FDSS Downlink Capacity in Urban Zone Near Digital Video Broadcasting Installations

RADIOENGINEERING, VOL. 12, NO. 2, JUNE 2003 31 FDSS Downlink Capacity in Urban Zone Near Digital Video Broadcasting Installations Bazil TAHA AHMED, Miguel CALVO RAMÓN, Leandro de HARO ARIET Departamento Sistemas, Señales y Radiocomunicaciones, ETSI Telecomunicación, Universidad Politécnica de Madrid, Ciudad Universitaria, Madrid, 28040, Spain [email protected].es Abstract. The FDSS macrocell downlink capacity is evaluated for macrocells that operate at the same frequency of the Digital TV station (DTV) and that are nearby the DTV installations. It has been founded that the cell capacity is not affected when the distance between the DTV installations and the macrocell is more than 25 km. For lower distance, the effect is high and the downlink vanishes at a distance less than 2.1 km. COFDM is a system of modulation well-suited to the propagation and interference environment of digital terrestrial television broadcasting (DVB-T) in the UHF bands. It uses a large number of carriers, each carrying a small part of the total coded data rate. The frequency spacing is carefully chosen to ensure "orthogonality" - the carriers do not crosstalk one to the other, despite having spectra, which overlap in the frequency domain [2]. Keywords DTV, FDSS, cell capacity. 1. Introduction Kaleh proposed an OFDM spread spectrum system that has carriers with disjoint frequency support and studied its performance in partial band Gaussian jamming [1]. He derived the optimal receiver and showed its performance and, in addition, showed a sub optimal receiver. The carriers in the system are orthogonal to each other and, most often are generated using the discrete Fourier transform. The previous mentioned system has the name of Frequency Diversity Spread Spectrum (FDSS). Fig. 2. The FDSS spectrum when M = 8. A [k] × ∑ Subchannel 0 × S (t) m m C [M-1] mfM-1 ×× C [0] mf0 ×× C [1] mf1 The aim of this article is to determine the FDSS macrocells downlink capacity, which operates in the same band of the digital TV. In this case the FDSS downlink is totally jammed by the digital TV signal. 2. The FDSS model The generation of the FDSS can be described as follows. A single data symbol with a time duration (Tb) is replicated into M parallel copies (M = 32 to 1024). For the m-th user, the i-th branch data of the parallel stream is multiplied by a chip cm [i] from a PN code or some orthogonal code of length M and then BPSK modulated on to a subcarrier (fi) spaced a part from its neighboring subcarriers by 1/Tb. Fig. 1 depicts the transmitter model. The transmitted Fig. 1. The FDSS transmitter model. 32 B. TAHA AHMED, M. CALVO RAMÓN, L. DE HARO ARIET, FDSS DOWNLINK CAPACITY IN URBAN ZONE NEAR … signal consists of the sum of the outputs of these branches. This process yields a multi-carrier signal with the subcarriers containing the coded data symbol with a processing gain of the FDSS system of M. FDSS spectrum can be viewed in terms of source bit rate; at a higher source bit rate, FDSS signal enlarges its bandwidth for a constant process gain. Fig. 2 shows a generalized FDSS spectrum in terms of source bit rate when M = 8. The receiver model can be shown in Fig. 3. R(t) × ∑ Subchannel 0 × C [M-1] m fM-1 ×× C [0] m f0 ×× C [1] m f1 Dm, M-1 Dm, 0 Dm, 1 ∫V0 Fig. 3. The FDSS receiver model. Fig. 4. The FDSS and digital TV (DVB-T) spectrum. 