MARONGIU, Teresa

Moeneclaey, Marc

Steendam, Heidi

Van Bladel, Marc

English

Ghent University Academic Bibliography

Some TDMA and OFDM/CDMA receiver structures for communication over the return path of the CATV networkMark Van Bladel, Heidi Steendam, Teresa Marongiu and Marc MoeneclaeyCommunication Engineering Lab, University of Ghent, BelgiumAbstractThis paper deals with the derivation and comparison of somepromising TDMA and OFDM/CDMA receiver structures forcommunication over the return path channel of the cable TV(CATV) network. Regarding TDMA, it is shown that theperformance substantially improves by using equalization. Giventhe slowly varying, frequency selective nature of the ingressnoise, OFDM seems an interesting possible access technique toemploy. Here it is considered in combination with CDMA. Withrespect to the performance in terms of SNR at the input of thedecision device, the TDMA structure gives better results than theOFDM/CDMA schemes. Also with respect to design complexityTDMA seems more interesting.1. IntroductionTelecom operators experience more and more competition thesedays from cable TV operators when it comes to offering newtelecommunication services. One can hereby think of fastinternet, home shopping, banking, video on demand, telephonyservices etc... In order to allow for this type of applications thenetwork configuration should evolve from a coaxial to a hybridfiber coax (HFC) structure.In the HFC architecture optical node units (ONU) are connectedto the headend by means of single mode optical fiber, while thepart between the ONU and a group of subscribers is over coaxialcable. This latter part of the structure is implemented as a treeand branch network. This network structure is now beingmodified to allow for bidirectional services. The return pathmust also be equipped with amplifiers. In general unity gain isdesired from the first return amplifier as seen coming from thesubscriber to the ONU. The scheme described here is shown inFigure 1.Head EndONUTrunk amplifier BridgerTapLine extenderFigure 1 : typical HFC network structureThe CATV operator can choose which frequency band to use forthe return path for the interactive services as well as the numberof channels and the channel bandwidth. Some operators onlywant to use the spectrum between 5 and 35 MHz, whereas othersgo from 5 to 60 MHz. Anyway, in this paper results arepresented for channels of 1 and 2 MHz bandwidth and withseveral values for the central frequency.Apart from the non-ideal channel transfer function (which is alsodifferent for every user), two very important impairments areencountered in this type of networks, namely ingress noise andimpulse noise. Ingress noise is slowly-varying noise, originatingfrom nonlinearities in the CATV system, from interferencegenerated by a device at the subscriber location or from signalsradiatively coupled to the return path from outside the network.Impulse noise can be caused by a whole range of phenomena. Itcan be generated by naturally occurring sources (like lightning)and man-made sources (engine ignitions, power switching,...). Itis assumed that most of the impulse noise can be counteracted byemploying Forward Error Correction [1].Various multiple access techniques can be considered toestablish reliable communications over HFC networks. Therehas recently been an increasing interest in using multi-carrierspread-spectrum (MC-SS) techniques for high data rateapplications [2-4]. An example of a MC-SS technique is thecombination of orthogonal frequency division multiplexing(OFDM) and code division multiple access (CDMA). Thecombination of OFDM and CDMA can be expected to give goodresults because the peaks in the ingress noise spectra affect onlya part of the carriers. As the CDMA part spreads the datasymbols of the different users over the carriers, all users cantransmit with comparable performances. The performance willalso depend on the number of active users in the system. In thispaper however, a fixed number of