Showing posts with label WCDMA. Show all posts
Showing posts with label WCDMA. Show all posts

Multimedia Broadcast Multicast Service, MBMS


Multimedia Broadcast Multicast Service, MBMS

A new service introduced in 3GPP Release 6 specifications is Multimedia Broadcast Multicast Service (MBMS). There are two high level modes of operation in MBMS, as given

 

1.       Broadcast mode, which allows sending audio and video. The already existing Cell Broadcast Service (CBS) is intended for messaging only. The broadcast mode is expected to be a service without charging and there are no specific activation requirements for this mode.

2.        Multicast mode allows sending multimedia data for the end users that are part of a multicast subscription group. End users need to monitor service announcements regarding service availability, and then they can join the currently active service. From the network point of view, the same content can be provided in a point-to-point fashion if there are not enough users to justify the high power transmission. A typical example in 3GPP has been the sport results service where, for example, ice hockey results would be available as well as video clips of the key events in different games of the day. Charging is expected to be applied for the multicast mode.

 

From the radio point of view, MBMS is considered an application independent way to deliver the MBMS User Services, which are intended to deliver to multiple users simultaneously. The MBMS User Services can be classified into three groups as follows

1. Streaming services, where a basic example is audio and video stream;

2. File downloads services;

3. Carousel service, which can be considered as a combination of streaming and file download. In this kind of service, an end user may have an application which is provided data repetitively and updates are then broadcast when there are changes in the content.

 

For MBMS User Services, an operator controls the distribution of the data. Unlike CBS, the end user needs first to join the service and only users that have joined the service can see the content. The charging can then be based on the subscription or based on the keys which enable an end user to access the data. The MBMS content can be created by the operator itself or by a third party and, as such, all the details of what an MBMS service should look like will not be specified by 3GPP, but left for operators and service providers. One possible MBMS high level architecture is shown in Figure, where the IP multicast network refers here to any server providing MBMS content over the Internet.
 
 
 
 

 

Push-to-Talk over Cellular (PoC)

Push-to-talk over cellular (PoC) service is instant in the sense that the voice connection is established by simply pushing a single button and the receiving user hears the speech without even having to answer the call. While ordinary voice is bi-directional, the PoC service is a one directional service. The basic PoC application may hence be described as a walkie-talkie application over the packet switched domain of the cellular network. In addition to the basic voice communication functionality, the PoC application provides the end user with complementary features like, for example:

      ·         Ad hoc and predefined communication groups;

·         Access control so that a user may define who is allowed to make calls to him/her;

·         ‘Do-not-disturb’ in case immediate reception of audio is not desirable.

With ordinary voice calls a bi-directional communication channel is reserved between the end users throughout the duration of the call. In PoC, the channel is only set up to transfer a short speech burst from one to possibly multiple users. Once this speech burst has been transferred, the packet switched communication channel can be released. This difference is highlighted in Figure.




 
 
The speech packets are in the PoC solution carried from the sending mobile station to the server by the OPRS/UMTS network. The server then forwards the packets to the receiving mobile stations. In the case of a one-to-many connection, the server multiplies the packets to all the receiving mobile stations. This is illustrated in Figure  The PoC service is independent of the underlying radio access network.

 

 
 


Differences between WCDMA and Second Generation Air Interfaces

Main differences between the third and second generation air interfaces are described. GSM and IS-95 (the standard for cdmaOne systems) are the second generation air interfaces considered here. Other second generation air interfaces are PDC in Japan and US-TDMA mainly in the Americas; these are based on TDMA (time division multiple access) and have more similarities with GSM than with IS-95. The second generation systems were built mainly to provide speech services in macro cells. To understand the background to the differences between second and third generation systems, we need to look at the new requirements of the third generation systems which are listed below:

       ·         Bit rates up to 2 Mbps;

·         Variable bit rate to offer bandwidth on demand;

·         Multiplexing of services with different quality requirements on a single connection, e.g. speech, video and packet data;

·         Delay requirements from delay-sensitive real time traffic to flexible best-effort packet data;

·         Quality requirements from 10 % frame error rate to 10_6 bit error rate;

·         Co-existence of second and third generation systems and inter-system handovers for coverage enhancements and load balancing;

·         Support of asymmetric uplink and downlink traffic, e.g. web browsing causes more loading to downlink than to uplink;

·         High spectrum efficiency;

·         Co-existence of FDD and TDD modes.


