1. Introduction

Most mobile phones have a screen size of 176 pixels x 220 lines with 16 bits (262 K) of color. The total number of bits to represent an image on the screen is 176 x 220 x 16 = 619,520 bits. A television quality video refreshes the screen 30 times (frames) per second. To feed video signals to such device, it is required to use a communication media with bandwidth of 619,520 x 30 = 18,585,600 = 18.6 Mega bits per second (Mbps). A video compression method along with skipping frames is often employed to reduce the required bandwidth. A video with 15 frames per second (fps) is acceptable for most applications in a small screen, which reduces the require bandwidth to a half, 9.3 Mbps. MPEG, H.263, H.264 and MJPEG are commonly used video compression methods for a motion video. Video compression methods except MJPEG reduce the bandwidth by sending the differences between frames. Therefore, the compression ratios vary widely depending on the amount of changes between frames. Higher compression can be achieved for scenes with least amount of changes between frames. MPEG can achieve 20:1 to 50:1 compression on the average without noticeable degradation in image quality for scenes in a video telephone commonly referred as talking head. With MPEG compression, the required bandwidth 9.3Mbps, can be further reduced to 186 Kbps ~ 465 Kbps. Existing video telephony services in Korea and Japan uses a 64 Kbps Circuit Switched Data (CSD) service in wireless network. With 64 Kbps, the frame rate changes from a few frames to 15 frames per second depending on the amount of the change in the video. The actual bandwidth required to transport the compressed video is higher due to transport overhead such as packet headers, error correction bits, and time stamps, etc. Figure 1.1 illustrates ranges of average data rates required for typical application in a mobile phone.

A streaming video is one-way downstream video from the head-end to a mobile device. It requires very little bandwidth in the upstream (reverse) link. Video telephony requires the same bandwidth for the upstream link as the downstream link. However, the required bandwidth for video telephony for each direction is typically lower than the bandwidth of a streaming video since a talking head has less changes than a steaming video with changing contents. The required bandwidth for video telephony in Figure 1.1 illustrates the combined bandwidth for upstream and downstream links. Since a cellular network has separate channels for the upstream from the downstream, the upstream has to support the same bandwidth to enable video telephony. Currently 3G and 3.5G networks have much lower upstream bandwidth compared to down-stream bandwidth (e.g., In EV-DO, the upstream peak date rate is154 Kbps compared to the downstream peak data rate, 2.4 Mbps). 3.5 G networks are adequate for streaming video but difficult to support video telephony services for multiple users. Emerging networks will offer higher bandwidth for upstream links.

We will discuss 3.5G cellular networks, issues associated with video over cellular network, applications and services, 3.5G mobile devices, related standards, and useful resources for video over cellular networks.