UMTS Handover

0 comments Posted by communications at 1:52 AM



There are following categories of handover (also referred to as handoff):

  • Hard Handover

    Hard handover means that all the old radio links in the UE are removed before the new radio links are established. Hard handover can be seamless or non-seamless. Seamless hard handover means that the handover is not perceptible to the user. In practice a handover that requires a change of the carrier frequency (inter-frequency handover) is always performed as hard handover.

  • Soft Handover

    Soft handover means that the radio links are added and removed in a way that the UE always keeps at least one radio link to the UTRAN. Soft handover is performed by means of macro diversity, which refers to the condition that several radio links are active at the same time. Normally soft handover can be used when cells operated on the same frequency are changed.

  • Softer handover

    Softer handover is a special case of soft handover where the radio links that are added and removed belong to the same Node B (i.e. the site of co-located base stations from which several sector-cells are served. In softer handover, macro diversity with maximum ratio combining can be performed in the Node B, whereas generally in soft handover on the downlink, macro diversity with selection combining is applied.

    Generally we can distinguish between intra-cell handover and inter-cell handover. For UMTS the following types of handover are specified:

  • Handover 3G -3G (i.e. between UMTS and other 3G systems)
  • FDD soft/softer handover
  • FDD inter-frequency hard handover
  • FDD/TDD handover (change of cell)
  • TDD/FDD handover (change of cell)
  • TDD/TDD handover
  • Handover 3G - 2G (e.g. handover to GSM)
  • Handover 2G - 3G (e.g. handover from GSM)

    The most obvious cause for performing a handover is that due to its movement a user can be served in another cell more efficiently (like less power emission, less interference). It may however also be performed for other reasons such as system load control.
  • Active Set is defined as the set of Node-Bs the UE is simultaneously connected to (i.e., the UTRA cells currently assigning a downlink DPCH to the UE constitute the active set).
  • Cells, which are not included in the active set, but are included in the CELL_INFO_LIST belong to the Monitored Set.
  • Cells detected by the UE, which are neither in the CELL_INFO_LIST nor in the active set belong to the Detected Set. Reporting of measurements of the detected set is only applicable to intra-frequency measurements made by UEs in CELL_DCH state.

    The different types of air interface measurements are:
  • Intra-frequency measurements: measurements on downlink physical channels at the same frequency as the active set. A measurement object corresponds to one cell.
  • Inter-frequency measurements: measurements on downlink physical channels at frequencies that differ from the frequency of the active set. A measurement object corresponds to one cell.
  • Inter-RAT measurements: measurements on downlink physical channels belonging to another radio access technology than UTRAN, e.g. GSM. A measurement object corresponds to one cell.
  • Traffic volume measurements: measurements on uplink traffic volume. A measurement object corresponds to one cell.
  • Quality measurements: Measurements of downlink quality parameters, e.g. downlink transport block error rate. A measurement object corresponds to one transport channel in case of BLER. A measurement object corresponds to one timeslot in case of SIR (TDD only).
  • UE-internal measurements: Measurements of UE transmission power and UE received signal level.
  • UE positioning measurements: Measurements of UE position.
    The UE supports a number of measurements running in parallel. The UE also supports that each measurement is controlled and reported independently of every other measurement.


    Further reading: 3GPP 25.331

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    Call Setup

    0 comments Posted by communications at 1:52 AM



    call setup diagram
    Basic Mobile Originating Call Diagram



    Further reading: 3GPP TS 25.303, 25.331

     

    Cell search procedure

    0 comments Posted by communications at 1:48 AM

    During the cell search, the UE searches for a cell and determines the downlink scrambling code and frame synchronisation of that cell. The cell search is typically carried out in three steps:

    Step 1: Slot synchronisation

    During the first step of the cell search procedure the UE uses the SCH's primary synchronisation code to acquire slot synchronisation to a cell. This is typically done with a single matched filter (or any similar device) matched to the primary synchronisation code which is common to all cells. The slot timing of the cell can be obtained by detecting peaks in the matched filter output.

    Step 2: Frame synchronisation and code-group identification

    During the second step of the cell search procedure, the UE uses the SCH's secondary synchronisation code to find frame synchronisation and identify the code group of the cell found in the first step. This is done by correlating the received signal with all possible secondary synchronisation code sequences, and identifying the maximum correlation value. Since the cyclic shifts of the sequences are unique the code group as well as the frame synchronisation is determined.

