Feasibility Study of Variable Multi-level QAM Modem for Wireless ATM Networks

Takashi OKADA  Toshiaki TAKAO  Tadashi SHIRATO  

IEICE TRANSACTIONS on Communications   Vol.E79-B   No.3   pp.316-327
Publication Date: 1996/03/25
Online ISSN: 
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Issue on Advanced Adaptive Radio Communication Technologies)
Category: Modulation, Demodulation
adaptive modulation,  variable multi-level,  modem,  wireless network,  ATM,  

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This paper describes a variable multi-level QAM modem applied to a wireless ATM transport network with the aim of effectively offering ATM network services over a terrestrial digital radio system. The concept of the wireless ATM transport network based on a Virtual Path (VP) capacity control system which optimizes both the number of channels and the multi-level QAM scheme for existing traffic variation is discussed. To achieve a hitless switch as a technical requirement of this network, we propose a modem configuration and a modulation scheme control (MSC) signal transmission. In this modem configuration, a multi-level control logic circuit in the modulator is used as the signal formatter. A modulated signal for the modulation scheme is maintained at a constant average power. Decision data and error signal selection for the received signal is carried out in the multi-level control logic circuit in the demodulator. The fluctuation of the demodulator loops due to modulation scheme switch can be reduced by using a fully digitized AGC loop and by converting the received signal to the condition of decision level constant. The MSC signal inserted into the first path data signal is transmitted without error by arranging the maximum amplitude of the signal point set. In this way, switching between the modulator and the demodulator is possible frame by frame. Finally, we present experimental results for a variable multi-level QAM modem employing four modulation schemes: QPSK, 16 QAM, 64 QAM, and 256 QAM. Through experiments, we prove that the modulation scheme is switched without fluctuation of the demodulator control loops by maintaining the signal condition of the decision level constant. The achievement of a hitless switch for multi-level QAM is also confirmed by experiments.