users is always considered.Hereafter, the performance on the return path is compared withthat of TDMA. In a TDMA system each channel is shared bymany users, whom are assigned time slots to in order to sendtheir information packets. As the signal-to-total noise ratio isdetermined by the total quantity of ingress noise within thechannel bandwidth and by intersymbol interference, theperformance of the TDMA system is user-independent.The organization of the rest of the paper is as follows. Section 2describes the derivation of a TDMA receiver structure andshows the benefit equalization can bring. In section 3 the basicOFDM/CDMA receiver scheme is presented. We also discussthe influence on the performance of adding certain filters to thescheme. Section 4 deals with the comparison between theTDMA and OFDM/CDMA receiver structures derived inprevious sections in terms of performance and complexity.Finally, some conclusions are drawn in section 5.2. Some TDMA receiver structuresIn this section we shall discuss the benefits and drawbacks ofsome TDMA receiver schemes. The numerical results presentedin this section are all obtained with the channel transfer functionwhose amplitude (deviation from the mean value) is given inFig. 2. In the simulations this function is normalized, such that atthe input of the receiver filter the signal has a power spectraldensity of -70 dBm/Hz. The ingress noise is modeled ascoloured Gaussian noise with a power spectrum equal to themeasured spectrum shown in Fig. 3. Note that this is a worstcase noise spectrum : measurements were carried out during 24hours and the values in the figure show the levels exceededduring only 0.1 % of the time.-30-25-20-15-10-5050 10 20 30 40 50 60 70frequency (in MHz)amplitude (in dB)Figure 2 : amplitude transfer function0.0 20.0  40.0  60.0  80.0 frequency (in MHz)-60.0-40.0-20.00.0noise level (in dBmV/sqrt(Hz))Figure 3 : ingress noise spectrumConsider now the receiver structure depicted in Fig. 4a. Thetransmit filter is a square root raised cosine filter. The receiverfilter is matched to both the channel, the ingress noise spectrumand the transmit pulse. This structure yields the theoreticaloptimal ratio of useful signal power Ps to noise power Pn at theinput of the decision device (matched filter bound). Table 1shows this optimal Ps/Pn for some channels, as well as the ratioof Ps to the intersymbol interference power Pisi. The calculationswere done with a roll-off factor of 0.3. In the fourth column thetotal ratio Ps/(Pisi+Pn) is given.Note that the transmission of uncoded QPSK over an AWGNchannel with a BER < 10-3 requires a Es/No ratio of at least 9.8dB. A bit error rate of 10-3 before channel coding corresponds toa BER of about 10-8 after coding.Figure 4aFigure 4bchannel(MHz)Ps/Pn(dB)Ps/Pisi(dB)Ps/(Pisi+Pn) (dB)9 - 11 22.3 0.8 0.813 - 14 26.7 7.1 7.021 - 23 29.1 11.3 11.347 - 48 29.9 12.4 12.350 - 52 30.5 11.3 11.3Table 1Given the relatively flat shape of the transfer function, the ISI ismainly caused by the receiver filter. This high amount of ISImakes any communication virtually impossible. In order toincrease the Ps/Pisi ratio we add decision feedback equalization tothe scheme (Fig. 4b). Sampling is performed at twice the symbolrate. The forward filter is implemented as a fractionally spacedfilter. At the output of the forward filter a decimation to thesymbol rate takes place. The equalizer filters reduce the effect ofthe ISI, but at the same time the Ps/Pn ratio at the input of thedecision device also decreases. Some numerical results for thiscase are summarized in Table 2.channel(MHz)Equalizer(FF/FB)Ps/Pn(dB)Ps/Pisi(dB)Ps/(Pisi+Pn)(dB)9 - 11 50/25 17.3 6.2 5.950/100 17.6 7.8 7.4∞ / ∞ 16.9 ∞ 16.913 - 14 25/50 22.5 15.6 14.850/100 22.1 19.6 17.7∞ / ∞ 25.1 ∞ 25.121 - 23 50/25 24.8 18.3 17.550/100 25.1 20.4 19.1∞ / ∞ 28.8 ∞ 28.847 - 48 25/50 27.1 21.1 20.150/100 25.8 25.6 22.7∞ / ∞ 29.7 ∞ 29.750 - 52 25/50 28.4 16.5 16.250/100 27.7 19.4 18.8∞ / ∞ 30.3 ∞ 30.3Table 2For each channel the figures