Analysis of Spectrum Band Sharing Between CDMA and Other Systems

SSC00-IX-7

Analysis of Spectrum Band Sharing Between CDMA

And Other Systems

Tarek Attia*, Peter Sweeney** and Martin Sweeting† University of Surrey, Guildford, Surrey, GU2 7 XH, United Kingdom

Tel: +44 1483 259278 Fax: +44 1483 259503 E-mail: [email protected]

Abstract ______

Many mobile satellite systems have been proposed to employ Code Division Multiple Access (CDMA) technique at L/S band. These bands have been allocated to the mobile satellite services on a primary basis since 1992, in addition to the existing services. Therefore, the mobile satellite services will be subject to share spectrum with each other and with existing services.

This paper discusses spectrum overlapping for two cases: i)- between a CDMA channel and narrowband signal channel; ii)- between CDMA channels sharing the same band. Analyses have been performed to increase the overall spectral efficiency in the frequency band, taking into account various factors such as the relative position of overlapping and the amount of overlapping. Positions of optimum overlapping for spectral efficiency are discussed. This spectrum sharing is referred to as a CDMA overlay, and has a potential to enhance both the spectral utilization and the flexibility of that utilization.

From the results, the best position of the multiple narrow band signals, to allow sharing spectrum with CDMA signal without loosing significant capacity for both of them, is at the edge of the CDMA channel. Further improvement can be achieved for CDMA system, especially when narrow band signals have more power compared to the CDMA system, by implementing a notch filter at the specific location of the narrow-band signal. For CDMA/CDMA spectrum sharing, the overall spectral efficiency may be increased significantly by carefully choosing the overlapping position.

______* PhD. Student at Surrey Space Center ** Senior lecture at Center for Communication Systems Research † Professor and Director of Surrey Space Center

Tarek Attia 141th Annual/USU Conference on Small Satellites I. Introduction the effect of the interference from CDMA ______systems to narrow-band system. The interference suppression inherent to CDMA Recently, there has been considerable interest in is not available to the other access methods. applying direct sequence spread spectrum (SS) FDMA and TDMA channels should be separate techniques to multiple access communications. and non-overlapping; the C/I ratio requirements This is partly due to its multiple access capability, give strong channel isolation, to avoid co-channel robustness against fading and anti-interference and adjacent channel interference. capability. Since direct sequence (DS) spread The capacity of a CDMA system is interference spectrum waveforms have a wide bandwidth, limited due to the internal code interference whenever the bandwidth exceeds the (interference from other users sharing the same coherence bandwidth of the channel the fading channel) and external interference. This implies tends to be frequency selective, and a rake that spectrum overlapping will result in the loss of Receiver can be used to enhance the system capacity for the CDMA system compared with a performance [1]-[4]. non-sharing system. The overlapping techniques Anti-interference capability in a DS-CDMA make use of multi-carrier code division multiple system is achieved by correlating the received access (MC-CDMA) waveforms. In such a signal with the predetermined spreading design, multiple narrow-band CDMA sequence thus allowing the inherent processing waveforms, each at a distinct carrier frequency, gain of the system to attenuate the interference are combined together to form a composite wide- [5]-[6]. Furthermore, an interference suppression band CDMA signal. Among the advantages of filter can be utilized to reject the narrowband such an approach is the ability to achieve the interference [7]. same types of system performance that a The radio frequency spectrum is a very limited conventional, single carrier, wide-band CDMA resource already heavily utilized by terrestrial signal would provide. Regarding the overlay, radio and satellite network's system in the MC-CDMA transmission can be especially VHF/UHF and L/S bands. Recently, many attractive, since frequency slots occupied by the mobile satellite systems have been proposed to narrowband waveform can be avoided altogether employ Code Division Multiple Access (CDMA) by simply not transmitting at the corresponding technique at L/S band. Those bands have been carrier frequencies. allocated to mobile satellite services on a primary The spectrum overlapping can be classified as basis since 1992, in addition to existing services. follows: With the expected wireless revolution in i)- Overlapping between CDMA channels. telecommunication the available spectrum should ii)-Overlapping between CDMA channel and a be used efficiently and flexibly. Methods of narrow band signal sharing the same band. sharing the frequency resource, not only amongst Also, the overlapping between CDMA channels the satellite networks but also the existing can be categorized as follows: terrestrial users of the bands, are the key to the 1-Overlapping between CDMA channels is made successful implementation of the communication by changing the frequency separation between systems. Accurate knowledge of the state of the CDMA channels without increasing the occupation of the frequencies is especially channel bandwidth. important to control the interference. 2-Overlapping between CDMA channels is made One significant advantage of spread spectrum by fixing frequency separation between the CDMA technique is its potential ability to coexist CDMA channels and increasing the channel with other CDMA systems and narrow-band bandwidth (i.e., each channel is widened to systems to improve the spectral efficiency. occupy a larger portion of the band). CDMA systems are relatively tolerant to narrow- A survey of the organization of this paper is as band interference due to their inherent follows: processing gain; spectrum spreading minimizes

