UMTS Layer Structure and Channels

In UMTS payload and control data transmission are carried out by channels, frames and slots in an efficient manner. UMTS uses WCDMA as a multiple access technique. UMTS allow both TDD and FDD transmissions. The version of UMTS which uses TDD is called as TD-UMTS.

In TDD Mode uplink and downlink are split in time with the base stations and then mobiles transmit alternatively on the same frequency. In FDD mode, the uplink and downlink are on different frequencies. The radio interface, Uu, is the interface between UE and UTRAN. It consists of three protocol layers as illustrated in Figure 9: Physical layer (layer #1), Data link layer (layer #2, lower part is called MAC and upper part is RLC), and Network layer (Layer#3, lower part is called RRC).

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Figure 9: Protocol Layers of UMTS

The physical layer in UTRAN performs the following functions:

  • Forward error correction, bit-interleaving, and rate matching
  • Signal measurements
  • Micro-diversity distribution/combining and soft hand off execution
  • Multiplexing/mapping of services on dedicated physical codes
  • Modulation, spreading, demodulation, despreading of physical channels
  • Frequency and time (chip, bit, slot, frame) synchronization
  • Fast closed-loop power control
  • Power weighting and combining of physical channels
  • Radio frequency (RF) processing.

The medium access control sublayer is responsible for efficiently transferring :-

  • Data for both real-time (CS) and non-real-time (PS) services to the physical layer
  • MAC offers services to the radio link control (RLC) sublayer and higher layers.
  • The MAC layer provides data transfer services on logical channels.
  • MAC is responsible for Selection of appropriate transport format (basically bit rate) within a predefined set, per information unit delivered to the physical layer.
  • Service multiplexing on random access channel (RACH), forward access channel (FACH), and dedicated channel (DCH)
  • Priority handling between data flow of a user as well as between data flows from several users
  • Access control on RACH and FACH
  • Contention resolution on RACH

Radio link control (RLC) sets up a logical link over the radio interface and is responsible for :-

  • Fulfilling QoS requirements
  • Segmentation and assembly of the packet data unit
  • Transfer of user data
  • Error correction through retransmission
  • Sequence integrity
  • Duplication information detection
  • Flow control of data

The Radio Resource Control (RRC) layer

  • Broadcasts system information, Handles radio resources (i.e., code allocation, handover, admission control, and measurement/control report), and controls the requested QoS.

  • The RRC layer offers the following services to the core network:

    (i) General control (GC) service used as an information broadcast service

    (ii) Notification (Nt) service used for paging and notification of a selected UE

    (iii) Dedicated control (DC) service used to establish/release connections and transfer messages.

Each layer provides service to other layer by different channels. These channels are classified in to three categories: Physical, Transport, and Logical.

Note : At UE side, The logical pieces of information mostly comes from higher layers (Application layer). In the MAC layer, these logical channels are multiplied with the Orthogonal Variable Spreading Factor (OVSF) codes (explained later). The OVSF code is allotted depending upon the current congestion in the network and type of data (simple e-mail/ web browsing/ multimedia transfer etc.) that has to be transmitted. After multiplication, these are called as transport channels. In the physical layer, these transport channels are mapped to a physical frequency channel of 5MHz and transmitted. At UTRAN side reverse process takes place. The channels related to layers are illustrated in following Figure 10:

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Figure 10

Till 2G, concept of transport channel did not arise. 3G services are multirate services due to which, services offered has exponentially increased. Thus different types of channels with different data rates are required. Hence, the definition of transport channel came into existence.

  • The radio interface between UE and UTRAN is Iu interface, The interaction between these two takes place by physical channels. Physical channels are defined differently for FDD and TDD. FDD identifies a physical channel by its carrier frequency, access code and the relative phase of the signal for the uplink. Similarly TDD identifies a physical channel channel by its carrier frequency, access code, the relative phase of the signal for the uplink and the time slot in which it is transmitted.
  • Logical channels are defined by the type of information they carry and come from higher layer. They are of two types: Control or Signaling and traffic channel.
  • The services offered by the MAC layer to physical layer at UE side and by physical layer to higher layers at UTRAN side are called Transport channels. These channels signify that how the information is transmitted on the radio interface.

