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