Interference on Signal Code Power

Description

Interference on Signal Code Power (ISCP) is a fundamental radio measurement defined in 3GPP specifications for UMTS/WCDMA systems. It is a measure of the power spectral density (expressed in dBm/3.84MHz) of the received wideband power within the 3.84 MHz chip rate bandwidth, after descrambling with the serving cell's scrambling code but before despreading with the channelization code of a specific physical channel. In essence, ISCP represents the interference power observed in the signal path of a specific cell, comprising noise and interference from other cells, other users within the same cell (intra-cell interference), and external sources.

Technically, ISCP is measured by the User Equipment (UE) on the downlink and by the Node B on the uplink. For the downlink, the UE measures the ISCP on the Common Pilot Channel (CPICH) or other dedicated physical channels. The measurement process involves tuning to the frequency, applying the cell's scrambling code to isolate that cell's signal space, and then measuring the power of the received signal before applying the orthogonal channelization code. Since the channelization codes are orthogonal, the power not aligned with the desired code appears as interference. This measured value is the ISCP. It is a critical input for the calculation of the Signal-to-Interference Ratio (SIR), where SIR = RSCP / ISCP (with appropriate unit scaling). RSCP (Received Signal Code Power) is the power of the desired signal.

Architecturally, ISCP measurements are reported by the UE to the UTRAN (Universal Terrestrial Radio Access Network) via Measurement Report messages as part of the Radio Resource Control (RRC) protocol. These reports are used by the network's Radio Resource Management (RRM) algorithms. Key RRM functions that rely on ISCP include fast power control, handover control, and admission control. For inner-loop power control in the uplink, the Node B continuously measures the SIR (using received signal power and ISCP) of a UE and sends Transmit Power Control (TPC) commands to maintain the target SIR, directly combating fading and interference.

Its role is central to the interference-limited nature of WCDMA. Unlike GSM which is noise-limited, UMTS capacity is primarily constrained by interference. Accurate ISCP measurement allows the network to dynamically assess the interference environment. For handover decisions, the UE reports ISCP (often as part of a quality measure like Ec/No, which is RSCP/ISCP) for neighboring cells alongside RSCP. This helps the network select a target cell not just based on signal strength, but on signal quality relative to the local interference. ISCP provides a real-time view of the 'pollution' in the radio environment, enabling the network to make intelligent decisions to preserve call quality and maximize system capacity.

Purpose & Motivation

ISCP was introduced as a core measurement for UMTS/WCDMA to address the fundamental challenge of managing a network where all users share the same frequency band and are separated by codes. In such a Code Division Multiple Access (CDMA) system, every user's signal is interference to every other user. Traditional signal strength measurements (like RSSI in GSM) were insufficient because they didn't distinguish between the desired signal's power and the interfering power within the same channel. The primary problem ISCP solves is providing a quantifiable measure of this interference floor, which is essential for power control and system stability.

The historical context is the shift from TDMA/FDMA systems (like GSM) to wideband CDMA for 3G. In GSM, capacity was added by new frequencies or timeslots. In WCDMA, capacity is a soft resource limited by the total interference. If interference rises too high, all connections degrade. Therefore, precise, fast measurement of interference (ISCP) was necessary to implement the fast closed-loop power control that is the hallmark of WCDMA. This power control ensures each UE transmits with the minimum necessary power to maintain the link, thereby minimizing its contribution to the overall interference seen by others—a concept known as interference averaging.

ISCP's creation was motivated by the need for a measurement that directly feeds into the SIR calculation, which is the key metric for link quality in a CDMA system. It addressed the limitation of not having a direct interference measurement for shared-bandwidth systems. By providing the interference component, ISCP enables the network to perform accurate handovers to cells with better signal-to-interference conditions, not just stronger signals, improving call drop rates and overall network performance in interference-heavy environments like cell edges or dense urban areas. It is a foundational metric for the RRM that makes WCDMA systems work efficiently.