Cyclic Delay Diversity

Cyclic Delay Diversity (CDD) is a simple approach to introduce spatial diversity to an Orthogonal Frequency Division Multiplexing (OFDM) based transmission scheme that itself has no built-in diversity. It also can be regarded as a Space-Time Code (STC).

But in contrast to that there is no additional effort in the receiver necessary, since the different codewords result in a changed channel impulse response in the receiver. They insert virtual echos and thus increase the frequency selectivity of the channel seen by the receiver.

Cyclic Delay Diversity (CDD) is a diversity scheme used in OFDM-based telecommunication systems, transforming spatial diversity into frequency diversity avoiding intersymbol interference.

Cyclic delay diversity is an elegant diversity technique for OFDM based transmission systems, which does not introduce additional effort in the receiver. For OFDMA systems with many users and slow fading channels, cyclic delay diversity cannot provide full diversity for one user. Hence, a new technique, called time-varying cyclic delay diversity, is introduced. With this technique the diversity can be increased, which leads to lower bit and frame error rates during an OFDMA transmission

In CDD, the signal on the second or each additional antenna is not delayed but cyclically shifted. Therefore, no inter-symbol interference can occur and thus there are no limits for the cyclic shift. Another advantage of CDD is that there is no additional complexity in the receiver. Also there is no rate loss even for a large number of antennasin contrast to other Space-Time Codes.

In principle,CDD shifts the TX-signal in time direction and transmit these modified signal copies over separate TX-antennas. The TX-antenna specific signal modifications, i.e. the time shifts, are inserted in cyclically, such that no additional inter symbol interference (ISI) occurs. CDD is capable to offer a larger degree of diversity since they increase the number of resolvable channel propagation paths. This additional diversity has to be exploited by the OFDM system itself by means of techniques, which guarantee a certain amount of Hamming distance for the data bearing signal, i.e. channel coding or spreading.

Spatial Multiplexing

Spatial multiplexing is a transmission technique in MIMO wireless communication to transmit independent and separately encoded data signals, so-called streams, from each of the multiple transmit antennas. Therefore, the space dimension is reused, or multiplexed, more than one time.

In spatial multiplexing different signals or data bits are transmitted through several independent (spatial) communication channels by multiple antennas and at the same time the receiving side also use multiple antennas for receiving signals-this way increase the date transmission rate which is in direct proportion to the number of antennas used for both transmission and receiving purpose. The higher the number of antennas, the higher the number of data transmission rate.

Multiple antennas are used to provide diversity gain (receive and transmit diversity )and increase the reliability of wireless links. With channel knowledge at the transmitter, multiple transmit antennas can also provide a power gain via transmit beam-forming. Multiple transmit antennas are used to induce channel variations, which can then be exploited by opportunistic communication techniques. The scheme can be interpreted as opportunistic beam-forming and provides a power gain as well.

Spatial multiplexing requires MIMO antenna configuration. In spatial multiplexing, a high rate signal is split into multiple lower rate streams and each stream is transmitted from a different transmit antenna in the same frequency channel. If these signals arrive at the receiver antenna array with sufficiently different spatial signatures, the receiver can separate these streams, creating parallel channels for free. Spatial multiplexing is a very powerful technique for increasing channel capacity at higher Signal to Noise Ratio (SNR). The maximum number of spatial streams is limited by the lesser in the number of antennas at the transmitter or receiver. Spatial multiplexing can be used with or without transmit channel knowledge.