the UEs can know where to puncture their PUSCH transmission. On the other hand, an SRS is not allowed to interfere with PUCCH. The parameters that define the SRS are bandwidth, duration, periodicity, location in the subframe, cyclic shift for the Zadoff-Chu sequence, and transmission comb. 3.4.3 Random Access Once a UE is synchronized to the downlink transmission after the cell search pro- cedure, it is able to request the eNodeB for resource allocation to begin the trans- mission or to recover from a loss of synchronism in the uplink. In addition, this mechanism allows the eNodeB to estimate and adjust the UE transmission timing within a fraction of the cyclic prefix.+ This process-namely, the random access pro- cedure-is based on the transmission of a special symbol (random access preamble) mapped on PRACH. Although PRACH can only be transmitted in certain time and frequency resources, the eNodeB is not explicitly prohibited from allocating these resources for data transmission in the uplink. Different UEs use different Zadoff-Chu sequences when transmitting SRSs. Because these sequences are orthogonal, they do not interfere with each other, although they may be trans- mitted in the same time/frequency resources. This is the reason that the random access procedure described in this section is referred to as "nonsynchronized." © 2009 by Taylor & Francis Group, LLC Radio-Interface Physical Layer Preamble Frequency Af/AfRA = 12 Random access Afra= 1.25kHz Data subcarrier Af = 15 kHz NZC = 839 BW RA = 1.08 MHz 72 Data subcarriers 864 RA subcarriers (13 guards + 839 Zadoff-Chu+12 guards) Figure 3.36 Time domain and frequency domain RA preamble structure. 3.4.3.1 Random Access Preamble The physical layer random access preamble, whose time-frequency structure is illus- trated in Figure 3.36, consists of a cyclic prefix of length TCP and a preamble part of length TPRE. The parameter values are listed in 3GPP 36.213 and depend on the frame structure and the random access configuration, whereas higher layers are in charg