Assignment 11: Problems on Multiuser CDMA Networks

Transcription

1 G. S. Sanyal School of Telecommunications Indian Institute of Technology Kharagpur MOOC: Spread Spectrum Communications & Jamming Assignment 11: Problems on Multiuser CDMA Networks Due date: Max. marks: A hand-held direct-sequence spread-spectrum (DS/SS radio is part of a cellular CDMA system. The system specifications are as follows: data and SS-code modulation is BPSK, data rate (R is 8000 bit/s, carrier frequency (f c is 1 GHz, chip rate (R ch is 25 Mchips/s, worst-case path loss (L p is db, gain of transmitting antenna (G t is 5 dbi, receiver figure-of-merit ( G r T is 18 db/k, occasional deep small-scale fading loss ( L o is 30 db, other losses (Lo are 4 db, required N o is 4 db. Assume link Margin, L M is unity. The required transmitted power (P t during deep small-scale feeding Hint: The link margin is given by, L M = and k is Boltzmann s constant. P ( tg t( Gr T Eb RkL No pl ol o reqd (a 2 W (b 4 W (c 3 W (d 1 W 1

2 2. Referring to question 1, the level can P t be powered down when there is no small-sacle fading (a 1 mw (b 2 mw (c 3 mw (d 4 mw 3. Referring to question 1, the processing gain, G p (a 33 db (b 35 db (c 37 db (d 39 db 4. Referring to question 1, the maximum number of users per cell is approximately: Hint: Assume the system is interference limited. i.e, (a 313 (b 629 (c 1258 (d 2516 (N o+. 5. Consider a CDMA direct-sequence cellular telephone system, where each user requires an of 6 db for acceptable voice quality. The chip rate (R ch is 3.68 Mchips/s, and the data rate (R is 14.4 kbit/s. Assume that the factors Nonsynchronous interference factor (γ, Voice activity factor (G v, and Outer-cell interference factor (H o are 1.5, 2.5 and / 8

3 respectively, and the transmission ceases during speech pauses. The number of users per cell can be supported are: Hint: By taking γ, G v, and H o into consideration, the number of users that can be supported are M = γgvgp (a 80 users/cell (b 160 users/cell (c 40 users/cell (d 320 users/cell ( Eb Io H o 6. Referring to question 5, if a powerful error-correcting code is used to lower the required are: (a 201 users/cell (b 100 users/cell (c 150 users/cell (d 250 users/cell by 1 db, the number of users per cell can be supported 7. A direct-sequence spread-spectrum system uses QPSK modulation for transmitting data. It is required that the bit-error probability (P b be 10 5 and that chip energy-to-interference power spectral density ratio, E ch / 30.4 db. The processing gain, G p Hint: Processing gain, G p = / (N o+ E ch / Note: is bit energy-to-interference and noise power spectral density ratio. Assume that b E (N o+. Note: For QPSK, with perfect synchronization and bit-error probability, P b = Q( 2 = 10 5 requires of 9.6 db. 3 / 8

4 (a 10 db (b 20 db (c 30 db (d 40 db 8. Referring to question 7, the minimum number of chips/bit required are: (a (b 5000 (c 8000 (d A direct-sequence spread-spectrum system with a processing gain of 20 ( db uses QPSK modulation for transmitting data. A rate 1 k 2 n errorcorrecting code is used, and the required bit-error probability is Assuming perfect synchronization, find the minimum values of the following parameters that are needed to support these requirements. Chip energy-to-interference spectral density ratio ( E ch Hint: Processing gain, G p = / (N o+ E ch / Note: is bit energy-to-interference and noise power spectral density ratio. Assume that b E (N o+. Note: For QPSK, with perfect synchronization and bit-error probability, P b = Q( 2 = 10 5 requires (a 10.4 db (b 10.4 db (c 5.2 db (d 5.2 db of 9.6 db. 4 / 8

5 10. Referring to question 9, channel bit energy-to-interference spectral density ratio ( E c Hint: Ec R c = E ch Hint: k n = R R c (a 6.6 db (b 3.3 db (c 6.6 db (d 3.3 db R ch = R 11. There is a rich set of signaling elements used in CDMA systems that are designed to IS-95 specifications: data bits, channel bits, Walsh waveforms, Walsh chips, spread-spectrum chips, and BPSK waveforms. Consider a reverse traffic channel that is carrying full-rate digitized speech (R at 9.6 N o+ kbits/s, with a received = 7 db (assuming that N o <<. The received power-to-interference power spectral density ratio ( P r Hint: Pr (a 11.7 db-hz (b 23.4 db-hz (c 46.8 db-hz (d 93.6 db-hz = ( R = ( Ec Rc = ( E w Io Rw = ( E wch Rwch = ( E ch Rch 12. Referring to question 11, the channel bit energy-to-interference power spectral density ratio ( E c Hint: For the reverse traffic channel, the code rate ( k n Hint: k n = R R c (a 4.4 db is / 8

6 (b 2.2 db (c 1.1 db (d 0.55 db 13. Referring to questions 11 and 12, the channel bit rate (R c (a 57,600 channel bits/s (b 7,200 channel bits/s (c 14,400 channel bits/s (d 28,800 channel bits/s 14. Referring to question 11, the Walsh waveform energy-to-interference power spectral density ratio ( E w Io Hint: Each 64-ary Walsh waveform corresponds to 6 channel bits. i.e, Rc R w = 6 (a 10 db (b 5 db (c 2.5 db (d 20 db 15. Referring to questions 11 and 14, the Walsh waveform rate (R w (a 9600 Walsh waveforms/s (b 1200 Walsh waveforms/s (c 2400 Walsh waveforms/s (d 4800 Walsh waveforms/s 6 / 8

7 16. Referring to question 11, the Walsh chip energy-to-interference power spectral density ratio ( E wch R Hint: A Walsh waveform is composed of 64 Walsh chips. i.e, w R wch = 1 64 (a 8.1 db (b 16.2 db (c 4.05 db (d 2.25 db 17. Referring to questions 11 and 16, the Walsh chip rate (R wch (a 614,400 Walsh-chips/s (b 307,200 Walsh-chips/s (c 153,600 Walsh-chips/s (d 76,800 Walsh-chips/s 18. Referring to question 11, the chip energy-to-interference power spectral density ratio ( E ch Hint: In IS-95, the spread-spectrum chip rate (R ch is Mchips/s. (a 28.2 db (b 7.05 db (c 14.1 db (d db 19. Referring to questions 11 and 18, the number of spread spectrum (SS chips correspond to one Walsh chip ( R ch R wch 7 / 8

8 (a 2 (b 4 (c 8 (d Consider a DSSS downlink with bandwidth expansion, G p = 100. Assume the system is interference-limited and there is no multipath on any user s channel. How many users can the system support under BPSK modulation such that each user has a BER less than Hint: Assume the system is interference limited. i.e, (N o+. Hint: For BPSK, bit-error probability P b = Q( 2, and yields P b = (a 16 (b 32 (c 21 (d 14 = 6.79 db 8 / 8