National Conference on Communication and Information Security, NCCIS 2007 Daffodil International University, Dhaka, Bangladesh, 24 November 2007 106 Analysis of Direct Modulation Technique Employing FQPSK-B Signals Md. Masud Rana 1 and D.M. Motiur Rahaman 2 1 Dept. of Electronics & Communication Engineering, Khulna University of Engineering & Technology, Khulna-920300, Bangladesh 2 Dept. of Electrical & Electronics Engineering, Rajshahi University of Engineering & Technology, Rajshahi-6204, Bangladesh E-mail: masudece@ece.kuet.ac.bd and mamaraece28@yahoo.com Abstract: The direct modulation architecture of recently standardized baseband filtered version of Feher-patented quadrature phase-shift keying (FQPSK-B) modulation is proposed. A continuous phase modulation (CPM) implementation alternative of a recently standardized class of FQPSK-B modulation is proposed. It is based on the fact that the FQPSK-B signal has quasi- constant amplitude and continuous phase characteristics, allowing CPM interpretation. Based on this, it derive the modulation index, premodulation filter and alternating change monitor differential encoder for the CPM generated FQPSK-B signal, to be fully compatible with the original I/Q-modulated one. Direct modulation architecture is simple and more cost efficient than the I/Q modulation architecture. With the proposed direct modulation architecture, FQPSK-B signals can be generated with existing FM modulation based transmitter system. The power spectral density of direct modulated FQPSK-B signals is practically the same with the I/Q modulated FQPSK-B signals and BER performance of coherent detection and non- coherent detection of direct modulated FQPSK-B signals are almost same with the I/Q modulated FQPSK-B. Key word: Feher-Patented Quadrature phase Keying (FQPSK), Voltage Controlled Oscillator (VCO), Phase Locked Loop (PLL), Alternating Change Monitor Differential Decoder(ACMDD). 1. Introduction The last few years, there have been researches of simple, low-cost and low-power wireless transceivers. The direct modulation architecture is considered as transmitter architecture for single chip transceiver. There are systems using direct modulation architecture such as Bluetooth DECT, which systems require low complexity, low cost and low power consumption [1-3]. In telemetry system, the needs for power and spectrum efficient modulation techniques also have been growing. Since FQPSK-B has been proposed, I/Q modulation have been used for transmitter implementation. Though I/Q modulation architecture have many advantages, I/Q modulation transmitter also has some problems and disadvantages. First, it may require multiple IF stages. It means that I/Q modulation architecture is complex and consumes more power. Second, it is difficult to achieve the gain match between I and Q signal paths because of gain mismatch by the low-pass filter (LPF) and mixer in I and Q signal paths. Third, the implementation of exact 90 o phase shifter is practically impossible, even though most I/Q modulation architecture assume exact 90 o phase shifter. If FQPSK-B transmitter can be implemented by direct modulation architecture, it removes the requirement for multiple IF stages including up-conversion mixers, IF BPF’s, and the exact 90 o phase shifter [5]. Since IF BPF’s aren’t required, FQPSK-B transmitter can be integrated in single chip. It is possible to achieve the low complexity, low cost and low power consumption in the transmitter. With the proposed direct modulation architecture, FQPSK-B signals can be generated with existing FM modulation based transmitter system. This paper presents the direct modulation architecture of FQPSK-B signals, which has been implemented with I/Q modulation. For the proposal of direct modulation architecture of FQPSK-B, it is shown that FQPSK-B modulation is a kind of CPM. Direct modulation architecture is realized by the CPM characteristics of FQPSK-B signals. The direct modulation of FQPSK-B signals requires ACMDE to produce the same phase behavior with I/Q modulated FQPSK-B signals. 2. Architecture of FQPSK-B Modulation FQPSK-B modulation is pulse-shaping terms, i.e., inter-symbol interference and jitter-free (IJF) signal encoder, cross correlator and low