• OFDM

Each user is allocated several carriers onto which their data is modulated using one (or more) possible modulation schemes.  Typical schemes chosen to modulate the individual carriers are BPSK, QPSK and QAM ( > 4QPSK).   After the individual data streams are modulated onto their respective carriers, the parallel data streams are re-combined for additional processing and transmission.  The orthogonal nature of the carriers permits them to be spaced very close together (as compared to the carriers in a classical FDMA transmission scheme) and thus results in a high spectral efficiency.

You can use our simple graphing calculator to explore orthogonal functions as follows:

Hit Add Function.

Plug in sin(x)/(x).  This is the first function.

Then Plug in sin(x + 3)/(x+3).  This is the second function.

Hit Rescale.

Set xMin = -10.0

Set xMax = 10.0

Set xScale = 1.0

Set yMin = -1.0

Set yMax = 1.0

Set yscale = 0.1

Notice that at the center frequency of each carrier, the effect of the orthogonal carrier is zero (no crosstalk).

Moreover, since each individual carrier has it's own modulation scheme on it, different modulation schemes can be used to deliver different Class/Quality of Services to different users.

Since the original data stream is now partitioned across several carriers, the effective symbol rate per carrier can be reduced.  This makes the data more resilient to the effects of ISI.   Additionally, guard symbols can be inserted into the individual data streams to make the data even more resistant to ISI.

Synchronization...

Phase noise can be a major cause of carrier frequency offset in any transmission scheme.  As compared to single carrier transmission schemes, OFDM is more susceptible to the effects of carrier frequency offset.  This is due to the fact that even very small frequency offsets can upset the delicate orthogonality amongst the multiple carriers.  If the carrier orthogonality is degraded, then OFDM is no longer resistant to ISI and no longer spectrally efficient.

Additionally, as the mobile moves around, the receiver will be subjected to the effects of Doppler Spread (as a function of mobile velocity and operating wavelength).  This also posses some strict requirements on the susceptibility of OFDM to carrier frequency offsets.

Peak to Average Ratio...

The nature of the orthogonal signals tends to increase the peak to average ratio (dynamic range) of the signals fed into the amplifiers.  This is not good as it necessitates the use of linear RF amplifiers in the transmitters.  Unfortunately, linear RF amplifiers tend to be inefficient.

• C(oded)OFDM

COFDM is a variation on OFDM that adds an additional layer of protection against harsh frequency selective fading in the operating channel.  To realize this extra protection, various types of Forward Error Correction (FEC) can be implemented.  These FEC codes can be of the block (Reed Solomon, Hamming, BCH, Cyclic, Linear) family, convolutional family (with Viterbi decoders) or "other" family.  One popular type in the "other" category is called Trellis Coded Modulation (TCM).  TCM actually combines the coding and the modulation schemes together such that coding gain is realized, but not at the expense of occupied bandwidth.  TCM achieves this by making the signal constellation denser in some areas and less dense in others.

Of course there is always a tradeoff, and TCM is not very adept at handling bursty (correlated) errors.  To overcome this deficiency, interleavers are employed to ensure that the errors presented to the decoder are uncorrelated.  The interleaver depth must be chosen judiciously to effect sufficient decorrelation in the errors without incurring inordinate amounts of interleaver latency.

• V(ector)OFDM

Cisco and the BWI Forum are putting forth a variation called V(ector)OFDM.  V-OFDM has a number of specific methods used to overcome some of the hurdles in OFDM via the use of spatial diversity.  Spatial diversity can be realized either with multiple antennas or a single antenna with multiple elements (array antenna).

• W(ideband)OFDM

Wi-Lan and the OFDM Forum are putting forth a variation called W(ideband)OFDM.   W-OFDM has a number of specific methods used to overcome some of the hurdles in OFDM (such as sensitivity to carrier frequency offset).  W-OFDM tries to find an optimal balance for the spacing between orthogonal carriers.  While very closely spaced carriers means high spectral efficiency, creating a little bit of extra space between carriers affords more tolerance against carrier frequency offset.

• Flash OFDM

Flarion has developed a technology called Flash OFDM that integrates Fast Frequency Hopping Spread
Spectrum technology with OFDM technology. In addition, Flash OFDM appears to have a tight coupling
between the Physical and MAC layers that is optimized for wireless IP data.

• MIMO OFDM

Iospan has developed a technology called MIMO (Multiple Input/Multiple Output) OFDM. MIMO OFDM
relies heavily on spatial multiplexing to increase the spectral efficiency.


The next installment in this series will explore digital modulation techniques in general.