    Step 3: Scrambling-code identification

    During the third and last step of the cell search procedure, the UE determines the exact primary scrambling code used by the found cell. The primary scrambling code is typically identified through symbol-by-symbol correlation over the CPICH with all codes within the code group identified in the second step. After the primary scrambling code has been identified, the Primary CCPCH can be detected and the system- and cell specific BCH information can be read.
    If the UE has received information about which scrambling codes to search for, steps 2 and 3 above can be simplified


    RCC States
    Structure of synchronization channel


    The Synchronisation Channel (SCH) is a downlink signal used for cell search. The SCH consists of two sub channels, the Primary and Secondary SCH. The 10 ms radio frames of the Primary and Secondary SCH are divided into 15 slots, each of length 2560 chips. Picture above illustrates the structure of the SCH radio frame.

    The Primary SCH consists of a modulated code of length 256 chips, the primary synchronization code (PSC) is transmitted once every slot. The PSC is the same for every cell in the system.

    The Secondary SCH consists of repeatedly transmitting a length 15 sequence of modulated codes of length 256 chips, the Secondary Synchronisation Codes (SSC), transmitted in parallel with the Primary SCH. The SSC is denoted csi,k in figure 20, where i = 0, 1, …, 63 is the number of the scrambling code group, and k = 0, 1, …, 14 is the slot number. Each SSC is chosen from a set of 16 different codes of length 256. This sequence on the Secondary SCH indicates which of the code groups the cell's downlink scrambling code belongs to.


    Summary of the process:
    Channel Synchronisation acquired Note
    Primary
    SCH     		Chip, Slot, Symbol
    Synchronisation     		256 chips
    The same in all cells
    Secondary SCH Frame Synchronisation,
    Code Group (one of 64)     		15-code sequence of secondary synchronisation codes.
    There are 16 secondary synchronisation codes.
    There are 64 S-SCH sequences corresponding to the 64 scrambling code groups
    256 chips, different for different cells and slot intervals
    Common Pilot CH Scrambling code
    (one of 8)     		To find the primary scrambling code from common pilot CH
    PCCPCH *) Super Frame Synchronisation,
    BCCH info     		Fixed 30 kbps channel
    27 kbps rate
    spreading factor 256
    SCCPCH **)
    		Carries FACH and PCH channels
    Variable bit rate

    *) Primary Common Control Physical Channel
    **) Secondary Common Control Physical Channel

    Further reading: 3GPP TS 25.211 25.213

     

    HSDPA in W-CDMA

    0 comments Posted by communications at 8:23 PM

    High Speed Downlink Packet Access (HSDPA) is a packet-based data service in W-CDMA downlink with data transmission up to 8-10 Mbps (and 20 Mbps for MIMO systems) over a 5MHz bandwidth in WCDMA downlink. HSDPA implementations includes Adaptive Modulation and Coding (AMC), Multiple-Input Multiple-Output (MIMO), Hybrid Automatic Request (HARQ), fast cell search, and advanced receiver design.

    In 3rd generation partnership project (3GPP) standards, Release 4 specifications provide efficient IP support enabling provision of services through an all-IP core network and Release 5 specifications focus on HSDPA to provide data rates up to approximately 10 Mbps to support packet-based multimedia services. MIMO systems are the work item in Release 6 specifications, which will support even higher data transmission rates up to 20 Mbps. HSDPA is evolved from and backward compatible with Release 99 WCDMA systems.


    Currently (2002) 3GPP is undertaking a feasibility study on high-speed downlink packet access.

    HSPDA Comparison

    HSPDA and CDMA2000 1xEV-DV Comparison


    Further reading:


    3GPP TS 25.855 High Speed Downlink Packet Access (HSDPA); Overall UTRAN description
    3GPP TS 25.856 High Speed Downlink Packet Access (HSDPA); Layer 2 and 3 aspects
    3GPP TS 25.876 Multiple-Input Multiple-Output Antenna Processing for HSDPA
    3GPP TS 25.877 High Speed Downlink Packet Access (HSDPA) - Iub/Iur Protocol Aspects
    3GPP TS 25.890 High Speed Downlink Packet Access (HSDPA); User Equipment (UE) radio transmission and reception (FDD)

     

    3G and LAN Date Speed

    0 comments Posted by communications at 8:21 PM

    Here are the theoretical maximum data speeds of 2G, 2.5G, 3G and beyond, and compared to LAN data speeds.

