Third-generation (3G) cellular networks offer unprecedented functionality and services for consumers, but also a host of challenges for handset designers. To provide the many services available, a mobile handset must work seamlessly across multiple wireless networks, such as wideband codedivision- multiple-access (WCDMA) and GSM/EDGE systems. Ideally, for reduced design cycle time and lower bill-of-material (BOM) costs, a practical transceiver solution should communicate across both networks as a WEDGE (WCDMA + EDGE) device. In the case of high-speed packetaccess capabilities, it would be considered a HEDGE solution.

In designing such a solution, it is important to understand the design specifications for each system and how they impact the transceiver’s operation in terms of compatibility across the other systems In practical terms, even if a transceiver has been optimized for use with a particular wireless standard, such as GSM, it must also work with a relatively high performance level with other wireless standards, such as WCDMA.

When specifying multiple WEDGE chipsets, for example, appropriate trade offs can be made to select the best solution for a given application. For the radio-frequency-integratedcircuit (RFIC) design team, the chipset must meet the type approval tests listed in the WCDMA and GSM standards. Each test case in these type approval procedures corresponds to a particular specification in the design of the chipset.

The challenge in building a WEDGE receiver, for example, is because of the differences in requirements for WCDMA and GSM systems. In order to understand the performance requirements of a WEDGE receiver, it is necessary to carefully review the requirements of each wireless communications standard first to discover common ground. Once understood, the requirements of each standard can be readily translated into design specifications for the WEDGE receiver. A direct-conversion receiver will be used for the example analysis.

The requirements for a Global System for Mobile (GSM) Communications handset receiver cover a number of different equipment designations, including GPRS and EDGE. A GSM receiver typically covers four frequency bands at 850, 900, 1800, and 1900 MHz. A GSM receiver must be able to demodulate an input signal at a level of -102 dBm and achieve a bit-error rate (BER) of better than than 10^-4. A GSM amplitude modulation (AM) suppression test requires a receiver sensitivity of -99 dBm with a -31-dBm Gaussian minimum-shiftkeying (GMSK) modulated blocker 6 MHz away. This test simulates the receiver operating in the desired channel and being jammed by the other inband channel. The worst-case signal blocker is a -23-dBm unmodulated tone 3 MHz away. This test evaluates the receiver’s reciprocal mixing performance. Intermodulation tests are set up with two signals at -49 dBm. One is a single tone 800 kHz away from the carrier while the other is a GMSK-modulated signal 1600 kHz from the carrier. The co-channel interference is -9 dBc. The adjacentchannel interference is specified at +9 dBc while the alternate-channel interference is -41 dBc.

Based on these requirements, the following receiver specifications can be derived: noise figure, input second- order intercept point (IIP2), input third-order intercept point (IIP3), image-rejection ratio (IRR), adjacentchannel rejection, alternate-channel rejection, phase noise, input carrier compression point (ICP), automatic- gain-control (AGC) range, and analog-to-digital-converter (ADC) signal-to-noise ratio (SNR). In working through the list, the noise figure is determined first. For GSM, the channel bandwidth is 170 kHz. In the static case, the SNR requirement is 6 dB, plus a 3-dB fading margin. Thus, the minimal required SNR is 9 dB. The thermal noise floor is -174 dBm/Hz. The receiver (Rx) sensitivity (RX_SEN) can be found by Eq. 1.

The only unknown in this expression is the noise figure, which can be solved as 10.7 dB in the static case. This is the minimum acceptable noisefigure performance needed in a GSM system although, in reality, most GSM receiver chipsets have about a 7 to 8 dB margin.

The IIP2 is determined by means of an AM suppression test, using a receive signal of -99 dBm. Given a 9-dB SNR requirement, the noise floor due to AM interference must be at least -99 dBm - 9 dB = -108 dBm. Because IIP2 is determined with a single tone instead of the standard two-tone test, a 3-dB correction factor is added. The expression for IIP2 is shown in Eq. 2.

The IIP3 can be determined from the receiver’s intermodulation requirements. The noise floor due to intermodulation is -108 dBm, just as in the case for IIP2. The expression for IIP3 is shown in Eq. 3.

The IRR can be calculated by determining the equivalent noise introduced by the image signal. With the signal sensitivity at -102 dBm and SNR of 9 dB, the total noise floor is essentially -111 dBm. The budget for IRR is selected so as to not cause a degradation of the noise floor by more than 0.1 dB. So, the total noise floor after image noise is added is -110.9 dBm. The total noise floor is the sum of the thermal noise floor and IRR noise floor. It can be expressed as in Eq. 4, with all terms converted from logarithmic form to linear form.

Using Eq. 4, the noise floor due to the IRR can be solved as -127.3 dBm. With a signal level of -102 dBm, the IRR is 25.3 dB.

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