A simple GMSK modulator for microwave and millimeter-wave frequencies

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Date: Feb. 1999
From: Microwave Journal(Vol. 42, Issue 2)
Publisher: Horizon House Publications, Inc.
Document Type: Article
Length: 3,392 words

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A simple hardware architecture for effective realization of Gaussian minimum-shift keying (GMSK) modulation at microwave and millimeter-wave frequencies is presented. The modulator is based on achieving continuous linear phase modulation of a carrier signal over the full 360 [degrees] range. The microwave circuitry contains only a single active component, is suitable for MMIC implementation and is readily extended to millimeter-wave frequencies. Performance results for a prototype GMSK modulator implementation at 18 GHz are also presented.

The demand for high data rate wireless communication systems is increasing for applications such as wireless asynchronous transfer mode traffic.(1) Other current or emerging high data rate systems include wireless local area networks and local-to-multipoint communication systems. Unfortunately, congestion in the traditional portions of the radio spectrum is forcing these emerging digital radio systems into the upper microwave and millimeter-wave bands where existing transceiver design technology is less mature. A high performance, cost-effective radio transmitter solution suitable for use at these frequencies is very attractive for the design of future systems.

One method of reducing the cost and complexity of the transmitter is to exploit the advantages of directly modulating the microwave or millimeter-wave carrier signal. Direct digital modulation at the transmit frequency removes the requirement for multiple stages of IF, up-conversion and filter circuitry, which makes an elegant modulator architecture at microwave or millimeter-wave frequency feasible.

A modulation method that has received considerable attention recently and is well suited to direct modulation is GMSK, which has been chosen as the modulation method for the GSM cellular system as well as the digital cordless telephone (DCS1800) PCS standard.(2) GMSK has a number of desirable properties that have lead to its increased popularity, including good spectral efficiency(3) and a nearly constant amplitude envelope. The constant envelope property makes the GMSK modulated signal fairly insensitive to nonlinearity in the transmitter. As a result, highly power-efficient transmitter designs are possible. GMSK modulation also has the advantage that it may be demodulated using coherent(3) or noncoherent(4) detection methods, leading to a simpler receiver structure.

Direct microwave GMSK modulation methods that have been proposed previously(3,5) result in hardware with complexity that may make implementation prohibitive at upper microwave and millimeter-wave frequencies. This article presents a simple and elegant hardware realization of direct GMSK modulation, which is suitable for implementation at these frequencies. The modulator is based on a novel circuit architecture for achieving linear phase control of a microwave or millimeter-wave carrier signal over the full 360 [degrees] range.[6,7] The microwave portion of the modulator contains a single active device operating at the desired transmit frequency. This configuration results in a simple, cost-effective hardware solution that requires very few components and is suitable for MMIC realization. Measurement results for a prototype GMSK modulator implementation at 18 GHz are presented. The architecture also can be easily extended into the millimeter-wave frequency range.

GMSK

Minimum-shift keying (MSK) is a special case in the family of constant envelope continuous phase modulation (CPM) signals.[8] When the carrier signal is modulated by the serial data...

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Gale Document Number: GALE|A54195211