UMTS / W-CDMA tutorial [5] - data, coding and handover

- packet data, speech coding and handover used within UMTS or Wideband CDMA cellular telecommunications system

This tutorial is in five pages dealing which address the different topics:
Introduction to UMTS / WCDMA [1]
The architecture of the UMTS / WCDMA elements [2]
The UMTS / WCDMA radio or air interface [3]
The UMTS / WCDMA channels [4]
The UMTS / WCDMA data coding and handover [5]

This final page of the UMTS / WCDMA tutorial looks at three elements of the system, namely the way packet data is carried, the way speech coding is accomplished and handover, including hard, soft and softer handover.

Packet data

Packet data is an increasingly important element within mobile phone applications. WCDMA is able to carry data in this format in two ways. The first is for short data packets to be appended directly to a random access burst. This method is called common channel packet transmission and it is used for short infrequent packets. It is preferable to transmit short packets in this manner because the link maintenance needed for a dedicated channel would lead to an unacceptable overhead. Additionally the delay in setting up a packet data channel and transferring the operational mode to this format is avoided.

Larger or more frequent packets have to be transmitted on a dedicated channel. A large single packet is transmitted using a single-packet scheme where the dedicated channel is released immediately after the packet has been transmitted. In a multipacket scheme the dedicated channel is maintained by transmitting power control and synchronization information between subsequent packets.

Speech coding

Speech coding in UMTS uses a variety of source rates. As a result, a variety of vocoders are employed including the GSM EFR vocoder. When a variety of rates are available, a system known as Adaptive Multi-Rate (AMR) may be employed where rate is chosen according to the system capacity and requirements. This scheme is the same as that used on GSM. The actual vocoder that is chosen is governed by the system.

The speech coding process can be combined with a voice activity detector. This is particularly useful because during normal conversations there are long periods of inactivity. In the same way that discontinuous transmission is applied to GSM, the same is also true for UMTS. It employs the same technique of inserting background noise when there is no speech as when the discontinuous transmission cuts out the transmission no background noise would otherwise be heard and this can be very disconcerting for the listener.

Discontinuous reception

One of the big issues with mobile phones in general is that of battery life. It is one of the key differentiators that people take into account when buying a phone and this gives a measure of its importance. Taking this into consideration when developing the UMTS / WCDMA standard a discontinuous reception or sleep mode was introduced. This mode allows several non-essential segments of the phone circuitry to power down during periods when paging messages will not be received.

To enable this facility to be introduced into the UMTS UE circuitry the paging channel is divided into groups or subchannels. The actual number of the paging subchannel to be used by a particular UE is assigned by the network. In this way the UE only has to listen for part of the time. To achieve this the Paging Indicator Channel (PICH) is split into 10 ms frames, each of which comprises 300 bits - 288 for paging data and 12 idle bits. At the beginning of each paging channel frame there is a Paging Indicator (PI) that identifies the paging group being transmitted. By synchronising with the paging channels being transmitted it is able to turn the receiver on only when it needs to monitor the paging channel. As the receiver, with its RF circuitry, will consume power, savings can be made by switching it off.

Handover

Within UMTS, handover follows many of the similar concepts to those used for other CDMA systems. There are three basic types of handover: hard, soft and softer. All three types are used but under different circumstances.

Hard handover is like that used for the previous generations of systems. Here, as the UE moves out of range of one node B, the call has to be handed over to another frequency channel. In this instance simultaneous reception of both channels is not possible and there must be a physical break.

Soft handover is a technique that was not available on the previous generations of mobile phone systems. With CDMA systems it is possible to have adjacent cell sites on the same frequency, and as a result it is possible for the UE to receive the signals from two adjacent cells at once, and they are also able to receive the signals from the UE. When this occurs and handover is affected it is known as soft handover.

The decisions about handover are generally handled by the RNC. It continually monitors information regarding the signals being received by both the UE and node B and when a particular link has fallen below a given level and another better radio channel is available, it initiates a handover. As part of this monitoring process, the UE measures the Received Signal Code Power (RSCP) and Received Signal Strength Indicator (RSSI) and the information is then returned to the node B and hence to the RNC on the uplink control channel.

If a hard handover is required then the RNC will instruct the UE to adopt a compressed mode, allowing short time intervals in which the UE is able to measure the channel quality of other radio channels.

Further pages from this tutorial
Previous
Page:   (1) >> (2) >> (3) >> (4) >> (5)
Return to Cellular Telecoms Index page