3. Digital Terrestrial Television Interference The basic formula for the median propagation loss given by Hata [3] is given by: [] () () () () [] () kmTV mTV MHz dh hah fdBL loglog55.69.44 log82.13 log16.2655.69 −+ +−− −+= (1) where fMHz is the frequency of the transmitted signal [MHz], hTV = 50 m is the antenna height of digital TV transmitter, d is the distance between the digital TV transmitter and the mobile receiver [km], and a(hm) is the mobile antenna height-gain correction factor given by: ( ) ( ) [ ] 97.475.11log2.3 2−= mm hha . (2) The total DTV path loss L in linear unit is given by: [ ] ( ) 10/ 10 dBL L= . (3) The DTV interference power is defined as: LGGPDTVP DTVDTV /)( 2int = , (4) where PDTV is the DTV transmitted power, GDTV is the DTV transmitter antenna gain, G2 is the mobile receiver antenna gain assumed to be 0 dB [4]. Fig. 4 shows the FDSS and the DTV spectrum where both spectra are displayed in frequency respect the central carrier (f0). 4. Urban Large City Macrocellular Loss We assume that the base station antenna height (h1) is 30 m and that the antenna gain is 15 dB. The macrocells are assumed to exist in urban (large city) zone. The propagation loss exponent n [3] is given by () 1 log655.049.4 hn − = . (5) For this case, the propagation loss between the base station and the user under consideration is given by [3]: [ ] () () () () [] () kmTV m MHzFDSS Rh hah fdBL loglog55.69.44 log82.13 log16.2655.69 1 −+ +−− − + = (6) where Rkm is the macrocell radius in km. The macrocell propagation loss L in linear unit is given by: [ ] ( ) 10/ 10 dBL FDSS FDSS L= . (7) The macrocell propagation loss LFDSS is the loss between two isotropic antennas. 5. The Macrocell Capacity We assume that the user under consideration is at the intersection of three macrocells (worst case condition). To calculate the ratio Eb/N0, we assume the following consternates: Pst(max) = 20 W [4] , where Pst(max) is the base station maximum transmitted power; RADIOENGINEERING, VOL. 12, NO. 2, JUNE 2003 33 Pusers(max) = 17 W [4] , nn R R R R Q      +       +≈ 62.2 6 2 32 , (11) where Pusers(max) is the maximum power transmitted to the all users. The difference between Pst(max) and Pusers(max) is due to the pilot signal and the common channels. where R is the cell radius. Now the total interference power is given by Psu(max) = 0.5 W for voice users [3] , NoiseFDSSPDTVPtotalP intintint ++ = )()()( , (12) Psu(max) = 2.0 W for data users [3] , where Psu(max) is the base station maximum transmitted power for one user. where Noise is the mobile receiver noise power. The ratio C/I is given by: The transmitted power for the user under consideration is given by: ( ) )(// totalPPIC intr FDSS = , (13) [ FNmaxPmaxPP uuserssuu ] α /)(),(min= (8) and the ratio Eb/N0 is defined as: FDSSpb ICGNE )/(/ 0 = , (14) where Nu is the cell capacity, α is the source activity factor and F is the power control reduction factor ≈ 0.5 [5]. where Gp is the FDSS process gain, which equals to M. Since LFDSS is calculated between two isotropic antennas, then the received desired signal level Pr between directive antennas is given by: From eqns. 8, 10, 13 and 14, we can notice that the ratio Eb/N0 is a function of the number of users. The macrocell capacity is calculated by increasing the number of users from one user and calculating Eb/N0 till it reaches the value (Eb/N0)req. The macrocell capacity will be the maximum number of users for which Eb/N0 ≥ (Eb/N0)req. FDSSubsr LPGGP / 2 = , (9) where Gbs is the base station antenna gain. Fig. 5. The urban zone macrocell performance (voice only users). Fig. 6. The urban zone macrocell performance (data only users 144 kbit/sec). The total multi-user FDSS interference signal is calculated using: () [] ( [] ) () () [] maxPmaxP L Q sectGG FPNP L Q sectGGFDSSP usersst bs suust bsint −⋅ ⋅ +− + ⋅ ⋅ +− ≈ )1( ,min )1( 2 2 φ α φ (10) 6. Numerical Results We assume the following general case: • sect = 0.4 (for non ideal three sectors), • φ = 0.5 [4], • Nrec = –100 dBm (assuming a noise figure of 7 dB), • PDTV ⋅ GDTV = 10 kW, • fMHz = 800. where sect = 1/3 is the sectorization factor for ideal three sectors, φ is the orthogonality factor of the downlink users, Q is the other cells interference [5] defined as First we study the case of