pp-1, NINETEENTH NATIONAL RADIO SCIENCE CONFERENCE, ALEXANDRL4, March, 19-2 1,2002

Mahmoud Ahmecl Attia Ali Department of Electronics and Electrical Communications Engineering

Faculty of Engineering, University of Tanta, EGYPT, Email: mahmoudasema hotmail.com

ABSTRACT

M-ary FCMA and FCMNDSNA systems are proposed for third generation mobiles. Either the low orbit satellites or ordinary base-stations can be used to switch information for various users all over the world. Huge population. large capacity, simp le access, universal identification, and higher spectral efficiency are amongst FCMA system characteristics. Moreover, are the features of new adaptive modulaticm and international roaming. Many design aspects of M-ary FCMA and FCMAlDSM4 for mobile ccm"mnications are presented. The optimum configuration of M-ary FCMA, permissible numkler of simultaneous mobile channels and the spectral efficiency are also evaluated.

1. LNTRODUCTION

Since 1989, the wide-band spread-spectrum technique that has been denoted FCMA is suggested for communications via satellites [ 11. FCMA. or frequency comb multiple access have been demonstrated and analyzed &om different aspects in many literatures [2- 151. They concentrate on general performance that achieves superior efficiency at lower bit signal to noise ratio. Initially, FCMA has been analyzed statistically and simulated assuming multiple access interference M N [2]. MAI and AWGN are modeled in a satellite channel where sub- optimum receivers have been checked against mnaxiinuni receiver. As a result, a logarithmic receiver was the optimum non-coherent detector for FCRAA [3,8]. The application of FCMA in very small aperture terminal (VSAT) satellite networks is proposed in [4]. It showed superior throughput compared to VFHMA or Aloha [5]. Another study ha:' suggested a rapid FCMA frequency error estimation at a low signal and heavy interference [tl]. A mathematical analysis of FCMA compared to RMA fbr identical code sequences has also been investigated in [7]. As an improvement for the performance of FCMA, Reed-Solomon codes have been analyzed [SI. Alternatively, good improvement has been simulated through controlling the number of common elements between various signatures [lo]. As Si result 0.4b/s/Hz efficiency was achieved with binary FCMA without coding. On fading channel, spectral efficiency of uncoded and coded FCMA has been investigated [ 1 11. Then the error rates o F non-coherent demodulation with R-S codes in fading satellite channel were evaluated [ 121, Analysis of FCMA over basic digital radio coherent demodulation has been proposed in [I 31 Then the performance of different coherent Mary FCIvfA in AWGN satellite channel has been investigated [ 141. The comparison of coherent and non-coherent detection has been indicated in [15]. What would FCMA offer for next-generation mobiles as W-CDMA? So. the applications of FCMA systems in mobile communications as an international system have been investigated. This paper presents a general performance of FCMA in mobile communications in contact with DS-C'DMA. But it does not intend to discuss the detailed design of the scheme 2 70

IcL"Ip-l NINETEENTH NATIONAL RADXO SCIIE:TW3E CONFERENCE, ALEXANDRIA, IUrrrch, 19-2 t ,2002

-- - _____ ---._-__ -- - Earlier in 1977, Cooper and Nettleton had proposed a spread spectrum radio system

using FHMA, Hudumurd coding, and DPSK and claimed higher eficicncy i.han FD-FM systems [16]. On the other hand, Viterbi first considered a fiequency hopping, FIIICDMA scheme for multiple access satellites by mobile-users R 171. Goodman et a!, using multilevel fiequency shifi keying MFSK, has suggested the same technique while 30% efficiency was reported [18]. Slow FHMA has been adopted for fading resist [19]. Others suggested wideband DS-CDMA for next-generation mobiles [20-2,:2].

The main architecture of FCMA is to share the comiriunicaiion spectral resources on discrete-quasi-orthogonal basis. Every mobile will be designed to receive a small set of' discrete FW carrier frequencies called the signature. If the signatures of mi, biles in any zone (cell) are fixed and unique, they are as identification. When a mobile mow; fiorn original zone, it could maintain its original signature or may be assigned a new om:, according to the design. The unique sets are selected from the total available discrete spectral resources of the system. 'The density of these resources along the RF \videband depends on tlw required data rate and the system capacity as well. The data of a piM.iculin mobile is then encoded along these discrete RF carriers simultaneously according to it predetermined or tnantiailq controlled discrete fiequency shifts. It is to be noticed that any signature-sliifi that reprewrrts a particular data wll lie in another set fkom the predetermined discrete spectra. 4 satellite or base station will accept signals from mobiles to direct them (i.e., propagation or ro t ihg capabilities) to all other mobiles in the same zone or any other zones in the vuorld.

