Subject: Contribution from ETRI and SK Telecom
Date: Fri, 22 Jun 2001 11:13:38 +0900 F
rom: "(Ilkyu Kim)" <igkim@ETRI.RE.KR>
To: 3GPP_TSG_RAN_WG1@LIST.ETSI.FR
(contribution for TSG_RAN_WG1 Rel'5 ad hoc meeting, Espoo, Finland, June 26-28, 2001)
Dear RAN1 colleagues, Here is a contribution on inter-frequency hard handover for next week's meeting in Finland. The main content of this document is the inter-frequency hard handover related problem of IS-95 CDMA based 2nd generation system in Korea. We believe that the WCDMA operators based on the current 3GPP's specifications would be faced with the same problem within one year after they launch the WCDMA services and the problem may be more severe than IS-95 based system.
We look forward to discussions on the inhancement of inter-frequency hard handover for Rel'5 specifications.
Best Regards, ILGYU KIM
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Agenda item: New Topics for Radio Link Performance Enhancements
Source: ETRI and SK Telecom.
Title: Improvement of inter-frequency hard handover for Rel'5
Document for: Discussion and decision
Summary:
In the last WG1 20th meeting in Korea, besides SFN detection related problem [1] of single receiver UE, we introduced that there is another severe problem at the inter-frequency hard handover procedure in current 3GPP's Rel'99 specification and proposed to make a new work item for Rel'5 or to include this issue in the existing inter-frequency measurement work item [2]. And as a candidate solution we proposed an efficient method, in which UE transmits a preamble using the uplink compressed mode before the handover execution and the target BS responds with an acquisition indicator [3].
In that meeting, detailed discussion could not be performed due to lack of time and unfamiliarity of WG 1 colleagues about this problem. So it was concluded that detailed discussion would be performed in the next meeting after WG1 colleagues study the new issue.
This contribution is the extension of Tdoc R1-01-532.
We believe that the WCDMA operators based on the Rel'99 specifications would be faced with the inter-frequency hard handover related problem within one year after they launch the WCDMA services.
In this contribution, in order to continue the discussion for the inter-frequency hard handover, we introduce the situations in IS-95 CDMA based 2nd generation cellular systems in Korea
We propose again to make a work item for the enhancement of inter-frequency hard handover for Rel'5 specifications or to deal with this item in the existing "improvement of inter-frequency and inter-system measurements" work item (RP-0016).
I. The situations in IS-95 based 2nd generation cellular systems in Korea
1.1 Intra-cell hard and inter-cell soft handover method
Figure 1 illustrates a handover situation, which may frequently take place in a CDMA network in which adjacent base stations use different number of carrier frequencies. Generally there is a common frequency allocation (so called "primary FA") in a CDMA network of an operator. Using this primary FA, UE acquires initial network synchronization as well as basic system information. After acquiring the initial network synchronization and basic system information using primary FA, the UE may change the frequency for paging or random access and so on.
In figure 1, f1 is the primary FA and the UE is communicating with home BS with non-primary FA (that is, with f2). In the situation of figure 1, when the UE moves to the handover region, the downlink pilot Ec/Io of home cell become small (that is, link quality becomes worse). When the link quality become worse, the UE performs intra-cell hard handover and then with common frequency, f1, ordinary soft handover is executed. This is so called "intra-cell hard and inter-cell soft handover method".
The basic concept of intra-cell hard and inter-cell soft handover method is to use the common frequency between adjacent BSs. And this approach is based on the fact that the path delays for the different frequencies are the same within home cell.
Figure 1. Intra-cell hard and inter-cell soft handover method
Because IS-95 (A) does not support the compressed mode, the UE cannot measure the signal strength of new f1 link from target BS. So this method should only rely on the UE's the old link(f2 link) quality measurement from home cell. And even though the path delay may be the same between the different frequencies within home cell, the channel fading and user capacity loading characteristics may be different. As a result, the call dropping rate is high when the UE is in handover situation. Even though the call dripping rate is high, this method is utilized optionally in current CDMA cellular network in Korea.
1.2 Dummy signal method
In this method, the target BS transmit dummy down link signal with f2. Using this signal, the UE can estimate the new link quality from the target BS indirectly. In order to cope with cell coverage, the signal at the f2 frequency of the target BS is only the replica of the signal at the f1 frequency and the transmit power is the same.
Figure 2. Dummy frequency method
IS-95 based system is synchronous system between BSs, so it is possible to estimate the round trip delay between UE and the target BS before handover execution. So the uplink handover search window is almost the same as multipath search window of target BS and it is typically 60~80 chips (in this case, the chip rate is 1.2288 Mcps).