Tarek Attia 142th Annual/USU Conference on Small Satellites The following section introduces a survey of the More details about the technique can be found in principle of the DS-CDMA technique, and [1]-[8]. section III presents the analysis of system performance of overlapping between the CDMA III. Analysis System Performance systems. In section IV, we present the analysis of Overlapping Between the of system performance of overlapping between DS-CDMA Systems the CDMA system and narrow band system, and ______numerical results are presents in section V. _ Finally, in section VI we present the conclusion. We are interested in exploring the optimum II. An Overview of The DS- overlapping technique between the adjacent MC- CDMA Techniques CDMA channels or overlapping between ______different CDMA systems sharing the same spectrum for minimum adjacent channel In this section basic principals of DS-CDMA are interference, which improves spectrum briefly reviewed. DS-CDMA signals are efficiency. separate, neither in frequency, nor in time. Every The channel overlapping technique is applicable mobile unit is assigned a unique code, which has only to CDMA because a certain amount of good orthogonal properties. Each cell uses the interference can be tolerated, since it can whole frequency bandwidth all the time (i.e., the operate on much lower C / I values (channel frequency reuse ratio is equal 1). The DS- isolation is not required)[9,10]. The spectrum of CDMA transmitter spreads the original data CDMA is determined by the chip pulses of the stream using a given spreading code in the time PN sequence and has a sinc-shape. The power domain. spectrum of each channel is concentrated at the Spreading is obtained by multiplying the middle of the band, results in power coming from transmitted signal with a PN sequence code. The the "tails" of the spectrum of the adjacent signal bandwidth is spread by the processing gain overlapping channel is small for small amount of ( PG ). The internal interference, from the other overlap. For an optimum choice of the users occupying the same CDMA channel, is permissible channel overlap, the net effect is a reduced by the PG and by the choice of reduction in total interference with considerable orthogonal code. The capability of suppressing improvement of spectrum efficiency and multi-user interference is determined by the increase in the capacity of the system. cross-correlation characteristic of the spreading codes. Also, a frequency selective fading 3.1- Analysis of Overlapping by Widened channel is characterized by the superposition of Channels Bandwidth several signals with different delays in the time domain. Therefore, the capability of In this technique, the channel overlapping distinguishing one component from other technique results in an increase in the bandwidth components in the composite received signal is of each channel which increases the processing determined by the auto-correlation characteristic gain, these result in a decrease in interference, of the spreading codes. DS-CDMA is a potential and hence, an increase in capacity. candidate for the next generation of mobile The adjacent CDMA channels are overlapped in communication, mainly due to its interference frequency. Although the adjacent channel reduction capabilities, which directly translates overlap permits adjacent channel interference, into higher capacity. Proper utilization of voice the larger bandwidth realized for each channel activity, frequency reuse, sectorization, soft hand increases the processing gain and hence reduces over and rake receiver capability of CDMA the interference. makes it attractive for cellular systems. We assume the power control is perfect and the However, imperfect power control may result in coded pulses have rectangular shapes. significant reductions in estimated capacity.

