Microwave Engineering Europe - July/August 2008 - (Page 14)

14 RF CMOS — BASEBAND Programmable transceiver IC minimises OEM inventory for femtocells By Ebrahim Bushehri, CEO, Lime Microsystems aseband ICs are increasingly becoming programmable, but making a multi-band multi-standard front end has been more of a challenge. Switching between different RF front-ends using a bank of RF MEMS switches was thought to be a solution until recently, as a solution in RF silicon was not forthcoming. However, opinions are changing as a result of companies developing programmable multi-band multi-standard transceiver ICs. A programmable transceiver, provided it’s sufﬁciently frequency-agile, removes the need for individual transceiver chips for each of the different standards or geographical territories. The chips can be reprogrammed rapidly and simply to function as a transceiver for various network conﬁgurations, bandwidths and data rates. This means OEMs don’t have to go back to the silicon vendors each time a new spectrum allocation appears and ask for the design and implementation of yet another transceiver IC, which would inevitably impact time to market. Once a product has been developed around a particular transceiver, this maximises design reuse, again shortening the production cycle. These factors allow OEMs to quickly develop new products to meet the market need. Today, RF MEMS is still dogged by reliability problems, while the programmable silicon approach is offering real beneﬁts to OEMs and system designers. Lime Microsystems has developed a single-chip transceiver, LMS6001, shown in Figure 1. The comapny’s transceiver can be digitally conﬁgured to operate as a WiMAX, 3G or LTE transceiver, at any frequency between 375 MHz B and 4 GHz, with user-selectable bandwidths. The chip combines LNA, PA driver, Rx/Tx mixer, Rx/Tx ﬁlters, synthesisers, Rx gain control, and Tx power control with a minimum requirement for external components. The key challenges in developing such a chip were achieving the required wide frequency range while maintaining the phase noise requirements of the synthesizer, multiple bandwidth ﬁltering with sharp roll-off at the baseband, along with linearity and noise ﬁgure budget for the transceiver chain. The baseband ﬁlter used within the design provides a very sharp roll-off for multiple bandwidths covering both WiMAX and LTE bands, the design of which is based on a modiﬁed transconductance ampliﬁer-capacitor (gm-C) cells. The wideband characteristics of the transceiver have been obtained by using a broadband synthesizer. The synthesizer is based on a fractional-N sigma-delta based architecture covering a continuous range between 375 MHz to 4 GHz. The sigma delta modulator is used with a very low oversampling ratio simplifying the design and relaxing the speciﬁcation of the analogue components within the synthesizer. The phase noise performance of the implemented synthesizer measured at the output nodes meet all requirements of the key broadband wireless standards including WIMAX and LTE standards. The frequency synthesizer employs a digitally controlled 40 MHz reference crystal oscillator. Reconﬁgurable design The zero-IF transceiver chip can handle OFDM modulation up to 64QAM, and supports both FDD and TDD full duplex, with a sensitivity of -70 dBm at 7 MHz bandwidth under 64QAM. Operating current is typically 300 mA under FDD operation at 1.8 V and 3.3 V, with a standby current of less than 1 mA with power-down modes being software-selectable. Modulated Tx RF output is -10 dBm. Based on LMS6001, a unique reference design has been developed for a reconﬁgurable multi-standard MicroTCA wireless transceiver system, which has 6 user-selectable channel bandwidths from 1.5 MHz to 28 MHz and can be digitally conﬁgured to operate in bands from 375 MHz to 4 GHz. The reconﬁgurable design Figure 2. Lime’s reference design board. supports a variety of network conﬁgurations – WCDMA/HSPA, WiMAX and LTE – as well as different bandwidths, and is the industry’s ﬁrst transceiver reference design to use the microTCA format. A Lime reference design board is shown in ﬁgure 2. Commercial considerations The bottom line is that programmable silicon will never grab the attention of femtocell makers unless it is also competitive on cost. The words ‘low-cost’ and ‘programmable silicon’ are not usually seen in the same sentence, but if you consider the problem in broader terms than just the per-unit cost, a convincing proposition begins to emerge. Programmable silicon has signiﬁcant implications for OEMs’ inventory. It will only be necessary to purchase one type of transceiver for an entire product range of small cell basestations. This is particularly useful for global OEMs shipping to different geographical locations. So if demand goes up in one country, you already have the silicon you need, no matter which country it is. Also, since one transceiver will be used in multiple product lines, OEMs will be able to take advantage economies of scale. These developments in programmable baseband and transceiver ICs are driving the small cell basestation sector closer towards a single bill of materials. This is the goal: a single end-product whose hardware can be fully reprogrammed to support any standard or frequency. We are not there yet, as power ampliﬁers and antennas still have a way to go, but the advantages in terms of logistical savings and time to market are apparent. Figure 1: Lime’s LMS6001 transceiver comes in a compact 68-pin QFN package. Microwave Engineering Europe ● July/August 2008 ● www.mwee.com http://www.mwee.com

Table of Contents Feed for the Digital Edition of Microwave Engineering Europe - July/August 2008

Microwave Engineering Europe - July/August 2008 Contents News Comment Cover Feature: Effective EM Simulations with Micro−λ Resolution in Macro-λ Objects — General Huygens Box Implementation RF CMOS: Programmable Transceiver IC Minimises OEM Inventory for Femtocells CAD/EDA: Software-Defined Radio Platforms CAD/EDA: Cadence Enhances RF Verification While AWR Delivers an Improved Microwave Office How to Meet the Design Challenges of WiMAX Power Amplifiers Products Calendar

Microwave Engineering Europe - July/August 2008

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