9.4 3G Services
9.8 Physical-Layer Procedures
9.8.4 Composition of the Physical Channels .1 Dedicated Physical Channel
The information from DCH transport channels is mapped onto the Dedicated Physi- cal Data Channel (DPDCH), which is multiplexed together with the Dedicated Physical Control Channel (DPCCH) that carries physical layer control information. Such multi- plexing is done differently in the uplink and downlink cases.
Uplink The composition of the dedicated physical channel in the uplink direction is shown in Figure 9.28. The DPDCH and DPCCH are multiplexed in I and Q components, respectively.
The uplink DPCCH is used to carry control information generated at the physical layer. This information consists of
• Known pilot bits to support channel estimation for coherent detection,
Tslot = 0.666 ms = 2560 chips DATA
Ndata = 10*2K bits(k = 0.6) DPDCH
PILOT Npilot bits
TFCI NTFCI bits
FBI NFBI bits
TPC NTPC bits DPCCH
Slot 0 Slot 1 Slot i Slot 14
Tframe = 10 ms = 38400 chips
Figure 9.28 Composition of DPDCH and DPCCH in the uplink direction [15].
Table 9.12 DPDCH fields [15].
Slot Format #i
Channel Bit Rate (kbps)
Channel Symbol
Rate (ksps) SF Bits/Frame Bits/Slot Ndata
0 15 15 256 150 10 10
1 30 30 128 300 20 20
2 60 60 64 600 40 40
3 120 120 32 1200 80 80
4 240 240 16 2400 160 160
5 480 480 8 4800 320 320
6 960 960 4 9600 640 640
• Transmit power control (TPC) commands,
• Feedback information (FBI), and
• An optional transport format combination indicator (TFCI). The transport format combination indicator informs the receiver about the instantaneous transport for- mat combination of the transport channels mapped to the simultaneously transmitted uplink DPDCH radio frame.
There is only one uplink DPCCH on each radio link.
The parameter k in Figure 9.28 determines the number of bits per uplink DPDCH slot.
It is related to the SF of the DPDCH asSF =256∕2k. The DPDCH spreading factor may range from 256 down to 4. The spreading factor of the uplink DPCCH is always equal to 256; that is, there are 10 bits per uplink slot. For DPDCH there are seven time-slot formats defined with channel symbol rate varying from 15 to 960 kbps.
Transmissions at variable bit rate can be obtained in the DPDCH channel simply by modifying the slot format or equivalently the spreading factor, as shown in Table 9.12, whenever the transport format is changed at the MAC layer. The physical channel allows the performance of this operation from frame to frame, although in practice the time between variations will be given by the TTI of the corresponding transport channel, which is an integer number of frames.
Variable transmission rate for DPDCH is accompanied by outer loop power control.
One of the QoS requirements is specified bit-error-rate, which depends on value of energy per bit-to-noise ratio, Eb/No, for a given modulation format. On the other hand, Eb/No relates to the bit rate and signal-to-noise ratio SIR by equation Eb/No=(W/Rb)*SIR, where W is a bandwidth of the transmitted signal, Rb is a bit rate. As a consequence, the requirement to fix Eb/No, leads to power adjustment of DPDCH when bit rate is changed. Figure 9.29 shows Tx power variations with variable bit-rate transmission. It should be noted that the control channel is transmitted at a constant power level with a constant bit rate and spread factor, SF=256.
Demodulation of uplink user data on DDDCH is performed coherently; that is, sim- ilar to the downlink. The difference with the downlink transmitter is that the users are distributed over cell area and, consequently, each user should generate a separate pilot signal for channel estimate. As shown in Figure 9.28, the pilot signal is embedded in
Third Generation Network (3G), UMTS 159
DPCCH DPDCH
TF1, Rb
TF2, 2xRb TF2, 4xRb
TF1, Rb TF0, no
DPDCH DPDCH
DPDCH
DPDCH
TTI
Figure 9.29 Power control and variable bit-rate transmission in the uplink direction.
uplink DPCCH. The number of pilot bitsNpilot varies from 3 to 8 bits depending on the slot format, which is configured by higher layers depending on transport channel composition [11].
