Third Generation Network (3G), UMTS
9.1 The WCDMA Concept
9.1.1 Spreading (Channelization)
Figure 9.5 shows the basic operations of spreading and despreading for a WCDMA sys- tem. User data bits have a rateRassuming the values of±1. The spreading operation, in this example, is the multiplication of each user data bit with a sequence of four code bits called chips. The resulting spread data is at a rate of 4×Rand has the same random (pseudonoise-like) appearance as the spreading code. In this case, we would say that we used a spreading factor (SF) of 4. This wideband signal would then be further processed and transmitted across a wireless channel to the receiving end.
During dispreading, the spread user data/chip sequence is multiplied, symbol by sym- bol, with the very same four coded chip sequence used in the spreading process of these symbols. As shown, the original user symbol sequence is then recovered, provided there is synchronization between the spread signal and the (de)spreading code.
The increase of the signalling rate by a factor of 4 corresponds to a widening (by a fac- tor of 4) of the occupied spectrum bandwidth of the spread user data signal. Due to this virtue, CDMA systems are more generally called spread spectrum systems. Despreading restores a bandwidth proportional to intended rateRfor the signal. In addition to widen- ing the signal spectrum, spreading sequence is coded in specific way thus introducing anorthogonality factorbetween different data streams (channels). Within the cell, the
Third Generation Network (3G), UMTS 125
0 1 0 0 1 1
time Data stream
OVSF code
Despreading:
applying OVSF code
Decoded data:
input stream OVSF code Spreading:
data OVSF code Tx side
Rx side
Figure 9.5 Combining data and spreading sequences with SF=4.
different channels are separated by achannelization code, also called theOrthogonal Variable Spreading Factor (OVSF) code. The OVSF handles the signal spreading, as illustrated in Figure 9.5. The OVSF possesses two important features:
• an orthogonality of the codes with same length
• and the fact that orthogonality is conserved between OVSFs of variable lengths.
The OVSF orthogonality property ensures that different users of the same cell do not interfere with each other. If a signal coded with a given OVSF is decoded with a different OVSF, the resulting signal is an average null signal producing an equal number of 1s (−1) and 0s (+1).
The length of the OVSF (also called theSpreading Factor, SF) refers to the number of chips for a single input bit/symbol: a bit coded with OVSF length 256 would be repre- sented by 256 chips, while a bit coded with OVSF length of 4 would be represented by four chips. Using a long OVSF has the advantage of adding redundancy to the transmit- ted information. The impact of this redundancy is seen in the spreading gain; that is, the ratio of user bits to transmitted chips.
The bit rate of the user signal represented in the chip sequence is related to the spread- ing factor. The orthogonal code families containing code sequence of different lengths are used to implement variable data rates. This is necessary even for a single UMTS user since a multibearer service could be available to a single user at the same time. Also, this is necessary to distinguish between control and traffic channels on the uplink from the same terminal.
The principle of implementation of different transmission rate is illustrated in Figure 9.6. This involves the ‘multiplication’ of every bit of the each data stream by a spreading code with a respective number of chips equal to the SF.
R1 R2 R3
SF 2 SF 4 SF 8
Figure 9.6 Spreading with OSVF codes.
SF1 SF2 SF4 SF8
C1 = {1}
C2,0 = {11}
C2,1 = {10}
C4,0 = {1111}
C4,1 = {1100}
C4,2 = {1010}
C4,3 = {1001}
C8,0 = {11111111}
C8,1 = {11110000}
C8,2 = {11001100}
C8,3 = {11000011}
C8,4 = {10101010}
C8,5 = {10100101}
C8,6 = {10011001}
C8,7 = {10010110}
Figure 9.7 OVSF code tree.
The different bit streams that are to be transmitted simultaneously are multiplied by different OVSF codes and then added together. The receiver that receives the sum of all chip streams must be in position to reconstruct each of the transmitted bit streams.
This is only possible if the code sequences of different chip streams are orthogonal to each other.The OVSF codes can be created through the use of code tree, as illus- trated in Figure 9.7. Each node of the tree has exactly two branches, each representing a double-length code. The codes of the same level (of the same SF value) have same length.
Each code with a spreading factor N is created from a code with spreading factorN/2.
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Consequently, a set of 2k spreading codes with a length of 2k chips are available at thekth level. Availability of OVSFs of a specific length is determined by the number of OVSFs of same length or shorter that are used, as well as by the number of longer OVSFs used.
An OVSF code is basically created though multiplication of a code of the next lower level of a code tree. The code being multiplied is called themother code. Exactly two next level double-length codes are created from a mother code by chaining the two copies of the mother code or chaining the mother code with a copy multiplied by−1. Based on the rules of creating the OVSF, the code of shorter length may be found in a longer one.
This means that two codes of different levels of the code tree are orthogonal to each other as long as one of the two codes is not the mother code of other one. Because of this limitation, the number of simultaneously usable codes depends on the bit rate and spreading factor.
The definition for the same code tree means that for transmission from a single source, from either a terminal or a base station, one code tree is used (with one scrambling code on top of the tree). This means that different terminals and different base stations may operate their code trees totally independently of each other; there is no need to coordinate the code tree resource usage between different base stations or terminals.