1. Field of the Invention
This invention relates generally to transceiver systems and methods, and more particularly to a technique for solving transmitter impairments when a system has both receiver and transmitter operating with the same local oscillator frequency but with independent phase and gain impairments.
2. Description of the Prior Art
Schemes utilizing the complex conversion (down conversion or up conversion) use two local oscillators (shifted by 90 degrees) and two mixers to form two orthogonal output signals in down conversion. One is referred to as Real and the other as Imaginary. In the up conversion case the complex baseband signal is up converted to a real signal in IF. Implementing this scheme using analog components (phase shifter and mixer) causes phase, gain and delay mismatches. The delay mismatch can be regarded as phase mismatch as long as the signal bandwidth is relatively small with respect to its center frequency.
Some systems include both a transmitter and a receiver operating in conjunction with the respective local oscillators and having the same frequency but with independent phase and gain errors. Such system exist for example in the case of WLAN (802.11), where possible implementation includes a complex up converter and a complex down converter, both having the same local oscillator frequency. This approach, being low cost and simple, is widely used in many applications such as the WLAN. Each of the converters needs the phase and gain mismatch to be corrected in case of limited analog performance of the converters. Each one must be optimized independently, as transmitter and receiver specifications are set separately due to interoperability requirements between different vendors.
In view of the foregoing, a need exists for a technique to solve transmitter impairments when a system has both receiver and transmitter operating with the same local oscillator frequency but with independent phase and gain impairments.
The present invention is directed to a technique for solving transmitter impairments associated with a transceiver system when the system has both receiver and transmitter operating with the same local oscillator frequency but with independent phase and gain impairments.
According to one embodiment, a method of repairing frequency complex up-conversion phase and gain impairments in a up/down conversion transceiver comprises the steps of:
According to another embodiment, an up/down conversion transceiver comprises:
According to yet another embodiment, an up/down conversion transceiver comprises:
According to still another embodiment, an up/down conversion transceiver comprises:
Other aspects and features of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the invention becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing figures thereof and wherein:
While the above-identified drawing figures set forth particular embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention.
Assuming a received signal is of the form:
S(t)=Si(t)cos(ω0t)−Sq(t)sin(ω0t)
And two local oscillators, then
L01(t)=cos(ω0t+θ/2)
and
L02(t)=sin(ω0t−θ/2)
where,
The resulting signals after down-conversion (assuming low-pass filters after the mixers) are:
i(t)=S(i)cos(ω0t+θ/2)≈S(t)cos(ω0t)−S(t)sin(ω0t)θ/2;
and
q(t)=S(i)A sin(ω0t−θ/2)≈S(t)A sin(ω0t)−S(t)A cos(ω0t)θ/2
The resulting signal after up-conversion is:
S(t)=Si(t)cos(ω1t+Δ/2)−Sq(t)B sin(ω1t+Δ/2)=Si(t)cos(ωit)cos(Δ/2)−Si(t)sin(ωit)sin(Δ/2)−{Sq(t)B sin(ω1t)cos(Δ/2)−sq(t)B cos(ω1t)sin(Δ/2)}
where,
Si(t) and Sq(t) are the real and imaginary components of the complex signal that is up converted and Δ<<1 and B≈1 are the phase and gain imbalance (ratio of the q and the i path's gains) of the unconverted paths. ω1 is the up-converted center frequency, which is the local oscillators' frequency. Under these assumptions,
