Remaining briefly [Under UMTS]
» RRC -
25.331 «
» Phy
procedures (FDD) - 25.214 «
»
Multiplexing and Channel coding (FDD) - 25.212 «
»
UE radio transmission and reception (FDD) - 25.101 «
» Phy
Measurements (FDD) - 25.215 «
» AMR speech codec
general description - 26.071 «
» HSDPA
overall description - 25.308 «
Articles till now have given fair idea about how UMTS has been designed and how it works in overall way. There are few important topics that still need to be looked at. We will take a quick look at them.
1) Radio access challenges (like fading, shadowing, multipath propagation, near-far effect etc.) and their mitigation.
In field, signal transmitted by sender reach receiver over multiple paths due reflections and also subjected to various interferences. The resultant effects are: fading, path loss, noise etc. The important techniques used to mitigate these impairments are diversity techniques and power control.
Diversity techniques:
Frequency diversity: Multiple or wide frequencies are used to transmit so that only smaller percentage of transmission suffer loss. W-CDMA due to its wideband nature inherently has frequency diversity.
Time diversity: A good example of time diversity technique is Interleaving, wherein parts of data frames are transmitted at different times so that with appropriate coding techniques lost part of data can be detected and recovered.
Multipath diversity: Due to signal reflections, the signal may be received at receiver at different times. Multipath diversity techniques makes use of these received transmissions to reconstruct the signals. Rake receiver is special receiver used in CDMA for this purpose. Another related technique is to use multiple antennas (separated in space), also known as space diversity.
2) Importance of power control and related techniques
In general, power control is important in wireless communications to take care of near-far effect (user near to base station need lesser power to transmit and receive than user far) and to maintain quality of transmission (radio conditions change all the time and so different powers levels are required at different times based on radio conditions). In CDMA world, this has more importance as transmissions occupy same time and frequency domain. Accuracy and speed of power control is important here.
In UMTS, power control is divided between RRC and PHY. RRC perform open loop power control and PHY perform inner loop or closed loop power control. RRC determine target SIR (Signal to Interference Ratio) which then controlled to the most possible accuracy by feedback mechanism by PHY. UMTS slot structure allow power control at the rate of 1,500 per second.
Note that different channels, depending on their use, may have different ways for power control. Let us take an example of DPDCH/DPCCH. PHY uses Pilot and TPC (Transmit Power Control) symbols of DPCCH for inner loop power control. Note that uplink and downlink require separate loops for power control as radio conditions differ in uplink and downlink. Receiver PHY checks pilot symbols and estimates SIR. Depending on difference between this SIR and target SIR, receiver informs sender to increase or decrease power through next TPC symbol that is to be sent to sender.
3) Paging procedure
Similar to GSM, UMTS also use concept of paging groups. UEs are divided in groups and UEs listen to only those paging "slots" assigned to group. This way UE can "sleep" between listening to paging periods (assigned to its group) instead of listening to paging at all time. This saves UE power and enhances battery duration.
GSM has physical channel of type paging (PCH). UMTS does one more addition: it has additional channel PICH. PICH broadcasts paging indicators (PIs). If paging is indicated, UE (in fact all UEs in group), listen to corresponding slot in PCH (S-CCPCH). Yes, PICH is ahead of S-CCPCH. Paging message has RNTI for UE identification.
4) AMR speech
AMR stand for Adaptive Multi-Rate. AMR speech codec allow speech coding rate be changed per 20 ms frame during voice call based on radio conditions. UMTS specify 8 possible rates (or modes).
AMR also support silence frame. Silence frame is used when there is no speech activity (pauses between words/sentences etc.). This saves radio resources, reducing interference to other users of radio. Silence frame contain "comfort noise" parameters. Comfort noise is that minor background noise - generated by UE locally - so that UE user do not feel that connection is broken during silence periods.
5) Measurements and Handover handling
During connected mode, RRC carry out measurements as per state. In states other than CELL_DCH, UE RRC does the cell re-selection based on measurements. In CELL_DCH (dedicated resources allocated), UE need to perform handover to select best cell. Similar to GSM, handover has three stages. First is doing the measurements and reporting by UE to RNC. Second stage is RNC taking a HO (and type of HO) decision. Third and last stage is executing HO.
UMTS support both soft and hard HOs. Soft HO is always intra-frequency HO. In soft HO. UE maintain connection with more than one cell (having same frequency carrier).
On network side, RNC with which UE does RRC connection establishment is known as Serving RNC (SRNC). Drift RNC (DRNC) is one which has active connection with UE. DRNC is possible only when DRNC has Iur connection with SRNC. DRNC act as relay between UE and SRNC. When new RNC (to which HO to be done) does not have Iur connection with SRNC, new RNC should made SRNC. This is called SRNS relocation.
6) HSDPA
Release 5 has introduced a important feature of HSDPA. We can take quick look at it to complete our understanding of release 5.
HSDPA (High Speed Downlink Packet Access) is meant to offer downlink (only) data rates of upto 10 Mbps. As it is meant only for downlink direction, HSDPA is perfect for services requiring high download data rates like streaming video on demand, FTP etc.
The principle of HSDPA is to have
1) Several downlink physical shared channels, both time and code multiplexed with constant TTI of 2ms. The channels are known as HS-PDSCHs - High Speed-Physical Downlink Shared CHannels. These channels map to one transport channel: HS-DSCH - High Speed-Downlink Shared CHannel.
2) Associated downlink control channels to carry information (usage, allocation etc. of related code and time slot of HS-PDSCH). H-RNTI is used for UE identification; H-RNTI stand for HS-DSCH RNTI. Control channels are known as HS-SCCH - High Speed-Shared Control CHannel.
3) Uplink control channel to send acknowledgements (and signal quality indication) for downlink data. The channels is known as HS-DPCCH (High Speed-Dedicated Physical Control CHannel). Note that this dedicated channel and will always have corresponding DPCCH.
For HS-DPSCHs, rather than fast power control, adaptive modulation and coding (AMC) is used. So based on signal quality indication received over HS-DPCCH, network can change modulation (QPSK, 16QAM) and/or coding. H-ARQ schemes are used for retransmissions along with AMC, making the system dynamic and efficient.
References: UMTS by Sanchez and Thioune, and WCDMA for UMTS by Holma and Toskala.
| Copyright © Samir Amberkar 2010 | § |
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