the underlying pseudo-random sequence is mapped to every other sub- carrier in the frequency domain over the OFDM symbol used for reference signal transmis- sion (see Fig. 4.3). Antenna ports 1000 and 1001 use even-numbered subcarriers in the frequency domain and are separated from each other by multiplying the underlying pseudo- random sequence with different length-2 orthogonal sequences in the frequency domain, resulting in transmission of two orthogonal reference signals for the two antenna ports. If the radio channel can be considered flat across four consecutive subcarriers, the two refer- ence signals will maintain orthogonality at the receiver. Antenna ports 1000 and 1001 are said to belong to CDM group 0, since they use the same subcarriers but are separated in the code-domain using different orthogonal sequences. Reference signals for antenna ports 1002 and 1003 belong to CDM group 1 and are generated in the same way using odd-numbered A Gold sequence is a type of binary sequence that is often used in telecommunication and satellite navigation. Gold sequences have bounded small cross-correlations within a set. A set of Gold sequences consists of 2nd - 1 sequences each with a period of 2nd - 1 A set of Gold sequences can be generated by taking two maximum-length sequences of the same length 2n - 1 such that their absolute cross-correlation is less than or equal to 2(n+2)/2 where n is the size of the linear feedback shift register used to generate the maximum length sequence. The set of 2nd - 1 (logical) exclusive OR of the two sequences in their various phases is a set of Gold codes. The highest absolute cross-correlation in this set of codes is 2nn22)/2 + 1 for even n, and 2(n+1)/2 + 1 for odd n. The exclusive OR of two Gold codes from the same set is another Gold code with arbi- trary phase. Chapter 4 subcarriers and are separated in the code domain within the CDM group and in the fre- quency domain between CDM groups. If more than four orthogonal antenna ports are needed, two consecutive OFDM symbols 