Post on 15-Jul-2015
transcript
Multi Input Multi Output(MIMO)
Contents- Diversity Definition - Why Diversity - Types of Diversity- Types of combining- MIMO Definition - Why MIMO ? - MIMO Advantages and disadvantages- Applications of MIMO
Diversity Definition
Diversity is a technique by which we transmit many copies of the signal or versions of the signal but effected with different fading over time, frequency or space.
Diversity Definition
Diversity means send the same message signal or replica of message signal by
using two or more communication channels with different characteristics.
Why Diversity
To overcome fading and To combat cochannel interference
(CCI) and To avoid error bursts .
Space diversity
1. Many copies of the transmitted signal effects with different fading over the space .
2. we use multi-antenna systems: At the transmitter ( transmit diversity) or At the receiver ( reception diversity) or At both of them( MIMO).
Frequency diversityFrequency diversity
This type of diversity used for the frequency selective channels as we will averaging or avoiding fading over the frequency by using:
Multi-carrier technique like OFDM. FHSS (frequency hope spread spectrum). DSSS (direct sequence spread spectrum).
Frequency Diversity
The signal is transmitted using several frequency channels or spread over a wide spectrum that is affected by frequency selective fading.
Time Diversity
Multiple versions of the same signal are transmitted at different time instants.
We averaging the fading of the
channel over time by using :- The channel coding and interleaving or Sending the data at different times
SISO
SISO stands for single input and signal output. It uses one antenna at the transmitter and one antenna at the receiver . SISO channel is more susceptible to problem caused by multipath effect however it is cheap to implement.
For SISO system the capacity is given by Shannon formula : C = B log2(1 + SNR).
SIMO
SIMO stands for single input and multiple outputs. It uses single antennas at transmitter and multiple antennas at the receiver. SIMO system is preferably used in uplink.
Combining techniques
The presence of reception diversity poses an interesting problem : how do we use effectively the information from all the antennas to demodulate the data.
Combining techniques
Combines the independent fading paths signals to obtain a signal that passed through demodulator.
The combining techniques can be applied to any type of diversity.
The combining techniques are linear as the output of is a weighted sum of the different fading signals of branches.
The combining techniques needs cophasing.
Selection combining (SC)
Selection combining used in spatial diversity systems involves the sampling of several antenna signals, and sending the largest one to the demodulator. Selection combining ( SC) is relatively easy to implement but not optimal because it does not make use of all the received signals simultaneously .
Selection combining (SC)
This is the simplest combining method. Consider a MR receiver system. In (SC), we select the signal coming into each of the MR antennas that has the highest instantaneous SNR at every symbol interval. The output of the combiner equal to that of the best incoming signal.
Selection combining (SC)
The advantage of SC is that it does not require any additional RF receiver chain.
All receive antennas share a single RF receiver chain. This keeps the cost down.
In practice the strongest signals are selected because it is difficult to measure SNR alone.
In maximal ratio combining (MRC), the signals from all of the several branches are weighted according to their individual SNRs and then summed. The individual signals are cophased before being summed.
Maximal ratio combining (MRC)
Maximal ratio combining (MRC)
In maximal ratio combining (MRC) the output is a weighted sum of all branches due to its SNR.
Maximal ratio combining (MRC)
Maximal-ratio combining produces an average SNR γM−− equal to the sum of the individual average SNRs where we assume that each branch has the same average SNR.
Maximal-ratio combining can produce an acceptable average SNR, even when none of the individual i γ is acceptable. It uses each of the M branches in a cophased and weighted manner such that the largest possible SNR is available at the receiver.
Equal gain combining (EGC)
Equal gain combining (EGC) is similar to maximal ratio combining (MRC) except that the weights are all set to unity. The possibility of achieving an acceptable output SNR from a number of unacceptable inputs is still retained. The performance is marginally inferior to maximal ratio combining.
Equal gain combining (EGC)
EGC is the same as MRC but with equal weighting for all branches.
The performance is marginally inferior to MRC, but the complexities of EGC implementation are much less than MRC.
MIMO Definition
The use multiple transmitters and receivers to transfer more data at the same time.
