Dual-mode reconfigurable wireless videophone transceivers are proposed for noise-, rather than interference-limited indoors and outdoors applications and their video quality, bit rate, robustness, and complexity issues are analyzed. A suite of fixed, but arbitrarily programmable low-rate, perceptually weighted vector quantized (VQ) codecs with and without run-length compression (RLC) are contrived for quarter common intermediate format (QCIF) videophone sequences. The 11.36-kb/s Codec 1 is Bose–Chaudhuri–Hochquenghem (BCH) (127,71,9) coded to a rate of 20.32 kb/s and this arrangement is comparatively studied along with the 8-kb/s Codec 2 and BCH(127,50,13) scheme, which has the same 20.32-kb/s overall rate. The source-sensitivity matched Systems 1–6 characterized in Table IV were contrived to comparatively study the range of system design options. For example, using Codec 1 in System 1 and coherent pilot symbol assisted 16-level quadrature amplitude modulation (16-PSAQAM), an overall signaling rate of 9 kBd was yielded, if the noise-limited channel had a signal-to-noise ratio (SNR) in excess of about 22 dB in the vicinity of the basestation or in indoors scenarios. In contrast, over lower quality outdoors channels near the fringes of the cell, the more robust 4-QAM mode of operation had to be invoked, which required twice as many time slots to accommodate the resulting 18-kBd stream and hence, reduced the total number of users supported. The robustness of Systems 2–4, and 6 was increased using automatic repeat requests (ARQ), again, inevitably reducing the number of users supported, which was between 6 and 16. In a bandwidth of 200 kHz, similarly to the Pan-European GSM mobile radio system’s speech channel, using systems 1, 3, 4, or 5, for example, 16 and eight videophone users can be supported in the 16- and 4-QAM modes, respectively, while in dual-mode cells the number of users is between eight and 16. The basic system characteristics are highlighted in Table IV. Index Terms—Fixed-rate video coding, QAM video communications, vector-quantized video coding, wireless video telephony

Hanzo, L

Streit, J

English

Southampton (e-Prints Soton)

