Key Technical Features of W-CDMA - 1.3.2.2

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First, W-CDMA relies on **Wideband Direct Sequence Spread Spectrum (DSSS)**. This means that user data is spread across a relatively broad 5 MHz frequency channel. This spreading is accomplished by multiplying the user's data bits with a much faster, pseudo-random sequence known as a **spreading code** or **chip sequence**. The wideband nature and spreading make the signal resilient to interference and allow for a high chip rate, enhancing the signal's processing gain.

To manage multiple users sharing this wide frequency, W-CDMA employs clever coding. In the downlink, **Orthogonal Variable Spreading Factor (OVSF) codes** are used. These codes are designed to be mathematically orthogonal to each other, ensuring that signals from different users within the same cell don't interfere. The "Variable Spreading Factor" aspect means the length of these codes can be adjusted dynamically. A shorter code (lower spreading factor) allows for higher data throughput for a user, while a longer code (higher spreading factor) provides more robustness for lower data rate services. Beyond separating users within a cell, **scrambling codes** are used to distinguish signals originating from different cells, helping the mobile identify its serving cell and manage inter-cell interference.

A paramount feature in W-CDMA is its **fast closed-loop power control**. This addresses the inherent "near-far problem" in CDMA, where a mobile close to the base station could overwhelm signals from more distant users. W-CDMA constantly and rapidly adjusts the transmit power of each mobile device, up to 1500 times per second. The base station measures the incoming signal strength and commands the mobile to adjust its power to the absolute minimum required. This precise power management is vital for minimizing overall interference in the network, which directly translates to higher system capacity.

For seamless mobility, W-CDMA introduces **soft handover** and **softer handover**. Unlike 2G systems that momentarily break a connection before making a new one (a hard handover), soft handover allows a mobile to simultaneously maintain connections with two or more Node Bs during a cell transition. The signals from these multiple paths are then intelligently combined at the RNC (Radio Network Controller) in a process called macro-diversity combining, significantly improving signal quality, reducing dropped calls, and boosting capacity at cell edges. Softer handover is a similar process but occurs between different sectors of the *same* Node B.

These technical features collectively enable one of W-CDMA's most significant advantages: **frequency reuse of 1**. This means that every single cell in a W-CDMA network can utilize the exact same 5 MHz frequency band. This is a profound improvement over 2G systems that required complex frequency planning and patterns (like 7/21) to prevent interference, thus maximizing the utilization of the limited radio spectrum.

Finally, W-CDMA was designed with inherent **Quality of Service (QoS) support**. It defines distinct QoS classes (Conversational, Streaming, Interactive, Background) to prioritize and manage different types of traffic based on their delay sensitivity and reliability requirements. This, along with **Voice Activity Detection (VAD)** which reduces transmission during silences in voice calls, and its native ability to support **simultaneous voice and data** services, made W-CDMA a robust and versatile foundation for the burgeoning mobile broadband era.

Key Concepts

• Fundamental Spreading (DSSS): How signals are spread.

• Coding Schemes (OVSF, Scrambling): How users and cells are differentiated.

• Power Control (Fast Closed-Loop): Critical for interference management.

• Seamless Mobility (Soft/Softer Handover): User experience and capacity benefits.

• Spectrum Efficiency (Frequency Reuse of 1): The major capacity gain.

• QoS Integration: Support for diverse services.

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• Examples

• Video Call Stability: While driving, a video call experiences no noticeable drops or pixelation, partly due to soft handover seamlessly managing the transition between cells.

• Efficient Bandwidth Use: In a crowded stadium, multiple users can still access the internet on a W-CDMA network because the system is efficiently separating them with OVSF codes and keeping interference low with fast power control, allowing frequency reuse of 1.

• Optimized Data Speed: A user downloading a large file gets a very fast connection (low spreading factor), while someone just checking email gets a slower, more robust connection (higher spreading factor) on the same channel, showcasing the variable spreading factor in action.

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• Flashcards

• Term: Wideband DSSS

• Definition: Spreading user data over a 5 MHz channel using a chip sequence.

• Term: OVSF Codes

• Definition: Orthogonal codes for user separation and variable data rates in W-CDMA downlink.

• Term: Fast Closed-Loop Power Control

• Definition: Rapid adjustment (1.5 kHz) of mobile transmit power to minimize interference.

• Term: Soft Handover

• Definition: Mobile maintains simultaneous connection with multiple Node Bs during transition.

• Term: Frequency Reuse of 1

• Definition: All cells use the same frequency band, enhancing spectral efficiency.

• Term: QoS Support (W-CDMA)

• Definition: Ability to prioritize different traffic types (e.g., voice, video, data) based on requirements.

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• Memory Aids

• "W-CDMA: Wide, Coded, Powered, and Soft\!": Remembers Wideband, Coding, Power Control, Soft Handover.

• "OVSF: One Variable Spreading Factor": Remembers its two roles for variable data rates and orthogonality.

• "Power Control: Precision for Performance": Emphasizes its quick and vital role.

• "Reuse 1 is Number 1 for Capacity": Highlights the biggest spectral efficiency gain.

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