of 20/20
S-72.333 Postgraduate Course in Radio Communications 2001-2002 Handover in WCDMA Uplink and Downlink. Performance of Handover Algorithms Kimmo Hiltunen, 39195V [email protected]

Handover in WCDMA Uplink and Downlink. Performance of ... · PDF [email protected] . S-72.333 Postgraduate Course in Radio Communications Handover in WCDMA Uplink

  • View
    219

  • Download
    5

Embed Size (px)

Text of Handover in WCDMA Uplink and Downlink. Performance of ... · PDF...

  • S-72.333 Postgraduate Course in Radio Communications

    2001-2002

    Handover in WCDMA Uplink and Downlink. Performance of Handover

    Algorithms

    Kimmo Hiltunen, 39195V [email protected]

  • S-72.333 Postgraduate Course in Radio Communications Handover in WCDMA Uplink and Downlink. Performance of Handover Algorithms _____________________________________________________________________________________

    Kimmo Hiltunen 2 26.2.2002

    Contents

    1 HANDOVER IN WCDMA...................................................................................... 3

    2 WCDMA INTRA-FREQUENCY HANDOVER.................................................. 3 2.1 HANDOVER ALGORITHM ..................................................................................... 4

    2.1.1 Mobile Station ............................................................................................ 4 2.1.2 Network..................................................................................................... 12

    2.2 SOFT HANDOVER GAINS ................................................................................... 13 2.2.1 Soft Handover Gain Against Shadow Fading ......................................... 13 2.2.2 The Impact of Soft Handover on WCDMA Capacity .............................. 13 2.2.3 The Impact of Soft Handover on WCDMA Coverage ............................. 15

    2.3 SOFT HANDOVER PROBABILITIES...................................................................... 15

    3 WCDMA INTER-FREQUENCY HANDOVER ................................................ 17 3.1 INTER-FREQUENCY HANDOVERS WITHIN WCDMA ........................................ 17

    3.1.1 Inter-Frequency Measurements. Compressed Mode .............................. 17 3.2 INTER-SYSTEM HANDOVERS BETWEEN WCDMA AND GSM.......................... 18

    4 SUMMARY............................................................................................................. 19

    5 REFERENCES ....................................................................................................... 19

  • S-72.333 Postgraduate Course in Radio Communications Handover in WCDMA Uplink and Downlink. Performance of Handover Algorithms _____________________________________________________________________________________

    Kimmo Hiltunen 3 26.2.2002

    1 HANDOVER IN WCDMA

    The handovers in WCDMA can be divided into two groups: soft and hard handovers. In case of a soft handover the mobile is simultaneously connected to several base stations, while in case of a hard handover the mobile is connected to one base station at a time. Soft handover is possible only between intra-frequency cells, i.e. cells using the same frequency. Hard handover, however, can be performed either between two intra-frequency cells, two inter-frequency (WCDMA) cells, or between WCDMA and GSM. In the latter case the handover is typically called as inter-system handover. Finally, intra-frequency handovers are Mobile Estimated Handovers (MEHO), while inter-frequency and inter-system handovers are Network Evaluated Handovers (NEHO). However, mobile measurements can be used to trigger the required inter-frequency and inter-system measurements.

    2 WCDMA INTRA-FREQUENCY HANDOVER

    For dedicated channels a soft handover is typically performed, while for shared and common channels a hard handover is the only possibility. However, the specifications allow hard handover even for dedicated channels [1]. Hard handover for dedicated channels may be required for example when inter-RNC handover is not possible due to Iur congestion.

