16-11-2010, 04:43 PM
The MIMO-OFDM System in the 4G Mobile Communication
Presented by
Deepak Narayanan.K
Roll no-17
S7 AE&I
College Of Engineering, Trivandrum
2007-11 batch
Presented by
Deepak Narayanan.K
Roll no-17
S7 AE&I
College Of Engineering, Trivandrum
2007-11 batch
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Previous Technology – 1G “First Generation”
Analog
Simplest type to wireless data
Average between 4,800 to 9,600 bps (bits per second)
2G
Digital
Digital data can be compressed and multiplexed much more effectively than analog voice encodings
Multiplexing -multiple analog message signals or digital data streams are combined into one signal
Modulation schemes in 2G
3G
3G allows simultaneous use of speech and data services and
higher data rates
At least 200 kbit/s peak bit rate
A greater number of users that can be simultaneously supported by a radio frequency bandwidth
CDMA
Code division multiple access
What is 4G?
Fourth Generation Technology
Faster and more reliable
–100 Mb/s (802.11g wireless = 54Mb/s, 3G = 2Mb/s)
Lower cost than previous generations
Multi-standard wireless system
–Bluetooth, Wired, Wireless (802.11x)
IPv6 Core
OFDM used instead of CDMA
Potentially IEEE standard 802.11n
MIMO-OFDM
Multiple Input Multiple Output
Orthogonal Frequency Division Multiplexing
OFDM
OFDM is a multi carrier system.
Single high-rate bit stream is converted to low-rate N parallel bit streams
Each sub-carrier can be modulated differently, e.g. BPSK, QPSK or QAM
To achieve high bandwidth efficiency, the spectrum of the sub-carriers are closely spaced and overlapped
OFDM symbols are generated using IFFT
Has the ability to avoid severe channel conditions like
fading, attenuation, interference etc…
Why is OFDM so popular for new broadband systems?
Most broadband systems are subject to
multipath transmission
With OFDM there is a simple way of dealing
with multipath
relatively simple DSP algorithms
How does OFDM solve the multipath problem?
Data is transmitted in parallel
longer symbol period
e.g. for N parallel streams, symbol period is N
times as long
Cyclic prefix
trick to avoid residual ISI
How are signals transmitted in parallel without interference?
Each subcarrier has a different frequency
Frequencies chosen so that an integral number of cycles in a symbol period
Signals are mathematically
orthogonal
Sub carrier
The spectrum of each OFDM subcarrier is a sin (x) /x function
Orthogonality
Sub carriers are orthogonal to each other
Nulls in each sub-carrier's spectrum land at the centre of all other sub-carriers (orthogonal)
The crosstalk between the subcarriers is eliminated
Guard bands are not required
High spectral efficiency
Sub carrier spacing is Δf=k/T
simplifies the design of both the transmitter and receiver
Transmitter & Receiver
Space time encoder
A method employed to improve the reliability of data transmission in wireless communication systems using multiple transmit antennas
We use Alamouti’s coding
How are OFDM signals generated?
Parallel data streams are used as inputs to an IFFT
IFFT output is sum of signal samples
IFFT does modulation and multiplexing in one step
Filtering and D/A of samples results in baseband signal
OFDM in a multipath environment - effect on one subcarrier
Received signal in one symbol period is not a sinusoid
Causes inter carrier interference (ICI)
Cyclic Prefix
cyclic prefix refers to the prefixing of a symbol with a repetition of the end. Although the receiver is typically configured to discard the cyclic prefix samples, the cyclic prefix serves two purposes.
As a guard interval , it eliminates the inter symbol interference from the previous symbol.
As a repetition of the end of the symbol
Some loss in efficiency as cyclic prefix carries no new information
In this paper MIMO-OFDM technique is studied for 4G wireless communication system.
We use Alamouti’s space time encoding.
This technique can provide high data rate transmission without increasing transmit power and expanding bandwidth
Using this technique we can efficiently use space resources and has a bright future
