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FDMA & narrowband/wideband systems

Verfasst: 7. Mär 2010 18:12
von phm
In the chapter on medium access (slide 3.12), it is written:
Narrowband Systems
In use are the following multiple access and duplexing schemes

Wideband systems
In use are the following multiple access and duplexing schemes

Why isn't there FDMA/TDD for narrowband systems? And why don't we use FDMA for wideband systems?

Re: FDMA & narrowband/wideband systems

Verfasst: 8. Mär 2010 23:00
von Matthias Hollick
phm hat geschrieben: ... Why isn't there FDMA/TDD for narrowband systems? And why don't we use FDMA for wideband systems?
The answer to this question is not straightforward, since the slide is a bit misleading (it should go out of the slideset one of these days or be updated). The given schemes are not the "only choices", but rather existing ones (other choices might as good or even better (e.g. consider schemes such as OFDMA) but are not listed).

The answer to the second part of the question first: Using FDMA typically results in a narrowband system (by design, you separate the channel in narrowband chunks, where each chunk has coherent channel/propagation conditions). However, in such a system you typically have a very high complexity in performing frequency planning for adjacent cells in cellular networks (due to interference in neighboring cells; at the same time, it can nicely allow for "orthogonal" transmission within a single cell).

- Some examples:
- In analog cellular networks (e.g. AMPS) FDMA/FDD has been used -- it is a very simple but robust scheme.
- In digital communications, DECT e.g. uses FDMA/TDD (uplink/downlink separated using time division, different frequencies to support multiple stations) and GSM uses a combination of FDMA/TDMA/FDD (frequencies for uplink/downlink; both frequencies and timeslots to separate multiple stations).
--> The DECT example is an example that is not given on the slide but apparently works. You would most likely not use this combiantion for celluar wide range networks, though (this slide was created with cellular systems in mind, hence FDMA/TDD did not make it onto the list of preferred options; TDMA/TDD in contrast is standardized for use in broadband wireless access technologies such as WiMAX).

Speaking from TDMA: TDMA sits somewhat between the chairs. Some people see it as "narrowband" likewise FDMA (for these, wideband is only spread spectrum like technologies such as CDMA), others see it already as "wideband". CDMA in contrast is a classical wideband technology.

An addition that helps in answering the first part of the question might be to look at the pros/cons of FDD and TDD. FDD splits frequencies for uplink and downlink, it requires some additional RF complexity, but is quite robust (but might be a bit inflexible for data communications, since it typically assumes uplink and downlink symmetry --- however, it is well suited for voice communication). TDD in contrast needs only a simple switch between RX and TX. It requires precise synchronization (and is hence easy to use in short-range communications), it allows for flexibility of uplink/downlink and thus supports asymmetric traffic. In TDD, the guard-times might decrease efficiency.

Hope this helps ...

Re: FDMA & narrowband/wideband systems

Verfasst: 9. Mär 2010 00:32
von phm
Thanks a lot for this detailed answer. It helped indeed!