    UMTS Frequencies

    Data Speed of Mobile Systems (top) and LANs (bottom)

     

    WCDMA Spreading

    0 comments Posted by communications at 1:03 AM


    TDD WCDMA uses spreading factors 4 - 512 to spread the base band data over ~5MHz band. Spreading factor in dBs indicates the process gain. Spreading factor 128 = 21 dB process gain). Interference margin is calculated from that:

    Interference Margin = Process Gain - (Required SNR + System Losses)

  • Required Signal to Noise Ration is typically about 5 dB
  • System losses are defined as losses in receiver path. System losses are typically 4 - 6 dBs
    UMTS Time slot

    Overview of Spreading Process

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    WiMAX Forum RF Network Engineer Certification Training Material Chapter 3, Antennas for WiMAX

    0 comments Posted by communications at 7:37 PM

    After the first and second chapter of my self learning, here we go the third chapter :) titled Antennas for WiMAX. Just like traditional Wireless System, there are 3 kinds of antenna used for transceiving Radio Access Signal i.e., Omnidirectional, Sectoral, and Point to Point. The use of those antenna depends on it’s application.

    1. Omnidirectional, broadcast 360 degree suitable for large area rural environment.

    2. Sectoral, broadcast on intended area: 60, 90, and 120 degree. This kind of antenna is suitable for urban environment which has lage population on a certain area.

    3. Point to Point Antenna is used for point to point communication, i.e., Microwave link, backbone etc.

    (pictures are filched from wimax.com). If you need to read WiMAX Application material, please visit this my previous post, here

    After we have discussed about those traditional antennas, now we are going to discuss advanced antenna technology in WiMAX.

    1. Antenna Diversity,

    Using two or more receiving antennas on a wireless device to eliminate multipath signal distortion. Typically, the signal from the antenna with the least noise (best SNR) is chosen, and the other antenna is ignored.

    There are three techniques in diversity scheme:

    1. Space Time Coding (Alamouti Code), is a method employed to improve the reliability of data transmission in wireless communication systems using multiple transmit antennas. STCs rely on transmitting multiple, redundantcopies of a data stream to the receiver in the hope that at least some of them may survive the physical pathbetween transmission and reception in a good enough state to allow reliable decoding. (resource: http://en.wikipedia.org/wiki/Space–time_code) Space time codes may be split into two main types:

    2. Antenna Switching, is a simple approach for capturing diversity gains. The purpose is not to combine signals from the multiple antennas available but simply to select the single antenna with the best channel gain at any given time. It is applicable to both DL and UL transmission.
    3. Maximum Ratio Combining (MRC), is a processing technique that estimates channel characteristics for multiple antennas and then apply weights to each antenna to maximize signal to noise ratio for the summed signal. MRC achieves diversity and array gain but does not involve active interference mitigation or spatial multiplexing in any way.
    Beside Antenna Diversity, there are other technique in WiMAX Antenna, i.e., MIMO and SAS. We are going to discuss it below.
    MIMO (Multiple Input Multiple Output)
    Multiple Tx/Rx chains and antennas in the base stations is now a well-established technique. Technological advances in high scale integration are making multiple Tx/Rx chains and antennas also economically viable for the mobile and stationary subscriber stations. The WiMAX Forum Mobility Task Group (MTG) defined profile specifies two MIMO versions called Matrix A MIMO and Matrix B MIMO. The IEEE 802.16 standard defines other MIMO classes, for example Matrix C MIMO, which may be adopted by the WiMAX Forum in future profiles. In this post we are going to disucuss Matrix A MIMO and Matrix B MIMO.
    Matrix A MIMO
    Matrix A MIMO implements the rate 1 Space-Time Coding scheme (commonly known as the Alamouti Code). This technique captures diversity gains by sending a single data stream in two parts out of two antennas, interleaved with transformed/conjugated versions of the same information, so that the receiver has
    higher probability of successfully extracting the desired signal. Matrix A achieves a spatial diversity order of two, but does not set out to achieve combining, interference mitigation, or spatial multiplexing gains.
    Matrix A MIMO delivers higher link robustness, reducing fade margin by 5 to 6 dB, with little or no degradation as subscriber mobility increases. The impact on end-user data rate is small; reduced fade margin may allow the use of marginally higher order of modulation, but it is not comparable to the 2x throughput gain achieved by comparable Matrix B MIMO through spatial multiplexing. Matrix A MIMO is useful in networks with light loading and relatively high subscriber mobility.
    We will now look at how Matrix A MIMO works in a little more detail.
    Consider a Matrix A MIMO system, consisting of 2 Transmit and 2 Receive antennas as depicted below.
    The signal received by one of the antennas at the receiver is a mixture of the signals transmitted from both of the transmit antennas. The receive signals can be expressed by the following simplified equations:
    Rx1(f) = (H1,1(f) x Tx1) + (H2,1(f) x Tx2)
    Rx2(f) = (H1,2(f) x Tx1) + (H2,2(f) x Tx2)
    The receiver sees a combination of the transmissions from the two transmit antennas and needs to recover the actual transmitted signals. MIMO systems achieve this by using coding schemes that define which signal should be transmitted and when in order to make it possible to recover the original signal. These coding schemes are called ‘Space-Time’ codes because they define a code across both space (antenna
    separation) and time (symbols).
    Matrix A MIMO is a Space-Time Block Code, so called because the code operates over a block of data. Block codes require less processing power to decode than convolutional codes.
    The following matrix defines how the code works:
    X is the output of the encoder and S1 and S2 are the input symbols into the encoder. ‘*’ denotes a complex conjugate of the symbol. The rows of the matrix represent the transmit antennas and the columns represent time. Each element of the matrix indicates which symbol is to be transmitted from which antenna and when.
    Picture below will guide us on how this matrix works.
    On the left hand side the binary bits enter a modulator, which converts binary bits into “symbols” according to the modulation scheme. These complex symbols are then fed into the Encoder, which maps the symbols onto the transmit antennas according to the matrix above.
    The code works with a pair of symbols at a time and it takes two time periods to transmit the two symbols. Therefore it has the same rate as the data stream that enters the encoder but the error performance of the system is improved due to the coded information transmitted by the system.
    In systems with high SNR performance, the improvement in the error rate achieved as a result of using Space-Time codes could be traded for higher capacity by using a higher order modulation than would otherwise be the case, resulting in marginal increases in throughput.
    Matrix B MIMO
    For channels with a rich multipath environment it is possible to increase the data rate by transmitting separate information streams on each antenna in the DL direction. Using sophisticated receiver technology, the different streams can be separated and decoded. For example, using 2 transmit and 2 receive Tx/Rx chains (and the associated antennas), up to twice the capacity of a single antenna system can be
    achieved. This is particularly useful in urban deployments where long reach is less important than high data rate at the end user device. In WiMAX, spatial multiplexing on the downlink is made possible using the Matrix B MIMO.
    The following matrix defines how the code works:
    X is the output of the encoder and S1, S2 are the input symbols into the encoder. The row of the matrix represent the transmit antennas; there is no time element because Matrix B MIMO operates over a single time interval. Each element of the matrix indicates which symbol is to be transmitted from which antenna. In this
    system 2 symbols are transmitted in a 1-symbol time duration thus providing a twofold capacity increase.
    The picture below will exlpain us about the matrix operational on the tx module.
    The theoretical upper band of capacity increases achieved by Matrix B MIMO is roughly proportional to the number of Tx/Rx chains used. A 4×4 system will have up to 4 times the capacity of a single antenna system.
    Therefore, the capacity gain delivered by Matrix B MIMO linearly depends on the number of Tx and RX antennas and can be expressed as:
    X = min (Tx Antennas, Rx Antennas)
    However, the number of Tx/Rx chains and antennas that can be used in the mobile subscriber (MS) devices are likely to be the limiting factor for the foreseeable future.
    Smart Antenna Systems (SAS)
    SAS combine antenna arrays with sophisticated signal processing to enhance SNR for higher throughput and link robustness while simultaneously reducing interference. Beamforming is an example of SAS. When receiving a signal, beamforming can increase the gain in the direction of wanted signals and decrease the gain in the direction of interference and noise. When transmitting a signal, beamforming can increase the gain in the direction the signal is to be sent and direct nulls at users that would otherwise be iterfered with.
    SAS typically delivers a +10 to 15 dB link budget improvement relative to a single antenna architecture. In the mobile WiMAX application, its active interference management can push the achievable net spectral efficiency into the 4-5bps/Hz range. SAS alone on the BS side provides operators wnith significant range benefits in the initial stages of network operation, and as their subscriber base grows and the
    network becomes interference limited, SAS can provide significant capacity benefits, especially when used in combination with spatial multiplexing techniques. SAS interference mitigation benefits start to diminish as subscriber mobility increases.
    For complete reading, you may read this Smart Antenna System whitepaper.
    Resource:
    www.wimax.com

     

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