A scrambling in uplink is user-specific and performed on the top of channelization of user data. Due to the fact that WCDMA is an asynchronous system, the uplink sig- nals from different users are not time synchronized in the NodeB receiver. Therefore, channelization code is only used to separate different channels from the same UE. Due to the lack of time synchronization, orthogonality of OVSF uplink codes between dif- ferent users cannot be achieved, therefore, all users in the cell can use the same set of channelization codes thus producing noise interference to each other. The user-specific scrambling effectively performs ‘whitening’ of the intra-cell interference on the uplink.
Downlink Channelization code spreading is applied to all physical channels on the downlink with the exception of SCH. Both I and Q branches are spread to the chip rate by the same real-valued channelization code Cch,SF,m; that is, the output for each input symbol on the I and the Q branches presents a sequence of SF chips multiplied by the real-valued symbol. The channelization code sequence is aligned in time with the symbol boundary. Application of the same OVSF code for I and Q branches is possible due to orthogonality of I and Q signal components by itself. In the example in Figure 9.30, the real-valued chip sequence on the Q branch is complex multiplied with j and summed with the corresponding real-valued chip sequence on the I branch, thus resulting in a single complex-valued chip sequence that is further scrambled by a complex-valued scrambling codeSdl,n.
I
downlink physical channel
→ S P
Cch,SF,m
j
Sdl,n
Q
I + jQ S
Modulation Mapper
Figure 9.30 Spreading for all downlink physical channels except SCH [14].
Different downlink Physical channels (point S in Figure 9.29)
Σ
Σ
G1
G2
GP
GS S-SCH
P-SCH
Figure 9.31 Combining downlink physical channels [14].
Different downlink channels are then combined together before channel shaping and RF modulation at the carrier frequency, as shown in Figure 9.31. Each complex-valued spread channel is separately weighted by a weight factor Gi. The complex-valued syn- chronization channels, primary P-SCH and secondary S-SCH, may also be separately weighted by corresponding weight factors Gpand Gs. All downlink physical channels are then combined using complex addition.
9.8.4.2 Common Downlink Physical Channels
Common Pilot Channel (CPICH) The CPICH is a fixed rate (30 kbps, SF=256) downlink physical channel that carries a predefined bit sequence. The CPICH is not mapped to a transport channel. With a predefined bit pattern the CPICH provides the estimate of phase reference for the other control channels on downlink. Figure 9.32 shows the frame structure of the CPICH.
There are two types of common pilot channels; Primary and Secondary CPICH. They differ in their use and the limitations placed on their physical features.
Primary Common Pilot Channel (P-CPICH) The P-CPICH has the following characteristics.
There is only one P-CPICH per cell, which is always transmitted with the same
Pre–defined bit sequence
Slot #0 Slot #1 Slot #i Slot #14
Tslot = 2560 chips , 20 bits
1 radio frame: Tf = 10 ms Figure 9.32 Frame structure for the common pilot channel [15].
Third Generation Network (3G), UMTS 161
channelization code, Cch,256, and is scrambled by the primary scrambling code. The mobile user can obtain the cell primary scrambling code during the synchronization procedure. The P-CPICH provides a phase reference for other downlink control channels, as well as a channel reference for cell selection and handover management.
The allocated power of the P-CPICH is broadcasted in the system message of the BCH channel, thus providing the means to a mobile terminal to estimate the path loss in given location within the cell. The neighbour cells with a measured CPICH power level comprise theMonitored Set, the one with highest level is normally selected as aserving cell. During soft handover, another set of neighbours is formed, namely the Active Set, comprised of cells to which the mobile has established a connection over the air interface.
