OFDMA Networks By Mohamad Awad
Outline Wireless channel impairments i and their effect on wireless communication Channel modeling Sounding technique OFDM as a solution OFDMA as an improved solution MIMO-OFDMA Conclusion
Propagation Mechanisms in wireless channels Three propagation mechanisms in two types of propagation channels Reflection Diffraction Scattering Indoor wireless channel Outdoor wireless channel Transmitter Receiver
Large Scale Fading Topographic and elevation data were obtained from Natural resources Canada.
Large-Scale Fading Vs. Small-Scale Fading Large-Scale Fading Partly- Coherent Summation of 2 multipaths Im Coherent Summation of 2 multipaths Small-Scale Scale Fading Im Incoherent Summation of 2 multipaths Receive ed Power Re Im Re Re Transmitter Receiver Separation Figure taken from : Wireless communication, principle and practice by T. S. Rappaport
Channel-Sounding? Propagation mechanisms: - Reflection. - Diffraction. - Scattering. power time Transmitter Measurement setting: - frequency. - line-of-sight vs no-line-of-sight. -environment. - transmitter - Receiver separation. - human presence. - floor Area. - extent of Clutters. - detection Threshold. Receiver e Power Channel Impulse Response Propagation Channel
Channel Impulse-Response h N N ( τ ) 2 = 2 δ ( τ τ ) ex a n n=1 ex Total number of multipaths n Norm malized Receiv ved Power (db B) a n-th multipath amplitude n τ excess delay ex τ n-th multipath excess delay n Receiver Detection Threshold Excess Delay (ns) τ max
Review: Convolution http://www.jhu.edu/signals/convolve/
Frequency-Flat Vs. Frequency-Selective Fading Transmitted Signal Channel Received Signal Time Dispersion and ISI ISI No Time Dispersion T s @ Tx τ max T s @ Tx τ max @ Rx time T s << τ max T s >> time τ max Frequency-Selective Fading Frequency-Flat Fading Bw B s @ Tx B w B s @ Rx B s @ Tx Frequency Frequency B s @ Rx B s >> B w B s << B w
Multicarrier Transmission
History of Multicarrier transmission A principle for multi-channel transmission over a bandlimited channel was proposed in 1966!! Why did take that long to be implemented? Combine DFT!?
What is OFDM? Orthogonal Frequency Division Multiplexing Method to multiplex many narrowband signals into an aggregate wideband data stream Normally, the subcarriers would need sufficient separation to avoid adjacent channel interference OFDM uses orthogonal (1/T sym ) subcarrier spacing to maintain subcarrier separation
Orthogonality
Orthogonal Carrier Spacing 1/Tsym 1.2 1 0.8 0.6 0.4 0.2 0-0.2-0.4 0 5 10 15 20 25 30
OFDM System Model
OFDM-TDMA MAC Frequency User 1 1 1.2 0.8 0.6 0.4 0.2 0-0.2-0.4 0 5 Sub-11 Sub 10 Sub-2 15 Sub-3 User 2 User 3 20 25 S b4 Sub-4 U User 4 30 Sub-5 Time Advantages Disadvantages Easy implementation High latency, latency # users large Simple resource allocation In-effecient power usage Low p processingg requirement q In-effecient in frequency utilization Low signaling overhead (channel gain)
Applications
Multi-user user diversity
Time Varying Channel The channel is generally time varying due to: User changes location. Object gets between the transmitter and receiver (bus) τ ex Transmitter Time τ ex τ ex
OFDMA Orthogonal Frequency Division Multiple Access. A subcarrier is exclusively assigned to a user. Exploits multiuser diversity. Dynamic subcarrier assignment Disadvantages of OFDM Frequency Sub-1 Sub-2 Sb3 Sub-3 Sub-4 User 1 User 2 User 3 Sub-5 User 4 High latency, # users large In-effecient power usage In-effecient in frequency utilization Time
OFDMA Resource Allocation Internet Channel gain QoS (rate, dropping rate, etc..) Base Station ( BS ) Maximize Throughput Assign power < Max Each subcarrier is exclusively assigned Guarantee QoS Time Freq New gains Allocated subcarriers Trans Power User # 1 User # 2
OFDMA MAC Frequency Sub-1 Sub-2 Sub-3 Sub-4 Sub-5 User 1 User 2 User 3 User 4 Time Advantages Simple implementation Resource allocation flexibility Adaptation to channel characteristics (adaptive scheme) Disadvantages Complicated Res. Alocation Signaling overhead (adaptive scheme) Better BER performance (adaptive scheme)
Space Diversity D Signal received by Antenna 1 Signal received by Antenna 2 Two antennas separated by several wavelengths will not generally experience fades at the same time Space Diversity can be obtained by using two receiving antennas and switching instant-by-instant to whichever is best Combined Signal
MIMO systems
MIMO Basic idea of MIMO: Improve quality (BER) and/or data rate (bits/sec) by using multiple TX/RX antennas Core scheme of MIMO: space-time coding (STC) Two main functions of STC: diversity & multiplexing
MIMO - OFDMA
Conclusion Wireless channel suffers from large scale fading and small scale fading After modeling the channel, communication systems are matched to the channel. OFDMA is one of advanced technologies to overcome the wireless channel impairments. OFDMA can be combined with MIMO to improve performance.
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