S(t)≈{Si(t)+BSq(t)Δ/2}cos(ω1t)−{Si(t)Δ/2+BSq(t)}sin(ω1t)
Inserting this into a complex down conversion with the phase and gain imbalance as described above yields,
i(t)≈S(t)cos(ω0t)−S(t)sin(ω0t)θ/2=└{Si(t)+BSq(t)Δ/2}cos(ω1t)−{Si(t)Δ/2+BSq(t)}sin(ω1t)┘cos(ω0t)−└{Si(t)+BSq(t)Δ/2}cos(ω1t)−{Si(t)Δ/2+BSq(t)}sin(ω1t)┘sin(ω0t)θ/2
and
q(t)≈S(t)A sin(ω0t)−S(t)A cos(ω0t)θ/2=└{Si(t)+BSq(t)Δ/2}cos(ω1t)−{Si(t)Δ/2+BSq(t)}sin(ω1t)┘A sin(ω0t)−└{Si(t)+BSq(t)Δ/2}cos(ω1t)−{Si(t)Δ/2+BSq(t)}sin(ω1t)┘A cos(ω0t)θ/2
The analysis herein below applies for a case where the up conversion and the down conversion are implemented with the same local oscillator frequency, meaning ω1=ω0, but the phase imbalances θ, Δ are independent. For this analysis it is also assumed ω1,0 are larger significantly than the signal bandwidth; therefore any terms around ω1 or 2ω1 are filtered with a low pass filter in the baseband. In this case,
The present inventors have discovered the phase and gain imbalance of the down-conversion can be addressed and canceled independently of the up-conversion impairment with external signal or noise. Therefore, in this analysis one can assume that A≈1 and θ≈0 after the correction algorithms. This yields,
i(t)≈½{Si(t)+BSq(t)Δ/2},
and
q(t)≈−½{Si(t)Δ/2+BSq(t)}.
As Si(t) and Sq(t) are independent, cross-correlating the two products (with zero delay) yields:
Riq(0)=E{i(t)q(t)}≈−⅛{Si2(t)+B2Sq2(t)}Δ.
Assuming again that the phase and gain imbalance are small, their estimation can be done as if decoupled; and the second order effect is negligible. Therefore, the phase imbalance estimate Δest is:
Δest≈−8Riq(0)/{Si2(t)+Best2Sq2(t)};
and the gain imbalance estimate Best is the empirical ratio in the receiver, since
Si(t)≈2i(t)−BSq(t)Δ/2,
BSq(t)≈−2q(t)−Si(t)Δ/2
and Δ is small enough to neglect its impact. We get,
Best=BSq(t)/Si(t)={−2q(t)−Si(t)Δ/2}/2i(t)−BSq(t)Δ/2}≈−q(t)/i(t)
Finally, we apply the gain and phase imbalance estimates to correct the transmitter up-converter impairments, and use modified real and imaginary inputs Sjmod(t) and Sqmod(t):
Sjmod(t)=Si(t)−Sq(t)Δest/2
Sqmod(t)=Sq(t)/Best−Si(t)Δest/2Best
Thus,
S(t)={└Si(t)−Sq(t)Δ/2┘+B└Sq(t)/B−Si(t)Δ/2B┘Δ/2}cos(ω1t)−S(t)={└Si(t)−Sq(t)Δ/2┘Δ/2+B└Sq(t)/B−Si(t)Δ/2B┘}cos(ω1t)={Si(t)−Si(t)Δ2/4}cos(ω1t)−{Sq(t)−Sq(t)Δ2/4}sin(ω1t)
And finally with error in the order of Δ2, we get
S(t)≈Si(t)cos(ω1t)−Si(t)sin(ω1t).
It is important to notice that the phase imbalance estimate algorithm of the up-converter and down-converter are the same; and measurements are done always in the receiver. This implies that the same hardware (HW) can be used for both, while in the case of the receiver impairments, the estimates of the phase and gain imbalance are used to correct forward the demodulated signals samples, while in the case of the transmitter, the estimates of the same mechanism in the receiver are fed back to the transmitter in order to correct the transmitted symbols.
The present inventors discovered the algorithm described herein above to have many significant benefits and characteristics:
In view of the above, it can be seen the present invention presents a significant advancement in the art of wireless transceiver applications. It should be apparent that the present invention represents a significant departure from the prior art in construction and operation. However, while particular embodiments of the present invention have been described herein in detail, it is to be understood that various alterations, modifications and substitutions can be made therein without departing in any way from the spirit and scope of the present invention, as defined in the claims which follow.