ISI vs ICI: Inter Symbol Interference and Inter Carrier Interference

This page explains the differences between two types of interference: Inter Symbol Interference (ISI) and Inter Carrier Interference (ICI).

Introduction to ISI and ICI

ISI and ICI are short for Inter Symbol Interference and Inter Carrier Interference, respectively. When radio frequency (RF) signals, composed of electromagnetic waves, are transmitted through the air, they can be affected by other RF signals, particularly those transmitted from nearby cellular towers. This RF interference is primarily categorized into two types: adjacent channel interference and co-channel interference.

For more details on these types of interference, refer to the article on RF interference types.

There are two main transmission methods for baseband information: Single Carrier (SC) and multiple carriers (based on OFDM). You can find more information on the differences between them in the SC vs OFDM article.

• Single Carrier (SC): 1 bit is carried by 1 carrier waveform.
• Orthogonal Frequency Division Multiplexing (OFDM): Multiple bits are transmitted in 1 symbol (over multiple carriers). A symbol in OFDM is a group of multiple carriers in the frequency domain.

Adjacent channel interference and co-channel interference are typically associated with single carrier based systems. ISI and ICI, on the other hand, are primarily concerns in OFDM based systems.

Understanding ISI (Inter Symbol Interference)

ISI due to multipath channel

In OFDM, transmission occurs symbol by symbol. Before transmission, symbols are packed with complex modulated data. For example, in WLAN 802.11a systems, one symbol consists of a 64-point FFT (Fast Fourier Transform). Assuming QPSK (Quadrature Phase-Shift Keying) modulation, each carrier in the group of 64 carriers is mapped with two bits of data information.

After the symbol is formed, a Cyclic Prefix (CP) is appended to each OFDM symbol. The CP is essentially a copy of the end of the OFDM symbol that is placed at the beginning. The CP length can be 1/4, 1/8, 1/16, or 1/32 of the OFDM symbol length.

The number of samples in the CP period is calculated as:

Number of Samples in CP period = CP_factor * (Number of samples in one FFT size)

Where CP_factor = 1/4, 1/8, 1/16, or 1/32.

No ISI due to guard interval

As the symbols travel from transmitter to receiver, the channel introduces delay spread in the time domain. This delay spread causes an OFDM symbol to spread out in time, potentially interfering with consecutive OFDM symbols. This phenomenon is ISI (Inter Symbol Interference).

Mitigating ISI: ISI can be mitigated or reduced by using the Cyclic Prefix (CP). The CP length is chosen to be longer than the channel delay spread. Figure-1 (described in original article) depicts ISI, where one OFDM symbol affects a consecutive symbol. Figure-2 (described in original article) shows how adding a “guard interval” (using the CP) between neighboring OFDM symbols eliminates ISI.

Tradeoff: However, adding a CP reduces the overall bit rate of the system due to the added redundancy. The difference between a CP and a general “guard time” is that the CP uses a copy of the end of the symbol, while a guard time can simply be unused samples added in between symbols.

Understanding ICI (Inter Carrier Interference)

ICI due to frequency offset

In OFDM, carriers are densely packed so the peak of one subcarrier is at a null of the other subcarriers. This is referred to as orthogonality. For OFDM to be an efficient modulation scheme, subcarriers should be orthogonal to each other.

ICI (Inter Carrier Interference) occurs when the subcarriers lose orthogonality. ICI can result from the following:

• Delay spread of the radio channel exceeding the CP interval (guard interval)
• Frequency offset at the receiver

Mitigating ICI: ICI can be reduced by estimating the frequency offset and correcting the subcarrier spacing accordingly.

Note: In the original article, it states “Cyclic Prefix(CP) eliminates ICI.” This is not quite right, CP helps with ISI, by reducing the channel delay spread between symbols.