EEE-752 Emerging Wireless Networks MIMO Riaz Hussain FA08-PCE-003 Ph.D. Student Department of Electrical Engineering COMSATS Institute of Information Technology Islamabad, Pakistan Riaz

Goals High data rate Quality Achieve “Channel Capacity (C)” Minimize Probability of Error (Pe) Real-life Issues Minimize complexity/cost of implementation of proposed System Minimize transmission power required (translates into SNR) Minimize Bandwidth (frequency spectrum) Used Riaz

Traditional Transceiver Single-Input-Single-Output (SISO) antenna system Theoretically, the 1Gbps barrier can be achieved using this configuration if you are allowed to use much power and as much BW as you so please! Extensive research has been done on SISO under power and BW constraints. A combination a smart modulation, coding and multiplexing techniques have yielded good results but far from the 1Gbps barrier channel User data stream Riaz

Before Moving On Coding Gain Diversity Diversity Gain Degree of freedom Array Gain Beam-forming Transit Beam-forming Riaz

Coding Gain It is the measure in the difference between the signal to noise ratio (SNR) levels between the uncoded system and coded system required to reach the same bit error rate (BER) levels Example: – If the uncoded BPSK system in AWGN environment has a Bit error rate (BER) of 10 − 3 at the SNR level 3dB, – and the corresponding coded (e.g., BCH) system has the same BER at an SNR level of 1.5dB, – then we say the coding gain = 3dB-1.5dB = 1.5dB Power Limited Regime: – With reference to QAM (2 x 2) Bandwidth Limited Regime – With reference to (M x M) QAM Riaz

Diversity Theme: – Improve the data reliability by utilizing two or more communication channels with different characteristics Helps in: – Reducing fading effect – Combating Co-channel interference – Avoiding error burst Riaz

Diversity Can Be Achieved Time Diversity: – Multiple versions of the same signal are transmitted at different times Frequency Diversity: – signal is transferred using several frequency channels or spread over a wide spectrum that is affected by frequency-selective fading Space Diversity: – signal is transferred over several different propagation paths. In the case of wired transmission, this can be achieved by transmitting via multiple wires. In the case of wireless transmission, it can be achieved by multiple antenna Polarization Diversity, etc. etc. Riaz

Diversity Combining technique applied to combine the multiple received signals of a diversity reception device into a single improved signal Selection Combining: Of the N received signals, the strongest signal is selected Switched Combining: The receiver switches to another signal when current signal drops below a predefined threshold Equal gain Combining: All the received signals are summed coherently Maximal-ratio Combining: The received signals are weighted with respect to their SNR and then summed Riaz

Antenna Diversity Receive (SIMO) Transmit (MISO) Both (MIMO) Riaz Courtesy of: Shivkumar Kalyanaramand: RPI Google: “Shiv RPI”

Receive Diversity Same mathematical structure as repetition coding in time diversity (!), except that there is a further power gain (aka “array gain”). Optimal reception is via matched filtering/MRC (a.k.a. receive beamforming). Riaz Courtesy of: Shivkumar Kalyanaramand: RPI Google: “Shiv RPI”

Array Gain Vs Diversity Gain Diversity Gain: – how much the transmission power can be reduced when a diversity scheme is introduced, without a performance loss --- measured in decibel – multiple independent channels between the transmitter and receiver, and is a product of the statistical richness of those channels Array gain – does not rely on statistical diversity between the different channels and instead achieves its performance enhancement by coherently combining the actual energy received by each of the antennas. – Even if the channels are completely correlated, as might happen in a line-of-sight (LOS) system, the received SNR increases linearly with the number of receive antennas, – Eg: Correlated flat-fading: – Single Antenna SNR: – Adding all receive paths: Riaz Courtesy of: Shivkumar Kalyanaramand: RPI Google: “Shiv RPI”

Receive Diversity: Selection Combinin g Pick max signal, but don’t fully combine signal power from all taps. Diminishing returns from more taps. Source: J. Andrews et al, Fundamentals of WIMAX Riaz

