Question 4.3 a) Can a 1.25 MHz band be used by as many mobiles as possible? (Hin
ID: 2268089 • Letter: Q
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Question 4.3 a) Can a 1.25 MHz band be used by as many mobiles as possible? (Hint: In CDMA, one mobile's signal is interference to other mobiles. The more users in the same cell talk simultaneously, the more interference.) If the answer to a) is NO, what would be the limit? If the answer to a) is YES, support your answer with argument. b) c) What is the relation between power control (both at mobile and at base station) and capacity in CDMA? d) How to calculate CDMA capacity? Compare that with TDMA?Explanation / Answer
In CDMA mobile radio system with CRAD , K equal to 8 mobiles are simultaneously transmitting signals of bandwidth B equals to 1.2MHz in the same frequency band of the same width B. These user signals can be distinguished by their different user specific signature sequences. Each mobile has a single transmitter antenna whereas Kn equals to one or two receiver antennas are deployed at the base station receiver. Hence transmission and reception takes place over 8. Kn equals to eight or sixteen different radio channels with the same time variant complex impulse responses (h^((k,kn) ) (,t),k=1…8,kn=1,2, , associated with the connection of mobile k, k=1…8 with receiver antenna kn=1,2. refers to distortion of the transmitted signals due to multipath propagation and t is the real time. It is assumed that the 8.Kn different radio channels are fading independently of each other.
Taking into account both CDMA and TDMA components, respectively, 96 different mobile users can be accommodated per frequency band of width B equal to 1.2MHz.Services with different gross bit rates between 16kbit/s and , theoretically, 1.536Mbit/s can be provided by allocating more than one time slot as well as more than one user specific signature sequence to a particular mobile. Due to the use of channel coding with code rate Rc equal to ½, the user bit rate can be varied between 8kbit/s and 0.768Mbit/s depending on the required service.
POWER CONTROL AND CAPACITY IN CDMA:
In CDMA, since all the mobiles transmit at the same frequency, the internal interference of the network plays a critical role in determining network capacity. Further, each mobile transmitter power must be controlled to limit the interference.
Power control is essentially needed to solve the near-far problem. The main idea to reduce the near-far problem, is to achieve the same power level received by all mobiles to the base station. Each received power must be at least level, so that it allows the link to meet the requirements of the system such that Eb/N0. To receive the same power level at the base station, the mobiles those are closer to the base station should transmit less power than the mobiles which are far away from the mobile base station.
When all mobile stations transmit the signals at the same power (MS), the received levels at the base station are different from each other, which depend on the distances between BS and MSs.
The received level fluctuates quickly due to fading. In order to maintain the received level at BS, a suitable power control technique must be employed in CDMA systems.
We need to control the transmission power of each user. This control is called the transmission power control (Control Power). There are two ways to control the transmission power. First is the open-loop (Open Loop) control and second is closed-loop (Closed Loop) control.
Effect of Power Control
By transmission power control, the user can obtain a constant communication environment regardless of the location. The user who is far from the base station sends a higher transmission power than the user who is nearer to the base station. Also by this transmission power control, you can reduce the effects of fading. This means that the variation of the received power due to fading can be suppressed by the transmission power control.
CAPACITY
In CDMA cellular systems with interference based admission control, the interference level resulting
from the connected users in the cell affects the capacity and coverage of the cell and any reduction in
interference converts directly into an increase in capacity. Also, It is wellknown for CDMA systems with
nonorthogonal users and singleuser detection that the coverage of a cell has an inverse relationship with the user capacity of the cell. Since all users share the same spectrum, power control is exercised in the reverse and forward links.
capacity formula
/= (1-)(W/R)F/(1+f)(Eb/Io)median Erlangs/Sector
=nominal noise to interference ratio W=Spread-spectrum bandwidth ,R=data rate,F=reduction factor,=probability of user being ON ,f=inter-cell interference factor ,(Eb/Io)median=SINR per user
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