FCMA is characterized by simplicity to access arid its huge population. A mobile set is able to contact any other mobile all over the world jus1 t)y ercotling its informarion along its signature. All mobile hardware is identical on its capability to :select any signature fiom the available spectral resources. The difference in hardware is fctr the receiving subsystem that should be adjusted to receive unique signature for each mer. If the receiver is based on digital signal processing of the wideband spectrum there will be no diffcrence ibr the mobile-unit hardware. This will also offer the dynamic and the efficient zillocation of signatures. Assume a fixed number of discrete and regular spectral resoirrces Ai along the total baiidwirlth. The number of mobile users including all permutation of:selec:ting n fiom Nis (11: srnzzll set of resources assigned fbr a user as a signature).

(1) N!

IIZ! ( N -- IZ)! Mobile population =

For 20MHz and 512 discrete spectra, pmnuta1.ions of 3 give more than 22 millions mobiles. The optinuim n of 8 spectra allows more lhan 10l7 mobiles. For nost pradical design aspects the above dimensions allow transrission of about 4Okbps in binary FCMA.

Rb = BWIN (2) A bandwidth of most mobile systems including GSlV is 2SMHnr. So, for 6.5Erbps

transmission rate ( 9' being 3846) and 8 discrete spectra per signature, pennulations cvould lead to more than IO2' mobiles. On the other hand, for one satellite transponder of 36h4Hz bandwidth, same rate ( N will be 5538) and signalinre size, mobile population will be 2.18~10". These figures show that FCMA system can be configured to include identification for people all over the world.

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NINETEENTH NATIONAL RADIO SCIENCE pq-q CONFERENCE, ALEXANDRIA, March, 19-21,2002

X: no of channels

3 DESIGN CONSTRAINTS

10 I 1123 I 11231 I "14974 1 22461 I 44922

One of the most powerhl tools of FCMA system is that its dimensions determine system performance. It depends on the system bandwidth SW, density of spectral resources N, size of signature n, number of common elements between signatures I, level of modulation M, as related to system capacity X, and permissible error probability Pb. Most configuration parameters are estimated by modifying the upper-bound relation given in [3]:

P < - 4(M -1)

3.1 Available Mobile Channels

Fig. 1 shows the relation between the number of mobile channels X, and the total number of discrete fiequencies N , for binary FCMA systems. It indicates a sharp increase of the number of possible mobile channels as FCMA spectra becoming more condense. However, to achieve this condense spectra only little values of data rate can be accommodated. Nevertheless, on formatting the M-ary FCMA systems for higher order of modulation M, the input data rate can be increased accordingly [3].

RM = Rb log, M (4)

No of Discrete Freauencies. N 1 E+o6

1EM5

1E+O4

1EM2

1EM1 1 10 100 lo00 loo00 loo000

1EN3

1E+02

1EM1

1E+OO

1E-01

135-02

Inmt Data Rate. R kbm

1 10 100 lo00 10000 lo0000

No of Channls, X No of Channels, X

Fig. 1 : X versus N for Binary FCMA/FSK Fig.2: X versus R for Binary FCMA/FSK

On dividing the bandwidth into only 90 spectral resources, the number ofmobile channels will only be 10. However, when N increases to 360000 resources the permissible simultaneous mobile channels could be increased to 44922. These results are summarized in Table. 1 assuming one transponder of bandwidth 36MHz.

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NINETEENTH NATIONAL RADIO SCIENCE pqq CONFERENCE, ALEXANDRIA, March, 19-21,2002

3.2 Input Rate & Wary FCMA

Fig.2 shows the relation between the input rate and the number of simultaneous channels for binary FCMA. As the rate decreases, the number of discrete fiequencies increases so that the number of mobile channels can be increased. When &, was 25kbps there is only 180 channels whereas if it reduces to 13kbps and 6.5kbps, the permissible number of channels will be doubled to 346 and 691 respectively. About 44922 simultaneous mobile channels could be accommodated if the rate is low as 0. lkbps. M a r y FCMA systems can be designed to accommodate the requirement for hture data rate. Table 2 summarises some examples for selecting a signature consists of 3 to 8 frequencies fiom a total of 5 12 spectral resources. It indicates that the data rate could be 34, 13,6, and lMbps in Ka, Ku, C, and DCS-1800 bands respectively.

