or data transmission. An example of reference signal allocation for two-antenna downlink transmission is illustrated in Figure 3.3. The complex values of the reference signals in the time-frequency resource grid are generated as the symbol-by-symbol product of a two-dimensional orthogonal A resource element corresponds to an OFDM subcarrier in the time-frequency grid. © 2009 by Taylor & Francis Group, LLC Radio-Interface Physical Layer Frequency Reference signal Not used Figure 3.3 Example of reference symbol allocation for two-antenna transmis- sion in downlink. sequence and a two-dimensional pseudorandom sequence. This two-dimensional reference signal sequence also determines the cell identity to which the terminal is connected. There are 504 reference signal sequences defined in the LTE specifica- tion, corresponding to 504 different cell identities. Additionally, frequency shifting is applied to the reference signals in order to provide frequency diversity. Reference signals are also used in the uplink to facilitate coherent demodulation (then called demodulation signals) as well as to provide channel quality information for frequency dependent scheduling (referred to as sounding signals). Demodulation signals are transmitted on the fourth OFDM symbol of each UL slot along the transmission bandwidth allocated to a particular user, whereas sounding signals utilize a larger bandwidth to provide channel quality information on other fre- quency subcarriers. The uplink reference signals are based on constant amplitude zero autocorrelation (CAZAC) sequences. Further details on CAZAC sequences can be found in Section 3.4.1.1. 3.1.3 Synchronization Signals The base station periodically sends synchronization signals in the downlink SO that the mobile terminals may be always synchronized. These signals also help the ter- minal during the cell search and handover procedures. Synchronization signals consist of two portions: Primary synchronization signals are used for timing and frequency acquisi- tion during cell search