are given for a few lengths offeedforward and feedback filter. The DFE yields a gain of a fewdB for the lower frequency channels, but even long equalizerfilters cannot do much about the high ISI levels. The figures arestill far from those we obtain with infinite length filters. Inchannels less affected by ingress the increase in Ps/(Pisi+Pn) islarger. The results given for the channels between 9-11 and 21-23 MHz clearly demonstrate the importance of the feedbackfilter.The receiver filter transfer function in Fig. 4 is quite complexand, as was discussed, a lot of ISI is introduced in this way.Therefore we now consider the receiver structure in Fig. 5. Thereceiver filter is a fixed filter matched to the transmitted pulse.Results are given in Table 3. The relatively flat amplitudecharacteristic in Fig. 2 now gives rise to high Ps/Pisi ratios in allcases. In frequency bands not too heavily affected by ingress,good values for Ps/Pn are obtained as well. Communication overchannels in the frequency band below 15 MHz still remainsproblematic in presence of a heavily peaked ingress spectrumlike that of Fig. 3.Figure 5channel(MHz)Ps/Pn (dB)Ps/Pisi(dB)Ps/(Pisi+Pn)(dB)9 - 11 2.5 32.8 2.513 - 14 14.9 40.3 14.821 - 23 27.6 47.9 27.647 - 48 27.5 39.8 27.250 - 52 29.7 42.8 29.5Table 3Attempting to obtain higher Ps/Pn ratios, we bring into thescheme of Figure 5 the decision feedback equalizer alreadyshown in Fig. 4b. Sampling is again performed at 2/T. As can beread from Table 4, even with a feedforward and a feedback filteras short as 10 taps we obtain an increase of more than 10 dB.Comparing the figures for the channels 13-14 MHz and 9-11MHz with those in Table 3, it is clear that the equalizercoefficients are adjusted such that in the first place theycompensate for the noise; as a matter of fact, for the channelbetween 9 and 11 MHz the Ps/Pisi ratio has even decreased. Thefeedback filter proves to be very useful to compensate for the(postcursor) ISI introduced by the peaks in the ingress spectrum.For the channels that already had a good Ps/(Pn+Pisi), theimprovement is relatively limited. The influence of the feedbackfilter has become less prominent in this case.channel(MHz)equalizer(FF/FB)Ps/Pn(dB)Ps/Pisi(dB)Ps/(Pisi+Pn)(dB)9 - 11 10/10 14.2 21.7 13.550/100 16.2 24.9 15.713 - 14 10/10 22.9 41.7 22.850/100 24.4 49.7 24.421 - 23 10/10 28.0 53.1 28.050/100 28.5 62.2 28.547 - 48 10/10 28.7 54.1 28.750/100 29.0 61.1 29.050 - 52 10/10 29.8 59.2 29.850/100 30.1 70.8 30.1Table 43. Some OFDM/CDMA receiver structuresThe basic transceiver structure of the OFDM/CDMAcombination is shown in Figure 6. At the receiver side, the datasymbols are first correlated with orthogonal CDMA sequencesand then spread over the orthogonal OFDM carriers. Thetransmit filter P(f) is a square root raised cosine filter withrespect to the interval T/N, N referring to the number of usersand T the duration of the OFDM symbol. The receiver filter ismatched to the transmit pulse. We assume a guard interval isinserted as a cyclic prefix, such that no inter-symbol interferenceoccurs. With the channel transfer function of Figure 2, a guardinterval of 8 samples is sufficient for all the channels we areconsidering. The main noise contributions are multi-userinterference (MUI), due to the loss of orthogonality between thedifferent users, and ingress noise. Simulations were done in thesame circumstances as for the TDMA structures. The number ofusers N equals 256.Table 5 shows the ratio of the useful signal power Ps to the MUIpower PMUI at the input of the decision device, as well as the ratioof  Ps to the ingress noise power Pn and the total ratioPs/(PMUI+Pn). The loss of orthogonality in this basic structure iscaused only by the channel characteristics. As the channeltransfer function is relatively flat, ingress noise is the dominantnoise mechanism.Figure 6channel (MHz)Ps/PMUI(dB)Ps/Pn(dB)Ps/(PMUI+Pn)(dB)9-11 26.8 1.4 1.313-14 31.6 13.8 13.721-23 42.0 26.5 26.447-48 31.6 27.1 25.850-52 36.9 28.5 27.9Table 