WCDMA: Uplink power control

Sunday, July 27, 2008

Uplink power control

The “near-far problem” resulting from using DS-CDMA for cellular was already known in the late 1970s and one envisaged solution was to use some form of power control in the uplink, that is the transmission from the mobile device to the network [3, 4]. Figure 9 shows two principles for uplink power control. Open-loop essentially means that the mobile device sets itstransmission power based on measurements of the received signal power, while closed-loop means the mobile device sets its transmission power based on explicit commands received from the network. Such commands are in turn based on measurements of the received signal made at the network side.

When IS-95 was defined, a combination of open-loop and closed-loop uplink power control was chosen and standardized as a solution to the “near-far problem”. This meant that the mobile device set its power based on both the received signal power as well as on commands received from the network. This scheme was later inherited by Cdma2000, a modification of the IS-95 standard. However, when the uplink power control solution was standardized for WCDMA, no
particular benefits of such a dual-loop solution could be identified and instead a pure closed-loop system was selected, but with an update rate twice as high (1500Hz versus 800Hz).

References : http://www.ericsson.com/
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History of spread spectrum and CDMA

This brief historical explanation begins with a look at Spread Spectrum (SS)
principles, since they are fundamental to the multiple access principle known as
CDMA.
The frequency bandwidth needed to transmit a given amount of information is directly
related to the information rate – the amount of information transmitted per unit of
time. The term “bandwidth” is now often used interchangeably with “bit rate” so this
appears intuitively clear to most people.
SS techniques are methods by which more frequency bandwidth is deliberately used
than motivated by the actual information rate (spectrum spreading). On one hand,
this implies a certain waste of valuable frequency spectrum; but on the other hand,
these techniques have advantages such as increasing the resistance to interference,
jamming and interception.

Technologies in WCDMA

In this section we describe some of the important innovations that constitute the WCDMA standard, developed in response to the challenging 3G requirements placed on it. We also include descriptions of how the technologies differ from those used in the IS-95 family of standards. In particular we discuss functions related to the air interface, which is the interface between the mobile device and the network.

Asynchronous network operation
Several of the requirements for WCDMA did not exist in legacy standards, including IS-95. One such requirement is asynchronous network operation. Early on, one of the fundamental technical principles shared by the Japanese and European WCDMA concepts (including CoDiT) was asynchronous operation; that is, the base stations should not have to be synchronized to each other. This was an opposite requirement to that of the IS-95 (and later Cdma2000) standard, which relies on GPS (Global Positioning System) for its proper operation. The UMTS requirement for an asynchronous system was fundamentally important because it led to a standalone and easily deployable system attractive to operators around the world. But it also gave rise to several new technological challenges for WCDMA, including cell search and handover, described below.

Cell search
Before engaging in any kind of communication with the network, a mobile device must first synchronize to the signals sent from base stations. Mobile devices need to find out which base station code to look for and then synchronize to it. Searching for all possible base station codes at all possible times is an arduous task that could potentially affect the time from power-on to placing a call. Being asynchronous, WCDMA could not employ the principle adopted in the IS-95/Cdma2000 standards, where a common base station code with unique GPS time-shifts was used throughout the system. Clearly there was a need for innovation in this area. The early Japanese version of WCDMA included a structure based on dual synchronization channels, but it was generally considered too complex. Extensive research was conducted to find an information encoding principle that would minimize the cell search time while still keeping the complexity of the search at a negligible level. Solutions were progressively proposed to the standard and soon the goals had been achieved [13]. One of the early major technical challenges of asynchronous CDMA-based 3G had been overcome.

Soft handover
In most older mobile systems, the mobile devices are connected to one base station at a time and handover is the process of seamlessly shifting a mobile device’s connection from one base station to another. In “soft handover” however, mobile devices communicate the same information through two or more base stations simultaneously (see Figure 7). In practice, soft handover is a prerequisite for any high-capacity DS-CDMA-based system operating with the same carrier frequency in neighboring cells. As early as 1988, Jan Uddenfeldt et al., of Ericsson, pioneered the fundamental principles of soft handover in some of Ericsson’s early patents in the digital cellular field [14].

Compressed mode

Already from the beginning, virtually all WCDMA mobile devices were expected to bedual-mode (to support GSM) as the initial WCDMA coverage would probably not beas wide as that of GSM. Therefore, handover of ongoing connections betweenWCDMA and GSM was crucial for a smooth early rollout of WCDMA4. For a securehandover from WCDMA to GSM, the mobile device needs to perform signalmonitoring on potential GSM target base stations in the area, even during an activecall on the WCDMA system. The problem is to find time for this, because the mobiledevice is “busy all the time” with sending and receiving on a given WCDMA carrierfrequency. A technology denoted “Compressed Mode,” originally developed in CoDiT[9, 16], was adopted in WCDMA to solve this problem.

WCDMA

Sunday, July 27, 2008

Uplink power control

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History of spread spectrum and CDMA

Early spread spectrum and CDMA development

Technologies in WCDMA

Compressed mode

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