5.1 Overview of the WCDMA Radio Interface

5.1.1  Protocol Structure of Radio Interfaces

From the perspective of protocol structure, the WCDMA radio interface is composed of the following three layers: Physical layer, medium access control layer and radio resource control layer. In terms of protocol layer, the WCDMA radio interface has three channels: Physical channel, transport channel and logical channel.

The circles among different layers/sub-layers in the Figure are Service Access Points (SAPs).
The physical layer provides data transmission services required by the upper layer. These services are accessed by using the transport channel through MAC sub-layer.
The physical layer provides services for the MAC layer through a transport channel, while the proprieties of transmission data determine what kind of transport channel should be used and how to transmit. The MAC layer provides the RRC layer with services through a logical channel, while the proprieties of the transmitted data determine the type of the logical channel. In the Medium Access Control (MAC) layer, the logical channel is mapped as a transport channel. MAC layer should select proper Transport Format (TF) for each transport channel, according to the transient source rate of logical channels. The selection of transmission format relates tightly to the transport format combination set of each connection (defined by receiver control module).
RRC layer also provides services for upper layers (non-access stratums) through Service Access Points (SAPs). The SAPs are used by the upper layer protocol and the RANAP of the Iu interface respectively on the UE side and the UTRAN side. All signaling of upper layers (including mobility management, calling control and conversation management) are compressed into RRC messages, and then are sent on radio interfaces.
The RRC layer configures such protocol entities of lower layers as physical channels, transport channels and logical channels by using the control interfaces between it and lower layer protocols. The RRC layer also uses control interfaces to control commands in real-time, for example, it requires the lower layers to perform specific measurement, and asks them to use the same interfaces to report measurement interfaces and error information.
Logical channel: Carrying user services directly. According to the types of the carried services, it falls into two types: Control channel and service channel.
Transport channel: It is the interface of radio interface layer 2 and physical layer, and is the service provided for MAC layer by the physical layer. According to whether the information transported is dedicated information for a user or common information for all users, it is divided into dedicated channel and common channel.
Physical channel: It is the ultimate embodiment of all kinds of information when they are transmitted on radio interfaces. Each kind of channel which uses dedicated carrier frequency, code (spreading code and scramble) and carrier phase (I or Q) can be regarded as a dedicated channel.
At the transmitting end, the data flows from MAC and upper layers are transmitted in radio interfaces, reused and mapped by channel coding, transport channel and physical channel, spread and modulated by physical channel, and then formed the data flows of radio interfaces to be transported on the radio interfaces. At the receiving end, it is a reverse process.
This chapter gives a brief introduction to logical channels and transport channels, and focuses on the process of physical channels and layers. By learning the process of physical channels and layers, we can deeply understand the operating principle of WCDMA radio interfaces, and get known to the WCDMA network planning.

5.1.2  Spreading Spectrum and Scrambling

On radio interfaces, after source coding and channel coding, the data flow continues to spread spectrum, scramble and modulate.

The code word used for spreading spectrum is called channelization code, for which OVSF (Orthogonal Variable Spreading Factor) code is used. The code word used for scrambling is called scramble, which adopts GOLD sequence.

1. Spreading spectrum and channelization code

Channelization code is used to distinguish the transmission from the same source, that is, different physical channels of the same terminal between the downlink connection and upper-link one of a sector. The spread spectrum/channelization of UTRAN is based on orthogonal variable spreading factor (OVSF) technology.
OVSF can change the spreading factor and keep the orthogonality between different spreading codes with various lengths. Code words can be selected from the code tree shown below. If one connection uses variable spreading factors, it can use correctly the code tree for dispreading according to the minimum spreading factor. Therefore, just select the channelization codes from the branch of the code tree directed by the minimum spreading factor code.

2. Scramble

Scramble is used to separate the terminals or BSs, and it is used after spreading spectrum, so it does not change the bandwidth of signals but only separate the signals from different sources. After scrambling, the problem that several transmitters use the same code word spreading spectrum is solved. Figure 5-2 shows the relation between spreading spectrum and channelization chip rate in UTRA. After the spread spectrum of channelization code, it already reaches chip rate, so the scrambling code does not affect the symbol rate.
The table below summarizes the functions and features of the scrambling codes and channelization codes.


Table 5-1 Functions and Features of the Scrambling Codes and Channelization Codes.

Channelization code
Scrambling code
Purpose
Uplinks: Distinguish physical data (DPDCH) and control channels (DPCCH).
Down links: Distinguish the down links of different users in the same cell.
Uplinks: Distinguish terminals
Down links: Distinguish cells
Length
4~256 chips (1.0-66.7 us)
The down links contain 512 chips
Uplinks: 10 ms = 38400 chips or =66.7 us = 256 chips
For advanced BS receipt, option 2 can be selected.
Down links: 10 ms = 38400 chips
Quantity of code words
Quantity of code words under a scrambling word is equal to that of spreading factors
Uplinks: Several million
Down links: 512
Code cluster
OVSF (Orthogonal Variable Spreading Factor)
Long 10 ms code: Gold code
Short code: Extended S (2) code cluster
Spreading spectrum
Yes, transport bandwidth is added.
No, transport bandwidth is not affected.

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