3.3 FCMA/DSMA & Adaptive Modulation

Mixing information over RF carriers makes up the transmission. Without modulation these RF centres constitute the unique signature of destination. On modulation they will be shifted up and down. The value of shift equals the rate of data whereas the number of shifts depends on the level of modulation. However, any frequency-shift should lead exactly to another spectra of FCMA discrete resources. Adaptive is to control this shift and the level of modulation. When spacing of spectra increases the data rate can be increased and vice versa. One could achieve large number of mobile channels at the required rate with the method of formatting. If the data is encoded so that the minimum frequency shift is twice the spacing between spectra the data rate can be doubled. Fig.3 shows that 6.4kbps can be achieved fiom 0.4 kbps rate by allowing 4 multiples of the fiequency shifts. This corresponds to 10946 mobile channels as shown in Fig.2 compared to 684 channels when the normal shift is made. The maximum shift will be one half the separation of RF carriers to allow equal shift on both sides. The separation between these different rates is attained using different direct sequence codes. So, this formatting has been termed FCMA/DSMA. Each data is spread by a particular code prior to formatting it over FCMA signatures. This offers high data rates in addition to lower error rates, and could be used to increase the systemxapacitg. The satellite or base station can be designed to allow different rates of transmission ih one or both direction on demand. Or it can be done in the mobile unit itself. The flexible design is very usefid to avoid large interference situations. Data rate can be adopted by controlling the level of modulation in Mary FCMA or the shift in FCMNDSMA.

3.4 FCMA Semi-Reused Signatures

The results indicated in Fig.1 and 2 were simulated assuming all permutations of signatures except coincidence. When there is some method to control the selection of signatures much more simultaneous channels can be accommodated. Examples for the design of FCMA systems at different scenarios of overlapping have been published in [ 101.

Applying these scenarios on mobile communications the number of possible simultaneous mobile channels is summarised in tables 3 to 8. Table 3 and 4 assume the bandwidth of most mobile systems (25MHz) whereas tables 7 and 8 indicated the case when the bandwidth is increased to 75MHz. The case of satellite transponder with 36MHz

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-

Channel 25kHz 13kHz

1 6.5kHz

Table4 Effect of Modulation Level on the Number of Mobile Channels in GSM (25MHz) M-FCMA I Resource I 717 1 617 1 517 I 417 1 317 I 217 1 117 1 05f7

4 755 I 881 I 1058 I 1322 I 1763 I 2644 I 5287 I 10574

Resource 717 617 517 417 317 2/7 1 I7 0.97 3000 313 365 438 547 730 1095 2189 4378 5769 602 702 842 1053 1403 2105 4200 8419 11538 1203 1403 I 1684 2105 2807 4210 8419 L 16839--

~ ~

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---..- ---- -- - - - bandwidth is illustrated in tables 5 and 6. Even nuinbered tal>llcs sutrunarise the effect of the information bandwidth on binary FCMA. Odd numbered tables indicate the elTect of' increasing the level of modulation when the channel bandwidth is 65kHz. As a conclusion, for infhite population the number of mobile channels is one-tenth the spectral rt:sources. On the other hand, for limited population and controlled overlapping the nuniber of mobjle channels could approach the spectral resourcw. A s indicated, limiting the number of c o m o r i elements between signatures has lead to a Xnnuted population [ IO]. A very simple solution is to use the signatures of minimum cross-correlation .For adjacent mobiles while highly correlated signatures for mobiles in long away areas as in Fig.4.

f + . ___-

Fig.3 : Controlling 'T'ransmission Rate Fig.4: Semi-reusing o f Signatures

This semi-reusing concept could be applied per country, district, city,, a larger or a smaller area according to the required capacity and existing tral'fic intensity. There is a basic difference of ordinary mobile reusing and FCMA reusing. Semi-reused FCh4 4 signatures are not identical. They are different, while having only higher va1iw:s of cross-correlation.

