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Motivation

Motivation. Wireless Communication Environment Noise Multipath ( ISI! ) Demands Multimedia applications  High rate Data communication  Reliability. Solutions. Some Possible Solutions Use more bandwidth ( limited resource! ) Use strong codes ( computational complexity! )

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Motivation

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  1. Motivation • Wireless Communication Environment • Noise • Multipath (ISI!) • Demands • Multimedia applications  High rate • Data communication  Reliability

  2. Solutions • Some Possible Solutions • Use more bandwidth (limited resource!) • Use strong codes (computational complexity!) • Use multiple antennas (hardware complexity!) • Choose Multiple Antennas • Multiple Antenna (Requires flat fading channel!) • Solution  Multiple Antenna + OFDM

  3. Implementation of STC + OFDM

  4. Space-Time Diversity Signal at jth antenna is Decision metric

  5. Alamouti’s Scheme Space-time code matrix Signal Received Estimates

  6. Adaptive OFDM (AOFDM) • Single user AOFDM =>Adapt. bits allocation/Optimal Power allocation • Multi-User AOFDM – Static TDMA/FDMA =>(Wastage of Frequency band(s)?) So, why not exploit spatial separation of users. and – Minimize the overall transmit power by allocating subcarriers to the users & bits transmitted on each subcarrier and power level transmitted on each subcarrier => (1. Overheads at Base Station! 2. What?? If fast fading channel!!)

  7. MU-OFDM with subcarrier, bit and power allocation

  8. Nonoptimal solution Subjected to the constraints C1: C2: Minimize Where k is the user index and n is the subcarrier index, is the number of bits of the kth user assigned to the nth subcarrier, is the set of all possible values of is the required received power (in energy per symbol) in a subcarrier for reliable reception of c bits/symbol when the channel gain is unity and is the magnitude of the channel gain of the nth subcarrier as seen by the kth user.

  9. Optimization Relax the requirement ck,n D to allow ck,n to be a real number within i.e. ck,n [0,M]. To deal with constraint C2, let new factor k,n[0,1] as a sharing factors of the nth subcarrier to kth user. The new optimization problem becomes where ck,n and k,n has to satisfy , Here is a lower bound to the minimum power P*T

  10. Problem definition • Incrementally correcting approaches for the following- • Dynamic MUMC system which will efficiently take care of users in and out of the system. • Correcting bit allocation strategy in time-varying MCM/MUMC system where time variations can be of various degrees – - Slowly fading - Medium fade - Fast fading

  11. Core system(s) to work on • At least adaptive MUOFDM as an adaptive MUMC system. • OFDM + STC and/or concatenation of any other coding technique to get more degree of freedom to work with.

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