GB2414365A

GB2414365A - Selecting a downlink transmit diversity technique (DTDT) on basis of link characteristic

Info

Publication number: GB2414365A
Application number: GB0411129A
Authority: GB
Inventors: Davood Molkdar; Mayank Kapadia; Nicholas Whinnett
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2004-05-19
Filing date: 2004-05-19
Publication date: 2005-11-23
Anticipated expiration: 2024-05-19
Also published as: GB0411129D0; GB2414365B

Abstract

A method of selecting a downlink transmit diversity technique (DTDT) from amongst a plurality of techniques for communication with a mobile station, characterised by the step of determining at least a first estimated value of a link characteristic and a radio network controller (RNC) arranged to carry out the method. The performance of a particular transmit diversity technique can be dependent on channel conditions and any additional enhancements used in the system. For example, in UMTS, the basic diversity techniques are the open loop (OL) technique and the two closed loop techniques, closed loop mode 1 (CL1) and closed loop mode 2 (CL2) and the standard enhancement is high-speed downlink packet access (HSDPA). HSDPA does not work well with the open loop technique but does work well with some closed loop techniques, e.g. CL1. In addition, the speed of mobile movement can affect a link condition and it is found that at high, medium and low speeds, the best performers are respectively, open loop(OL), closed loop mode 1 (CL1) and closed loop mode 2 (CL2). The invention thus enables the selection of the most appropriate diversity technique. In one embodiment, mobile speed can be presumed by identifying a mobility designation given to the active cells of the mobile, i.e. the mobility designation given according to the location of the cell, e.g. urban, suburban or rural. Open loop techniques include space time transmit diversity (STTD), orthogonal transmit diversity (OTD), delay diversity, time switch transmit diversity (TSTD), phase sweep transmit diversity (PSTD) and some multiple input multiple output (MIMO) techniques and closed loop techniques include selection transmit diversity (STD), transmit adaptive array (TxAA) and closed loop MIMO schemes.

Description

24 1 4365 c Apparatus and method of selecting a downlink transmit

diversity technique

Technical Field

The invention relates to an apparatus and method of selecting a downlink transmit diversity technique. In particular, it relates to an apparatus and method of selecting a downlink transmit diversity technique given the nature of the current link, and any available enhancements.

Background

The standardization of data transmission in wireless communications, whether packet or circuit switched, is expected to result in current wireline services becoming viable to deploy in wireless communication networks.

Typically in such services, the bandwidth of the uplink and downlink channels is asymmetric (for example, asymmetric digital subscriber line or ADSL) with the downlink requiring the greater bandwidth and thus also being the limiting requirement in a fixed-capacity network.

To reduce this imbalance and thus increase overall capacity in a wireless communication system, downlink transmit diversity techniques (DTDTs) of varying complexity and for varying conditions have been standardized Transmit diversity techniques can be broken down into two categories; open loop methods that do not rely on feedback from the receiver to the transmitter, and closed loop methods that do rely on such feedback.

There are many open loop methods of varying complexity and performance including space time transmit diversity (STTD) CE12924EP/GBRl/DRE/Molkdar where diversity is obtained by space-time coding; orthogonal transmit diversity (OTD) where diversity is obtained by splitting data and sending from multiple antennas using orthogonal codes; delay diversity where diversity is obtained by sending a delayed replicas of a common signal from multiple antennas; time switch transmit diversity (TSTD) where diversity is obtained by switching transmissions between antennas, and phase sweep transmit diversity (PSTD) where diversity is obtained by sweeping a phase offset between antennas. Certain forms of multiple input multiple output (MIMO) schemes (where both the transmitter and receiver have multiple antennas) also fall into this category.

Closed loop methods rely on feedback from the receiver to the transmitter to provide the transmitter with information that allows it to adjust phase and/or gain between antennas such that the signal is enhanced at the receiver. Several methods exist e.g. Selection transmit diversity (STD) where the best antennas is/are selected for transmission, or transmit adaptive array (TxAA) where amplitude and/or phase adjustments are made to optimise the received signal at the receiver. Closed loop MIMO schemes are also feasible.

Closed loop methods vary in complexity and performance according to the format of information fed back and the algorithms used to determine this information.

In parallel with such techniques, enhancements independent of transmit diversity have also been developed to mitigate the higher download bandwidth requirement. Of these, some enhancements complement some DTDTs but not others.

CE12924EP/GBRI/DRE/Molkdar In general, there are thee factors to consider in the use of a DTDT: i. A given technique may not perform equally in all channel conditions, and different techniques can work better than others in different conditions; il. Only a given technique or group of techniques may provide an additional benefit when used in conjunction with one of the transmit diversity independent enhancement techniques; and iii. It is not practical to combine different DTDTs over a given link since they all have their own unique signal characteristics.

Thus for a wireless data transmission, it is preferable to select one DTDT given the current link and any available enhancements.

The purpose of the present invention is to address the above problem.

Summary of the Invention

The present invention provides a method of selecting a downlink transmit diversity technique (DTDT) from amongst a plurality of DTDTs for communication with a mobile station (MS).

In a first aspect, the present invention provides a method of selecting a downlink transmit diversity technique, as claimed in claim 1.

CE12924EP/GBRI/DRE/Molkdar In a second aspect, the present invention provides a radio network controller, as claimed in claim 18.

Further features of the present invention are as defined in the dependent claims.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawing, in which:

Brief description of the drawings

Fig. 1 is a block diagram illustrating a decision logic in accordance with an embodiment of the present invention.

