We introduce an improved image transmission scheme over wireless channels with flat Rayleigh fading. The proposed scheme jointly optimizes bit power and modulation level to maximize the peak signal-to-noise ratio (PSNR) of the reconstructed image and hence improves the perceptual quality of the received image. In this optimization process, the significance of bits with regard to the overall quality of the image is exploited. The optimality of the proposed algorithm is demonstrated using the Lagrange method and verified through an iterative offline exhaustive search algorithm. For practical implementation, a look-up table is used at the transmitter for assigning the bit power and modulation level to each bit stream according to the received signal-to-noise ratio (SNR) observed at the receiver. The proposed scheme has low complexity since the look-up table is computed offline, only once, and used for any image which makes it suitable for devices with limited processing capability. Analytical and simulation results show that the proposed scheme with jointly optimized bit power and variable modulation level provides an improvement in PSNR of about 10 to 20？dB over fixed power fixed modulation (16-QAM). A further reduction in complexity is achieved by using the average signal-to-noise ratio rather than the instantaneous SNR in selecting the system parameters. 1. Introduction Recent advances in wireless communications have made it more possible to transmit multimedia traffic (voice, image, video, and data) over high-speed wireless links [1, 2]. This is expected to attract more users of wireless networks with diverse applications such as video on demand, mobile TV, and medical services. Such applications, however, pose many challenges to system design since they are both power- and bandwidth-hungry applications with strict time requirements. It is therefore imperative to develop new transmission schemes that use radio resources as efficient as possible, especially for mobile users. A key point in the design of multimedia systems is how to tailor the transmission and reception schemes according to the type of traffic being sent and the dynamics of the wireless medium. Several alternatives, which can be employed either separately or jointly, have been investigated in the past, such as channel coding, source coding, power and rate control, and adaptive modulation. Channel coding is an essential constituent of any digital communication system since it allows the system to use less signal power to achieve a target bit error rate (BER). However, channel coding
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