Agilent Technologies ESG Guia do Utilizador descarregar pdf (Página 368)

352 Chapter 15

W-CDMA Uplink Digital Modulation for Receiver Test

Understanding the PRACH

PRACH transmission.

Signatures

A signature is a series of 16 bits that is repeated 256 times within a single preamble. These

signatures are shown in the 3GPP standards. The signature is the first of two parts used in

the spreading of the preamble. It points to a node within the channelization codes that

corresponds to a spread factor (SF) of 16. This node, within the SF=16 group, has sub-trees

(OVSF codes) that are used in the channelization/spreading of the message part.

A base station can simultaneously receive 16 different UEs, and all base stations use the

same 16 signatures. Using the BCH (broadcast channel), a base station tells each UE which

signature to use and then uses the signature to determine which UE it is communicating

with.

Scrambling Codes

The scrambling code, in most cases, is unique to the base station and it identifies the base

station that the UE is communicating with. In an actual UE to base station transmission, the

base station tells the UE which scrambling code to use. This scrambling code is then used in

connection with the signature to uniquely identify the UE to the base station. The addition of

the scrambling code is the second part of the spreading.

There are 8192 (0 to 8191) different scrambling codes. The value selected as the scrambling

code is actually the seed to generate the code sequence. The same scrambling code is used for

both the preamble and message part. However the preamble uses only the first 4096 chips of

the selected scrambling code.

Message Part

The message part is transmitted as part of the PRACH after the UE receives the AICH from

the base station. It contains the information necessary for the UE to establish a link with the

base station. This information is transmitted in two parts, a control part and a data part.

These two parts form the message part. Each part has its own power level that influences the

overall message part power. See “Power Control” on page 354 for more information.

There are three groups of parameters that the ESG lets you set for controlling the message

part: control part, data part, and the message part as a whole. The message part can be set to

transmit within either a 10 or 20 ms time period called a TTI (transmission time interval).

Structure

Each 10 ms message part is divided into 15 slots (two slots per access slot). Each of the 15

slots carry the two parts that compose the message part, the control part and the data part.

These two parts are transmitted in parallel. This is similar to the DPCH (dedicated physical

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Agilent Technologies ESG Guia do Utilizador Página 368