GSM also covers services and core network aspects, and this GSM platform will be used together with the WCDMA air interface: see the next section regarding core networks.
 
 
 
 
 
 

Link Budget in WCDMA / UMTS

Link budget planning is part of the network planning process, which helps to dimension the required coverage, capacity and quality of service requirement in the network. UMTS WCDMA macro cell coverage is uplink limited, because mobiles power level is limited to (voice terminal 125mW). Downlink direction limits the available capacity of the cell, as BTS transmission power (typically 20-40W) has to be divided to all users. In a network environment both coverage and capacity are interlinked by interference. So by improving one side of the equation would decrease the other side. System is loosely balanced by design. The object of the link budget design is to calculate maximum cell size under given criteria:

Type of service (data type and speed)
Type of environment (terrain, building penetration)
Behavior and type of mobile (speed, max power level)
System configuration (BTS antennas, BTS power, cable losses, handover gain)
Required coverage probability

What is HARQ and explain cell breathing.

Hybrid automatic repeat-request(HARQ):Data and voice blocks are transmitted in 10 ms time-intervals called frames. At each frame and for each transmission the SIR is evaluated and used to derive the Block Error Rate (BLER).  Each block is then considered correct or erroneous
according to a random experiment based on the BLER. For the packet switched traffic only, an ideal ARQ procedure is adopted, i.e., the transmitted block is kept in the transmitting queue in
case of error and is cancelled otherwise

Cell breathing is the constant change of the range of the geographical area covered by a cellular telephone transmitter based on the amount of traffic currently using that transmitter. When a cell becomes heavily loaded, it shrinks.

How many codes are required to get 7.2Mbps in HSDPA?

5 codes and qpsk= 1.8mbps;
5 codes and 16 qam= 3.6 mbps;
10 codes and 16 qam= 7.2 mbps;
15 codes and 16 qam= 14.4;
15 codes and 64 qam= 21.1mbps;
15 codes and 16 qam(mimo)=28mbps;
16 codes and 64 qam(mimo)=42.2mbps



WCDMA System message

SIB1 : NAS Information. UE Timers and counters to be used in RRC Idle & Connected State,
SIB2: List of URA Identities
SIB3: Parameters for Cell Selection and Reselection
SIB4: Same as SIB3 but used in Connected State
SIB5: Configuration parameters of common physical channels in a cell. PCH and PICH Info (CPCH)
SIB6: Configuration of Common and Shared physical channel
SIB7: Contains fast changing UL interface params and dynamic. As this is changes often so controlled by timer
SIB8: Used in FDD . Static CPCH info of cell. Used in Connected mode only
SIB9: CPCH info. As it changed often, controlled by timers connected mode only.
SIB10 : DRAC Procedure, used when CELL_DCH controlled by timer
SIB11 : Measurement control information to be used in CELL
SIB12: Same as SIB11 but used in connected mode only
SIB13: For ANSI-41 . It also has 4 associated SIBS 13.1 to 13.4. Reference to subblocks. Used when System is ANSI-41.
SIB14: Parameters for common and dedicated physical DPCH UL outer loop power control info for TDD
SIB15
: Assistance info for UE positioning. Used to reduce signaling by position. 15.1 to 15.5 sub sibs.
SIB16: Predefined channel conf. used while hand over. Radio Bearer transport channel, physical channel parameter to be stored by UE in idle/connected mode. Several occurrences but UE does not bother.
SIB17: Shared channel info for TDD only
SIB18: PLMN Identities of neighboring cells. Used in Shared Access N/w with the cell reselection process


WCDMA Capacity Management Parameter

compModeAdm: Absolute admission limit for the number of radio links in compressed mode in a cell.
dlCodeAdm: Parameter that defines in percentage the absolute admission limit for DL code usage
pwrAdm: Parameter that defines in percentage the absolute admission limit for DL power utilization.
sf8Adm: Defines the absolute admission limit for the number of RLs with SF=8 (PS384) in DL.
sf16Adm: Defines the absolute admission limit for the number of RLs with SF=16 (PS128 RAB) in DL.
sf32Adm: Defines the absolute admission limit for the number of RLs with SF=32 (PS64) in DL.
sf4AdmUl: Absolute admission limit for the number of RLs with SF=4 in UL (PS384/HS)
sf8AdmUl: Defines the absolute admission limit for the number of RLs with SF=8 in UL.

sf16AdmUL: Parameter that defines absolute admission limit for the number of RLs with SF=16 in UL.

