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 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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