Receive Beamforming: Maximal Ratio Combining (MRC) Weight each branch SNR: MRC Idea: Branches with better signal energy should be enhanced, whereas branches with lower SNR’s given lower weights Source: J. Andrews et al, Fundamentals of WIMAX Riaz

Selection Diversity vs MRC Source: J. Andrews et al, Fundamentals of WIMAX Riaz

Antenna Diversity Receive (SIMO) Transmit (MISO) Both (MIMO) Riaz

Riaz Tx Diversity Similar gain as achieved by using multiple receive antenna can also be obtained by using multiple transmit antenna Alamouti : Orthogonal space-time block code (OSTBC). – 2 × 1 Alamouti STBC Rate 1 code: – Data rate is neither increased nor decreased; – Two symbols are sent over two time intervals. – Goal: harness spatial diversity. Don’t care about ↑ rate Alamouti Code:

Alamouti Scheme Over two symbol times: Projecting onto the two columns of the H matrix yields: Receive Signal Receiver: Project on columns of H Channel response is same over two symbol time Linear Algebra manipulation Alamouti:OSTBC: Orthogonal Space-Time Block Code

Transmit Diversity Gain Like MRC, but 3dB (i.e. ½) lower power Two outputs at two symbols time

Antenna Diversity Receive (SIMO) Transmit (MISO) Both (MIMO) Riaz

MIMO: w/ Repetition or Alamouti Coding Transmit the same symbol over the two antennas in two consecutive symbol times (at each time, nothing is sent over the other antenna). – ½ symbol per degree of freedom (d.f.) MRC combining w/ repetition: Alamouti scheme used over the 2 × 2 channel: – Sends 2 symbols/2 symbol times (i.e. 1symbol/d.f), – Same 4-fold diversity gain as in repetition.

Spectral Efficiency Spectral efficiencies of some widely used modulation schemes The Whole point: Given an acceptable Pe, realistic power and BW limits, MIMO Systems using smart modulation schemes provide much higher spectral efficiencies than traditional SISO Schemeb/s/Hz BPSK1 QPSK2 16-QAM4 64-QAM6 Riaz

MIMO offers – significant increases in data throughput and link range – without additional bandwidth or transmit power – achieves this by higher spectral efficiency (more bits per second per hertz of bandwidth) and link reliability or diversity Riaz

MIMO System Model y = Hs + n User data stream Channel Matrix H s1 s2 sM s y1 y2 yM y Transmitted vector Received vector.... h1 1 h1 2 Where H = h11 h21 …….. hM1 h12 h22 …….. hM2 h1M h2M …….. hMM.. ……... MT MR hij is a Complex Gaussian random variable that models fading gain between the ith transmit and jth receive antenna Riaz

Functions of MIMO Precoding Spatial Multiplexing Diversity Coding Riaz

Mathematical Description Riaz

MIMO-OFDM Multiple Input, Multiple Output Orthogonal Frequency Division Multiplexing is a technology developed by Iospan Wireless that uses multiple antennas to transmit and receive radio signals. MIMO-OFDM will allow service providers to deploy a Broadband Wireless Access (BWA) system that has Non-Line-of-Sight (NLOS) functionality. Specifically, MIMO-OFDM takes advantage of the multipath properties of environments using base station antennas that do not have LOS. According to Iospan, "In this environment, radio signals bounce off buildings, trees and other objects as they travel between the two antennas. This bouncing effect produces multiple "echoes" or "images" of the signal. As a result, the original signal and the individual echoes each arrive at the receiver antenna at slightly different times causing the echoes to interfere with one another thus degrading signal quality. The MIMO system uses multiple antennas to simultaneously transmit data, in small pieces to the receiver, which can process the data flows and put them back together. This process, called spatial multiplexing, proportionally boosts the data-transmission speed by a factor equal to the number of transmitting antennas. In addition, since all data is transmitted both in the same frequency band and with separate spatial signatures, this technique utilizes spectrum very efficiently. Riaz

References Shivkumar Kalyanaraman: RPI lectures Harish Ganapathy: MIMO Systems Riaz