Questions on different multiplexing schemes used in Digital Communications

Hi,

I know this might take a while to answer,but can someone please explain the
basic differences between OFDM vs OFDMA, TDM vs TDMA, FDD vs TDD? Also,
what is the difference between a symbol and a subcarrier?

Any help would be appreciated!

Thank you,
Clay

On 1/17/13 10:07 PM, Clay wrote:
>
> I know this might take a while to answer,but can someone please explain the
> basic differences between OFDM vs OFDMA, TDM vs TDMA, FDD vs TDD?

alfabet soup.

> Also, what is the difference between a symbol and a subcarrier?

Vlad might pick on you for that question.



-- 

r b-j                  rbj@audioimagination.com

"Imagination is more important than knowledge."

Re: OFDM v OFDMA - In OFDMA the bandwidth is split up into different
frequency bands and each user can transmit/receive their data in a
particular band. 

Re: FDD v TDD - FDD requires paired spectrum where the user transmits on
one frequency bandwidth and receives, perhaps simultaneously,  on a
different bandwidth. In TDD, the user transmits and receives within the
same frequency bandwidth but at different times. 

A symbol is a duration in the time domain and a subcarrier is a frequency
domain parameter. In OFDM, the bandwidth contains a certain number of
subcarriers each modulated with its own data during a symbol period. 

On Thursday, January 17, 2013 9:07:06 PM UTC-6, Clay wrote:
> Hi, I know this might take a while to answer,but can someone please explain the basic differences between OFDM vs OFDMA, TDM vs TDMA, FDD vs TDD? Also, what is the difference between a symbol and a subcarrier? Any help would be appreciated! Thank you, Clay

Clay,
Here is my "shot" at it. Think about CDM and CDMA - code division multiplex and code division multiple access. so..........

OFDM - orthogonal frequency division multiplex
OFDMA - orthogonal frequency division multiple access

Same for TDM and TDMA

Maurice

>Hi,
>
>I know this might take a while to answer,but can someone please explain
the
>basic differences between OFDM vs OFDMA, TDM vs TDMA, FDD vs TDD? Also,
>what is the difference between a symbol and a subcarrier?
>
>Any help would be appreciated!
>
>Thank you,
>Clay
>

Communication can be multiple ones to many(broadcast) or many to
many(mobile phone). To achieve this multiple approach, systems can separate
users through any one or more of the following methods:

multiple wires (space division), same frequency,same time.
same wire (or space),same frequency, but multiple time slots(TDM)
same wire(or space),same time, multiple frequencies(FDM)
same wire(or space), same time, one frequency but different code used like
keys(CDM)

You can view ofdm as a special case of frequency division e.g. carriers are
chosen according to certain rules while FDM is free to choose any
affordable band.

"M" stands for multiplexing (implying multiple information sources) and
when additional features are added to enhance multiple users then ...MA
is used meaning Multiple Access.

for example. TDM implies fixed slots to given users while TDMA allows using
same slots for different users.

FDD and TDD are similar but applies to the case of duplex(tx,rx) as two
users that need be separated.

Kadhiem

>Hi,
>
>I know this might take a while to answer,but can someone please explain
the
>basic differences between OFDM vs OFDMA, TDM vs TDMA, FDD vs TDD? Also,
>what is the difference between a symbol and a subcarrier?
>
>Any help would be appreciated!
>
>Thank you,
>Clay
>

regarding last question, the difference between symbol and subcarrier is
similar to that between and apple and a homeless person. However, there is
some connection that possibly you have come across in ofdm. A subcarrier is
used to modulate one symbol.
In ofdm bits are mapped to symbols according to any of QAM methods. An ofdm
frame is then constructed from a set of symbols. This frame is assumed in
frequency domain and in order to generate multiple subcarriers, ifft is
applied. Anytime the symbol is +1 it generates a carrier at its location
within ifft bin and when -1 it generates same carrier at opposite phase.
The amplitude of carrier depends on symbol level e.g. for qpsk it will be
only one level(+/-1) but for 16QAM it will be +/-1,+/-1/3 and so on.

Kadhiem