348 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 46, NO. 2, MAY 1997TABLE IIIAVERAGE PSNR PERFORMANCE OF CODECS 1 AND 2FOR THE “MISS AMERICA” AND “LAB” SEQUENCESFirst, the 396-entry activity table containing the binary ﬂagsis grouped in 2 2 blocks and a 4-b symbol is allocatedto those blocks that contain at least one active ﬂag. These4-b symbols are then run length encoded and transmittedto the decoder. This concept requires a second active tablecontaining ﬂags in order to determine which of thetwo by two blocks contain active vectors. Three consecutiveﬂags in this table are packetized to a symbol and then runlength encoded. As a result, a typical 396-b active/passivetable containing 30 active ﬂags can be compressed to lessthan 150 b. The motion vectors do not lend themselves to runlength encoding.If at this stage of the encoding process the number of bitsallocated to the compressed motion-activity tables as well asto the active MV’s exceeds half of the total number of bitsavailable for the encoding of each frame, which is 800 in caseof the 8-kb/s Codec 2, then some of the blocks satisfyingthe initial motion-active criterion will be relegated to themotion-passive class.The VQ blocks are handled using a similar procedure.Depending on the required number of bits per frame andthe free buffer space, an appropriate number of active VQblocks is chosen and the corresponding compressed tablesare determined. If the total number of bits “overspills,” orif there are too many bits left unused, a different numberof active blocks is estimated and new tables are determined.The disadvantage of such a high compression scheme is thestrongly increased vulnerability of the transmission burst. If atransmission error corrupts one of the RL encoded tables, it islikely that a codeword of a different length is generated anddecoding becomes impossible. This will force the decoder todrop the entire frame.The average PSNR performances of Codecs 1 and 2 aresummarized for the MA sequence and for the higher activity“lab sequence” in Table III.III. BIT SENSITIVITYIn order to apply source-sensitivity matched protection thevideo bits were subjected to sensitivity analysis. In [11] wehave consistently corrupted a single bit of a video codedframe and observed the image PSNR degradation inﬂicted.Repeating this method for all bits of a frame provided therequired sensitivity ﬁgures and on this basis bits havingdifferent sensitivities can be assigned matching FEC codes.This technique, however, does not take adequate account ofthe phenomenon of error propagation across image frameboundaries. Therefore in this treatise we propose to use themethod suggested in [18], where we corrupted each bit ofthe same type in the current frame and observed the PSNRdegradation for the consecutive frames due to the error eventin the current frame.Fig. 12. PSNR degradation proﬁle for Bits 1 and 12 of the in Frame 21.Fig. 13. Integrated PSNR bit sensitivities of Codec 1.As an example, Fig. 12 depicts the PSNR degradationproﬁle in case of corrupting all “No 1” bits, the most signiﬁcantbit (MSB) of the PFU and all “No 12” Bits, one of the addressbits of the MV, in each block of frame 21. In the ﬁrst case, theMSB’s of all PFU blocks are corrupted, causing a scatteredpattern of artifacts across the image. Those blocks will bereplenished by the PFU exactly every 18 frames, revealed inthe “staircase” effect in Fig. 12. The impact of the corruptedMV is randomly distributed across the frame and it is alsomitigated continuously by the PFU.In order to quantify the overall sensitivity of any speciﬁc bitwe have integrated (summed) the PSNR degradations over theconsecutive frames, where they have had a measurable effectand averaged these values for all the possible occurrencesof the corresponding bit errors. These results are shown inFig. 13 for the 13 MV bits and 17 VQ bits of an 8 8 block,as well as for the four partial forced update bits. The VQ-index sensitivity depends on the codebook-contents as well ason the codebook order. We decreased the sensitivity of theVQ indexes by reordering the codebook using the simulatedannealing algorithm.Accordingly, starting from a random codebook order, weinvoked a recursive algorithm selecting two codebook entriesat random and evaluating the potential gain in terms of code-book robustness, if the candidate entries were swapped. At theSTREIT AND HANZO: VIDEOPHONE SYSTEMS FOR INDOORS AND OUTDOORS APPLICATIONS 349Fig. 14. Improvement of the codec robustness due to reordering the code-book entries.commencement of our codebook rearranging algorithm we al-lowed codebook entry swaps even if they decreased the overallcodebook robustness. This assisted the algorithm in emergingfrom a local minimum. Throughout the optimization processwe decreased the probability of such “disadvantageous” swapsand allowed the algorithm to settle to a minimum. In Fig. 14we the demonstrated the increase of the CB robustness forthree “simulated annealing” attempts starting from differentinitial assignments. As seen in the ﬁgure, all attempts resultedin a robustness increase of up to 20%. The impoved robustnessis revealed by comparing the two sets of PSNR degradationbars associated with the corresponding bit positions 27–34in Fig. 13. Having characterized a range of VQ-based videocodecs let us now relate their performance to that of