One of the main air interface for 3G system is referred as wide band CDMA (WCDMA). It is one of the air interface used with UMTS mobile communication standard which allows communication between UE and Node B. Below table shows some of the key parameters of W-CDMA air interface.

a) Processing gain of the WCDMA system: It provides the robustness against self interference. This helps in reusing the available 5 MHz carrier frequencies over geographically close distances. Users in different cells are distinguished only by different codes. Consider an example, Speech signal with a bit rate of 12.2 kbps is spreaded with a CDMA code of chip rate 3.84 MCps provide a processing gain of 25 dB $(10 × log_{10} (3.84e6/12.2e3))$. After de-spreading, the signal power needs to be typically a few decibels above the interference and noise power. The required power density over the interference power density after de-spreading is designated as $E_b= N_0$, where $E_b$ is the energy or power density per user bit and $N_0$ is the interference or noise power density. The amount of interference is mostly determined by the codes that are used in the different cells. For speech service, $E_b = N_0$ is typically in the order of 5.0 dB and the required wideband signal to interference ratio (SIR) is therefore 5.0 dB minus the processing gain = −20.0 dB. That is the signal power can be 20 dB under the interference or thermal noise power and the WCDMA receiver can still detect the signal. The wideband SIR is also called the carrier-to-interference ratio (C/I).

b) High variable bit rates (up to 2 Mbps): The wide band in W-CDMA refers to the channel bandwidth of 5 MHz, which is four times of CDMA 1(1.25 MHz) and 25 times that of GSM (200 KHz). Such a wider bandwidth provides data rates ranging from 8 kbps to 2 Mbps on a single W-CDMA 5 MHz radio channel. In W-CDMA system, the use of a variable spreading factor and multicode connections is supported. This arrangement is illustrated in Figure 11. WCDMA supports highly variable user data rates; in other words, the concept of obtaining bandwidth on demand (BoD) is well supported. The user data rate is kept constant during each 10 ms frame. However, the data rate among the users can change from frame to frame by changing the spreading factor (Number of chips /bit rate).

c) Spectrum efficiency: The processing gain together with the wideband nature of WCDMA enables to employ frequency reuse of 1 between different cells of a wireless system (i.e., a frequency is reused in every cell per sector). Frequency planning is not necessary. Each cell requires 2$\times$5 MHz spectrum only, a two or three layer network can be deployed with 2$\times$15 MHz spectrum allocated to service providers. WCDMA makes very efficient use of available spectrum.

d) System capacity and coverage: WCDMA system provides improved capacity due to improved coding gain, voice activity, cell sectorisation and reuse of the spectrum in every cell. The capacity can be further improved by employing hierarchical cell structure, adaptive antenna arrays and multi user detection.

e) Improved Voice Capacity: 3G wireless service operators have been allocated 2$\times$15MHz spectrum. It can handle 192 voice calls per sector or 576 voice calls per cell having 3 sector configuration.

f) Multiple services per connection and service flexibility: WCDMA allows circuit switched and packet switched connection on the same channel, therefore variety of services (mix of voice and data services) with mixed data rates (ranging from 8 Kbps - 2 Mbps) can be delivered with specified QoS.

g) Fast Service Access: A new random access procedure has been developed to support instant access to multimedia services. In this procedure connection between a mobile user and a BS is established in a few tenth milliseconds.

h) Asynchronous Radio Access: WCDMA does not depend upon expensive GPS system for base stations synchronization, It has its own internal synchronization system makes the overall system implementation easier.

i) Seamless Access: Dual mode mobile phones provide seamless handover and roaming access with mapping of services between GSM and IMT 2000 networks.

j) Quality of Service: WCDMA system exploits the advantage of multipath fading to enhance the link performance and voice quality. Thus provides the robust operation in fading environment too. The probability of dropped call decreases due to use of RAKE receiver and advanced signal processing techniques. Each user selects the three strongest multipath signal, combines them coherently and produces an enhanced signal.

k) Multicode Transmission: The simultaneous transmission of two or more CDMA channels by the same mobile user is referred as multicode transmission. Due to use of two or more channels, the peak to average ratio of the transmitted waveform increases and hence affects the efficiency of power amplifier of the mobile phone.

l) Power Control: Many users share the same wideband 5 MHz carrier for their communications in adjacent cell per sector, the multiple access interference from many system users may occur, if power control in a particular cell is not employed. Hence, there is a stringent requirement of power control in WCDMA. In a cellular CDMA system, a strong signal received at a BS receiver from a nearer mobile phone will mask the weak signal received from another mobile station (MS). This interference is known as the near–far effect as shown in Figure 12.

The solution to power control in WCDMA is fast closed loop power control, also shown in Figure 13. In closed loop power control in the uplink, the BS performs frequent estimates of the received SIR and compares it to a target SIR. If the measured SIR is higher than the target SIR, the BS will command the MS to lower the power; if it is too low, it will command the MS to increase its power. The cycle ‘measure command-react’ is executed at a rate of 1,500 times per second (1.5 KHz) for each MS and thus operates faster than any significant change of path loss could possibly happen and indeed even faster than the speed of fast Rayleigh fading for low to moderate mobile speeds. Thus, closed loop power control will prevent any power imbalance among all the uplink.

m) WCDMA uses two types of code for spreading and multiple access: Channelization or spreading codes and scrambling codes. Spreading codes spread the signal by increasing the occupied bandwidth in accordance with the basic principle of CDMA. Whereas scrambling codes do not lead to bandwidth expansion but help to distinguish between cells and/or users.

