Orthogonal Frequency Division Multiplexing [Under Modulation >> Digital modulation]
In FDM, frequencies are divided and used individually. The interference is controlled by adequate separation between frequencies (known as guard band). GSM used it this way. But the spectral efficieny is less due to guard band.
In OFDM, multiple information signals are transmitted using "orthogonal" (time synchronised) carriers. Though overlapping in frequency domain, they do not cause intercarrier interference due to orthogonality property. Needless to say, we have better spectral efficiency in OFDM.
How is it done ?
A base carrier frequency is chosen which has integer number of cycles during a symbol time (which in turn relate to bandwidth/data rate of information signal). The infomation signals have relatively much slower rate compared to carrier signal. Further carrier signals (called as subcarriers in OFDM) are chosen in such a way that they will have integer number of cycles during the symbol duration. This result into subcarriers being orthogonal to each other.
In mathematical terms, if (sub)carriers are spaced by k/T where k is an integer (typically 1) and T is symbol duration, the subcarriers are orthogonal. 1/T is information rate for a single carrier and so for N subcarriers, it would be N/T or N multiplied by subcarrier spacing.
One way to look at why above way of orthogonality achieve less interference is that during sample period, the modulated signals are "in sync" (they do not change). And so the effect on each other is calculable (or known) can be filtered considering much slower rate of information signals. Note that sample period could mean single bit (like in BPSK) or two bits (like in QPSK), or depending on modulation used; important point being modulated signal not changing during sample period.
The important point in OFDM is to keep frequency and time synchronisation intact between transmitter and receiver. Note that individual carriers themselves can use different modulation schemes like FSK, PSK, QAM. Time and Frequency synchronisation over a "symbol time" is the key here to maintain orthogonality property. Also, the bit rates for subcarriers may be different depending on modulation scheme (like BPSK transfer 1 bit whereas QPSK transfer 2 bits during the same time).
OFDMA (OFDM for multiple access) is preferred choice for today's front runner standards like WiFi (802.11a), WiMAX (802.16), LTE (3GPP), DVB-H, DSL etc.
More on OFDM and OFDMA in later articles.
References: In addition to Wikipedia page on OFDM, check out Thesis from Lawrey, and Complex2Real site OFDM Tutrorial.© Copyright Samir Amberkar 2010
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