UMTS / WCDMA radio air interface
- the air interface, frequencies and power control used within UMTS or Wideband CDMA, WCDMA, cellular telecommunications system
UMTS WCDMA tutorial includes:• UMTS WCDMA Tutorial
• UMTS 3G history
• UMTS WCDMA network architecture
• UMTS / WCDMA radio or air interface
• CDMA multiple access technology
• UMTS / WCDMA modulation schemes
• UMTS WCDMA channels
• UMTS TDD
• UMTS WCDMA handover / handoff
Physical layer within UMTS / WCDMA 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 Uplink and Downlink
When looking at the radio air interface and its associated properties, it is necessary to define the directions in which the transmissions are occurring. Being a full duplex system, i.e. transmitting simultaneously in both directions, it is necessary to be able to define which direction is which.
- Uplink; This may also sometimes be known as the reverse link, and it is the link from the User Equipment (UE) to the Node B or base station.
- Downlink; This may also sometimes be known as the forward link, and it is the link from the Node B or base station to the User Equipment (UE).
The terms Uplink and Downlink are the terms that are used with UMTS, and especially within Europe. The terms forward link and reverse link are more commonly used with the CDMA2000 technologies and also within North America.
There are currently six bands that are specified for use for UMTS / WCDMA although operation on other frequencies is not precluded. However much of the focus for UMTS is currently on frequency allocations around 2 GHz. At the World Administrative radio Conference in 1992, the bands 1885 - 2025 and 2110 - 2200 MHz were set aside for use on a world wide basis by administrations wishing to implement International Mobile Telecommunications-2000 (IMT-2000). The aim was that allocating spectrum on a world wide basis would facilitate easy roaming for UMTS / WCDMA users.
Within these bands the portions have been reserved for different uses:
- 1920-1980 and 2110-2170 MHz Frequency Division Duplex (FDD, W-CDMA) Paired uplink and downlink, channel spacing is 5 MHz and raster is 200 kHz. An Operator needs 3 - 4 channels (2x15 MHz or 2x20 MHz) to be able to build a high-speed, high-capacity network.
- 1900-1920 and 2010-2025 MHz Time Division Duplex (TDD, TD/CDMA) Unpaired, channel spacing is 5 MHz and raster is 200 kHz. Transmit and receive transmissions are not separated in frequency.
- 1980-2010 and 2170-2200 MHz Satellite uplink and downlink.
UMTS carrier frequencies are designated by a UTRA Absolute Radio Frequency Channel Number (UARFCN). This can be calculated from:
UARFCN = 5 x (frequency in MHz)
UMTS uses wideband CDMA as the radio transport mechanism. The UMTS channels are spaced by 5 MHz.
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.
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