LTE & LTE Advanced

9Uplink Physical Channels

9.1. PRACH physical channel

The Physical Random Access Channel (PRACH) contains a preamble used by the mobile when it needs to carry out random access.

The random access is the first step of the mobile connection to the evolved Node B (eNB) entity.

The PRACH physical channel contains a signal using an Orthogonal Frequency Division Multiplexing (OFDM) different from other Physical Uplink Control Channel (PUCCH) and Physical Uplink Shared Channel (PUSCH).

9.1.1. Time structure of the preamble

The preamble is made of a Cyclic Prefix (CP) in duration T_CP and of a sequence of T_SEQ in duration.

The duration of the sequence T_SEQ determines the energy collected by the eNB entity in reception.

During the random access, the mobile uses the zero value for timing advance.

The guard time keeps the reception of the PRACH physical channel from disturbing the following subframes for a localized mobile at the edge of the eNB entity (Figure 9.1).

Figure 9.1. Structure of the PRACH physical channel: guard time

The guard time is equal to the maximum Round Trip Time (RTT) and determines the maximum value of the radius of the cell:

R(km) = guard time (μs) / 6.6

Table 9.1 describes the format of the different preamble.

Table 9.1. Structure of the PRACH physical channel: duration of each field

Note.– T_s = 0.032552 _μs

For the mobile located at the edge of the cell, the ...