Bell Northern Research, LLC, Exhibit 2006 Page 1 of 7
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`WCDMA
`FOR UMTS
`
`Radio Access for Third Generation
`Mobile Communications
`
`Third Edition
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`Edited by
`
`Harri Holma and Antti Toskala
`Both of
`Nokia, Finland
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`Bell Northern Research, LLC, Exhibit 2006 Page 2 of 7
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`Copyright # 2004
`
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`British Library Cataloguing in Publication Data
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`A catalogue record for this book is available from the British Library
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`ISBN 0-470-87096-6
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`Typeset in 10/12pt Times by Thomson Press (India) Limited, New Delhi.
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`Bell Northern Research, LLC, Exhibit 2006 Page 3 of 7
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`Introduction to WCDMA
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`55
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`Figure 3.7. Block diagram of the CDMA Rake receiver
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`estimator uses the pilot symbols for estimating the channel state which will then be removed
`by the phase rotator from the received symbols. The delay is compensated for the difference
`in the arrival times of the symbols in each finger. The Rake combiner then sums the channel-
`compensated symbols, thereby providing multipath diversity against fading. Also shown is a
`matched filter used for determining and updating the current multipath delay profile of the
`channel. This measured and possibly averaged multipath delay profile is then used to assign
`the Rake fingers to the largest peaks.
`In typical implementations of the Rake receiver, processing at the chip rate (correlator,
`code generator, matched filter) is done in ASICs, whereas symbol-level processing (channel
`estimator, phase rotator, combiner) is implemented by a DSP. Although there are several
`differences between the WCDMA Rake receiver in the mobile and the base station, all the
`basic principles presented here are the same.
`Finally, we note that multiple receive antennas can be accommodated in the same way as
`multiple paths received from a single antenna: by just adding additional Rake fingers to the
`antennas, we can then receive all the energy from multiple paths and antennas. From the
`Rake receiver’s perspective, there is essentially no difference between these two forms of
`diversity reception.
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`3.5 Power Control
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`Tight and fast power control is perhaps the most important aspect in WCDMA, in particular
`on the uplink. Without it, a single overpowered mobile could block a whole cell. Figure 3.8
`depicts the problem and the solution in the form of closed loop transmission power control.
`Mobile stations MS1 and MS2 operate within the same frequency, separable at the base
`station only by their respective spreading codes. It may happen that MS1 at the cell edge
`suffers a path loss, say 70 dB above that of MS2 which is near the base station BS. If there
`were no mechanism for MS1 and MS2 to be power-controlled to the same level at the base
`station, MS2 could easily overshout MS1 and thus block a large part of the cell, giving rise to
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`56
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`WCDMA for UMTS
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`Figure 3.8. Closed loop power control in CDMA
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`the so-called near–far problem of CDMA. The optimum strategy in the sense of maximising
`capacity is to equalise the received power per bit of all mobile stations at all times.
`While one can conceive open loop power control mechanisms that attempt to make a
`rough estimate of path loss by means of a downlink beacon signal, such a method would be
`far too inaccurate. The prime reason for this is that the fast fading is essentially uncorrelated
`between uplink and downlink, due to the large frequency separation of the uplink and
`downlink bands of the WCDMA FDD mode. Open loop power control is, however, used in
`WCDMA, but only to provide a coarse initial power setting of the mobile station at the
`beginning of a connection.
`The solution to power control in WCDMA is fast closed loop power control, also shown in
`Figure 3.8. In closed loop power control in the uplink, the base station performs frequent
`estimates of the received Signal-to-Interference Ratio (SIR) and compares it to a target SIR.
`If the measured SIR is higher than the target SIR, the base station will command the mobile
`station to lower the power; if it is too low it will command the mobile station to increase its
`power. This measure–command–react cycle is executed at a rate of 1500 times per second
`(1.5 kHz) for each mobile station 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 signals received at the base station.
`The same closed loop power control technique is also used on the downlink, though here
`the motivation is different: on the downlink there is no near–far problem due to the one-to-
`many scenario. All the signals within one cell originate from the one base station to all
`mobiles. It is, however, desirable to provide a marginal amount of additional power to
`mobile stations at the cell edge, as they suffer from increased other-cell interference. Also on
`the downlink a method of enhancing weak signals caused by Rayleigh fading with additional
`power is needed at low speeds when other error-correcting methods based on interleaving
`and error correcting codes do not yet work effectively.
`Figure 3.9 shows how uplink closed loop power control works on a fading channel at low
`speed. Closed loop power control commands the mobile station to use a transmit power
`proportional to the inverse of the received power (or SIR). Provided the mobile station has
`enough headroom to ramp the power up, only very little residual fading is left and the
`channel becomes an essentially non-fading channel as seen from the base station receiver.
`While this fading removal is highly desirable from the receiver point of view, it comes at
`the expense of increased average transmit power at the transmitting end. This means that a
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`Introduction to WCDMA
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`57
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`Figure 3.9. Closed-loop power control compensates a fading channel
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`mobile station in a deep fade, i.e. using a large transmission power, will cause increased
`interference to other cells. Figure 3.9 illustrates this point. The gain from the fast power
`control is discussed in more detail in Section 9.2.1.1.
`Before leaving the area of closed loop power control, we mention one more related control
`loop connected with it: outer loop power control. Outer loop power control adjusts the target
`SIR setpoint in the base station according to the needs of the individual radio link and
`aims at a constant quality, usually defined as a certain target bit error rate (BER) or block
`error rate (BLER). Why should there be a need for changing the target SIR setpoint? The
`required SIR (there exists a proportional Eb=N0 requirement) for, say, BLER ¼ 1% depends
`on the mobile speed and the multipath profile. Now, if one were to set the target SIR setpoint
`for the worst case, i.e. high mobile speeds, one would waste much capacity for those
`connections at low speeds. Thus, the best strategy is to let the target SIR setpoint float
`around the minimum value that just fulfils the required target quality. The target SIR
`setpoint will change over time, as shown in the graph in Figure 3.10, as the speed and
`propagation environment changes. The gain of outer loop power control is discussed in detail
`in Section 9.2.2.1.
`Outer loop control is typically implemented by having the base station tag each uplink
`user data frame with a frame reliability indicator, such as a CRC check result obtained
`during decoding of that particular user data frame. Should the frame quality indicator
`indicate to the Radio Network Controller (RNC) that the transmission quality is decreasing,
`the RNC in turn will command the base station to increase the target SIR setpoint by a
`certain amount. The reason for having outer loop control reside in the RNC is that this
`function should be performed after a possible soft handover combining. Soft handover will
`be presented in the next section.
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`58
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`WCDMA for UMTS
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`Figure 3.10. Outer loop power control
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`3.6 Softer and Soft Handovers
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`During softer handover, a mobile station is in the overlapping cell coverage area of two
`adjacent sectors of a base station. The communications between mobile station and base
`station take place concurrently via two air interface channels, one for each sector separately.
`This requires the use of two separate codes in the downlink direction, so that the mobile
`station can distinguish the signals. The two signals are received in the mobile station by
`means of Rake processing, very similar to multipath reception, except that the fingers need
`to generate the respective code for each sector for the appropriate despreading operation.
`Figure 3.11 shows the softer handover scenario.
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`Figure 3.11. Softer handover
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