In this section, different types of channel on both uplink and downlink are presented.

Physical Channels: Between UE and UTRAN, physical channel is the basic physical resource. It carries payload data and governs the physical characteristics of the signal. It is identified by specific code and frequency. It consists of radio frames and time slots. The length of a radio frame is 10 ms and one frame consists of 16 time slots, each of 625μsec. There are two physical channels, Dedicated Physical Channel (DPCH) and Common Physical Channel (CPCH). They are also distinguished by data and control information.

A) Dedicated Physical CHannel (DPCH):

  • It is a user dedicated, point-to-point channel between UE and node B (present in UTRAN). These channels carry dedicated channels at various rates up to 2 Mbps.
  • For UL a long code is used to identify the channel.
  • The UL channel uses different data streams transmitted on I and Q branch.
  • There are two types of DPCH, Dedicated Physical Data CHannel (DPDCH) and Dedicated Physical Control CHannel (DPCCH).

    (i) Dedicated Physical Data CHannel (DPDCH): It is used to carry actual user data and signaling information generated at layer 2 (there may be none, one, or several DPDCHs).

    (ii) Dedicated Physical Control CHannel (DPCCH): It is used to carry control information generated at layer 1 (pilot bits, transmit power control (TPC) commands, feedback information (FBI) commands, and optional transport format combination indicator (TFCI).

These two channels exist for uplink (UL- DPDCH) as well as down link (DL-DPDCH).

  • The UL-DPDCHs are I/Q and code multiplexed. The DPDCH and DPCCH are transmitted at the same time and on the same carrier by using the I- and Q- branch, respectively. For each layer 1 connection, there is only one UL DPCCH. DPCCH rate and power remain constant. The UL DPCH carries $10 \times 2^k$ (k= 0, 1, . . . , 6) bits per slot and may have a spreading factor (SF) 256 and 4.
  • The DL- DPDCH is time multiplexed. The DL DPDCH carries $10 \times 2^k$ (k = 0, 1, . . . , 7) bits per slot and may have spreading factor (SF= 512/2$^k$) from 512 to 4 .For DL channels two codes are used, one to identify the cell and the other to identify a particular channel within that cell.

B) Common Physical CHannels (CPCH): These channels are shared between users.

  • Uplink (UL-CPCH) are Physical Random Access CHannel (PRACH), Fast Uplink Signaling CHannel (FAUSCH) and Physical Common Packet CHannel (PCPCH).

    (i). Physical Random Access CHannel (PRACH): It is not only a common physical channel, but also a logical channel used to carry the RACH. A slotted ALOHA approach is used for medium access. Data packets may be transmitted via the PRACH, too.

    (ii) Physical Common Packet CHannel (PCPCH): This channel is used to carry CPCH. Information is transmitted after a preamble. Initially, several access preambles are transmitted with ascending transmission power until the BS receives the necessary signal strength. After the BS acknowledges reception, another preamble is transmitted to detect eventual collisions with packets from other MSs that try to access the PCPCH at the same time. Before user data are transmitted, a power control preamble of length 0 or 8 time slots can be transmitted. Only then are the actual data transmitted; the length of this transmission period is a multiple of the frame duration – i.e., 10ms.

    (iii) Fast Uplink Signaling CHannel (FAUSCH)

  • The DL Common Physical CHannels (DL-CPCH) include the following channels: Primary common control physical CHannel (PCCPCH), Secondary Common Control Physical CHannel (SCCPCH), Physical Synchronization CHannel (PSCH), Common Pilot CHannel (CPICH), Physical Downlink Shared CHannel (PDSCH).

