents the image portion that will fall inside the RX band based on Himage which is not nullified due to the non-ideal matching between I and Q paths, so for example if finterferer=2*fIF, the second portion of term b becomes:
Ai(t)/2·e+j·(φi(t))**Himage
which is centered at 0 frequency, ie falling inside the wanted RX channel and the amplitude of this interference depends on the level of the interfence max(Ai(t)) and the image rejection magnitude abs(Himage).
In the case where the Local RF oscillator is lower than the Wanted channel frequency, ie fLO=fwanted channel−fFI, the Low Side Low IF, the low IF receiver I,Q vector output can be expressed as:
I
out(t)+j·Qout(t)=Aw(t)/2·e−j·φw(t)**Hwanted+Ai(t)/2·e−j·(φi(t)+2·n·finterferer·t)**Hwanted (term a′)+Aw(t)/2·e+j·(φw(t)+4·n·fFI·t)**Himage+Ai(t)/2·e+j·(φi(t)+2·n·finterfer·t+4·n·fFI·t)**Himage(term b′)
The term b′ represents the image portion that will fall inside the RX band based on Himage which is not nullified due to the non-ideal matching between I and Q paths, so for example if finterferer=2*fIF (same interferer location as in the High Side case), the second portion of term b′ becomes:
Ai(t)/2·e+j·(φi(t)+2·n·4fFI·t)**Himage
which is centred at 4·fIF frequency, ie falling outside the wanted receiver channel thus resulting in removing this contribution term during the time where the receiver is operating in Low side Low IF Mode.
Since the local oscillator frequency fLO alternates between High Side and Low Side, the outputs alternate between the values indicated and the average impact of the interferers present is reduced. It is not necessary to know on which side of the wanted frequency band the interferer frequency occurs, which is often not even possible, provided that the alternation between High Side and Low Side occurs several times while the interferer is on the same side of the wanted frequency band.
It will be appreciated that, instead of the implementation shown in
It will also be appreciated that in certain applications, it is not necessary to alternate the local oscillator frequency between the two LO frequencies at each successive time-slot, provided that the alternation occurs often compared to the fluctuations of the interferers that is to say many times in the same block (of 2000 slots in the case of GSM and EDGE).
Number | Date | Country | Kind |
---|---|---|---|
03293343.4 | Dec 2003 | EP | regional |
Number | Date | Country | |
---|---|---|---|
Parent | 10596910 | Nov 2006 | US |
Child | 12705836 | US | |
Parent | PCT/EP04/14913 | Dec 2004 | US |
Child | 10596910 | US |