Subject: [NEXTEL-1] Article: Comprehending the Technology Behind the UMTS Wideband CDMA Physical Lay

From: Marcel <[email-masked]>
Date: Mon, 03 Feb 2003 04:11:24 -0500

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From: "syedmhassan <[email-masked]>" <[email-masked]>
Subject: Article: Comprehending the Technology Behind the UMTS Wideband CDMA Physical Lay

Comprehending the Technology Behind the UMTS Wideband CDMA Physical 
Layer

By Louis Litwin, RF Design, Nov 1, 2002
 
[For a copy of this article in PDF format, which displays figures 
and equations, click here. Requires Adobe Acrobat Reader, free 
download.] 
The physical layer of the universal mobile telecommunications system 
(UMTS) wideband code-division multiple access (WCDMA) standard uses 
direct sequence spread spectrum (DSSS) modulation with a chip rate 
of 3.84 Mcps. The channel bandwidth is 5 MHz and this wider 
bandwidth (compared to 2G systems such as IS-95, which have a 
bandwidth of 1.25 MHz) has benefits such as higher data rates and 
improved multipath resolution.

For example, WCDMA has a multipath resolution of approximately 78 m 
(meters) versus IS-95's resolution of approximately 244 m.

Error control coding options include rate 1/2 and 1/3 convolutional 
coding as well as rate 1/3 turbo coding. The data rates supported 
range from a few kb/s to 2 Mb/s. The physical layer supports two 
modes of operation: FDD and TDD.

FDD and TDD operational modes 
The frequency-division duplex (FDD) mode carries the uplink and 
downlink channels on separate frequency bands of 5 MHz each. This 
mode is typically used for large outdoor cells because it can 
support a larger number of users than TDD mode. In time-division 
duplex (TDD) mode, the transmissions share the same frequency band 
by sending the uplink and downlink channels during different time 
slots. The TDD mode does not support as many users as the FDD mode, 
and hence, TDD mode is more suitable for smaller cells. TDD mode 
also is more suited for carrying asymmetric traffic compared to FDD 
mode.

The uplink and downlink transmissions in FDD mode are assigned fixed 
and equal frequency bands. This assignment works well when carrying 
voice traffic since such traffic tends to have uplink and downlink 
transmissions of approximately equal size. However, TDD mode is more 
efficient for carrying asymmetric traffic, such as Internet data.

Internet data tends to have downlink transmissions that are much 
larger than the uplink transmissions.

For example, a user's HTTP request for a Web page requires a brief 
uplink transmission, whereas downloading the Web page might require 
a large downlink transmission to carry text, audio and video. TDD 
mode is more suited to this asymmetric type of transmission because 
the ratio of the uplink and downlink bandwidths can be modified by 
assigning more or fewer time slots to each link.

Thus FDD mode, with its fixed uplink/downlink allocation and larger 
capacity, is typically used in large outdoor cells that primarily 
carry voice traffic.

On the other hand, TDD mode is typically used for indoor cells since 
such cells tend to be smaller and have a higher tendency to carry 
asymmetric data traffic. Because of the popularity of FDD mode, the 
remainder of this article will only focus on the WCDMA FDD mode.

Radio frame format 
The basic transmission element in the FDD physical layer is a radio 
frame. A radio frame has a duration of 10 ms, and it is broken down 
into 15 time slots of 0.667 ms each. Each time slot contains 2,560 
chips and thus the entire 10 ms radio frame contains 38,400 chips.

The purpose of the time slot structure is to provide a timing 
framework for determining when various events can occur. For 
example, a user's data rate can change for every frame, and power 
control commands are sent every slot (thus giving WCDMA a power 
control rate of 1,500 Hz). The data in WCDMA is modified by both 
spreading and scrambling codes prior to transmission.

The next sections will discuss these codes.

Spreading codes 
Spreading codes are an integral part of the DSSS modulation process. 
The codes are used to spread the spectrum of the transmitted signal 
by multiplying the data bits by the higher frequency spreading code, 
thus increasing the data rate of the signal.

The spreading codes used in WCDMA are called orthogonal variable 
spreading factor (OVSF) codes, and the spreading factor can vary 
from SF == 4 to SF == 512. These codes are formed from a code tree 
where all codes at a given spreading factor are orthogonal to each 
other. The use of OVSF codes allows various codes of different 
lengths to be orthogonal to each other.