Tarek Attia 143th Annual/USU Conference on Small Satellites In the case of non-overlap, orthogonal code and adjacent channel interference (I o ) ext and is single cell; carrier to interference ratio is given given by: by [4]: P / n Kn P = (I ) + P (1-7) (C / I ) = (1-1) T o ext o n (K - 1).P / n n K n The carrier to interference ratio for the CDMA Where C is the carrier power, I is the channel after overlap is given by: interference power. Po (C / I )o = (1-8) Kn is the number of users sharing the channel K o .PT - Po and P is the total power of one CDMA channel. n We assume the threshold level of E b No of the

The E b No is equal to: Detector is the same in both cases then, (Eb / No)th = PGo.(C / I)o (1-9) E / N =PG .(C / I) Substituting from Eq.(1-8) into Eq.(1-9), b o n n (1-2) (C I ) o

Where PGn is the processing gain without we obtain: overlapping. By using Eq.(1-1) and Eq.(1-2) the Po (Eb / No )th = PGn.(BWo / BWn ). (1-10) number of users is equal to; Ko.PT - Po

PG n The factor BWon represents the ratio K n = 1 + (1-3) ( E b / N o ) th BWo BWn , the capacity Ko can be obtained by

Where (E b No)th is the threshold value of the simplifying Eq.(1-10), we obtain: detector. PG P K = (1+ BW . n ) o o on E / N .P (1-11) We assume both (E b No )th and PGn are set ( b o)th T to 6.8dB and 127 respectively. The bandwidth of The improvement of the overlapping system can the channel before overlapping is BWn and be obtain by dividing the capacity of the system in case of overlapping with respect to the BWo is the bandwidth of the channel after overlapping. The amount of interference coming Capacity of the system of non-overlapping, and is from the adjacent channel after overlapping is obtained from Eq.(1-3) and Eq.(1-11): K o denoted by (I o ) ext . K = (1-12) on K The relation between the processing gain of the n CDMA channel after and before overlap is given All the values of this equation are known except P and P which are given by the by: 2 T integration of the curve of desired channel after (PGo / PGn)= (BWo / BWn) (1-4) increasing the bandwidth of the channel before and after filtering respectively and this is done by Where, PG is the processing gain after o the simulation. increasing the channel bandwidth. The total interference has two components; one from the internal interference (I ) from users 3.2- Analysis of Overlapping by o int Changing Channel Separation Between sharing the CDMA channel, and the other from adjacent channel and is given by: CDMA Channels I T = (I o ) + (I o ) (1- int ext The spectral shaping of the signal is achieved by 5) a root-raised cosine filter at the transmitter. Po (I o )int = Po - (1-6) The effect of filtering will be evaluated by K o assuming raised cosine filter with different roll- Where Ko is the number of users in the CDMA off factors. channel after overlap, Po is the power of the The frequency characteristic of raised cosine CDMA channel after overlap. filter are given by [2]: The total power of the channel is the net of the power of the desired channel ( Po ) and the

Tarek Attia 144th Annual/USU Conference on Small Satellites ìTcp f £ f1 3.3- A Simplified Model for the Block ï p p Diagram of CDMA System HR( f ) = íX f1 f f2 ï (1-13) îï0 f ³ f2 The simulation block diagram of a CDMA system is shown in Fig. (1). Where Assuming, the two CDMA systems are identical and have a perfect power control for both of æ p æ Rcp(1- b )öö X =T cos2ç ç f - ÷÷ them. Then the interference effect of CDMA cp ç ç ÷÷ è 2b Rcp è 2 øø system 2 to the other CDMA system 1 can be estimated as follows; b = Roll-off factor. f = (1- b )R / 2 Interference power of CDMA 2(IC2 ) 1 cp I sk = (1-14) Interference power of CDMA1(IC1) f =(1 + b )R / 2 2 cp Where Rcp is the chip rate W s / 2 2 IC 2 = K . ò PC 1. H R ( f - f sep )df (1-15) -W s / 2

Spreading Raised QPSK UP Inp. signal modulator cosine filter modulator converter

Channel

N.B.Signal +

Original QPSK Despreading Raised Down signal demodulator demodulator cosine filter converter

Figure (1) Block diagram of CDMA system

Pc1 is the power of each CDMA user signal W s / 2 is the frequency offset between 2 f sep I = P . H ( f )df (1-16) C 1 ò C1 R carrier frequencies -W s / 2 Ic1 is the internal code interference of W s / 2 2 ò H R ( f - fsep )df a single CDMA signal. I sk -Ws / 2 The total frequency efficiency without any Is1 = = Ws / 2 (1-17) spectrum overlapping is given by: K 2 ò H R ( f )df Actual capacity of the channel -W / 2 eff1 = s Total frequency of the channel Where (1-18) 2.K .Rb I sk is the loss of capacity due to overlapping = 2.Rcp .(1 + b ) k is the number of users in CDMA channel With overlapping the total frequency efficiency Ws =R cp(1+b ) is given by:

