(a) Explain what is meant by near-far problem and how it could be mitigated in C
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Question
(a) Explain what is meant by near-far problem and how it could be mitigated in Code Division Multiple Access (CDMA) systems. (5 marks) (b) With the aid of a diagram, explain how subcarriers in an Orthogonal Frequency Division Multiplexing (OFDM) transmission system are spaced to avoid causing interference to each other. (5 marks) (c) Suppose that a cellular system is allocated 20 MHz of spectrum with a full duplex channel bandwidth of 50 kHz. (i) Calculate the number of channels per cell if the system is employing a frequency reuse factor of 4. (3 marks) (ii) Using the Erlang B Table shown in Table 1, determine the number of subscribers that can be supported per cell at a grade-of-service (GoS) of 0.1% if the offered traffic per subscriber is 0.02 Erlangs. (4 marks) (iii) Further determine the mean holding time in seconds if each subscriber generates on average 2 calls per hour. (3 marks)Explanation / Answer
Please post as three seperate questions
a) Near-far problem is one of the major problems that hurts mobile communications badly. In a CDMA system, mutual interference will determine the majority of SN ratio of each user.The CDMA near far problem arises because handsets may be anywhere within the particular cell boundaries. Some handsets will be close to the base station, whereas others will be much further away.In a near far situation the signal of the handset which is closer to the base station may dominate the signal of other handsets that are distant from the same bs
Suppose user A is far away from the receiver and user B is close to the receiver, then there will be big difference between desired signal power and interfered signal power. Desired signal power will be much higher than the interfered signal power and hence SN ratio of user A will be smaller and communication quality of user A will be severely degraded.
It could be mitigated by applying power control mechanisms diversity techinques soft handovers, multi-user receivers and near far resistance receivers .
Power control is essentially needed to solve the near-far problem. The main idea 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.
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