Since the 3.84 Mcps chip rate is held constant, higher data rates 
are obtained by using shorter spreading codes and lower data rates 
are obtained using longer spreading codes. The number of available 
codes of a given length is equal to the spreading factor.

For example, there are 512 spreading codes at a spreading factor of 
SF == 512. Decreasing the spreading factor increases the data rate 
but reduces the number of users that can be supported because fewer 
codes are available at the shorter spreading factors. Shorter 
spreading codes also result in a lower correlation gain at the 
receiver and thus the transmit power must be increased in order to 
maintain the same energy per bit across all spreading factors.

Scrambling Codes 
Scrambling codes are pseudorandom sequences that also are multiplied 
by the data.

However, unlike spreading codes, the scrambling codes do not change 
the data rate of the signal. Instead, the codes are overlaid on top 
of the spread data as a mask to identify the source of the data. The 
typical long scrambling codes are formed by truncating Gold codes to 
a length of 38,400 chips (e.g., a frame length). These codes are 
used when the base station has implemented a rake receiver.

In addition, short scrambling codes can be used when the base 
station has implemented multiuser detection (MUD) algorithms. The 
shorter scrambling code length simplifies the implementation of 
these MUD algorithms.

Code usage 
These spreading and scrambling codes are used in different ways 
depending on whether the transmission is on the uplink or downlink. 
On the uplink, the signals from different cellular phones, or mobile 
terminals (MTs), are separated by overlaying the signal with 
different scrambling codes. On the uplink, there are a total of 16.8 
million possible scrambling codes that can be used. The base station 
(BS) assigns these codes to each MT in its cell. A given MT will 
simultaneously transmit several different physical channels on the 
uplink, and these channels are separated by using different codes 
from the OVSF code tree. The range of spreading factors on the 
uplink goes from SF == 4 to SF == 256. The same OVSF code tree is used 
by all MTs, and thus, it is possible that multiple MTs will be 
transmitting using the same spreading code. However, it is the 
different overlaid scrambling codes used by the MTs that allow the 
BS to differentiate the signals from the MTs in its cell.

On the downlink side, the base stations separate themselves by 
transmitting with different scrambling codes. Thus all the downlink 
signals in a given cell will be overlaid with the same scrambling 
code. Unlike the uplink, the number of possible scrambling codes is 
only 512. This number is intentionally kept small because doing so 
reduces the complexity of the cell search procedure performed by the 
MTs.

The downlink physical channels sent to the MTs in the cell are 
separated by transmitting the channels for different MTs using 
spreading codes taken from a single code tree. The range of 
spreading factors for the downlink goes from SF == 4 to SF == 512. 
Because the spreading codes for all MTs in a given cell are taken 
from the same code tree, trade-offs must be made when assigning 
codes to various MTs. If a high data rate code with a short 
spreading factor is assigned to an MT, this assignment prevents all 
of the codes further down that branch in the code tree from being 
used (because codes from a given branch are not orthogonal to one 
another). Therefore, although a large number of users can be 
supported in a given cell at low data rates, only a few users can be 
supported at high data rates.

Uplink dedicated physical channels 
There are two types of physical channels — dedicated and common. A 
dedicated channel is assigned to and used by only one user. A common 
channel is shared by several (possibly all) users in a cell. On the 
uplink there are two dedicated physical channels.

The dedicated physical data channel (DPDCH) and the dedicated 
physical control channel (DPCCH) are I/Q multiplexed (i.e., one is 
transmitted on the real channel and one on the imaginary channel) on 
the uplink. The DPDCH carries data dedicated for one user. For 
example, this channel would be used to carry voice traffic as well 
as audio/video data from a specific user.

Typically there is only one DPDCH, however it is possible to 
transmit six simultaneous DPDCHs when the spreading factor is SF == 
4. The six DPDCHs are carried on three spreading codes by 
transmitting three DPDCHs on the real channel and three DPDCHs on 
the imaginary channel.

The DPCCH is used to carry physical layer control information 
associated with the DPDCH. Pilot bits are carried on the DPCCH in 
order to aid the base station with channel estimation and carrier 
recovery for the DPDCH. The DPCCH also carries power control 
commands, feedback information as well as information about the 
format of the data being carried over the DPDCH. Regardless of the 
number of DPDCH channels, there is always only one DPCCH.

