OFDMA, OFDM vs CDMA Comparison

Comparing the advantages and disadvantages of OFDM, OFDMA and CDMA

4G/LTE technology uses OFDMA
4G/LTE technology uses OFDMA

Both OFDM (orthogonal frequency-division multiplexing) and OFDMA (orthogonal frequency-division multiple access) are used in many modern wireless systems including AC-MIMO Radio, 802.11ac and 4G/LTE. CDMA is widely used in 3G networks. Here we give a comparison of the advantages and disadvantages of OFDMA / OFDM and CDMA.

Both CDMA and OFDM / OFDMA have their own advantages and disadvantages and therefore an OFDMA / OFDMA CDMA comparison can be beneficial.

When undertaking an OFDM CDMA comparison, or more correctly an OFDMA CDMA comparison, it is necessary to take a look at all the advantages and disadvantages. Often the OFDM CDMA choice will not be easy, but many systems these days are tending to opt for the OFDM / OFDMA solution over the CDMA solution.

OFDM and OFDMA advantages and disadvantages

The tables below outline some of the advantages and disadvantages of OFDM and OFDMA. These need to be considered when looking at an OFDM CDMA comparison.

First the advantages and disadvantages of OFDM will be detailed in the table below:

OFDM ADVANTAGES OFDM DISADVANTAGES
  • OFDMA can easily adapt to severe channel conditions without the need for complex channel equalisation algorithms.
  • It is robust when combatting narrow-band co-channel interference. As only some of the channels will be affected, not all data is lost and error coding can combat this.
  • Inter-symbol interference, ISI, is less of a problem with OFDM because low data rates are carried by each carrier.
  • Provides high levels of spectral efficiency.
  • Relatively insensitive to timing errors.
  • Allows single frequency networks to be used, significantly improving spectral usage – particularly important for broadcasters.
  • OFDM is sensitive to Doppler shift – frequency errors offset the receiver and if not corrected the orthogonality between the carriers is degraded.
  • Sensitive to frequency timing issues.
  • Possesses a high peak to average power ratio – this requires the use of linear power amplifiers which are less efficient than non-linear ones and this results in higher battery consumption.
  • The cyclic prefix used causes a lowering of the overall spectral efficiency.

It is also necessary to look at the advantages of OFDMA when undertaking an OFDM CDMA comparison.

OFDMA ADVANTAGES OFDMA DISADVANTAGES
  • Provides flexibility of deployment across a variety of frequency bands with little need for modification, which is important as spectrum becomes more fragmented, especially for LTE and LTE Advanced systems.
  • When used in a cellular system, using different carrier permutations between the two cells can minimise interference from neighbouring cells.
  • Interference within the cell in a cellular system is averaged by using allocation with cyclic permutations.
  • A single frequency network can be used to provide excellent coverage and good frequency re-use.
  • Offers frequency diversity by spreading the carriers all over the used spectrum.
  • It has a relatively high sensitivity to frequency offsets as this degrades the orthogonality between the carriers.
  • Sensitive to phase noise on the oscillators as this degrades the orthogonality between the carriers.
  • Requires complex electronics to run the software – DSP including FFT algorithms for forward error correction. This is always active regardless of data rate, leading to higher power consumption, though the system can hibernate when idle.
  • Resistance to selective fading is lost if only a few carriers are assigned to each user.
  • In a cellular system, co-channel interference from neighbouring cells is more complicated to combat than with CDMA as allocation of carriers needs to be coordinated.
  • The fast channel feedback information and adaptive sub-carrier assignment is more complex than CDMA fast power control.

OFDM CDMA comparison

Both OFDM and CDMA have significant benefits.

OFDM ADVANTAGES CDMA ADVANTAGES
  • OFDM combats multipath interference more effectively, with simpler equalisation undertaken on a carrier by carrier basis.
  • OFDMA with MIMO offers higher spectral efficiency than CDMA using a RAKE receiver.
  • No cell breathing, as additional users connect to the base station.
  • Can be used to provide a single frequency network.
  • Spectrum is aggregated relatively easily.
  • Scaled according to requirements relatively easily.
  • Easier to implement than OFDM based systems.
  • CDMA’s wide bandwidth and continuous signal make equalising the overall spectrum difficult and processing-intensive, unlike OFDM’s discrete carriers.
  • Not as easy to aggregate spectrum as for OFDM.

While both CDMA (DSSS) and OFMD with its variants of OFDMA and SC-FDMA have advantages, many of the systems being implemented now are erring towards the use of OFDMA. With the increase in processing power, it is possible to generate and demodulate OFDM signals with relative ease. As increasing bandwidths are needed, OFDM is being used as it is relatively easy to scale and lends itself to the high data rates being required for many applications.

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