Tarek Attia 145th Annual/USU Conference on Small Satellites 2.K.(1 - Isk ).Rb Assuming the effect of the filter within the eff2 = (1-19) bandwidth of the narrowband signal is very fsep + Rcp.(1 + b ) small, the interference produced by the Where narrowband signal after despreading is given as R is the bit rate b follows; Then the improvement in the spectral efficiency W s / 2 is given by: 2 I N = ò PTn . H R ( f - fsep )df (1-21) eff2 - eff1 -W s / 2 eff12 = (1-20) eff1 Where The overlapping of CDMA/CDMA system can PTn = PT . H R ( fsep ) (1-22) achieve an improvement in the spectral The interference produced by the CDMA signal efficiency and the amount of the spectral from other users after despreading is given as efficiency can be evaluated. follows; W s / 2 2 IV. Analysis of Sharing Between IC = K . ò PC . H R ( f )df (1-23) DS-CDMA and Narrowband -W s / 2 Signal The amount of interference accumulation of ______multiple narrowband signals sharing the bandwidth with respect to single CDMA signal is One of the most attractive features of direct given by; sequence code division multiple access (DS- M CDMA) system is the ability to overlay the åInterference powerof NBS (INBS(n)) spread spectrum waveforms on the top of n=1 ITC = (1-24) narrowband signal of communication systems. Interference powerof CDMA1 (Ic1) DS-CDMA signals, spread over a sufficiently Where wide bandwidth, have a very low power spectral Ws /2 2 density, so the additional degradation to the I NBS(n) = k. ò PT (n). HR( f - fsep(n))df (1-25) narrowband systems can be made small. -Ws /2 Furthermore, to eliminate the interference from Where the DS-SS signals to a narrowband user, one can I (n) is the nth narrowband interference just stop transmitting at this frequency, which NBS On the other side, the effect of the interference occupies the same spectrum as the narrowband from CDMA system sharing the frequency band user; in this case there is no interference from with narrowband signal will depend on the the CDMA to the existing narrowband system frequency separation between of them. The [11-13]. On the other hand, the DS-CDMA is smaller the frequency separation, the larger the inherently resistant to narrowband interference, amount of interference signal from CDMA to because the despreading operation has the effect narrowband signal. of spreading the narrowband interference over It is very important to evaluate the bit error rate the wide bandwidth. However, the narrowband (BER) performance, in order to investigate the interference signal should be rejected before frequency-sharing problem of narrow band despreading especially when the power of signal and DS-CDMA systems. interference is sufficiently large when compared with the power of the DS-CDMA. Bit Error Rate of Sharing System From Figure (1), the narrowband signal The received signal at the receiver is multiplied experiences an attenuation by the square-root by the PN-Sequence, then the narrowband raised-cosine filter before despreading, the level interference signal is spread out and its power of attenuation depends on the frequency offset spectral density is reduced, the total average from the center frequency of the CDMA and the interference power is determined by the value of bandwidth of the narrowband signal with respect processing gain and interference power. In the bandwidth of the CDMA signal. narrowband signals assuming an AWGN channel,

