Spread spectrum CDMA systems for subband image transmission

This paper investigates the application of spread spectrum code-division multiple access (SS-CDMA) techniques to subband image transmission over a bandlimited radio channel. For the transmission of images over SS-CDMA AWGN channels, a subband coding scheme that divides the image information into a number of independent data streams using an analysis filter bank, each of which is multiplied by its unique signature PN code, enables the transmission of these data streams via multiple parallel virtual channels created by their corresponding PN codes. With a sufficiently large number of streams, the total signal is able to fit within the narrow radio channel bandwidth even though the total bandwidth of all the signals may exceed the channel bandwidth. At the receiver, each received signal is separately recovered at the decoder by multiplying its PN code and integrating over the code length in order to obtain the desired subband. All the recovered subbands are then reassembled by a synthesis filter bank into a close reproduction to the original image. Additionally, for color subband image transmission, color images are first transferred to luminance (Y) and two chrominance components (I, Q). Each component is then decomposed independently into several subbands for SS-CDMA transmission. Therefore, a number of additional PN codes are required to support the transmission of the chrominance signals over the CDMA channels whereas the luminance signal was treated in the same manner as monochrome pictures. Moreover, SS-CDMA allows more than one image to be transmitted and be accessed simultaneously at the same limited channel bandwidth. If all the product signals for different images are transmitted over the SS-CDMA AWGN channel with perfect power control, the performance is the same as if the data were transmitted in a single stream. However, the quality of the farther received images would be degraded when the near-far problem occurs. The image quality of both the monochrome and color subband image transmission via SS-CDMA AWGN channels is evaluated for both perfect and imperfect power control.