In order to exploit the nonuniformly distributed channel capacity over the cell area, the intelligent 7.3-kB programmable videophone transceiver of Table I is proposed, which is capable of exploiting the higher channel capacity of uninterfered, high-channel-quality cell areas, while supporting more robust, but lower bit-rate operation in more interfered areas. The system employed an enhanced H.263-compatible video codec. Since most existing wireless systems exhibit a constant bit-rate, the video codec's bit-rate fluctuation was smoothed by a novel adaptive packetization algorithm, which is capable of supporting automatic repeat request (ARQ)-assisted operation in wireless distributive video transmissions, although in the proposed lowlatency interactive videophone transceiver, we refrained from using ARQ. Instead, corrupted packets are dropped by both the local and remote decoders in order to prevent error propagation. The minimum required channel signal-to-interference-plus-noise ratio (SINR) was in the range of 8-28 dB for the various transmission scenarios of Table I, while the corresponding video peak-signal-to-noise ratio (PSNR) was in the range of 32-39 dB. The main system features are summarized in Table I. Index Terms-H.263-based video communications, interactive wireless video, QAM-based video transmission, video communications in interference-limited environments, video tranceivers.

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Abstract-The video performance of a 155-Mbps wireless asynchronous transfer mode (WATM) proposal and that of a 2-Mbps Universal Mobile Telecommunications System (UMTS)concept is evaluated for a range of low-to high-quality video application scenarios, various propagation conditions, and video bit rates using the H.263 video codec, assisted by a novel packetization and packet acknowledgment scheme. Orthogonal frequency-division multiplexing is invoked over the highly dispersive channels for conveying high-rate video signals. Various binary Bose-Chaudhuri-Hochquenghem and turbo codes are investigated comparatively, with the conclusion that due to the high error resilience of the video packetization and acknowledgment scheme, the increased power of the higher complexity turbo codec does not translate to substantially improved overall system robustness, although the bit error rate and acknowledgment flag error rate are significantly reduced. The whole range of video resolutions and system parameters is summarized for reasons of space economy in Tables II-IV. The required channel signal-to-noise ratio for near-unimpaired video quality is about 16 dB for the inherently lower quality, lower resolution video frame formats, but slightly higher, about 18 dB, for the high-definition formats, where the error-induced subjective video degradations become more objectionabl over the highly dispersive worst case channels used.Index Terms-H.263-based wireless video communications, orthogonal frequency-division multiplexing (OFDM)-based video.

Second-generation (2G) mobile radio standards have not been designed with video communications in mind, although the employment of error-resilient, constant-bit-rate proprietary video codecs over these systems is realistic. The third-generation (3G) systems I. STATE-OF-THE-ART SYSTEM COMPONENT DEVELOPMENTS A. Introduction and OutlineThe subject of mobile radio communications has reached a state of maturity over the past two decades, as indicated by the excellent monographs by Jakes This contribution attempts to summarize a range of recent advances in the field of bandwidth-efficient wireless video communications. In this section, we commence our discourse with a brief overview of the wireless scene and, in particular, by considering the video transmission capabilities of the existing and future wireless systems. The geographical variation of the cellular channel capacity is characterized by Section II, as a motivation for invoking burst-by-burst adaptive transceivers [27]- [29]. The nature of co-channel interference (CCI)-which is the most dominant channel impairment in wireless systems-is the topic of Section III, where it is shown that the geographic variation of CCI justifies the employment of multimode transceivers, which are discussed in Section IV. Low-rate video compression aspects are reviewed in Section V, and the performance of the ITU H.263 codec is analyzed in some depth. Our discussions in Sections VI and VII are focused on the performance of reconfigurable video transceivers. Specifically, in Section VI, no power control is used-which is typically the case in cordless telephones, such as DECT and CT2-while in Section VII, power control is employed, which is characteristic of cellular systems, such as GSM [30], etc. Section VIII proposes a near-instantaneously adaptive or burst-by-burst adaptive code-division multiple-access (CDMA) scheme, which can be invoked in the context of the forthcoming 3G systems, in order to enhance their performance. Lastly, our discussions are concluded in Section IX with a range of system design guidelines. Let us now commence our overview of the range of existing and forthcoming wireless systems and their video transmission capabilities. B. Second-Generation (2G) Wireless SystemsThe existing 2G wireless systems now constitute a mature technology. Although they have not been designed with video communications in mind, with the advent of the specially designed error-resilient, fixed-rate video codecs [27]-[35] proposed by Streit et al., it is nonetheless realistic to provide videophone services over these low-rate schemes. For low-latency interactive videophony these systems have

The video performance benefits of burst-by-burst adaptive modulation are studied, employing a higher-order modulation scheme when the channel is favorable, in order to increase the system's bits per symbol capacity and conversely, invoking a more robust lower order modulation scheme when the channel exhibits inferior channel quality. It is shown that due to the proposed adaptive modem mode switching regime, a seamless video-quality versus channel quality relationship can be established, resulting in error-free video quality right across the operating channel signal-to-noise ratio (SNR) range. The main advantage of the proposed burst-by-burst adaptive transceiver technique is that irrespective of the prevailing channel conditions, the transceiver achieves always the best possible source-signal representation quality-such as video, speech, or audio quality-by automatically adjusting the achievable bitrate and the associated multimedia source-signal representation quality in order to match the channel quality experienced. This is achieved on a near-instantaneous basis under given propagation conditions in order to cater for the effects of path loss, fast-fading, slow-fading, dispersion, co-channel interference, etc. Furthermore, when the mobile is roaming in a hostile outdoors or even hilly terrain-propagation environment, typically low-order low-rate modem modes are invoked, while in benign indoor environments, predominantly the high-rate high source-signal representation quality modes are employed.

Abstract-This contribution studies the impact of adaptive quadrature amplitude modulation (AQAM) on network performance when applied to a cellular network, using adaptive antennas in conjunction with both fixed channel allocation (FCA) and locally distributed dynamic channel allocation (DCA) schemes. The performance advantages of using adaptive modulation are investigated in terms of the overall network performance, mean transmitted power, and the average network throughput. Adaptive modulation allowed an extra 51% of users to be supported by an FCA 4-QAM network, while in conjunction with DCA, an additional 54% user capacity was attained.

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