Tarek Attia 146th Annual/USU Conference on Small Satellites the BER of CDMA without QPSK modulations The degree of destruction depends on the are used is given by [14]: bandwidth of the notch filter and the performance of the band sharing depends on the æ ö ratio of the bandwidth of the notch filter to the ç ÷ 1 ç Eb ÷ bandwidth of the DS-CDMA signal. Pe = erfc (1-26) 2 ç 2 k - 1 ÷ In the frequency sharing system, the BER of ç N o + .V .Eb . ÷ è 3 PG ø narrowband signal with DS-CDMA signals is given by: In the frequency sharing system, the BER of æ ö DS-CDMA with narrowband signals is given by; ç ÷ 1 ç 1 ÷ Pe = erfcç ÷ 2 N P (1-29) æ ö ç o + C ÷ ç ÷ ç ÷ Eb PT .PG 1 ç Eb ÷ è ø Pe = erfc (1-27) 2 ç 2 k - 1 P ÷ ç N + .V .E . + T ÷ ç o b ÷ è 3 PG Pc .PG ø V. Results Where ______P is the power of the narrowband interference T It is obviously from Fig. (2) that, the relative signal. capacity increases when BW also increases. PG is the processing gain on V is the voice activity factor The capacity improvement reduces with increasing BW after the optimum value. The It can be observed from Eq.(1-27), that the on performance of CDMA signal depends on the reason is that the amount of the reduction in the power of the narrow band interference and the interference caused by the larger processing bandwidth ratio of CDMA to narrow band gain, due to increasing CDMA channel signal. When the narrow band signal level is bandwidth, is less than the adjacent channel comparable to the CDMA signal. An active interference due to overlapping. For N=4 the narrow band interference rejection technique optimum value of the capacity improvement can be employed at the CDMA receivers at the ( Kon) is 1.21 at BWon =1.37 and the optimum locations of the narrow-band waveforms would value of the capacity improvement ( Kon) is 1.18 minimize interference to the CDMA network. at BWon =1.33 for N=5. It is clear for N=5, The BER of DS-CDMA with narrowband signals where N is the order of the filter, the optimum by using a notch filter is given by: value of the capacity improvement reduces æ ö because the slope of the roll-off factor filter ç ÷ 1 Eb (1 - e ) increases and becomes more flat. P = erfcç ÷ e 2 ç 2 k -1 ÷ (1-28) Fig.(3) shows that the optimum overlapping ç No + .V .Eb. ÷ techniques depends on the roll-off factor of the è 3 PG ø raised cosine filter. For roll-off factor ( b =1), Where e is the bandwidth ratio of the notch the maximum efficiency improvements are filter to CDMA signal. 13.4%, at optimum overlapping between the It is observed that the rejection of the narrow adjacent CDMA channels ( f /W =0.71). band signals can significantly improve the bit sep s error rate performance of the CDMA system For b =0.8, maximum efficiency improvement is and the total capacity of co-existence. However, 11.8% at optimum overlapping ( fsep/Ws = 0.74). the improvement of the performance of the DS- As b decreases the maximum efficiency CDMA can result in some spectral distortion due improvement reduces. to the notch filter, which destroys the auto- There is no capacity improvement for all values correlation and cross-correlation characteristic of the roll-off factor of the raised cosine filter for of spreading code of the signal passing through ( f /W <0.48), because the amount of it. sep s interference from the adjacent channel is

Tarek Attia 147th Annual/USU Conference on Small Satellites significant with respect to efficiency existence. As the power ratio of the total DS- improvement. CDMA to narrow band signal increases the BER From Fig. (4), the overlapping between the of narrow band signal degrades, but when the adjacent CDMA channels causes a loss of the power of narrow band signal is large enough capacity of the CDMA channel, and the amount compared to DS-CDMA signal, we can ignore of loss depends on the roll-off factor of raised the effect of DS-CDMA signal on the BER of cosine filter, which reduces with increasing the narrow band signal. roll-off factor at ( fsep /Ws >0.5). The loss of capacity is approximately the same for different roll-off factors at the optimum overlapping of VI. Conclusion each case and, equals 0.04. ______Fig. (5), it can be seen that a greater number of narrow band signal channels may be achieved The with greater roll-off factor of the raised-cosine feasibilit filter with insignificant effect on the CDMA y of system, especially when the narrow band signal CDMA/ channels share the edge part of the CDMA CDMA spectrum. For instance, when the ratio between spectru the bandwidth of CDMA signal to narrow band m signal (Ws /BW1 )=50; the internal code overlapp interference from single CDMA signal equals the ing and effect of 13 narrow band signal users sharing the CDMA edge part of CDMA channel at b =1.0 and the with number of narrow band signal users sharing the narrow edge parts of CDMA channel reduce with band decreasing the roll-off factor ( b =0.2) and the system is number of narrow band users equals 6. studied Also, It can be observed that the interference of for CDMA to narrow band signal depends on the spectru value of energy, which lies in the narrow band m sharing in the communication systems. The signal, and this value is significantly dependent of results show the feasibility of improving channel the frequency offset. capacity and efficiency of the spectrum by the It can be observed from Figs. (6,7), that the overlapping techniques. performance of CDMA signal depends on the For the case of widened channel bandwidth of power ratio of the narrow band interference to the CDMA channel, the overlapping is tested the CDMA signal, and the bandwidth ratio of under different degrees of channel overlap and CDMA signal to the narrowband signal. For different order of the filters. The best result instance, when the power ratio equals -5dB the shows that at the optimum degree of channel effect of narrow band signal on the performance overlap; channel capacity increases up to 21%. of CDMA signal is more significant than power For the case of fixed channel bandwidth, the ratio equals 0dB. Furthermore, when the optimum overlapping between CDMA systems bandwidth ratio equals 4 the effect of narrow depends on the filtering Roll-off factor and can band signal on the performance of CDMA signal be achieved an improvement of the spectrum is more significant than bandwidth ratio equals efficiency up to 13.4%. 64 or 32. It can be observed that the number of From Fig. (8), when we employed a notch filter narrowband signal users sharing with CDMA at the receiver. It is clear, that the rejection of channel depends on the ratio between the the narrow band signals can be significantly bandwidth of CDMA signal to narrowband improved the bit error rate performance of the signal, and the offset of narrow band signal from CDMA system and the total capacity of co- the center frequency of CDMA channel. The