Uplink channel topology 
The physical random access channel (PRACH) is used to carry random 
access transmissions. For example, random access transmissions are 
used when the MT wishes to set up a connection with the base station 
in order to place an outgoing call. The random access procedure is 
based on the slotted ALOHA protocol and it uses open-loop power 
control.

The open-loop power control works as follows. First, the MT 
transmits a PRACH preamble to the base station. If the BS does not 
acknowledge the preamble, the MT transmits the preamble again but at 
a higher power. This process continues until the received signal 
strength at the BS is strong enough for reception. At this point, 
the BS sends an acknowledgement to the MT.

Following this acknowledgement, the MT sends the PRACH message at 
the same power level as the last preamble transmission. Since this 
power level is only valid for a very short period, the PRACH message 
part can only be one or two frames in duration. The PRACH message 
part consists of both a data and control channel.

The physical common packet channel (PCPCH) is a shared channel that 
carries packet-based data transmissions. The channel access is based 
on the carrier sense multiple access with collision detection 
(CSMA/CD) protocol where the MT first senses the medium to see if it 
is free before transmitting. If it detects a collision, the MT backs 
off for a random amount of time before re-trying the transmission.

Downlink channel topology 
The dedicated physical channel (DPCH) is the downlink equivalent of 
the DPDCH and DPCCH channels. It is a single channel that carries 
both dedicated data information as well as control information. It 
can be viewed as a time-multiplexed version of the DPDCH and DPCCH 
channels.

The common pilot channel (CPICH) is a continuous downlink pilot 
signal that contains a known training sequence scrambled by the 
current cell's scrambling code. This channel is used by the MT as a 
reference signal for a variety of signal processing operations, such 
as carrier synchronization and channel estimation.

The common control physical channel (CCPCH) is used to carry several 
upper-layer transport channels. The CCPCH carries the broadcast 
transport channel that is used to send network and cell-specific 
information to all MTs in a cell. The CCPCH also carries the forward 
access transport channel which sends information to an MT after the 
MT makes a random access attempt on the PRACH uplink physical 
channel.

The paging channel is also carried on the CCPCH. The paging channel 
carries pages from the base station to an MT to notify the MT that 
the base station has information for the MT. For example, the base 
station will use the paging channel to page the MT in order to 
notify it of an incoming call.

The synchronization channel (SCH) is the first channel that an MT 
looks for at start-up. The SCH is a sparse channel that is only 
active during the first 256 chips of each slot. The SCH is made up 
of two subchannels, primary and secondary. The primary SCH carries 
the same signal for all cells in the system and it is used by the MT 
to obtain chip, symbol and slot synchronization. The secondary SCH 
is different for each cell. The secondary SCH channel carries a 
pattern of secondary synchronization codes (SSCs) that repeat every 
frame. Once the MT receives this sequence, it will have frame 
synchronization.

In addition, the pattern will identify which scrambling code group 
the current cell's scrambling code belongs to. There are 64 
scrambling code groups, and each group contains eight scrambling 
codes. Once the MT has determined the current cell's scrambling code 
group, the search for the current cell's scrambling code will be 
narrowed to the eight codes in that group.

The physical downlink shared channel (PDSCH) is used to carry the 
downlink shared transport channel. This channel carries control 
information for several users that share the channel.

Indicator channels 
There are also other types of channels, known as indicator channels. 
There are several of these channels, and they are used to carry 
specific indicator messages. For example, there is a paging 
indicator channel (PICH) that carries paging indicators. This 
channel is used to notify all MTs in a given paging group that there 
is a page from the base station for an MT in that paging group.

Another indicator channel is the acquisition indicator channel 
(AICH). The AICH is used by the base station to notify the MT that 
it has received the MT's PRACH preamble during the open-loop power 
control phase of a random access transmission.

Mobile terminal Rx processing 
After power-up, the mobile terminal has to perform several 
operations before voice/data communications can begin. First, the 
receiver needs to implement automatic gain control (AGC) in order to 
scale the received signal power and prevent clipping at the analog-
to-digital (A/D) converter. This process first can be performed on 
the SCH channel, and later the descrambled CPICH pilot can be used 
once the cell's scrambling code is acquired.

