FAQ on WCDMA Physical Layer (Layer 1)

Edited By Zahid Ghadialy (zahidtg@yahoo.com)

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 slot timing 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 synchronization code. 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 in every 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 way that 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 can determine the code group used as well as the frame boundaries (i.e., frame synchronization).

Reference: Introduction to 3G Mobile Communications - Juha Korhonen

What is RACH and how does it work?

The Random Access Channel (RACH) is an uplink transport channel. The RACH is always received from the entire cell. The RACH is characterized by a collision risk and by being transmitted using open loop power control. The Random Access Channel (RACH) is typically used 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 structure of the physical RACH for signalling purposes is the same as when using the RACH for user data transmission.

In UTRA the RACH procedure has the following phases:

  • The terminal decodes the BCH to find out the available RACH sub-channels and their scrambling codes and signatures.
  • The terminal selects randomly one of the RACH sub-channels from the group its access class allows it to use. Furthermore, the signature is also selected randomly from among the available signatures.
  • The downlink power level is measured and the initial RACH power level is set with the proper margin due to the open loop inaccuracy.
  • A 1 ms RACH preamble is sent with the selected signature.
  • The terminal decodes AICH to see whether the base station has detected the preamble.
  • 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 is retransmitted in the next available access slot.
  • When an AICH transmission is detected from the base station, the terminal transmits the 10 ms or 20 ms message part of the RACH transmission.

The RACH 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 RACH, the spreading factor and thus the data rate may vary; this is indicated with the TFCI on the DPCCH on PRACH. Spreading factors from 256 to 32 have been defined to be possible, thus a single frame on RACH 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 with the lowest rates, especially as RACH messages do not use methods such as macro-diversity as in the dedicated channel.

Reference: WCDMA for UMTS: Radio 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 the MAC layer. SFN is independent of the UE contexts, but associated with the Radio Link. CFN is associated with a UE context. The RRC layer maintains the mapping between CFN and various (for each RL) 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-RAT measurements. 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 compressed mode must be used if the frequency to be measured is close to the uplink frequency used by the UTRAN air interface (i.e., frequencies in TDD mode/GSM 1800/1900 band). Otherwise interfrequency 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 measurements while 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 the downlink frequency to be measured, then compressed mode must be employed in the uplink to prevent interfrequency interference.

Reference: 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 application could 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 common channels 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 be introduced 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 are shared evenly between two different scrambling codes.

Introduction to 3G Mobile Communications - Juha Korhonen
WCDMA for UMTS: Radio Access for Third Generation Mobile Communications - Harri Holma

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