Tarek Attia 148th Annual/USU Conference on Small Satellites best location of narrowband signal to share spectrum with CDMA system is at the edge of the CDMA channel, where interference with each other is minimized. Also we can see that the frequency sharing system without notch filtering is interference limited and the BER of narrowband signal performance improves as its power increases, while the BER performance of the DS-CDMA signal degrades. Further improvement is possible in the frequency sharing system with notch filtering, especially when the power of narrowband signal is comparatively large with respect to the CDMA signal. The BER performance of DS-CDMA signal is significantly improved by rejecting Narrowband signals. REFERENCES

1. J.Proakis, Digital Communications, McGraw- Communication and Spread Spectrum Systems, Hill, New York, 1989. Macmililan, New York, 1985. 2. G.L. Turin, " Introduction to Spread Spectrum 6. R.L.Picholtz, D.l.Shililing and L.B.Milstien, Antimultipath Techniques and Their Application "Theory of Spread Spectrum Communications- to Urban Digital Radio." Proc. IEEE, vol.68, A tutorial." IEEE, Trans. Comm., vol. Com.- 30, Mar.1980. No.5, May 1982. 3. T.Eng and L.B. Milstien, " Coherent DS-DMA 7. L. Milstien,” Interference Rejection Techniques Performance in Nak-Agami Multi-path fading." in Spread Spectrum Communications." Proc. IEEE Trans. Communication. Vol.43, PP.1134- IEEE, vol.76, Jun.1988. 1143, Mar.1995. 8. Klein S. Gilhousen, Irwin M. Jacobs, Roberto 4. Jhong S. Lee, Leonard E. Miller, CDMA Padovani, Lindsay A. Weaver, " Increased Systems Engineering Handbook, Artech House, Capacity Using CDMA for Mobile Satellite New York, 1998. Communication,” IEEE Journal on Selected 5. R.E. Ziemer and R.L.Peterson, " Digital

Tarek Attia 149th Annual/USU Conference on Small Satellites Areas in Communications, vol. 8, No.4, May Spread Spectrum," IEEE Transaction on 1990. Vehicular Technology, vol.48, No.2, pp.397-403, 9. S. Kondo and L. B. Milstien, "On The Use of Mar.1996. Multi-carrier Direct Sequence Spread Spectrum 13. Shiro Kondo and Laurence B.Milstein," Multi- Systems." in Proc. IEEE MILCOM'93, Boston, carrier DS- CDMA Systems in The Presence of pp.52-56, Oct. 1993. Partial Band Interference," IEEE transactions 10. Laurence B. Milstein, " The CDMA Overlay on communications, vol.44, No.2, pp. 238-246, Concept." IEEE Military Communication Feb.1996 Conference, 1996, pp.476-480. 14. M. B. Pursley, "Performance Evaluation for 11. K.S.Gilhousen, I. M. Jacob, R. Padovani, Viterbi, Phased-Coded Spread-Spectrum Multiple access L. A. Weaver, and C. A.E. Wheatly, "On the Communication-Part I: System Analysis,” IEEE capacity of a cellular CDMA system," IEEE Trans. Communication, vol.25, No8, August Transactions on Vehicular Technology, vol.40, 1977. pp.303-312, May1991. 12. Eckhard Papproth and Ghassan Kawas Kaleh, "A CDMA Overlay Using Frequency Diversity

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