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Home » GATE Study Material » Electronics and Telecommunication » Cellular Telecommunications and Cell Phone Technology » UMTS / WCDMA basics tutorial

UMTS / WCDMA basics tutorial

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UMTS / WCDMA basics tutorial

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


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.

 

Frequencies


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.

     

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 channels are spaced by 5 MHz. The modulation that is used is different on the uplink and downlink. The downlink uses quadrature phase shift keying (QPSK) for all transport channels. However the uplink uses two separate channels so that the cycling of the transmitter on and off does not cause interference on the audio lines, a problem that was experienced on GSM. The dual channels (dual channel phase shift keying) are achieved by applying the coded user data to the I or In-phase input to the DQPSK modulator, and control data which has been encoded using a different code to the Q or quadrature input to the modulator.

 

Spreading


The data to be transmitted is encoded using a spreading code particular to a given user. In this way only the desired recipient is able to correlate and decode the signal, all other signals appearing as noise. This allows the physical RF channel to be used by several users simultaneously.

The data of a CDMA signal is multiplied with a chip or spreading code to increase the bandwidth of the signal. For WCDMA, each physical channel is spread with a unique and variable spreading sequence. The overall degree of spreading varies to enable the final signal to fill the required channel bandwidth. As the input data rate may vary from one application to the next, so the degree of spreading needs to be varied accordingly.

For the downlink the transmitted symbol rate is 3.84 M symbols per second. As the form of modulation used is QPSK this enables two bits of information to be transmitted for every symbol, thereby enabling a maximum data rate of twice the symbol rate or 7.68 Mbps. Therefore if the actual rate of the data to be transmitted is 15 kbps then a spreading factor of 512 is required to bring the signal up to the required chip rate for transmission in the required bandwidth. If the data to be carried has a higher data rate then a lower spreading rate is required to balance this out. It is worth remembering that altering the chip rate does alter the processing gain of the overall system and this needs to be accommodated in the signal processing as well. Higher spreading factors are more easily correlated by the receiver and therefore a lower transmit power can be used for the same symbol error rate.

The codes required to spread the signal must be orthogonal if they are to enable multiple users and channels to operate without mutual interference. The codes used in W-CDMA are Orthogonal Variable Spreading Factor (OVSF) codes, and they must remain synchronous to operate. As it is not possible to retain exact synchronisation for this, a second set of scrambling codes is used to ensure that interference does not result. This scrambling code is a pseudo random number (PN) code. Thus there are two stages of spreading. The first using the OSVF code and the second using a scrambling PN code. These codes are used to provide different levels of separation. The OVSF spreading codes are used to identify the user services in the uplink and user channels in the downlink whereas the PN code is used to identify the individual node B or UE.

On the uplink there is a choice of millions of different PN codes. These are processed to include a masked individual code to identify the UE. As a result there are more than sufficient codes to accommodate the number of different UEs likely to access a network. For the downlink a short code is used. There are a total of 512 different codes that can be used, one of which will be assigned to each node B.

 

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.

 

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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