3.5 Local, National or international Calls

Local calls are established by re-broadcasting uplink signatures via downlirlk signatures. For adjacent, tandem, national, or international calls the situat.iori is differmi. Outgoing calls require different ctoss-links or routs between satellites or base stations. A n y satellite can be interconnected to four adjacent satellites (or base stzttions) as for low orbit iridium. So, the incoming signatures could be separated into five sections. The first arid most big section (local signatures) will be propagated back to the same tone. 'I'he other four sections ( naticonal or international) are routed to the proper direction. On advance, a satellite or base station can regist the used resources and the uplink signatures outgoing to all directionst. Separation can then be accomp1isht:d from registered information by controlling a bank of H I T filters in each direction. New call may be initiated by sending ii partic:ular code to the re-l)roadcast.ing centre across predefined RF resources. If destination has been recognised it will be re-broadcasted to the proper direction.. The code includes the sender ,and de srinatiotx identification while re- broadcasting centre adds its location. On connection, re-.broadcasting centre (satellite or base station) intercepts uplink signatures, amplifies and transla1 es thin back over dovmlink signatures.

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---_ --- 4 SPECTRAL EFFICENCY

FCMA systems are characterized by the possibility to adjust the efficiency as needed Fig.5 indicates the efficiency of Mary FCMA, in b/s/Hz for a11 permutation of 7 discrete fiequency from 25MHz bandwidth. Two approaches could be used to improve this basic efficiency. First is to control the common elements between signatures. Results indicated in Fig.6 show that the spectral efficiency may increased to 5.3 b/s/Hz. This is about ten times the basic technique. The second is to control the minimum frequency shift for a particular user and henceforth multiples the efficiency by the factor of shifts using FCMA/DSMA format. However, the second criteria could be better used to achieve adaptive modulation. This allows new services requirements with larger bit raies.

Efficiency: b/s/Hz ~- Efticiencv, b/s/Hz 0.5

0.4

0.3

0.2

0.1

0.0 1

6 .U

5.0

4.0

3.0

2.0

1 .O

n n "." 10 100 1000 10000 1000001000000 0 5 10 15 20 25 30 35 10 35 50 55 60 65 70

Information rate R b/s Level of Modulation, M

Fig.5. Efficiency of M-ary F C W S K Fig.6: Effect of the Levcl of Modulation

5 CONCLUSION

FCMA systems have been investigated for mobile communications with simple access, adaptive modulation, and universal identification. Semi-frequency reusing, adaptive modulation and huge capacity, efficiency, and population are amongst FCMA features. Results indicate that binary FCMA provides 3 743 5 simultaneous mobile-channels per transponder for 2.1 8 ~ 1 0 ~ ~ population at low-rate of data. M a r y systems allow up to 6 1000 mobile-channels with 5.3blsMz spectral eficiency on controlling the elements of signatures at 6.5kbps. By increasing the level of modulation the data rate could be as high as 34, 13, 6, and 1MHz for Ku, Ku, C, DCS-1800 bands respectively. The adaptive technique FCMA/DSMA allowed a variable data rate system whereas the isolation has been achieved through the direct sequence codes. 'The rate in one or both sides could be set manually or on demand. Dividing the incoming spectrum into a local and four outgoing routs allows international roaming with minimum interference. This paper presented the performance of FCMA including MAI. Fading has a little degradation since FCMA is a frequency diversity technique. Fading has been solved using RS codes It has been concluded that, to achieve

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NINETEENTH NATIONAL RADIO SCIENCE p-q-q ~1 ~

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most FCMA features for mobile system the discrete spectrum should be very COndensed with controlled-shift and very high modulation level.

6 REWRENCES

[I] T. J. Stevenson and K. W.-Yates, “ A New Multiple Access Scheme for Packet Satellites”, F’roc. of USRI Inter. Symposium on Signals, Systems and Elec. ISSSE‘89, Germany, pp.57O-S73, September 18-20,1989.

[2] M A. k Ali and S. J. Braithwaite, “Performance Analysis of FCMA with CO-channel Interference”, Proceeding of the Third Bangor Symposium on Com, U.K, pp. 229-232, May 29-30,1991.

131 M. A. A. Ali, Some Investigations on the Performance of Frequency Comb Multiple Access FCMA for Communications Systems, Egypt: University of Mmoufia, PhD. Thesis, November 199 1.

[4] R Boreli and K. W. Yates, “ FCMA Error Probability in AWGN and Multiple Access Noise”, Proceeding of IEEE International Cderence on Communications, Geneva: Switzerland, pp. 1108-1 11 1, 1993.

[5] T.J. Stevenson, A. Watson, ‘Throughput Performance of Frequency Comb Multiple Access (FCMA)”, IEEE Global Telecom. Conference, Vo1.2, pp. 1273-1277, 1993.

