Image rejection receiver architecture for multi ...

signal processing

Image rejection receiver architecture for multi-standard TDMA applications Getting the most out of any product design is important. Using design optimized building blocks can extract the most out of your receiver. By Aristodimos Pneumatikakis, Dermentzoglou Lampros, Aggeliki Arapoyanni and I. Moisiadis

M

ulti-standard radio frequency (RF) transceivers seem to become more and more of a favorite in wireless communication systems. Multimode terminals, however, are the precursors for the next generation of mobile terminals described by the concept of software radio [1]. Receiver planning is often the essence of considerations, leading to an abundance of trade-offs. Such trade-offs include the number and the frequency positioning for the different IF stages, linearity, noise figure (NF) and gain distribution, hardware and power consumption expenses. Attention to system specifications requirements, as they are defined in standardization documents, are always a priority. [2]. Simulation results of non-ideal building block models provide optimal building block specifications that are compatible to the system requirements. These specifications ensure the reliable performance essential for a first silicon run. Two receiver architectures are optimized and compared: the single down conversion and the double IF–high first IF superheterodyne. There are two main concerns for these topologies:

Parameter GSM9000 Max. Rx level –23 Min. Rx level –102 NF 9 dB C/I 12 dB C/(N+I) 9 dB RBER 2% Image rej. 68 dB

multi-standard operation and image rejection. The first leads to maximum common receiver path, reducing component multiplicity. Drivers for this are the global system for mobile communications, (GSM), digital cellular systems (DCS) and personal communications systems (PCS) standards. This article will address all of these issues from both a market perspective and an operational perspective based upon timedivision multiple access (TDMA) and gaussian minimum shift keying (GMSK) modulated channels of equal bandwidth. The second concern is covered using two different approaches: the superheterodyne receiver with a high first IF to facilitate multi-standard operation, or the single down-conversion receiver employing an image rejection mixer [3]. The two architectures are discussed and optimized using simulation in the following sections. As a result, the feasibility of each one, given multi- standard operation, is extracted.

System requirements and tests

Multi-standard operation is guaranteed when the topology is capable of fulfilling the GSM/DCS/PCS system requirements provided in the standardization documents. The fulfillment is confirmed by the success in a number of tests, determined by the performance goals given in Table 1. The tests can be grouped in the following four categories. The blocking (desensitization) characteristics of the receiver determine its ability to operate DCS1800 PCS1900 successfully under strong interferers. –26 –26 They are specified –100 –102 separately for inband and out-of11 dB 9 dB band. The desired 12 dB 12 dB channel is set at 9 dB 9 dB the frequency f 0 . Two out-of-band 2% 2% and eight in-band 68 dB 68 dB ranges are specified for unmodulat-

Table 1. Multi-mode receiver performance goals.

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ed blockers of certain levels. The blocking profile for each one of the standards is presented in Figure 1 [4]. The adjacent channel selectivity is a measure of the capability of the receiver to receive a wanted modulated signal without exceeding a given degradation due to the presence of an unwanted modulated signal in the adjacent channel. Nominal frequencies and power levels of useful signals and interferers are given in Table 2. The intermodulation response rejection for a receiver mainly determines its linearity requirements. The receiver must maintain a bit error rates residual bit error rate (RBER) of less than 2% or carrier-to-interferer (C/I) greater than 12 dB for a minimum wanted signal F0 at –99 dBm (GSM/PCS) or –97 dBm (DCS), in the presence of a continuous sinewave F1 and a modulated tone F2, both at –43 dBm (GSM) or –49 dBm (DCS/PCS) [4]. The frequencies F0, F1 and F2 are such as: F0=2F1 –F2

(1)

and |F2–F1| = 800 kHz

(2)

Images are signals that can be folded into the signal frequency band due to mixing or sampling. The mixing images lie twice the IF from the signal, and are as many as there are mixing stages. The sampling images, given that subsampling is employed, exist 1/2 of the sampling frequency away from both sides of the signal. The image blocker can be considered as a spurious response exception of –43 dBm [4]. For this reason the blocking requirement is relaxed to 68 dB at the frequency in which the image is applied.

Single down conversion receiver The receiver architecture depicted in Figure 2 is the single down conversion. In this receiver topology the IF has been set at 75 MHz, a value which is convenient for further conversion of the sig-

May 2000

Duplexers for single down conversion and double conversion high IF1

Passband Maximum insertion loss Maximum inband ripple Attenuation: Image frequency (GHz) Frequency 1 (GHz) Frequency 2 (GHz

GSM 900 925 to 960 MHz 4.3 dB 2.6 dB N/A 35dB @f>3.0 28 dB @f0.98

DCS 1800 1.805 to 1.880 GHz 2.7 dB 1.7 dB N/A 37 dB @ f>4.5 12 dB @1.92