5To improve the system performance, a filter is inserted as shownin Figure 7. It can be proven that the filter which optimizes thetotal ratio Ps/(PMUI+Pn), contains N2 coefficients only dependingon the channel characteristics and the ingress noise. To reducethe complexity of this filter, an alternative filter, containing onlyN coefficients, is proposed in Figure 8. This filter can be seen asa frequency domain equalizer,Figure 7Figure 8Tables 6 and 7 summarize the results for the filter of Figure 8optimizing the Ps/PMUI ratio and for the one that optimizes thePs/Pn ratio, respectively. When maximizing the Ps/PMUI ratio, ahuge amount of ingress noise will obstruct reliablecommunication, and optimizing the Ps/Pn ratio will engender ahigh amount of MUI, as the used filter strongly affects theorthogonality between the users.channel(MHz)Ps/PMUI(dB)Ps/Pn(dB)Ps/(PMUI+Pn) (dB)9-11 50.5 0.9 0.913-14 34.3 13.7 13.721-23 63.2 26.5 26.547-48 34.2 27.1 26.350-52 60.1 28.5 28.5Table 6channel(MHz)Ps/PMUI(dB)Ps/Pn(dB)Ps/(PMUI+Pn)(dB)9-11 0.5 18.5 0.513-14 6.8 25.4 6.721-23 10.6 27.7 10.547-48 11.4 28.7 11.350-52 11.2 29.2 11.2Table 7The performance of the filters shown in Figures 7 (a) and 8 (b),that maximize Ps/(PMUI+Pn), can be found in Table 8. As can bederived from the results in Table 8, we find that, although thereis an enormous increase in complexity of the filter shown inFigure 7 compared to the alternative filter of Figure 8, the gainin the ratio Ps/(PMUI+Pn) is small.channel (MHz)filter Ps/PMUI(dB)Ps/Pn(dB)Ps/(PMUI+Pn)(dB)9-11 a 10.0 10.1 7.0b 9.8 9.3 6.613-14 a 20.6 18.0 16.1b 20.4 17.9 16.021-23 a 51.3 26.5 26.5b 51.8 26.5 26.547-48 a 48.1 27.1 27.1b 34.0 27.1 26.350-52 a 59.9 28.5 28.5b 57.2 28.5 28.5Table 8A comparison of the figures in Tables 5 and 8 shows us that inregions highly affected by ingress noise the Ps/Pn ratio increasesand at the same time, the Ps/PMUI ratio decreases. For channels inthe frequency band under 15 MHz reliable communicationremains problematic, while for channels above 15 MHz, wherethe ingress spectrum is less peaked, a good signal to total noiseratio is obtained.Figure 9As the ingress noise can not be influenced by the system, anadditional improvement of the performance can only beachieved by reducing the multi-user interference. In Figure 9, afeedback section was entered in the structure shown in Figure 8,eliminating the multi-user interference. Assuming perfectdecisions, the signal after elimination is still disturbed by theingress noise and needs an additional filter to diminish the effectof the ingress. Analog considerations as made for the filter ofFigure 8 allow to implement this second filter as a filter with Ncoefficients, which need to be optimized to ingress only, as allmulti-user interference is eliminated. Table 9 shows the resultsobtained with elimination of the multi-user interference.channel(MHz)Ps/PMUI(dB)Ps/Pn(dB)Ps/(PMUI+Pn)(dB)9-11 ∞ 18.5 18.513-14 ∞ 25.4 25.421-23 ∞ 27.7 27.747-48 ∞ 28.7 28.750-52 ∞ 29.2 29.2Table 9Comparing the results in Tables 8 and 9 we notice a majorperformance improvement for the channels severely affected byingress noise. However, for channels disturbed by less peakedingress, only a marginal gain in performance is obtainednotwithstanding the increased complexity of the structure.In practice, all filters are implemented as adaptive filters to meetthe variation in time of the ingress noise. The dependency of thesystem performance on the number of carriers was notexamined. However, it can be expected that increasing N,considering a fixed used bandwidth, will ameliorate the systemperformance of the structure shown in Figures 8 and 9. This canbe explained by noticing that if N increases, the carrier spacingwill decrease such that the noise experienced by a carrierbehaves more white and makes the filter of Figure 8 converge tothe complex filter of Figure 7. Nevertheless, it should be notedthat increasing the number of carriers also leads to a rise inreceiver complexity.4. System