UMTS Physical Layer & Radio Interface

- the air interface, frequencies and power control used within UMTS or Wideband CDMA, WCDMA, cellular telecommunications system

The Physical layer is one of the key elements that differentiates the various cellular systems that operate. Although there is very much more to a cellular system than just the physical layer, it is one of the aspects that gains a high level of visibility.

The UMTS physical layer is totally different to that employed by GSM. It employs a spread spectrum transmission in the form of CDMA rather than the TDMA transmissions used for GSM. Additionally it currently uses different frequencies to those allocated for GSM.

UMTS physical layer signal format

One of the chief elements of the UMTS physical layer or radio interface is the signal format that has been adopted.

The UMTS physical layer utilises direct sequence spread spectrum format to enable a multiple access scheme called Code Division Multiple Access, CDMA to be used.

Using CDMA, multiple users share the same channel, but different users are allocated different codes, and in this way the system is able to distinguish between the different users.

      Read more about UMTS CDMA

The CDMA signal is 5MHz and in view of this, the UMTS physical layer is often referred to as Wideband CDMA, W-CDMA. This compares to the US based cdmaOne and cdma2000 systems that use a 1.25 MHz bandwidth.

UMTS transmitted signal characteristics

One key element of the UMTS physical layer is the definition of the transmitted signal characteristics. It is necessary to define the overall signal bandwidth and shape so that interference is minimised for adjacent channels and users.The pulse shaping applied to the transmitted signals is root raised cosine filtering with a roll-off-factor of 0.22.

The nominal carrier spacing is 5MHz, and the carrier centre frequencies are normally divisible by 5, but the carrier frequency can be adjusted in increments of 200kHz. Accordingly the centre frequency of UMTS carriers are indicated with an accuracy of 200kHz. This adjustment can be used to provide operators with a more flexible use of their available spectrum.

One important characteristic of the signal is the way in which the signal spreads out either side of the central area, and affecting other channels. It is never possible to have complete isolation or infinite filtering and therefore spectral masks are defined showing elves that must be achieved for compliance with the standard.

The spectral mask showing the levels for ACLR for the UMTS physical layer RF signal
UMTS physical layer spectral mask

Shown in the diagram of the UMTS physical layer signal is the Adjacent Channel Leakage Ratio. This is a measure of the signal level that appears in adjacent channels. ACLR1 is the level in the channel one up or down from the signal, and ACLR2 is two channels up or down.

The requirements are not surprisingly more stringent for base stations / NodeBs than for the handsets or UEs.


ACLR Requirements for UMTS Physical Layer
  ACLR1 ACLR2
UE / handset* 33dB 43dB
Base station 45dB 50dB

* ACLR values for handsets with power classes of 21dBm and 24dBm.

Synchronisation

The level of synchronisation required for the WCDMA system to operate is provided from the Primary Synchronisation Channel (P-SCH) and the Secondary Synchronisation Channel (S-SCH). These channels are treated in a different manner to the normal channels and as a result they are not spread using the OVSFs and PN codes. Instead they are spread using synchronisation codes. There are two types that are used. The first is called the primary code and is used on the P-SCH, and the second is named a secondary code and is used on the S-SCH.

The primary code is the same for all cells and is a 256 chip sequence that is transmitted during the first 256 chips of each time slot. This allows the UE to synchronise with the base station for the time slot.

Once the UE has gained time slot synchronisation it only knows the start and stop of the time slot, but it does not know information about the particular time slot, or the frame. This is gained using the secondary synchronisation codes.

There is a total of sixteen different secondary synchronisation codes. One code is sent at the beginning of the time slot, i.e. the first 256 chips. It consists of 15 synchronisation codes and there are 64 different scrambling code groups. When received, the UE is able to determine before which synchronisation code the overall frame begins. In this way the UE is able to gain complete synchronisation.

The scrambling codes in the S-SCH also enable the UE to identify which scrambling code is being used and hence it can identify the base station. The scrambling codes are divided into 64 code groups, each having eight codes. This means that after achieving frame synchronisation, the UE only has a choice of one in eight codes and it can therefore try to decode the CPICH channel. Once it has achieved this it is able to read the BCH information and achieve better timing and it is able to monitor the P-CCPCH.

UMTS power control

As with any CDMA system it is essential that the base station receives all the UEs at approximately the same power level. If not, the UEs that are further away will be lower in strength than those closer to the node B and they will not be heard. This effect is often referred to as the near-far effect. To overcome this the node B instructs those stations closer in, to reduce their transmitted power, and those further away to increase theirs. In this way all stations will be received at approximately the same strength.

It is also important for node Bs to control their power levels effectively. As the signals transmitted by the different node Bs are not orthogonal to one another it is possible that signals from different ones will interfere. Accordingly their power is also kept to the minimum required by the UEs being served.

To achieve the power control there are two techniques that are employed: open loop; and closed loop.

Open loop techniques are used during the initial access before communication between the UE and node B has been fully established. It simply operates by making a measurement of the received signal strength and thereby estimating the transmitter power required. As the transmit and receive frequencies are different, the path losses in either direction will be different and therefore this method cannot be any more than a good estimate.

Once the UE has accessed the system and is in communication with the node B, closed loop techniques are used. A measurement of the signal strength is taken in each time slot. As a result of this a power control bit is sent requesting the power to be stepped up or down. This process is undertaken on both the up and downlinks. The fact that only one bit is assigned to power control means that the power will be continually changing. Once it has reached approximately the right level then it would step up and then down by one level. In practice the position of the mobile would change, or the path would change as a result of other movements and this would cause the signal level to move, so the continual change is not a problem.

By Ian Poole


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