. 7.3.2 Symbols and Chips The phenomenon of spreading (and de-spreading) is shown in Figure 7.4. The signal is 'spread' by modulating the original signal (referred to as data in the figure). Usually, BPSK (binary phase-shift keying) modulated signal is used as the original signal. means that the signal modulation is performed a second time on the original signal. This original data signal (obtained after BPSK modulation) is then modulated by multiplying it by a sequence of bits (the wide-band spreading signal), thus converting the original BPSK narrowband signal into a spread signal of wider bandwidth. Each bit of the origi- nal signal (also known as a 'symbol') is multiplied by a sequence of bits called 'chips'. These chips are a part of the signal that is used to multiply the original data, the 'code' signal. Symbol Spreading Spread Signal De-Spreading Figure 7.4 Spreading and de-spreading The spread signal is then transmitted to the receiver. At the receiving end, the received signal is then again multiplied by the same code to retrieve the original data. The original data stream can be obtained only if the code signals are identical. In the radio network, there are many subscribers transmitting and receiving signals at the same time. The spreading codes achieve separation between these signals. Here we can define the term 'spreading factor' (also known as 'processing gain'), which is the number of chips used by one symbol. Processing gain can also be defined as the ratio between the chip rate and the bearer bit rate. The processing gains, together with the wideband nature, 3G RADIO NETWORK PLANNING AND OPTIMISATION suggest a frequency re-use of 1 between different cells/sectors. Thus, we can come up with some important information here: One WCDMA carrier is 5 MHz. With the guard band taking 1.16 MHz of bandwidth, the effective bandwidth is 3.86 MHz. One bit of the base-band signal is called a symbol. One bit of the code signal is called a chip. The chip rate is defined to be 3.84 Mcps (million chips per secon