RF Optimization Processes


Network Optimization process involves the following activities:


  • FIRST SET THE CRITERION (GOAL) OF OPTIMIZATION PROCESS
    • BASELINE & TARGET KPI’s.
    • DELIVERABLES
  • CONDUCTING A BASELINE PHYSICAL AUDIT
  • REMOVING ALL SERVICE AFFECTING ALARMS
  • IDENTIFYING POOR COVERAGE AREAS
  • IDENTIFYING CAPACITY CONSTRAINTS & OVERUTILIZED CELLS
  • VARIOUS KPIs with Root-Cause-Analysis of problems.
    • Frequency Plan (BCCH & TCH)
    • Neighbor plan
  • CONDUCTING A GSM SYSTEM PARAMETERS AUDIT
  • Deliverables of an Optimization activity:
    • Baseline Drive test comparison with post implementation results.
    • Statistical comparison of baseline & improved network.
    • Parameter Audit report.
    • Physical parameter inconsistencies.
    • Frequency & neighbor plan inconsistencies
    • Recommendations for
      • Coverage
      • Capacity
      • Physical Optimization
      • Location Area Optimization.

WCMDA Paging

Paging process in WCMDA or HSDPA system is called WCMDA Paging. The Paging Channel (PCH) is a downlink transport channel. The PCH is always transmitted over the entire cell. The transmission of the PCH is associated with the transmission of physical-layer generated Paging Indicators, to support efficient sleep-mode procedures.

Paging Channel selection
System information Block Type 5 (SIB 5) defines common channels to be employed in Idle mode. The UE may use Discontinuous Reception (DRX) in idle mode in order to reduce power consumption. When DRX is used the UE needs only to monitor one Page Indicator, PI, in one Paging Occasion per DRX cycle.
The Paging Indicator Channel (PICH) is a fixed rate (SF=256) physical channel used to carry the paging indicators.
Paging Procedure:

Basically two types of Paging Procedures in WCMDA Paging.

1) Idle Mode Paging
2) Dedicated Mode Paging
Idle mode paging is used when UE is in idle mode so we can say UE is in Cell PCH or URA PCH.PAGING TYPE 1 message on an appropriate paging occasion on the PCCH.

UE dedicated paging procedure is used to transmit dedicated paging information to one UE in connected mode in CELL_DCH or CELL_FACH state. PAGING TYPE 2 message on the DCCH using AM RLC.

WCDMA Question for Interview

1.         What is the WCDMA technology?
2.         What is the different between WCDMA & GSM?
3.         What is the different between CDMA & WCDMA?
4.         What is architecture & Interface of WCDMA?
5.         What are the channel concepts of WCDMA Network?
6.         What are the Logical, transport & physical channel in WCDMA?
7.         What is the channel mapping in WCDMA?
8.         What is the cell search procedure?
9.         What are the RRC and what are the RRC states?
10.       What is location registration?
11.       What is the admission control & congestion control?
12.       What is the power control?
13.       UE TX Power
14.       What are the types of handover?
15.       What is Handover procedure?
16.       What are the handover Parameter?
17.       When Events 2A, 2B, 2C & 2E occur?
18.       When Event 1A-1F events occur?
19.       What are theSIR, RSCP, RSSIand EC/NO?
20.       What is the CPICH?
21.       What is the channelization and scrambling code?
22.       What is the code tree?
23.       What is the spreading factor?
24.       What is the cell breathing?
25.       What is the Pilot Pollution? 
26.       What are the near far effects?
27.       What are the noise rise effects?
28.       What is compressed mode?
29.       What is the processing Gain?
30.       What is the Pole capacity?
31.       What are Rake receiver and WCDMA reception issue?
32.       What is the different between CELL_PCH & URA_PCH?
33.       What are the types of measurement?
34.       What is the paging? What are types of paging and why paging is required?
35.       What is the cell update procedure and what are the various cell update cause?
36.       What is active set?
37.       What is virtual active set?
38.       What is the monitoring & detected cell set?
39.       What are the various types of RNC?
40.       What is the channel concept and channel mapping in WCDMA Network?
41.       What is the SIB?
42.       What are the idle mode and cell search parameter?
43.       What are the admission control & congestion control parameter

Thanks to read Article and Best of luck for your interview.

