The communication between UE and the Node B takes place via air interface. WCDMA is one of the air interface for UMTS networks. It is totally different from the air interface (TDMA) used in GSM. The traffic can vary from 8Kbps to 2 Mbps data and can be either circuit switched or packet switched. The UMTS air interface (WCDMA) has certain key features, which are listed in Table 1.
- UMTS air interface is based on DS CDMA, that is, user data bits are multiplied with a spreading sequence (consists of random bits called chips) and chip rate is much higher than symbol rate or data rate. This process spreads the user data signal to wider frequency band. The ratio of chip rate to data rate is called as spreading factor or processing gain. This spreading is used for distinguishing between different UEs, and also between UEs and some control channels.
- In addition, WCDMA also shows a hierarchical time slot structure similar to GSM: the time axis is divided into frames of 10 ms, each of which is subdivided into 15 slots of 0.667 ms. A radio frame consists of 38400 chips and each time slot consists of 2560 chips. A super frame has 72 frames of total 720ms.
- The chip rate of 3.84 Mcps (Mega chips per second) leads to a carrier bandwidth of approximately 5 MHz and spreading factors are in the range of 4 - 512.
- The configuration of frames and time slots is different for uplink and downlink as illustrated in Figure 8.
- Time slots in WCDMA are not used for user separation but also to support periodic function. For example in downlink the dedicated physical data channel (DPDCH) is time multiplexed and in uplink dedicated physical data channels are I/Q (i.e., DPDCH on I branch and DPCCH on Q-branch) and code multiplexed.(DPDCH and DPCCH Channels are explained in later section).
- The frame and slot structure is illustrated in Figure 8.
|1||Multiple Access method||DS CDMA|
|2||Frequency Spectrum||uplink : 1920 MHz - 1980 MHz
downlink : 2110 MHz - 2170 MHz
|3||Channel Spacing/ Separation / Bandwidth||5 MHz|
|5||Chip Rate||3.84 Mcps|
|6||Frame Length||10 ms with 15 time slots|
|7||Service multiplexing||Multiple Services with different QoS Requirements Multiplexed on one Connection|
|8||Coding technique||Orthogonal Variable Spreading Factor (OVSF)|
|9||Service type||Multirate and multiservice|
|10||Spreading modulation||Balanced QPSK (downlink), Dual-channel QPSK(uplink) , Complex spreading circuit|
|11||Data modulation||QPSK (downlink), BPSK (uplink)|
|12||Multiuser Detection, Smart Antennas||Supported by Standard, Optional in Implementation|