LTE:Why is the Cyclic Prefix (CP) needed?

Question

I came to know 5G radio is going to use something called Filter OFDM unlike 4G radio which was using cyclic prefix OFDM.

Answer 1

The guard period after each rectangular pulse carrying the modulated data symbol is a simple and efficient method to deal with multi-path reception.
The cyclic prefix (CP) simply consists of the last part of the following symbol.

The size of the cyclic prefix field depends on the system and can even vary within one system. Cyclic prefixes are used by all modern OFDM systems and their sizes range from 1/4 to 1/32 of a symbol period. Most receiver structures use the cyclic prefix to make an initial estimation of time and frequency synchronization (pre-FFT synchronization, non-data assisted synchronization).

A receiver typically uses the high correlation between the cyclic prefix and the last part of the following symbol to locate the start of the symbol and begin then with decoding. In multi-path propagation environments the delayed versions of the signal arrive with a time offset, so that the start of the symbol of the earliest path falls in the cyclic prefixes of the delayed symbols. As the CP is simply a repetition of the end of the symbol this is not an inter-symbol interference and can be easily compensated by the following decoding based on discrete Fourier transform.

Of course cyclic prefixes reduce the number of symbols one can transmit during a time interval. This method to deal with inter-symbol interference from multi-path propagation is theoretically sub-optimal. CDMA with RAKE receiver for instance provides a much better efficiency. On the other hand non-ideal implementations of RAKE receivers also degrade system performance drastically but still require a lot of hardware capacity for the basic implementation.

The rectangular pulse with cyclic prefix requires far less hardware, so the free capacity can be used to implement other performance optimization techniques like MIMO.

Source: http://www.teletopix.org/4g-lte/what-is-cyclic-prefix-in-lte

Answer 2

Cyclic prefix is often used in conjunction with modulation in order to retain sinusoids' properties in multipath channels. It is well known that sinusoidal signals are eigenfunctions of linear, and time-invariant systems. Therefore, if the channel is assumed to be linear and time-invariant, then a sinusoid of infinite duration would be an eigenfunction. However, in practice, this cannot be achieved, as real signals are always time-limited. So, to mimic the infinite behavior, prefixing the end of the symbol to the beginning makes the linear convolution of the channel appear as though it were circular convolution, and thus, preserve this property in the part of the symbol after the cyclic prefix.
In telecommunications, the term cyclic prefix refers to the prefixing of a symbol with a repetition of the end. Although the receiver is typically configured to discard the cyclic prefix samples, the cyclic prefix serves two purposes.

• As a guard interval, it eliminates the intersymbol interference from the previous symbol.
• As a repetition of the end of the symbol, it allows the linear convolution of a frequency-selective multipath channel to be modelled as circular convolution, which in turn may be transformed to the frequency domain using a discrete Fourier transform. This approach allows for simple frequency-domain processing, such as channel estimation and equalization.

In order for the cyclic prefix to be effective (i.e. to serve its aforementioned objectives), the length of the cyclic prefix must be at least equal to the length of the multipath channel. Although the concept of cyclic prefix has been traditionally associated with OFDM systems, the cyclic prefix is now also used in single carrier systems to improve the robustness to multipath propagation.

Answer 3

In order to reduce the ISI (Inter Symbol Interference) and ICI (Inter Carrier Interference) in time dispersive environments.

Insertion of cyclic prefix prior to transmission improves robustness in time-dispersive channels and Spectral efficiency loss.The Length of cyclic prefix is 4.7microsec in normal case.The CP is copy of the last part of the symbol in order to preserve the subcarrier orthogonality. This is possible since the FFT is a cyclic operation, but it is required that the time dispersion of the radio channel is shorter than the CP length.