The basic signal format for UMTS is WCDMA, but this can utilise a variety of different forms of modulation.
The modulation used for UMTS WCDMA is dependent upon a variety of factors including spectrum efficiency, signal to noise ratio, error correction and a number of other variable.
WCDMA modulation schemes
There are several considerations that were taken into account when making the choice for the overall format for the UMTS WCDMA modulation formats. Some of the considerations were:
- It is necessary to ensure that the data is carried efficiently over the available spectrum, and therefore maximum use is made of the available spectrum, and hence the capacity of the system is maximised.
- The modulation scheme should be chosen to ensure that the efficiency of the RF power amplifier in the handset or UE is made as high as possible. By enabling the power amplifier to be maximised, less battery power is consumed for the same transmitted power. As battery power is of particular importance to users, this is a key requirement.
- The modulation format should be chosen to avoid the audio interference caused to many nearby electronics equipment resulting from the pulsed transmission format used on many 2G systems such as GSM
As the uplink and downlink have different requirements, the exact format for the modulation format used on either direction is slightly different.
UMTS modulation schemes for both uplink and downlink, although somewhat different are both based around phase shift keying formats. This provides many advantages over other schemes that could be used in terms of spectral efficiency and other requirements.
Note on PSK - Phase Shift Keying:
Phase shift Keying, PSK is a form of modulation used particularly for data transmissions. If offers an effective way of transmitting data. By altering the number of different phase states which can be adopted, the data speeds that can be achieved within a given channel can be increased, but at the cost of lower resilience to noise an interference.
The UMTS modulation format for the downlink is more straightforward than that used in the uplink. The downlink uses quadrature phase shift keying, QPSK.
The QPSK modulation used in the downlink is used with time-multiplexed control and data streams. While time multiplexing would be a problem in the uplink, where the transmission in this format would give rise to interference in local audio systems, this is not relevant for the downlink where the NodeB is sufficiently remote from any local audio related equipment to ensure that interference is not a problem.
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.
The different WCDMA modulation formats for uplink and downlink are required as a result of the different requirements and conditions experienced. In later versions of 3G UMTS, much higher order modulation levels were used to enable the higher data rates to be achieved.
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