    In this paper the main focus is on intra-frequency soft handover. Soft handover has a special importance in CDMA based systems, due to its close relationship to power control. CDMA systems are interference-limited meaning that their capacities are closely related to the amount of interference the systems can tolerate. Due to its effective frequency reuse factor of one, a CDMA system cell is affected, not only by the interference within its own cell, but also by the interference from its neighboring cells. To alleviate level of interference, and thus increase the capacity and quality, CDMA systems employ power control. Power control attempts to solve the near/far problem by adjusting the transmit power so that the target C/I is evenly satisfied. The fundamental idea behind the power control is to restrain from transmitting more power than necessary in order to limit the interference. With power control, each mobile and base station is disciplined to transmit just enough power to meet the target C/I level. But in order for power control to work effectively, the system must ensure that each mobile station is connected to the base station with the strongest signal at all time, otherwise a positive power feedback problem can instabilize the entire system. Thus, an instantaneous handover from the current cell to the new cell would be required when the signal strength of the new cell exceeds the signal strength of the current cell. This is not, however, feasible in practice. Since in soft handover the mobile is connected to either two or more base stations, its transmission power can be controlled according to the cell, which the mobile station receives with the highest signal strength. A mobile station enters the soft handover state when the signal strength of a neighboring cell exceeds a certain threshold but is still below the signal strength of the current cell.

    Another fact in favour of the soft handover is that it offers gain both in system capacity and coverage. First of all, the handover, hard or soft, reduces the required log-normal shadow fading margin. This is because the shadow fading is partly uncorrelated

  • S-72.333 Postgraduate Course in Radio Communications Handover in WCDMA Uplink and Downlink. Performance of Handover Algorithms _____________________________________________________________________________________

    Kimmo Hiltunen 4 26.2.2002

    between the sectors, and by making a handover the mobile can select a better base station. Soft handover gives an additional macro diversity gain against fast fading by reducing the required Eb/N0 relative to a single radio link, due to the effect of macro diversity combining. All these different kinds of soft handover gains are discussed further in Chapter 2.2.

    In the uplink direction, one typically separates the scenario where the mobile is communicating with different sites (soft handover) from the scenario where the mobile is communicating with sectors belonging to the same site (softer handover). In case of softer handover the combining of the links is performed internally within the site. Basically the question is about a similar RAKE processing (maximum ratio combining) as in case of multipath/antenna diversity. In case of soft handover selection combining is performed within the RNC. This is done by selecting the best received frame from all the RBS in the Active Set on a frame-by-frame basis.

    In the downlink direction the same signal is transmitted from all RBS within the current Active Set. The links are maximum ratio combined within the mobile stations RAKE receiver. However, opposite to the uplink, in the downlink the soft handover creates more interference to the system, since the new base station now transmits an additional signal for the mobile station. Therefore, it is possible that the mobile station is not able to collect all the energy that the base station transmits due to a limited number of RAKE fingers. Thus, the soft handover gain in the downlink depends on the gain of macro diversity and the loss of performance due to increased interference.

    2.1 HANDOVER ALGORITHM

    As mentioned, the intra-frequency soft handover is a MEHO algorithm. This means, that the mobile station monitors the air interface following the instructions supplied by the RNC, and reports the measurement results back to the RNC. The RNC makes then the final decision on which cells to add, remove, or replace.

    Next, the soft handover algorithms implemented within the mobile station and the network are presented in more detail.

    2.1.1 Mobile Station

    A simple overview of the UE (User Equipment) model is presented in Figure 1 [2]. All the intra-frequency measurements are performed on the downlink Primary Common Pilot Channel (P-CPICH). The definitions of the different possible measurement quantities (i.e. Ec/I0, path loss and RSCP, Received Signal Code Power) and their performance requirements can be found in [2], [3] and [4]. First, the measurement samples are filtered with the Layer 1 filter in order to average out of the impact of multipath fading. Furthermore, in order to give the operator the possibility to have a better control over the accuracy of the measurements, a special higher layer (i.e. layer 3) filter model is specified in [1]:

    ( )2

    1

    2

    1k

    nnn

    a

    XaFaF

    =

    +=

    (2.1)

  • S-72.333 Postgraduate Course in Radio Communications Handover in WCDMA Uplink and Downlink. Performance of Handover Algorithms _____________________________________________________________________________________

    Kimmo Hiltunen 5 26.2.2002

    where Xn is the latest received measurement result from the physical layer measurements, Fn is the filtered output from the layer 3 filter, k is the L3 filter parameter supplied by the operator.

    The physical layer measurement results are sampled once every measurement period. The measurement period and the accuracy for a certain measurement is defined in [4]. With the help of the layer 3 filter the operator can easily increase the accuracy of the measurements, with the price of an increased delay.