MIMO technology takes advantage of a radio wave phenomenon called multipath where transmitted information bounces off walls and other
objects, reaching the receiving antenna multiple times via different angles and at slightly different times.
Configurations overview SISO : Stands for Single Input
Single Output SIMO : Stands for Single Input
Multi Output (reception diversity)
MISO : Stands for Multi Input Single Output ( transmit diversity)
MIMO : Stands for Multi Input Multi Output ( transmit and reception diversity)
MIMO Configurations
MIMO configuration can described by :-
N (Transmitter) *N (Receiver)
Most common MIMO configuration is: 2*2, 2*3, 2*4, and 4*4
Why MIMO?
MIMO can exploit multiple transceivers at both the enhanced node B (base station BS) and the user equipment (UE)
So we can increase the data rates of the mobile system.
Why MIMO?
MIMO increase data rate via Spatial ( space) Multiplexing
by allowing to transmit different streams of data simultaneously on the same resource block(s) by exploiting
the spatial dimension of the radio channel.
Why MIMO?
MIMO increase the robustness of data transmission via Transmit Diversity
Each transmit antenna transmits the same stream of data. This increases the signal to noise ratio at the receiver side and thus the robustness of data transmission especially in fading scenarios
Why MIMO?
MIMO enhance link reliability in challenging propagation conditions when the signal strength is low and multipath conditions are challenging. Thus, MIMO lower bit error rate
MIMO Advantages
Major advantages of MIMO Higher capacity. Increase data rate. Lower bit error rate. Increased coverage. Improved position estimation.
MIMO Applications
MIMO provides high speed wireless communication link to support wide range of applications without the expansion of the available bandwidth or increase of transmitted power.
MIMO Applications Communication network applications such as
broadcasting network, cellular network, satellite communication.
Narrowband Applications where limited bandwidth and lower data rate, higher performance required ( since space-time block coding (STBC) is attractive).
Pager, text messaging applications such as blackberry.
BER for BPSK modulation with Selection combining (SC) in BER for BPSK modulation with Selection combining (SC) in Rayleigh channelRayleigh channel
0 5 10 15 20 25 30 3510
-5
10-4
10-3
10-2
10-1
Eb/No, dB
Bit E
rror Rate
BER for BPSK modulation with Selection diveristy in Rayleigh channel
nRx=1 (sim)
nRx=2 (sim)
BER for BPSK modulation with Equal Gain Combining (EGC) in Rayleigh channel
0 5 10 15 20 25 30 3510
-5
10-4
10-3
10-2
10-1
Eb/No, dB
Bit E
rror Rate
BER for BPSK modulation with Equal Gain Combining in Rayleigh channel
nRx=1 (sim)
nRx=2 (sim)
BER for BPSK modulation with Maximal Ratio Combining (MRC) in Rayleigh BER for BPSK modulation with Maximal Ratio Combining (MRC) in Rayleigh channelchannel
0 5 10 15 20 25 30 3510
-5
10-4
10-3
10-2
10-1
Eb/No, dB
Bit E
rror Rate
BER for BPSK modulation with Maximal Ratio Combining in Rayleigh channel
nRx=1 (sim)
nRx=2 (sim)
Comparison among no diversity, Alamouti and max ratio combining Comparison among no diversity, Alamouti and max ratio combining (MRC)(MRC)
0 2 4 6 8 10 12 14 16 18 20
10-4
10-3
10-2
10-1
100
Eb/No (dB)
BER
Transmit vs. Receive Diversity
No Diversity (1Tx, 1Rx)
Alamouti (2Tx, 1Rx)Maximal-Ratio Combining (1Tx, 2Rx)
0 2 4 6 8 10 12
10-4
10-3
10-2
10-1
100
Eb/No (dB)
BER
G2-coded 2x2 System
No Diversity (1Tx, 1Rx)
Alamouti (2Tx, 1Rx)Maximal-Ratio Combining (1Tx, 2Rx)
Alamouti (2Tx, 2Rx)
Comparison among no diversity, Alamouti transmit diversityComparison among no diversity, Alamouti transmit diversity and max ratio combining (MRC) reception diversity and MIMOand max ratio combining (MRC) reception diversity and MIMO