An overview of the LTE physical layer--Part II

Downlink Reference Signals
To allow for coherent demodulation at the user equipment, reference symbols (or pilot symbols) are inserted in the OFDM time-frequency grid to allow for channel estimation. Downlink reference symbols are inserted within the first and third last OFDM symbol of each slot with a frequency domain spacing of six sub-carriers (this corresponds to the fifth and fourth OFDM symbols of the slot in case of normal and extended cyclic prefix, respectively) as shown in Figure 6 for an LTE system with one antenna in normal CP mode. Furthermore, there is a frequency domain staggering of three sub-carriers between the first and second reference symbols. Therefore, there are four reference symbols within each Resource Block. The user equipment will interpolate over multiple reference symbols to estimate the channel. In case of two transmit antennas, reference signals are inserted from each antenna where the reference signals on the second antenna are offset in the frequency domain by three sub-carriers. To allow the user equipment to accurately estimate the channel coefficients, nothing is transmitted on the other antenna at the same time-frequency location of reference signals.

The reference symbols have complex values, which are determined according to the symbol position as well as of the cell. LTE specifications refer to this as a two-dimensional reference-signal sequence, which indicates the LTE cell identity. There are 510 reference signal sequences corresponding to 510 different cell identities. The reference signals are derived from the product of a two-dimensional pseudo-random sequence and a two-dimensional orthogonal sequence. There are 170 different pseudo-random sequences corresponding to 170 cell-identity groups, and three orthogonal sequences each corresponding to a specific cell identity within the cell identity group.

Uplink Reference Signals
There are two types of reference signals for uplink in LTE. The first is Demodulation Reference Signals (DM-RS) which are used to enable coherent signal demodulation at the eNodeB. These signals are time multiplexed with uplink data and are transmitted on the fourth or third SC-FDMA symbol of an uplink slot for normal or extended CP, respectively, using the same bandwidth as the data.

The second is Sounding Reference Signal (SRS) which is used to allow channel dependent (i.e. frequency selective) uplink scheduling as the DM-RS cannot be used for this purposes since they are assigned over the assigned bandwidth to a UE. The SRS is introduced as a wider band reference signal typically transmitted in the last SC-FDMA symbol of a 1 ms subframe as shown in Figure 7. User data transmission is not allowed in this block, which results in about 7% reduction in uplink capacity. The SRS is an optional feature and is highly configurable to control overhead--it can be turned off in a cell. Users with different transmission bandwidth share this sounding channel in the frequency domain.

SYNCHRONIZATION SEQUENCES
A User Equipment wishing to access the LTE system follows a cell search procedure which includes a series of synchronization stages by which the UE determines time and frequency parameters that are necessary to demodulate downlink signals, to transmit with correct timing and to acquire some critical system parameters.

There are three synchronization requirements in LTE: symbol timing acquisition by which the correct symbol start is determined; carrier frequency synchronization, which mitigates the effect of frequency errors resulting from Doppler shift and errors from electronics; and sampling clock synchronization.

There are two cell search procedures in LTE: one for initial synchronization and another for detecting neighbor cells in preparation for handover. In both cases, the UE uses two special signals broadcast on each cell: Primary Synchronization Sequence (PSS) and Secondary Synchronization Sequence (SSS). The detection of these signals allows the UE to complete time and frequency synchronization and to acquire useful system parameters such as cell identity, cyclic prefix length, and access mode (FDD/TDD). At this stage, the UE can also decode the Physical Broadcast Control Channel (PBCH) and obtain important system information.