comparisonHaving another look at the figures in Tables 4 and 8, weimmediately notice a difference in performance. Even for aTDMA structure with short equalizer filters, we find for thechannels below 15 MHz a signal-to-noise ratio more than 6 dBhigher than in Table 8. In other words, it is mandatory to takemeasures to eliminate the multi-user interference in theOFDM/CDMA scheme in order to obtain signal-to-overallinterference ratios more or less similar to that for TDMA (seeFigure 9 and Table 9). In that case the OFDM/CDMA schemegives even better results than the TDMA structure for channelsimpaired by strong ingress noise peaks.Let us compare now the system complexity. In the TDMAscheme with equalization one obtains good results withrelatively short filters. The basic structure of the OFDM/CDMAscheme is clearly more complex (multiplication with PNsequence, (I)FFT, …). Concerning the adaptivefiltering/equalization, we have already indicated that the muchsimpler structure of Fig. 8 gives almost as good results as thefilter with N2 coefficients in Fig. 7. With the filter of Fig. 8 thereare N multiplications per OFDM symbol, where N is the numberof users. The structure depicted in Figure 9 is definitely the mostcomplex one. Filters 1 and 2 in the figure can be N-tap filters,whereas the filter to eliminate the MUI must be a NxN filter.Typical values for N range from 64 to 512. For TDMA goodresults were obtained with equalizer filters with 50 taps.5. ConclusionsIn this paper some receiver schemes for communication over thereturn path of the CATV network were discussed. A TDMAstructure suitable for this type of communication was derived.We indicated the profit decision feedback equalization couldbring in combating the effect of the peaks in the ingress noisespectrum. Next some structures were considered where the chipsof a PN sequence are put on the carriers of an OFDM structure.Filters to compensate for the ingress noise and multi-userinterference can improve the performance. However, it wasshown that several adaptive filters are required to obtain aperformance comparable to that of the TDMA solution. Themain advantage of the TDMA scheme is its much highersimplicity from an implementation point of view.AcknowledgmentThe authors would like to express their gratitude to the Flemishgovernment by means of the IWT and the other partners in theHybrid Fiber Coax Systems project, i.e. the research departmentof Alcatel Alsthom in Belgium, Barco, Integan and the Intecdepartment of the University of Ghent. Part of this work hasbeen carried out within the framework of the European ACTSInteract project. The first author therefore would also like tothank the partners in this project, namely Alcatel SESA (E),Barco (B), Bosch (D), CCETT (F), EBU (B), Integan (B), ITC(GB), Nozema (NL), Retevision (E), SAT (F), TBS (F), TDF(F) and UPM (E). The last author gratefully acknowledges thefinancial support from the Flemish Fund for Scientific Research(FWO-Vlaanderen).References[1] “CATV Return Path Characterization for ReliableCommunications”, C. Eldering, N. Himayet and F. Gardner,IEEE Communications Magazine, August 1995, pp.62-69[2] “Wideband CDMA System for Personal RadioCommunication”, Fukusawa , A., Sato , T., Takizawa, Y. et al.,IEEE Comm. Mag., vol. 34, no. 10, Oct 96, pp. 116-123[3] “New Concepts and Technologies for Achieving HighlyReliable and High-Capacity Multimedia WirelessCommunications Systems”, Morinaga, N., Nakagawa , M.,Kohno, R., IEEE Comm. Mag., vol. 35, no. 1, Jan 97, pp. 34-40[4] “Performance of Orthogonal Multicarrier CDMA in aMultipath Fading Channel”, Sourour , E.A., Nakagawa , M.,IEEE Trans. on Comm., vol. 44, no. 3, Mar 96, pp. 356-367

Some TDMA and OFDM/CDMA receiver structures for communication over the return path of the CATV network.

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Some TDMA and OFDM/CDMA receiver structures for communication over the return path of the CATV network.

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