WCDMA RAN ( Radio Access Network)

WCDMA RAN

WCDMA RAN is a part of the 3rd generation (3G) mobile system, and comprises:
  • OSS-RC
  • RNC
  • RANAG
  • RBS
WCDMA RAN has interfaces towards the Core Network (CN), and towards the external Network Management Systems (NMS).
WCDMA RAN provides Radio Access Bearers (RAB) between the CN and the subscriber's User Equipment (UE) for speech, data, and multimedia services.

The Network Elements (NEs) RNC, RANAG, and RBS provide and manage the data links between WCDMA RAN and the UE. The links between the NEs in WCDMA RAN carry the user data within WCDMA RAN. These physical links are also used to carry O&M data. See figure for an overview of WCDMA RAN. 


Fading


FADING in Telecomunication
          The communication between the base station and mobile station in mobile systems is mostly non-LOS.
          The LOS path between the transmitter and the receiver is affected by terrain and obstructed by buildings and other objects.
          The mobile station is also moving in different directions at different speeds.
          The RF signal from the transmitter is scattered by reflection and diffraction and reaches the receiver through many non-LOS paths.
This non-LOS path causes long-term and short term fluctuations in the form of log-normal fading and rayleigh and rician fading, which degrades the performance of the RF channel

LONG TERM FADING
          Terrain configuration & man made environment causes long-term fading.
          Due to various shadowing and terrain effects the signal level measured on a circle around base station shows some random fluctuations around the mean value of received signal strength.
          The long-term fades in signal strength, r, caused by the terrain configuration and man made environments form a log-normal distribution, i.e the mean received signal strength, r, varies log-normally in dB if the signal strength is measured over a distance of at least 40l.
          Experimentally it has been determined that the standard deviation, s, of the mean received signal strength, r, lies between 8 to 12 dB  with the higher s generally found in large urban areas.

RAYLEIGH FADING
          This phenomenon is due to multipath propagation of the signal.
          The Rayleigh fading is applicable to obstructed propagation paths.
          All the signals are NLOS signals and there is no dominant direct path.
          Signals from all paths have comparable signal strengths.
          The instantaneous received power seen by a moving antenna becomes a random variable depending on the location of the antenna.







RICEAN FADING
          This phenomenon is due to multipath propagation of the signal.
          In this case there is a partially scattered field.
          One dominant signal.
          Others are weaker.






DOPPLERS SHIFT
          Dopplers shift is the shift in frequency due to the motion of mobile from the actual carrier frequency.
          Consider a mobile moving at a constant velocity v along a path segment having a length d between points X and Y while it receives signal from a remote source S.
          The Change in frequency due to dopplers shift is given by
                        fd = (v/l) * cos(f)
          It can be seen from the above equation that if the mobile is moving towards the direction of arrival of wave the dopplers shift is positive I.e. the apparent received frequency is increased. .


Inter-RAT Handover ( IRAT Handover)


The 2G/3G inter-RAT handover involves the handover from GSM to UMTS and the handover from UMTS to GSM. The handover is controlled mainly by the network. For MSs in dedicated mode, inter-RAT handovers can be performed, including the emergency handover, better cell handover, inter-RAT load handover, and inter-RAT service handover.

Inter-RAT Handover from UMTS to GSM
MSs in dedicated mode can be handed over from a UMTS cell to a GSM cell. The handover decision and handover procedure are controlled by the RNC. The BSS considers the incoming handover from UMTS to GSM as a common inter-BSC handover.
 The parameter Inter-RAT In BSC Handover Enable determines whether inter-RAT handover from UMTS to GSM is enabled. If Inter-RAT In BSC Handover Enable is set to No, the BSS rejects all the requests for the handover from UMTS to GSM.