[6] S. Reisenfeld & A. Kumar, ‘Trequency Error Estimation for Frequency Comb Multiple Access Demodulation”, Electronic Letters, V.30, n .9 ,28~ April, pp. 685-686, 1994.

[7] M A. A. Ali, “A Comparison between FCMA and RMA Schemes Using Identical CodeSequence” Proceeding of the Twelfth National Radio Science Con!krence, NRSC’95, Egypt: Alexandria, no. C3 1 ,

[8] M. A A. Ali, ‘Demodulation of FCMA Signal Corrupted by Multiple Access Interference, Power Change, and AWGW, Proceeding of the Second Engineering Conference, Mansoura’97, Egypt: El-Mansoura, April 8-10, 1997.

[91 M. F. Marie, A. Abou-El-- and M. A. A. Ali, “Computer Simulation of Reed-Solomon Codes in FCMA System”, Proceeding of the Fifteenth National Radio Science Conference NRSC’98, Helwan, EGYPT, No. C25, pp. 1-8, February 24-26, 1998.

[ 101 M. A. A. Mi, “ Improvement in FCMA Efficiency with Limited Interleaving of Signatures”, Proceeding of the 4th IEEE Inte. Cderence on Electronics, Circuits, and Systems ICECS’97, May-3 1 to June-3, 1998.

[11] A. Abou-El-Azm, M A. A. Ali, and M. F. Marie ‘‘Spectral Efficiency of Uncoded and Coded FCMA System Over Fading Channels”, Electronic Engineering Bulletin, Faculty of Electronic Engineering

[ 121 M. A. A. Ali, A. Abou-El-fiztn, and M. F. Marie, “Error Rates of Non-coherent Demodulation FCMA with Reed Solomon Codes in Fading Satellite Channel”, Proceeding of the 1999 EEE 49* Vehicular Technology Conference, VTC99, Houston, Texas, USA, Vol. 1, pp. 92-96, May 16-20,1999.

1131 M. A. A. Ali ‘Performance Analysis of Binary Frequency Comb Multiple Access over Three Radio Coherent Demodulation Techniques”, Electronic Engineering Bulletin, Faculty of Electronic Engineering. Menod, EGYPT, No. 18, July 1999.

[14] M. A A. Ali, “On the Spectral Efficiency of Different Coherent Techniques for Frequency Comb Multiple Access System”, Proceeding of the Sixteenth National Radio Science Conference NRSC’99, Ain Shams University, Cairo, EGYPT, No. C7, pp.1-5, February 23-25, 1999.

[ 151 M. A. A. Ali ‘Coherent and Non-coherent Detection of Binary FCMA”, Electronic Engineering Bulletin, Faculty of Electronic Engineering, MenouC EGYPT, No. 18, July 1999.

[16] G. R Cooper and R W. Nettleton, “A Spread Spechum Technique for High Capacity Mobile Communications”, IEEE Vehicular Technology Conference Record pp.98- 103, 1977.

[ 171 A. J. Viterbi, “A Processing Satellite transponder for Multiple Access by Low-Rate Mobile Users”, Inter. Conference on Digital Satellite CO”. MONTREAL,, pp. 166-174, October 23-25, 1978.

11 8) D. J. Goodman, P.S. Henry, and V. IC Prabhu, “ Frequency-Hopped Multilevel FSK for Mobile Radio”, The Bell System Technical Journal, Vo1.59, No.7, pp.1257-1275, September 1980.

[ 191 D. Verhulst, M. Mouly, and J. Szpirglas, “Slow Frequency Hopping Multiple Access for Digital Cellular Radiotelephone ”IEEE Journal cm Selected Areas in CO”., Vol. SAC-2, No.4, pp.563-574, July 1984.

[20] D. Grillo, A. Sasaki, R A. Skoog, and B. Warfield, eds., “Issue on Personal Comm.-Senices, Architecture, and Performance Issues”, IEEE Jour. on Selected Areas in Communications, Vol.15, N0.8, 1997.

[21] N. J. Muller, Desktop Encyclopedia of Telecommunications, McGraw-Kill Com. and Inc., USA, 1998. [22] Fumiydci Adachi, Mamoru Sawahashi, and Hirohito Suda, “Wideband DSCDMA for Next-Generation

pp.1-8, Narch 21-23, 1995.

Menod EGWT, NO. 17, pp. 1 - 10, J a n w 1 999.

Mobile Communications Systems ”, IEEE Communications Magazine, pp. 56-69, September 1998.

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