Chapter 2 Multi-Band/Multi-Mode Radio Systems
2.3 A Review of Existing Multi-Mode/Multi-Band Radio Architectures
2.3.1 Multi-Band Receiver Architectures
In practice, the high selectivity required in narrow-band communications (Table 2.2) can hardly be achieved with any practical filter at radio frequencies. Therefore, the desired selectivity is often achieved in multiple stages. The carrier frequency of the received signal is lowered in one or more steps allowing for filtering and demodulation at lower frequencies. In a common process known as down-conversion, the carrier frequency of a modulated signal can be reduced by multiplying (or mixing) it with a constant tone signal of a local oscillator (LO)4. Unfortunately, in a multiplier, the image of the main signal with respect to the LO frequency is down-converted along with the main signal to the same frequency causing an unwanted cross-talk (Figure 2.2). Usually, distinct receiver architectures vary in the number of down-conversion stages, the method to remove the unwanted image signal, as well as the choice of placement and value of gain and filtering stages.
Figure 2.2: Illustration of cross-talk caused by the image signal in the receiver
Most of today’s wireless receivers use any of these general architectures: heterodyne, homodyne, low-IF, and image-rejection receivers such as the ones originally proposed by Weaver  or Hartley . Similarly, all these architectures have been employed in the multi-band receivers for various applications. In the following sections, advantages and disadvantages of each of these architectures for multi-band and multi-mode systems are briefly discussed.
Similarly, the carrier frequency can be increased in a mixing process (up-conversion). LO fLO -fLO desired signal image signal fRF -fRF -fRF+fLO fRF-fLO |fRF-fLO| RF IF
Heterodyne receivers achieve a large selectivity by down-converting the received RF signal in multiple steps (usually two), and by extensive filtering at each intermediate-frequency (IF) section before converting the analog baseband signal to a digital one for demodulation and further signal processing. Filtering at RF and IF stages is usually achieved via external SAW filters that attenuate the energy at the image frequency band for reduced cross-talk. In a multi-band heterodyne system, there will be a larger number of external image-rejection filters resulting in a larger footprint, higher power consumption, and a higher overall cost for the system. A simplified schematic of a triple-band receiver for CDMA phone standards integrated with GPS capability (quad-mode radio) that uses a heterodyne architecture is shown in Figure 2.3 ().
Figure 2.3: A multi-band / multi-mode receiver based on Heterodyne architecture 
The cross-talk caused by the image signal can also be eliminated if a pair of multipliers with in-phase and quadrature-phase LO signals are used in the down-version stage.
Homodyne (also known as zero-IF or direct-conversion) receivers perform the radio frequency down-conversion in only one step using a pair of high dynamic-range in-phase
GPS SAW High-Band Duplexer Low-Band Duplexer IF SAW Filters Baseband I & Q Outputs IF Mixer RF Mixer LNA Band-Select Filter
and quadrature mixers. Image-frequency and interference rejections as well as amplitude control are performed at dc for the most part. Unfortunately, several sources of noise, including self-mixing of the LO signal due to leakage, component mismatches, and low- frequency noise of devices contribute to an undesired amount of signal energy at the zero frequency ,. A direct-conversion scheme requires extra circuitry for the removal of the so-called dc-offset signal.
The aforementioned problem of dc-offset can be eliminated in a low-IF architecture, where the received RF signal is down-converted to a very low frequency instead of zero frequency. However, low-IF architectures necessitate low-frequency building blocks such as filtering and gain stages with a higher bandwidth to further process the signal. A higher bandwidth in many of these blocks including analog-to-digital converters is usually gained at the price of larger power consumption. Final down-conversion to zero frequency that might be necessary in some of the low-IF implementations can be done at the digital domain.
Many prefer the added expense of baseband signal-processing in these systems for the benefit of reduced number of off-chip components which usually translates to a cheaper overall cost compared to heterodyne architectures. Figure 2.4 shows a simplified schematic of a quad-band mobile-phone receiver for GSM standards that uses a direct-conversion architecture . Note that in-phase and quadrature-phase paths can be generated by phase shifting the RF signal (Figure 2.4), or more commonly, the LO signal .
Figure 2.4: A direct-conversion multi-band receiver for mobile-phone applications 
The required high-selectivity in most standards is achieved through bulky SAW filters in heterodyne receivers or through high dynamic-range wide-band baseband circuits in direct- conversion and low-IF architectures. Hence, architectures that select the narrow-band channel of interest while rejecting the image-frequency before baseband without the use of any IF SAW filters are desirable. Down-converting the RF signal in orthogonal phases (in- phase and quadrature) and combining the outcome constitutes the basis for image-reject architectures -. Usually, the image-rejection of these architectures is limited to the matching of integrated components and can be enhanced with digital calibration . An example of such an image-reject receiver for a dual-band system, where clever frequency planning allows for sharing of many circuit blocks, is shown in Figure 2.5 (). The propagation of the one of the desired inputs and its corresponding image signal along the receiver chain is also depicted to illustrate the image rejection principle of the Weaver down-converter. Baseband I & Q Outputs Tx/Rx Switch 90°°°° 90°°°° GSM850 GSM900 DCS1800 PCS1900 SAW Filters DCOC DCOC DC-Offset Correction LO1 LO2
Figure 2.5: Dual-band receiver based on Weaver’s image-rejection scheme