Inter-RAT Handover from GSM to UMTS
The parameter Inter-RAT In BSC Handover Enable determines whether the inter-RAT handover from GSM to UMTS is enabled. If Inter-RAT In BSC Handover Enable is set to NO(No), the BSS rejects all the requests for the handover from GSM to UMTS and does not select a UMTS cell as the target cell.


In dedicated mode, an MS obtains the list of neighboring UMTS cells and other information from the Measurement Information. Then, the MS reports the measurement result to the BSS through the measurement report. After receiving the measurement result, the BSS determines whether to initiate the inter-RAT handover from GSM to UMTS based on the measurement result and the handover algorithm.

HSDPA

High-Speed Downlink Packet Access (HSDPA) is an enhanced 3G (third-generation) mobile telephony communications protocol in the High-Speed Packet Access (HSPA) family, also dubbed 3.5G, 3G+, or Turbo 3G, which allows networks based on Universal Mobile Telecommunications System (UMTS) to have higher data-transfer speeds and capacity. As of 2013 HSDPA deployments can support down-link speeds of up to 42.3 Mbit/s. HSPA+ offers further speed increases, providing speeds of up to 337.5 Mbit/s with Release 11 of the 3GPP standards

Capacity
It is the maximum throughput that the RBS can deliver to one cell. The capacity is shared by all HSDPA users.

System Capacity
It is the average capacity per cell for a cluster of cells. For system capacity calculation it is assumed that the load is homogenously distributed and HSDPA is deployed in all cells.

Dedicated Channel Traffic
DCH traffic is defined as the traffic carried by dedicated transport channels such as speech, PS or CS radio bearers i.e. on channels other than HSDPA.

RBS Load
It is the percentage of the maximum available RBS power that is used in the downlink.

Power Margin
Power margin saves a part of the RBS power to cater for power variations, due to the dynamic UE behavior when users move and experience varying channels conditions.
For HSDPA it is assumed that no power margin is needed and RBS may use 100% of the available power in a system with HSDPA.

Shared Channel Transmission
Shared channel transmission means that a set of radio resources are dynamically shared among multiple users.
The sharing is done in time and code domain

Fast Radio Dependent Scheduling
Scheduling is the function that determines which UE to transmit to at a given time instant.
Three scheduling algorithms are implemented.
1. Proportional Fair Scheduling
2. Round Robin Scheduling
3. Maximum Channel Quality Indicator
Proportional Fair Scheduling
The algorithm uses information about fading peaks to prioritize users with good radio conditions
It also takes delay into account promoting users that have not been given any data for a long time
Round Robin Scheduling
The algorithm gives every user same amount of radio resources (TTI).
The algorithm is fair for all users from a resource point of view but bit rate varies.

Max CQI (Channel Quality Indicator)
UE sends CQI in the UL to aid rate adaptation and scheduling
The algorithm maximizes system throughput by prioritizing users with good radio channels
The CQI report estimates the number of bits that can be transmitted to the UE using a certain assumed power with a block error rate of 10%

High-order Modulation
HS-DSCH uses 16 QAM if the UE category permit.
This allows twice as high data rates to be transmitted as compared to QPSK
2 ms TTI
Transmission Time Interval for HSDPA is short when compared to R99
It is 2 ms for HS-DSCH for R99 it is 10-40 ms

Fast Link Adaptation
As opposed to R99 RBs, HS-DSCH is transmitted with constant power within the TTI.
Transmission rate is controlled by adaptive channel coding.
Data rate depends on radio conditions (CQI)
Fast Hybrid ARQ with soft combining
In hybrid automatic repeat request scheme, the received blocks that cannot be decoded are buffered and soft combined with later received transmissions of same information bits. Hybrid ARQ protocol terminates in Node B which means short RTT (typically 12 ms

HSDPA Power
The RBS power available for HSDPA is determined dynamically, depending on R99 power usage
At least 25% of the average power can be used for HSDPA

HSDPA Channel Structure

New Physical and Transport channels are introduced in HSDPA:

 Transport Channel
High Speed Downlink Shared Channel (HS-DSCH)

Physical Channels
High Speed Physical Downlink Shared Channel (HS-PDSCH)
High Speed Shared Control Channels (HS-SCCH)
High Speed Dedicated Physical Control Channel (HS-DPCH)
Associated Dedicated Channel (A-DCH)