    (i) Primary Common Control Physical CHannel (PCCPCH): It carries Broadcast Channel (BCH), and it is thus critical that it can be demodulated by all MSs in the cell. Thus, it uses a constant, high spreading factor - namely, 256 and transmits at a rate of 30 kbps. There is no power control associated with the CCPCH and therefore the power control bits do not have to be transmitted. The P-CCPCH has an idle period in each frame; this idle period is 256 chips long.

    (ii) Secondary Common Control Physical CHannel (SCCPCH): It carries Forward Access Channel (FACH) and Paging channel (PCH). The main difference between PCCPCH and SCCPCH is of data rate and spreading factor. In SCCPCH, data rate and spreading factors are variable, i.e. It is transmitted when data is available and spreading factor range is from 256 to 4.

    (iii) Physical Synchronization CHannel (PSCH): It does not relate to any logical channel. It is used for cell search. It consists of two sub-channels: primary SCH transmits a modulated code of 256 chips once every slot, and secondary SCH transmits repeatedly 15 codes of 256 chips for synchronization. It is multiplexed with the P-CCPCH.

    (iv) Common Pilot CHannel (CPICH): This channel has a constant spreading factor of 256. The CPICH consists of a primary and a secondary CPICH. The primary CPICH serves as a reference for phase and amplitude for common channels in the entire cell. The primary CPICH is transmitted all over the cell and the secondary might be transmitted in selected directions. The primary and secondary pilot channels differ in the spreading and scrambling codes used; the primary CPICH always uses the primary scrambling code and a fixed channelization code, so that there is only one such code per cell. CPICHs are particularly important for establishing a connection as during this period the pilots of the dedicated channels are not available. Furthermore, pilot channels provide an indication of signal strength at the MS and are therefore important for handover procedures. The cell size of a BS can be varied by varying the transmit power of pilot channels. By reducing transmit power the area over which the BS provides the strongest signal is decreased. This reduces the traffic load of a BS.

    (v) Physical downlink Shared CHannel (PDSCH): It carries DSCH; This is shared by users based on code multiplexing; It is associated with DPCH.

    (vi). Acquisition Indicator CHannel (AICH): It carries acquisition indicators, which provides feedback about whether synchronization was successful or not.

    (vii) Page Indicator CHannel (PICH): It carries a page for UE, has fixed rate of transmission and, Spreading Factor of 256.

  • Transport Channels

    In UTRAN transport channels can be either common (i.e., shared between users) or dedicated channels. They provide data transfer services to layer #2, MAC sublayer.

    The dedicated transport channels are as follows:

    (i) Dedicated CHannel (DCH): Dedicated channels are present in both the uplink and the downlink. They are used to transmit both higher layer signaling and actual user data. As the position of the UE is known when a dedicated channel is in use, smart antennas, as well as fast power control and adaptation of the data rate on a frame-by-frame basis, can be used

    (ii) Fast Uplink Signaling CHannel (FAUSCH)

    (iii) ODMA (opportunity driven multiple access) Dedicated CHannel (ODCH)

    The common transport channels are as follows:

    (i) Broadcast CHannel (BCH): This channel is used in downlink. Both cell-specific and network-specific information is transmitted on it over the entire cell with low fixed bit rate. For example, the BS uses this channel to inform all UEs in the cell about free access codes and available access channels. This channel has to be transmitted with relatively high power, as all UEs within the cell have to be able to receive it. Thus, on this channel neither power control nor smart antennas are implemented.

    (ii) Paging CHannel (PCH): This is also a channel that can be found only in the downlink. It is used to tell an MS about an incoming call, SMS message, data sessions, or required maintenance or re registartion. Since attenuation of the channel to the UE, as well as the location of the UE, is not known the PCH is transmitted with high power and without employing smart antennas. Depending on whether the current cell of the UE is known, paging information is either transmitted in only one cell or several cells.