voice only users. We assume the following [3]: 34 B. TAHA AHMED, M. CALVO RAMÓN, L. DE HARO ARIET, FDSS DOWNLINK CAPACITY IN URBAN ZONE NEAR … • Gp = 512, • α = 0.5, • (Eb/N0)req = 7 dB. Fig. 5 shows the macrocell capacity as a function of the distance between the user under consideration and the DTV transmitter for two different cell radii (1 and 1.5 km). For a cell radius of 1 km, we can notice that the capacity is null when the distance is less than 2.1 km and it increases with the distance. When the distance is 25 km or more we get the maximum possible capacity. Secondly, we study the case of data only users (UDD 144 kbit/sec). We assume the following [3]: • Gp = 53, • α = 1.0, • (Eb/N0)req = 3 dB. Fig. 6 shows the macrocell capacity as a function of the distance between the user under consideration and the DTV transmitter for two different cell radii (1 and 1.5 km). For a cell radius of 2.1 km, we can notice that the capacity is null when the distance is less than 1 km and it increases with the distance. When the distance is 15.5 km or more we get the maximum possible capacity. 7. Conclusion The macrocell downlink capacity has been calculated for urban zones near to DTV installations. It has been found that the downlink capacity vanishes very near to the DTV transmitter. At a distance of 2.1 kilometres, for the data users case and for the voice users case the capacity begins to increase from zero when the cell radius is 1 km. At a distance of 25 km or more, the DTV effect is null. References [1] KALEH, G. Frequency Diversity Spread Spectrum Communications to Counter Bandlimited Gaussian Interference. IEEE Transactions on Communications. 1996, vol. 44, no. 7, p. 886 - 893. [2] ETS 300 744. Digital Video Broadcasting. March 1997. [3] CALVO-RAMÓN, M. Third Generation IMT-2000 (UMTS) Mobile Communication Systems. Fundación Airtel Vodafone, 2002. In Spanish. [4] HOLMA, H., TOSKLA, A. WCDMA for UMTS. New York: John Wiley and Sons, 2000. [5] LEE, J. S., MILLER, L. E. CDMA Systems Engineering Handbook. London: Artech House, 1998. About Authors... Bazil TAHA AHMED was born in Mosul, Iraq, in 1960. He received the B.Sc. and M.Sc. degrees in telecommunication engineering from the University of Mosul, in 1982 and 1985, respectively. From 1985 to 1998, he was teaching at the Electrical Engineering Department at Mosul University. He is currently working toward the Ph.D degree at the Technical University of Madrid. His research interests include W-CDMA capacity and E. M. Wave propagation in micro-cellular and macro-cellular environments. The results of his research activity may be found in several presentations in international conferences and in published papers. Miguel CALVO RAMÓN was born in Pueyo de Jaca, Huesca, Spain on June 10 1949. He got the “Ingeniero de Telecomunicación” degree from the “Escuela Técnica Superior de Ingenieros de Telecomunicación” of the Universidad Politécnica de Madrid in 1974 and the “Doctor Ingeniero de Telecomunicación” degree from the same University in 1979. He works as Professor in the “Señales Sistemas y Radiocomunicaciones” (Signals, Systems and Radiocommunications) department where he obtained the Catedrático (Full Professor) position in 1986. Leandro de HARO ARIET was born in Barcelona, Spain, on May 17th, 1962. He got the "Ingeniero de Telecomunicación" degree in 1986 and the "Doctor Ingeniero de Telecomunicación" degree ("Apto cum laude") in 1992, both from the "Escuela Técnica Superior de Ingenieros de Telecomunicación" of the "Universidad Politécnica de Madrid" (Dpt. of "Señales, Sistemas y Radiocomunicaciones"). Since 1990 he develops his professional career in the "E.T.S. Ingenieros de Telecomunicación" of the "Universidad Politécnica de Madrid" (Dpt. of "Señales, Sistemas y Radiocomunicaciones") as Assistant Professor on the Signal Theory and Communications area.