3G Evolution Outline 4 - Lunds tekniska högskola

Department of Electrical and Information TechnologyJohan Löfgren

3G EvolutionChapter:

2009-03-19 3G Evolution - HSPA and LTE for Mobile Broadband 1

4OFDM Transmission


Outline

• OFDM Signals• Conceptual OFDM System• Multipath Channel and Cyclic Prefix• Frequency Domain Model• Channel Coding• OFDM Parameters• Channel Estimation• User Multiplexing and OFDMA• OFDM and Multicast Transmission• Summary


Outline



Subcarriers in Time Domain

• A number of different, orthogonal frequencies are transmitted simultaniously– Requires full period for each subcarrier

• The number of carriers varies– Number of carriers denoted Nc


OFDM Symbol in Time Domain

• Combined symbol looks random– Gives good spectral effiency


Subcarrier Orthogonality

• In the frequency domain the orthognality is seen by zeros– All other subcarriers are zero when one subcarrier peaks


Outline



Conceptual Transmitter and Receiver

• The transmitter can be seen as a number of single carrier systemadded together

• The receiver is in the same way thought of as a set of independent correlators, one for each subcarrier

tfje 02π

tfje 12π

tfj cNe 12 −π

tfje 02π−

tfje 12π−

tfj cNe 12 −− π

∫K

∫K

∫K

)(tx )(tr


IFFT/FFT Implementation

• In an actual OFDM system, there is only one transmit path– The structure with orthogonal subcarriers enables an

implementation using an IFFT that translates the different frequencies into a combined time signal

• The receive side is also simplified– An FFT unit replaces the Nc correlators– The different subcarriers still have to be detected separately

• The IFFT/FFT implemenation requiers sufficient sampling– If so, the OFDM signal can be described mathematically as

∑∑∑−

=

−

=

−

=

Δ ====1

0

/2'1

0

/21

0

2)(N

k

NknjN

k

Nknjk

N

k

fnTkjksn eaeaeanTxx

k

ccs πππ

⎩⎨⎧

≤≤≤≤

=NkNNka

ac

ckk 0

0 '


Outline



Multi-path Components

• Often there are many reflections of a signal sent between a transmitter and a receiver– These are seen as different paths for the signal to travel

• Different paths have different delays which might pose a probelm


Cyclic Prefix

• Since the paths do not arrive simultaniously the orthogonality between the different subcarriers will be destroyed.– When integrating over a full period the non-wanted subcarriers do not

cancel• The solution is to pro-long the symbol with a so called cyclic prefix (CP)

– Copy of the final piece of each sub-carrier inserted in beginning– Restores the orthogonality


Outline



Frequency Domain Model

• With a sufficient CP it is possible to decouple the Nc different subcarrieres– Each subcarrier is treated

separately– Independent channel

parameters and noise• The receiver can equalize the

independently– One-tap equalizer– Maximum Likelihood Estimation

(per subsymbol)

0n

1n

1−cNn1−cNH

1H

0H 0*H

1*H

1*

−cNH


Time Frequency Grid

• The different subsymbols are thus time and frequency independent– They can be illustrated in a time frequencey grid– Each square is an indepedent subsymbol


Outline



Fading and Channel Coding

• Another effect due to multi path is frequency fading– Some frequencies will be much weaker due to canceling wave fronts

• This may lead to poor SNR at certain frequencies– Problem in OFDM since each subcarrier have different data

• Calls for channel coding– Channel coding spreads the information over many subcarriers


Outline



OFDM Parameters

• When dessigning an OFDM system a number of different parameters have to be considered

– Subcarrier bandwidth

– Number of subcarriers

– Length of cyclic prefix


Subcarrier Bandwidth

• The subcarrier bandwidth (or spacing) is affected by– Cost of cyclic prefix

• A small spacing leads to long symbol durationhence less cost of cyclic prefix

– Doppler spread• A small bandwidth leads to high relative doppler spread

thus sensitivity to movement

• The system designer has to trade off these contradicting demands depending on the system– How mobile is the system?– What data rates are needed?– et c.


Number of subcarriers

• When the subcarrier separation is decided the number of subcarriers can be calculated– If the system has a limited bandwidth this will give the maximum

number of subcarriers• Some slack is needed to ensure low enough out-of-band transmission

– If instead a certain througput is needed, the number of subcarriers can be calculated based on constellation and coding

• This will then set the total bandwidth of the system


Cyclic Prefix Length

• The cyclic prefix should be long enough to ensure orthogonality of the system– Multipath spread has to be considered

• Longer transmission distances normally call for longer CPs– However at long distances, e.g. at cell edges, other effects may limit

the performance and therefore it is possible to allow a system which is not completely orthogonal everywhere

• Some systems support multiple cyclic prefix lengths to handle different environments


Outline



Channel Estimation

• In the receiver the channel needs to be equalized so that the transmitted information can be correctly interpreted

• For this to be possible, the channel must be known• Since the channel constantly changes due to movement of

both the mobile unit and the surroundings, the channel cannot be known once and for all

• The channel has to be re-estimated every once in a while• So called pilots are used to estimate the channel

– These are data that are known to the receiver


Channel Estimation

• Since the system should also transmit unknown data, pilots cannot be transmitted continously– Pilots are placed in different, strategic places in the time frequency grid– The other values have to be interpolated from the known values

• Simple linear interpolation• MMSE estimation• FIR estimation


Outline



User Multiplexing and OFDMA

• Many users can share the same bandwidth in a system– FDMA– TDMA– CDMA

• In OFDM an advanced FDMA scheme, that uses the orthogonality of the system is available.– Different users are allowed to use different subsets of the

subcarriers– Both in downlink and uplink

• An uplink like this is refered to as OFDMA• The uplink requires some coordination so that the information of the

different units reaches the base station at approximately the same time (within the cyclic prefix) and same power


User Multiplexing and OFDMA

• The subcarriers for each user can either be bundled together or distributed– Bundled subcarriers may lead to simpler implementation,

e.g. a smaller FFT size, but is prone to fading dips


Outline



OFDM and Multicast Transmission

• Often many users want the same information• To not waste bandwidth the users can listen to the same

subcarriers for this information– Requires that the system is robust enough to handle the worst user

condition– To improve worst case at the cell border a user can listen to more

than one base station at a time

• In OFDM all base stations can transmit the same information at the same frequency and time


Outline



Chapter summary

• A correctly designed OFDM system can be decoupled into a number of single carrier systems– This makes OFDM an attractive choice for new systems– However the this comes at a price of wasted power and bandwidth

for the cyclic prefix– It also requires channel coding to avoid information loss due to

frequency fading

• It is important to know what conditions the system is designed for since different environments call for different system parameters– Subcarrier separation– Number of subcarriers– Length of cyclic prefix


Chapter summary

• The channel needs to be known in order to extract the information– In OFDM correlation between different subcarriers and time

instances can be used in order to estimate the channel– The pilots must thus not be available on all subcarriers

• In multi user systems OFDM can be used as an access method where the frequency spectrum is divided between the users.– This is an advanced type of FDMA, that uses the orthogonality of

the OFDM subcarriers– To make this possible in the uplink, OFDMA, certain care has to be

taken to ensure that the information from different units reaches the base station at approximately the same time (within the cyclic prefix) and same power

Documents

3G Evolution Outline 4 - Lunds tekniska högskola