    (iii) Random Access CHannel (RACH): The RACH is only used in the uplink. The UE uses it to initialize a connection to the BS. It can employ open-loop power control, but no smart antennas, as the BS must be able to receive signals on the RACH from every UE within the cell. As it is a random access channel, collisions might occur. Therefore, the structure of bursts in the RACH is different from that of other channels.

    (iv) Forward Access CHannel (FACH): This downlink channel is used to transmit control information to a specific UE registered on the system. However, as the FACH is a common channel and therefore received by more than one UE, explicit addressing of the desired UE is required (in-band identification, UE-ID at the beginning of the packet). In contrast with this, a dedicated control channel has an implicit addressing of the desired MS: the MS is specified by the carrier and the spreading code used. The FACH can also transmit short user information packets. It can employ smart antennas, as information is transmitted to one specific, localized mobile. There may be more than one FACH per cell as they may carry packet data.

    (v) Common Packet CHannel (CPCH): The CPCH is an uplink channel, and can be interpreted as the counterpart of the FACH. It provides additional capability beyond that of the RACH. It is a contention-based random access channel used for transmission of bursty data traffic. It can transmit both control and data packets. If the FACH and the CPCH are used together closed-loop power control is possible.

    (vi) Downlink Shared CHannel (DSCH): the DSCH is a downlink channel similar to the FACH. It sends mainly control data, but also some traffic data, to multiple UEs. Explicit addressing has to be used on this channel, as the same CDMA code is used for all UEs. The reason for using the same code for multiple MSs lies in the limited amount of short spreading codes. Under normal circumstances, one spreading code would be permanently reserved for one UE, even if the traffic is bursty. The cell would thus quickly run out of codes (remember that there are very few codes that can be used for high-data-rate traffic). On the DSCH one short code is used for several mobiles and data are multiplexed in time. The DSCH supports fast power control, use of smart antennas, and rate adaptation from transmission frame to transmission frame.

  • Logical channels

    These channels provide service from Layer 2 MAC sublayer to RLC sublayer. They are of two types: Logical Control Channels and Logical Traffic Channel.

    The Logical Control Channels are as follows:

    (i) Broadcast Control CHannel (BCCH): This is a downlink channel for broadcasting system and control information to UEs relevant to cell, such as radio channel of neighboring cells etc.

    (ii) Paging Control CHannel (PCCH): This channel is used in downlink to transfer page and notification information. It is used when: (a) network does not know the location of cell of the UE, and (b)UE is in cell connected state (using sleep mode).

    (iii) Common Control CHannel (CCCH): It is Bidirectional channel to transfer control information between network and UE, it is used: (a) by UE without RRC connection with the network, and (b) by UE using common transport channel to access a new cell after cell resection.

    (iv) Dedicated Control CHannel (DCCH): It is a Point-to-point bidirectional channel to transmit dedicated information between a UE and network. The channel is established through RRC connection setup procedure.

    (v) ODMA Control CHannel (OCCCH): It is Bidirectional channel to transmit control information between UEs.

    (vi) ODMA Dedicated control CHannel (ODCCH): It is a Point-to-multipoint bidirectional channel to transmit dedicated control information between mobiles. This channel is established through RRC connection setup procedure.

    The Logical Traffic Channels are as follows:

    (i) Dedicated Traffic CHannel (DTCH): It is a Point-to-point traffic channel, dedicated to one UE to transfer user information. A DTCH can exist in both UL and DL.

    (ii) ODMA Traffic CHannel (ODTCH): It is a Point-to-point channel dedicated to one UE to transfer user information between UEs. An ODTCH can exist in relay link. It is used as a point-to-multipoint unidirectional channel to transfer dedicated user information for all or a group of specified UEs.

UMTS assures backward compatibility with the second generation TDMA based GSM and 2.5 G TDMA technologies such as GPRS and EDGE. The network structure and bit level packaging of GSM data is retained by UMTS with additional capacity.


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