Next, the receiver needs to acquire timing synchronization. Timing 
synchronization can be achieved from the SCH channel. The MT will 
search for the strongest SCH signal that it can find, and that 
signal determines which cell the MT will initially communicate with. 
Since the SCH channel is periodic, the receiver can correlate 
against the primary SCH to derive a timing error. Based on this 
channel, the receiver can achieve chip, symbol and slot 
synchronization.

However, since the primary SCH channel transmits the same sequence 
in all slots, the receiver cannot yet determine the location of 
frame boundaries. Once the secondary SCH channel is acquired, the 
receiver will have frame synchronization because the channel carries 
a pattern that repeats every frame.

Based on this secondary SCH pattern, the MT will know which 
scrambling code group the current cell's scrambling code belongs to. 
There are 512 scrambling code groups and the Secondary SCH indicates 
which group the current cell's code is in. Since there are only 
eight scrambling codes in a group, the MT can determine which 
scrambling code is being used by correlating against the CPICH pilot 
channel using each of the eight scrambling codes in the cell's code 
group. The current cell's code is identified by looking for the 
largest correlation peak out of all eight correlations.

After the cell's scrambling code has been determined, all downlink 
channels can be descrambled and acquired. The MT will first use the 
CPICH pilot channel to derive an error for carrier frequency 
synchronization. This step removes any residual frequency errors 
that result from a frequency offset between the oscillators at the 
base station and the MT.

Next, the MT uses the CPICH pilot channel to generate an estimate of 
the multipath channel that the downlink signal has travelled 
through. This channel estimate is used by the rake receiver to 
determine which channel paths to assign rake fingers to.

Getting signal strength up 
The rake receiver is used to constructively combine energy from 
various channel paths in order to allow the receiver to work with a 
stronger signal. Thus, instead of suppressing the energy from other 
multipaths, the rake receiver actually combines this energy.

The rake fingers are each assigned to a given channel path, and they 
correlate against the signal corresponding to that channel delay. 
The channel estimate value for that path is used to weight the 
signal prior to combining the signals from multiple paths. This 
weighting is an important step, because it aligns the phases of all 
of the signals so that they can be constructively combined. Without 
this step, it is possible that the phases of the various signals 
could be such that the signals could be destructively combined, and 
in such a case, the signal power of the resulting signal would be 
lessened.

The rake receiver is used to first acquire the broadcast transport 
channel that is carried on the CCPCH physical channel. This channel 
will inform the receiver of which random access codes are available 
in the cell. The MT will use one of the available codes to begin a 
random access procedure on the PRACH channel. Once it has contacted 
the base station, the MT will register itself with the base station 
so that the network knows the location and capabilities of the MT. 
The base station also will assign the MT to a paging group.

The MT will then go into sleep mode where most of the receiver is 
shut down to conserve battery power. The MT will wake up if the user 
wishes to make a call.

In addition, the MT will periodically wake up to monitor the paging 
indicator channel. If this channel indicates that there is a page 
for the MT's assigned paging group, the MT will decode the paging 
channel to see if the page is actually for the MT. The base station 
uses pages to contact the MT when there is data for the MT. An 
example would be an incoming phone call for the MT. The receiver 
would then acquire the DPDCH and DPCCH channels to receive the data.

Conclusions 
This article provided a basic overview of some of the main features 
of the WCDMA physical layer standard. In addition, some insight was 
given into the signal processing that must be performed by the 
receiver after power-up. The interested reader can learn more about 
the various details of the standard from the references listed below.

References 
Holma, H. and Toskala, A., WCDMA For UMTS: Radio Access for Third 
Generation Mobile Communications, Wiley, NY, 2001.

Ojanpera, T. and Prasad, R., Wideband CDMA For Third Generation 
Mobile Communications, Artech House Publishers, MA, 1998.

Prasad, R., Mohr, W., and Konhauser, W., Third Generation Mobile 
Communication Systems, Artech House Publishers, MA, 2000.

About the author 
Louis Litwin is a member of the technical staff with Thomson 
Multimedia Corporate Research, where he is working on 3G CDMA 
technology for mobile applications. Litwin received his master's 
degree in electrical engineering from Purdue University, and his 
bachelor's degree in electrical engineering (summa cum laude) from 
Drexel University.

His professional interests include wireless digital communications 
with a particular focus on adaptive equalization, error control 
coding and synchronization. He can be reached at [email-masked]
 
Source:
http://rfdesign.com/ar/radio_comprehending_technology_behind/index.htm

Best regards
Syed Hassan

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