Wide-band systems will be the next significant generation in wireless communications. Those include both wireless local area networks and cellular systems with a large coverage area. They will provide a higher data rate and access to internet and video services, for example. Although most of the data processing is performed digitally, also the requirements and possibilities to implement the analog part of the radio receiver will be different compared to the second-generation narrow-band receivers. Direct conversion architecture is a distinct candidate for wide-band systems because some non-idealities involved in baseband signal processing are significantly relaxed. The requirements and feasibility of direct conversion in wide-band systems are analyzed in this work. The main emphasis is on cellular systems based on direct sequence code division multiple access, but the same principles are generally valid in all receivers for different applications. The basic principles and design methods involved in receiver design are overviewed as well as the most common radio architectures. In a detailed analysis, the fundamental limitations of the direct conversion architecture are analyzed in wide-band signal processing. Especially, the effect of envelope distortion is characterized both with respect to the specific modulation and to the implementation of a downconversion mixer. Downconversion mixer is the key component in direct conversion because it transfers the radio frequency signal immediately down into the baseband after a relatively small gain at the preceding signal processing blocks, which do not provide filtering of the unwanted radio channels within the system band. Both switching mixersand subsampling mixers are analyzed. Direct Conversion Receivers in Wide-Band Systems consists of four different circuit implementations. A subharmonic sampler operating up to 2 GHz is implemented with a GaAs MESFET technology. The second IC is a CMOS low-noise amplifier with an optimized interface to a subsampling mixer. Two BiCMOS implementations of the wide-band direct conversion receiver are given. The first consists of four different chips: RF front-end, analog baseband circuitry and two analog-to-digital converters. In the second chip, all blocks from the low-noise amplifier to the A/D converters are placed on the same die. In that case, an excellent isolation is required between rail-to-rail clock signals and the sensitive RF input.

This book is based on my doctoral thesis at the Helsinki University of Technology. Several different projects during five years guided me from the basics of the RF IC design to the implementations of highly integrated radio receiver chips. Sharing time and effort between IC and system issues is not always straightforward. I have been lucky to follow both topics and share experiences with diligent and enthusiastic people having different specialities. As a result, this book will cover a wide range of different topics needed in the design of highly integrated radio receivers. Experiences from the first receiver prototypes for the third generation cellular systems form the basis of this book. Most of the issues are directly related to the early proposals of European and Japanese standardization organizations. For example, the chip rate was originally set to 4. 096 Mcps in a wide-band CDMA channel. I have kept that number in the book in most of the examples although it has been later changed to 3. 84 Mcps. I hope that the readers will accept that and the possible other incompabilities to the latest specifications. At least in the research phase the changes even in the most essential requirements are definitely not a rare incident and IC designers should be able to react and modify their designs as soon as they can.