Secondary Common Pilot Channel (S-CPICH) The secondary pilot appeared in the cell only in special cases when the transmit diversity or multiple narrow beam antennas are con- figured in the cell. In the case of transmit diversity in the MIMO mode and of beam forming with different antennas, there may be several S-CPICH per cell transmitted with an arbitrary channelization code of SF=256. The S-CPICH can be scrambled by either the primary or a secondary scrambling code. The S-CPICH can be used as a phase reference for the second, third or fourth transmit antenna by UEs configured in MIMO mode or in MIMO mode with four transmit antennas.
Synchronization Channel (SCH) The Synchronization Channel (SCH) is a downlink signal used for cell search. The SCH consists of two subchannels, the Primary and Secondary SCH. The 10 ms radio frames of the Primary and Secondary SCH are divided into 15 slots, each with a length of 2560 chips. Figure 9.33 illustrates the structure of the SCH radio frame.
The Primary SCH transmits the Primary Synchronization Code (PSC) once in every slot in a frame, as shown in Figure 9.33. The Primary Synchronization Code (PSC) is denotedcpin Figure 9.33. The PSC code is the same for every cell in the system and is therefore known to the UE and has a length of 256 complex chips with identical real and imaginary values. The primary SCH is a first channel in the cell which mobile termi- nal has to acquire in order to be time synchronized with the base station. The Primary Synchronization Code sequencecp has an autocorrelation correlation function with a sharp peak at zero time shift that ease PSC detection with a matched filter. Since the Primary PCH repeats the PSC sequence as shown in Figure 9.33, the correlator output
Primary SCH Secondary SCH
256 chips 2560 chips
One 10 ms SCH radio frame acp
acsi,1 acp
acsi,14 acp
Slot #0 Slot #1 Slot #14
acsi,0
Figure 9.33 Structure of the synchronization channel [16].
will contain peak replicas at every time slot in SCH radio frames. As a result, by reading the Primary SCH, the mobile terminalwill acquire both time slot and chip level synchro- nization with the base station transceiver.
The Secondary Synchronization Codes (SSC) are transmitted in parallel with the Pri- mary SCH. The S-SCH contains a sequence of 15 modulated code wordsci,ks each of length 256 chips, wherei=0, 1,…, 63 is the number of the scrambling code group and k=0, 1,…, 14 is the slot number. The autocorrelation properties of code wordci,ks are similar to PSC. Each SSC sequence is chosen from a set of 16 different codescs,j(j=0,…, 16) of length 256. Total 64 code groupsSi(I=0,…, 63) is constructed by composition of 16 codescs,j. Each codeSiof the group of 64 codes identifies to which scrambling code groups the base station belongs. The codeSipresents a sequence of 15 code wordsci,ks , one per time slot as shown in Figure 9.16. As an example, withI=17, the SSC sequence may look like [14]:
S17= {c17,0s ,c17,1s ,…,c17,14s }
= {cs,1,cs,11,cs,14,cs,4,cs,13,cs,2,cs,9,cs,10,cs,12,cs,16,cs,8,cs,5,cs,3,cs,15,cs,6} (9.1) It is important that the SSC codeSiis unique such that none of the 64 codesSican be generated by cyclical rotation of another codeSn, withn≠i. This property allows synchronization at the frame level. When a mobile detects the SSC sequence, it knows the position of the first time slot in the radio frame; therefore, frame synchronization is achieved.
The detection of SSC codeSidetermines one of the 64 code group that the base station belongs to. A total of 512 scrambling codes are defined for the WCDMA system, they are grouped in 64 groups associated with 64 SSC codes. Each group contains eight scram- bling codes. Once the UE detects the sequenceSi, it knows the specific group of eight scrambling codes. In the next step, the mobile has to attempt to descramble the Primary Common Control Physical Channel (P-CCPCH) with eight possible scrambling codes and choose one with a high correlation. Knowing the scrambling code mobile can then read the system information message by decoding the Broadcast Transport Channel transmitted at P-CCPCH.