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FAQ on WCDMA Physical Layer (Layer 1)

Edited By Zahid Ghadialy ([email protected])

Last Updated: 18/08/2006

What happens when the Mobile is switched on? How does it find the Scrambling code to

camp on?

When the mobile The synchronization procedure starts with downlink SCH synchronization.The UE knows the SCH primary synchronization code, which is common to all cells. The slottiming of the cell can be obtained by receiving the primary synchronization channel (P-SCH)

and detecting peaks in the output of a filter that is matched to this universal synchronizationcode. The slot synchronization takes advantage of the fact that the P-SCH is only sent during the

first 256 chips of each slot. The whole slot is 2,560 chips long. This is depicted in Figure above.Thus the UE can determine when a slot starts, but it does not know the slot number yet (there

are 15 slots in each frame), and thus it does not know where the radio frame boundary may be.

Thereafter the UE correlates the received signal from the secondary synchronization channel (S-SCH) with all secondary synchronization codes (SSC), and identifies the maximum correlation

value. The S-SCH is also only sent during the first 256 chips of every slot. One SSC is sent inevery time slot. There are 16 different SSCs, and they can form 64 unique secondary SCH

sequences. One sequence consists of 15 SSCs, and these sequences are arranged in such a waythat in any nonzero cyclic shift less than 15 of any of the 64 sequences is not equivalent to some

other sequence. This means that once the UE has identified 15 successive SSCs, it candetermine the code group used as well as the frame boundaries (i.e., frame synchronization).



R eference: Introduction to 3G Mobile Communications - Juha Korhonen

What is RACH and how does it work?

The R andom Access Channel (R ACH) is an uplink transport channel. The R ACH is always

received from the entire cell. The R ACH is characterized by a collision risk and by beingtransmitted using open loop power control. The R andom Access Channel (R ACH) is typicallyused for signalling purposes, to register the terminal after power-on to the network or to perform

location update after moving from one location area to another or to initiate a call. The structureof the physical R ACH for signalling purposes is the same as when using the R ACH for user data

transmission.

In UTR A the R ACH procedure has the following phases:

y The terminal decodes the BCH to find out the available R ACH sub-channels and

their scrambling codes and signatures.y The terminal selects randomly one of the R ACH sub-channels from the group its

access class allows it to use. Furthermore, the signature is also selected randomlyfrom among the available signatures.

y The downlink power level is measured and the initial R ACH power level is setwith the proper margin due to the open loop inaccuracy.

y A 1 ms R ACH preamble is sent with the selected signature.y The terminal decodes AICH to see whether the base station has detected the

preamble.y In case no AICH is detected, the terminal increases the preamble transmission

power by a step given by the base station, as multiples of 1 dB. The preamble isretransmitted in the next available access slot.

y When an AICH transmission is detected from the base station, the terminaltransmits the 10 ms or 20 ms message part of the R ACH transmission.

The R ACH procedure is illustrated in Figure above, where the terminal transmits the preamble

until acknowledgement is received on AICH, and then the message part follows. In the case of data transmission on R ACH, the spreading factor and thus the data rate may vary; this is

indicated with the TFCI on the DPCCH on PR ACH. Spreading factors from 256 to 32 have been defined to be possible, thus a single frame on R ACH may contain up to 1200 channel



symbols which, depending on the channel coding, maps to around 600 or 400 bits. For the

maximum number of bits the achievable range is naturally less than what can be achieved withthe lowest rates, especially as R ACH messages do not use methods such as macro-diversity as in

the dedicated channel.

R eference: WCDMA for UMTS: R adio Access for Third Generation Mobile Communications -Harri Holma

What is the significance of SFN and CFN?

SFN is the frame number used by the physical layer. CFN is the frame number used by theMAC layer. SFN is independent of the UE contexts, but associated with the R adio Link. CFN is

associated with a UE context. The RR C layer maintains the mapping between CFN and various(for each R L) SFNs.

What is compressed mode and is it necessary for the UE to support compressed mode?

Compressed mode is needed if the UE needs to perform Inter-Frequency or Inter-R ATmeasurements. More details on what compressed mode is and how its performed can be seen in

Compressed Mode Tutorial.

Compressed Mode is performed in Uplink(UL) as well as in Downlink(DL). Uplink compressedmode must be used if the frequency to be measured is close to the uplink frequency used by the

UTR AN air interface (i.e., frequencies in TDD mode/GSM 1800/1900 band). Otherwiseinterfrequency interference may affect the results. Downlink compressed mode is not necessary

if the UE has dual receivers. In that case one receiver can perform interfrequency measurementswhile the other handles the normal reception. Note however, that double receivers in the UE do

not remove the need for uplink compressed mode. If the uplink frequency is close enough to thedownlink frequency to be measured, then compressed mode must be employed in the uplink to

prevent interfrequency interference.

R eference: Introduction to 3G Mobile Communications - Juha Korhonen

Why is secondary scrambling code needed?

For each primary scrambling code there is a set of 16 secondary scrambling codes. They can be

employed while transmitting channels that do not need to be received by everyone in the cell.They should be used sparingly because channels transmitted with secondary scrambling codes

are not orthogonal to channels that use the primary scrambling code. One possible applicationcould be in sectored cells, where separate sectors do not have to be orthogonal to each other.

The secondary downlink scrambling codes can be applied with the exception of those commonchannels that need to be heard in the whole cell and/or prior to the initial registration. Only one

scrambling code should be generally used per cell or sector to maintain the orthogonality between different downlink code channels. With adaptive antennas the beams provide additional

spatial isolation and the orthogonality between different code channels is less important.



However, in all cases the best strategy is still to keep as many users as possible under a single

scrambling code to minimise downlink interference. If a secondary scrambling code needs to beintroduced in the cell, then only those users not fitting under the primary scrambling code

should use the secondary code. The biggest loss in orthogonality occurs when the users areshared evenly between two different scrambling codes.

R eference:

Introduction to 3G Mobile Communications - Juha KorhonenWCDMA for UMTS: R adio Access for Third Generation Mobile Communications - Harri

Holma

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