• Spreading codes in WCDMA are Orthogonal Variable Spreading Factor (OVSF) codes: If different users require different data rates, so that codes of different length need to be used for the spreading. Orthogonal Variable Spreading Factor (OVSF) codes are a class of codes that fulfills these conditions; they are derived from Walsh Hadamard codes. Following are the Hadamard matrix of order 1, $H_{had}^{(1)}$ i.e.( $H_{2 \times 2}$) and $H_{had}^{(2)}$ i.e. ( $H_{4 \times 4}$) of order 2.

$\mathbf{H}_{\mathrm{had}}^{(1)}=\left(\begin{array}{rr}{1} & {1} \\ {1} & {-1}\end{array}\right)$, $\mathbf{H}_{\mathrm{had}}^{(2)}=\left(\begin{array}{rrrr}{1} & {1} & {1} & {1} \\ {1} & {-1} & {1} & {-1} \\ {1} & {1} & {-1} & {-1} \\ {1} & {-1} & {-1} & {1}\end{array}\right)$

Writing all code words of different Walsh-Hadamard matrices at different levels form a code tree as illustrated in Figure 14. All codes within one level of the tree (same duration of codes) are orthogonal to each other. Codes of different duration (1, 1) and (1, 1, 1, 1) are only orthogonal if they are in different branches of the tree. They are not orthogonal to each other if one code is a “mother code” of the second code – i.e., code (1,1) lies on the path from the “root” of the code tree to code (1,1,-1,-1). Examples of such codes are $(p_{2,2} , p_{4,4} ) , ( p_{2,2} , p_{4,3} ), (p_{2,1} , p_{4,2}), ( p_{2,1} , p_{4,1} )$ in Figure 14, whereas codes $( p_{2,1} , p_{4,3} ), ( p_{2,1} , p_{4,4} ) ( p_{2,2} , p_{4,1}), ( p_{2,2} , p_{4,2} )$ are orthogonal to each other.

Example 1: If a chip sequence is changed from (1,1) to (1,-1,-1,1), what will the change in spreading factor for speech service of bit rate of 12.3 Kbps ?

Spreading factor or processing gain is the ratio of chip rate and symbol rate. Chip rate is number of chips per seconds. And numbers of chips are changed from 2 to 4 therefore the spreading factor will also double considering symbol rate does not change.

Example 2: If a chip sequence is changed from (1,-1,-1, 1) to (1,1) , what will be the change data rate ?

Data rate = chip rate / spreading factor and number of chips are changed from 4 to 2 therefore the spreading factor will also double considering symbol rate does not change.

• For scrambling codes, a long and a short code exist. Both are complex codes, and are derived from real-valued codes in accordance with the following expression:

$C_{\text {Scrambler}}(k)=c_{1}(k) \cdot\left(1+j \cdot(-1)^{k} \cdot c_{2}(2\lfloor k / 2\rfloor)\right)-----(1)$

Here, k is the chip index and $c_1$ and $c_2$ are real-valued codes. For short codes $c_1$ and $c_2$ are two different members of the very large Kasami sets of length 256 . It is worth noting that the duration of the short code equals symbol duration only for spreading factor 256. Otherwise, a “short” code in WCDMA is not a short code. The long code is a Gold code, a combination of two Pseudo Noise (PN)-sequences that each have length $2^{25} − 1$. The I- and Q- part, $c_1$ and $c_2$ in above Eq. 1 are versions of the same Gold sequence, shifted relative to each other. The codes are truncated to a length of 10ms – i.e., one frame.

n) Hand over:

Intra frequency Handover

A connection handover between two BSs on the same carrier frequency is performed as a soft handover. The MS has a connection to both BSs during handover. Thus, signals from both BSs are used during this time and the Rake receiver processes them similarly to two paths of a multipath signal with two or more fingers. As Base Stations use different scrambling codes in WCDMA, the Rake receiver in the MS has to be able to apply different codes in each finger. For soft handover, the MS has to acquire synchronization with other BSs. This synchronization process is similar to the one described above, apart from the fact that the MS has a priority list for code groups. This list contains the code groups used by the neighboring cells for handover, and is continuously updated. Cell selection for soft handover is based on signal strength measurement after despreading or wideband power (Received Signal Strength Indication RSSI). No particular algorithms are specified in the standard. However, it is suggested to divide cells into the active cell and neighboring candidate cells that provide good signal strength. Softer handover is a special case of soft handover, in which the MS switches between two sectors of the same BS. Algorithms and processes are similar to those used for soft handover with the difference that only one BS is involved.

Inter frequency Handover

This kind of handover takes place in the following situations:

• Two Base Stations employ different carriers.
• The MS switches between hierarchy layers in the hierarchical cell structure.
• Handover to other service providers or systems.
• The MS switches from TDD mode to FDD mode or vice versa.

Inter frequency handover is a hard handover during which the connection between the MS and the old Base Station is first interrupted before the establishing a new connection with a new Base Station. There are two ways of measuring signal strength, on other frequencies while the old connection is still active:

(i) Two receiver feature: The MS may have two receivers, so that one can measure the signal strength on other frequencies while on the second one data is continually received; and

(ii) Transmission in compressed mode: In this mode, the data which is normally transmitted within a 10-ms frame is compressed to 5 ms. The time that is freed up can then be used for measurements of signal on other frequencies. Compression can be achieved, for example, by puncturing the data stream or reduction of the spreading factor.