the DCT-and QT-based benchmarker of [9], [10] as well as to theCCITT H.261 and the MPEG2 standard codecs.IV. COMPARISON OF BENCHMARK CODECSLet us now compare the 11.36-kb/s QT scheme of [10], theDCT codec arrangements of [9] and the currently proposedVQ codecs in quality and error resilience terms. As a com-parative basis we will also refer to two widely used standardcodecs, namely the MPEG-2 and H261 codecs, which have aﬂuctuating bit rate and hence, are unsuited for videophonyover ﬁxed-rate mobile channels. In our PSNR comparisonshown in Fig. 15 we adjusted the parameters of the H261 andMPEG-2 codecs to provide a similar video quality associatedwith a similar average PSNR performance to our ﬁxed-ratewireless video codecs. The corresponding bit rate was abouttwice as high for the two standard codecs, exhibiting a randomﬂuctuation for the H261 codec. The MPEG codec exhibitsthree different characteristic bit rates, corresponding to the so-called intra (I), bidirectional (B), and predicted (P) frames indecreasing order from around 8000 b/frame, to about 1800and 1300, respectively.Scrutinizing the above ﬁve codecs in Figs. 15 and 16 showsthat our codecs achieve a similar PSNR performance to theMPEG-2 and the H.261 codecs at less than half the bit rate,although our ﬁxed-rate DCT and VQ codecs reach their steady-state video quality due to the ﬁxed bit-rate limitation only afterFig. 15. PSNR versus frame index performance of the proposed adaptivecodecs and two standard codecs.Fig. 16. PSNR degradation versus BER for the proposed codecs.some 2 s. Furthermore, the delay of our codecs and that ofthe H-261 codec is 100 ms, while that of the MPEG-2 codecextends to several frames due to the P-frames. The vectorquantized codec constitutes the best compromise in terms ofquality, compression ratio and computational demand, closelyfollowed by the DCT codecs [9].Speciﬁcally, the proposed VQ codec achieved a similar DFDrepresentation quality and video quality to the DCT codecof [9] by allocating an 8-b codebook index to all VQ-activeblocks, whereas the DCT codec used a total of 12 b per activeDFD block, yet introducing slightly more low-pass effects.Therefore the proposed VQ codec was able to support 38,rather than 30 motion-active and DFD-active blocks, as wehave seen in [9] for the DCT codec, providing an overallimproved activity tracking and video quality.In robustness terms we concluded that best error resiliencewas achieved by the currently proposed VQ Codec 1, followedby the DCT Codec 1 of [9], while the corresponding Codec 2schemes and the QT codec of [10] were more error sensitive.The VQ codec’s lower error sensitivity in comparison to thatof the DCT codec, was partly a consequence of the reducedcodebook index sensitivity due to using simulated annealing.This exhibited itself also in Fig. 13. In contrast, in the DCTcodec [9] the corruption of the low-frequency DCT coefﬁcientsintroduced large video degradations. Due to its variable-lengthSTREIT AND HANZO: VIDEOPHONE SYSTEMS FOR INDOORS AND OUTDOORS APPLICATIONS 355Fig. 21. PSNR versus channel SNR performance of the 16-QAM/9-kBd mode of Systems 4 and 5 over various channels.Fig. 22. PSNR versus channel SNR performance of Systems 2 and 6.VII. SUMMARY AND CONCLUSIONSFollowing the comparative study of a variety of VQ-basedconstant-rate video codecs, an 8- and an 11.36-kb/s schemewas earmarked for further study in the context of varioussystems. It was demonstrated that the proposed VQ codecsslightly outperformed the identical-rate DCT and QT codecs.The various studied reconﬁgurable mobile videophone ar-rangements are characterized by Table IV. The video codecsproposed are programmable to any arbitrary transmission ratein order host the videophone signal by conventional mobileradio speech channels, such as the Pan-European GSM system,the IS-54, or IS-95 systems as well as the Japanese digitalcellular system.In Systems 1, 3, and 6 the 11.36-kb/s Codec 1 was invoked.The BCH(127,71,9) coded channel rate was 20.32 kb/s. Uponreconﬁguring the 16- and 4-QAM modems depending on theBCH decoded frame error rate due to the prevailing noise andinterference conditions experienced, signaling rates of 9 and18 kBd, respectively, can be maintained. Hence, dependingon the noise and interference conditions, between eight and16 videophone users can be accommodated in the GSMbandwidth of 200 kHz, which implies minimum and maximumuser bandwidths of 12.5 and 25 kHz, respectively.In Systems 2, 4, and 5 the RL-coded 8-kb/s Codec 2 per-formed similarly to Codec 1 in terms of PSNR, but it was moreerror sensitive. The lower rate allowed us, and the lower ro-bustness required us to use the stronger BCH(127,50,13) FECcodecs. Further system features are summarized in Table IV.As expected, the 4-QAM/18-kBd mode of operation of allsystems is more robust than the 16-QAM/9-kBd mode, but itcan only support half the number of users. Explicitly, withoutARQ, eight or 16 users can be supported in the 4-QAM/18-kBd or 16-QAM/9-kBd modes. When ARQ is used, thesenumbers are reduced by two in order to reserve slots forretransmitted packets. A compromise scheme is constitutedby Systems 2 and 6, which retransmit only the corruptedClass One bits in case of decoding errors and halve thenumber of bits per modulation symbol, facilitating higher

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