Even though the relatively smaller uplink handover search window, the call dropping rate of the above procedure is high. The main reason is the inaccuracy of initial Tx power of the UE after frequency change. The initial transmit power of UE after frequency change should only rely on the open loop power estimate of the signal from target BS. So, the received power at the target BS may be excessively large in some cases but may be very small in other cases This inaccurate initial power causes severe uplink synchronization delay for the UE or severe performance degradation for other UEs in the target BS.
(In the new IS-95 based cdma2000 1x system, the dummy frequency may not be required because it support for the UE to measure the new downlink frequency using a method which similar with compressed mode of WCDMA, but in the uplink, the situation is the same with IS-95 (A) system)
1.3 Dummy signal + intra-cell hard and inter-cell soft handover method
This method is the hybrid method which combines the intra-cell hard and inter-cell soft handover method and dummy frequency method. That is, the UE estimates the new link quality from target cell using the dummy signal and then intra-cell hard handover is performed firstly and ordinary soft handover is performed. This method was devised in order to solve the problem of dummy pilot method and the one of intra-cell hard and inter-cell soft handover method. So the call dropping rate of this method is somewhat less than the ones from previous methods.
Figure 3. Hybrid method
Even though it is, the call dropping rate is much higher than ordinary soft handover case. And when many users are in hard handover situation, the performance of f1 link of home cell grows worse. And the use of this method is very restrictive, because there should be common frequency between adjacent cells. So this method could not used in the hierarchical cell structure or between different operators.
II. Situation of 3GPP's WCDMA specifications
The inter-frequency hard handover procedure of current 3GPP's specification is conceptually similar with dummy pilot method of IS-95 based CDMA system in Korea. The difference is that the WCDMA UE uses the compressed mode for the measurement of the new link instead of dummy frequency. Figure 4 is the general inter-frequency hard handover procedure in WCDMA.
Figure 4. General inter-frequency hard handover in WCDMA
The WCDMA UE can measure the new down link frequency from target cell using the compressed mode so the dummy frequency may not be required in WCDMA systems. But in the uplink, the situation is the same because the WCDMA target BS should find the uplink signal after the UE changes its uplink frequency and the initial transmit power should rely only on the open loop estimation based on the CPICH_RSCP of the target BS. Due to the estimation error and up/down link power unbalancing, the received DPCCH power at the target BS be excessively large in some cases but may be very small in other cases. This inaccurate initial power causes severe uplink synchronization delay for the UE or severe performance degradation for other UEs in the target BS. And if we consider the uplink handover search window of WCDMA, the situation becomes more severe.
Basically the chip rate of WCDMA is 3.125 times as IS-95 CDMA (that is, 3.84/1.2288). So the basic uplink handover search window of WCDMA should be almost 3 times than IS-95 based systems. As stated in the previous page, typical uplink handover search window for IS-95 base system is 60~80 chips. So the one for WCDMA may be 187.5~250 chips. This is only based on the assumption that the network can estimate the round trip delay between UE and target BS before the handover execution. Unfortunately in the WCDMA, it is basically impossible to estimate the round trip delay so that the uplink handover search window should be the sum of maximum round trip delay and delay spread [2]. If we assume the coverage of target cell is 10 km, then the uplink handover search window for that cell should be 300~400 chips. And if we assume the coverage of the cell as 40 km, then the search window should be larger than 1000 chips.
We believe that in addition to the inaccurate initial transmit power after frequency change of UE, the wide uplink handover search window of WCDMA will make the situation more difficult than IS-95 CDMA based 2nd generation systems. So the WCDMA operators based on the current Rel'99 specifications would be faced with the inter-frequency hard handover related problem within one year after they launch the WCDMA services.
III. Conclusions
In addition to the SFN detection related problem of single receiver UE, we feel that the current specification did not fully consider the inter-frequency handover related problem mentioned in this contribution. So in order to improve the performance of inter-frequency hard handover, we propose again to make a work item for the enhancement of inter-frequency hard handover for Rel'5 specifications or to deal with this item in the existing "improvement of inter-frequency and inter-system measurements" work item (RP-0016).
<Reference>
[1] Siemens AG, "Neighbor Cell SFN detection for Handover preparation"; TSG-RAN WG1
meeting #13; Tdoc R1-00-0689, Tokyo, Japan, May 22-25, 2000
[2] ETRI, "Enhancement of inter-frequency hard handover for Rel'5", TSG-RAN WG1
meeting #20, Tdoc R1-01-0532, Busan, Korea, May. 21-25, 2001.
[3] ETRI, "A method for enhancement of inter-frequency hard handover in Rel'5", TSG-RAN
WG1 meeting #20, Tdoc R1-01-0623, Busan, Korea, May. 21-25, 2001. |