Detailed description

A method of selecting a downlink transmit diversity technique (DTDT) from amongst a plurality of DTDTs is disclosed. In the following description, a number of specific details are presented in order to provide a thorough understanding of the present invention. It will be apparent, however, to a person skilled in the art that these specific details need not be employed to practice the present invention. In other instances, well known methods, procedures and components have not been described in detail in order to avoid unnecessarily obscuring the present invention.

The general principle of the present invention is to select or reselect the most appropriate transmit diversity technique for a given link (where "link" refers to communication between one or multiple base stations and a CE12924EP/GBRI/DRE/Molkdar mobile station), dynamically reconfiguring if the link characteristics change beyond a certain threshold.

The process (discussed in more detail later) is illustrated in Fig. 1 for a universal mobile telecommunications system (UMTS), using the DTDTs standardized for UMTS.

In the UMTS standard, there are two basic diversity techniques; open loop (OL) 161 and closed loop (CL) 171- 172. Closed loop has two forms; closed loop mode 1 (CL1) 171 and closed loop mode 2 (CL2) 172.

UMTS also has a standardised enhancement technique, known as high-speed downlink packet access (HSDPA).

Simulation has shown that HSDPA does not benefit from the use of OL, while it does benefit from the use of CL1.

In the UMTS standard, CL2 was not supported with HSDPA as there was a concern that the amplitude adjustment allowed by CL2 (which is not used by CL1) could interfere with the 16QAM demodulation required for HSDPA).

Specifically, for HSDPA each wireless communication device or mobile station (MS) is assigned a "serving cell" from which it will receive HSDPA data on a downlink. The MSs send reports of downlink quality back to the serving cell.

Based on these reports, inter alla, the network scheduler decides when and how much data to send to each MS. The aim of HSDPA is that MSs will be scheduled when the channel fading conditions are such that the instantaneous signal strength at a given MS is at a peak, known as "upfade" scheduling.

CE12924EP/GBRI/DRE/Molkdar However, most downlink transmission diversity techniques try and smooth out the variations in the signal in order to remove the signal strength troughs, and in so doing also remove the peaks. This impairs upfade scheduling. The OL diversity technique is an example of this, and is therefore not desirable in conjunction with HSDPA.

By contrast, HSDPA works with some closed loop transmission diversity techniques as they generally create more peaks as well as removing the troughs.

It is assumed that a cell my perform any of the considered DTDTs. The relative performance of the different diversity techniques OL, CL1 and CL2 is a function of MS speed. OL is the best performer at high speeds, CL1 at medium speeds and CL2 at low speeds: OL does not require any feedback from the MS and so is insensitive to rate of change of the channel and hence MS speed.

CL1 has a fast feedback mechanism (as the applied setting is determined from feedback sent over 2 timeslots), but a relatively coarse granularity of adjustment (resolution = 90 degrees in phase, and no amplitude adjustments).

CL2 by contrast has a slower feedback mechanism (as the applied setting is determined from feedback sent over 4 timeslots), but finer granularity of adjustment than CL1 (resolution = 45 degrees in phase, plus amplitude adjustments).

CE12924EP/GBRI/DRE/Molkdar Thus, and without loss of generalization, one way to characterize the link channel is in terms of MS speed.

One possible method of inferring MS speed is to consider the type of cell serving the MS.

The 3rd generation partnership project (3GPP) rules that the downlink transmit diversity technique shall be common in all cells in an MS's active set, and so preferably MS speed is considered in respect of all the cells in the active set. Clearly however the serving cell alone may be contemplated under different rules.

Consequently in an embodiment of the present invention, a presumption of MS speed may be made by identifying the mobility designation of the MS's active set of cells.

A cell's mobility is a fixed characteristic defined when it is set up, and may be low, medium or high, typically corresponding to urban, suburban and rural geographical areas. The rationale is that urban users are likely to be pedestrian and suburban users will be in speed- controlled zones, while rural users are mostly higher speed traffic (e.g. on motorways).

An alternative or complementary method of estimating MS speed is by direct or indirect measurement. These measurements include any or all of the following; i. MS active set dynamics. The rate of active set change and the number of cells that change in the active set is indicative of the MS's speed; CE12924EP/GBRI/DRE/Molkdar ii. Idle mode activities: cell selection reselection dynamics. Information about the MS's movements prior to making a call may be collected (with an appropriate forgetting factor) to provide an indication of likely MS speed when a call is instigated) iii. Layer change dynamics in hierarchical cell structure (HCS) (pica, micro and macro). As with active set dynamics, the rate of layer change and movement within a layer are indicative of MS speed; iv. Closed loop power control. Information about the rate of step change of power level is indicative of MS speed; and v. MS positioning. If the MS and/or network are equipped with an MS positioning system, this can indicate MS speed.

In an embodiment of the present invention, it is the set of cell mobility designations obtained from the MS's active set are used, in conjunction with knowledge of any enhancement techniques active for the communication, to decide a downlink transmission diversity technique as follows: If at step 100 the MS's active set only includes non- diversity (ND) cells, then use ND 151 irrespective of whether or not transmission is in HSDPA mode.

If at step 110 the MS's active set only includes cells where OL would be preferred or a combination of non- diversity cells and cells where OL would be preferred (i.e. high mobility cells supporting transmission diversity), CE12924EP/GBRI/DRE/Molkdar then use OL 161 irrespective of whether or not transmission is in HSDPA mode.

If at step 120 the MS's active set includes a mix of cells where OL and CL would be preferred (i.e. a mix of high and medium and/or low mobility cells) use OL if transmission is in a non-HSDPA mode, and CL1 171 if transmission is in HSDPA mode.

If at step 130 the MS's active set only includes a mix of cells where CL1 and CL2 would be preferred (i.e. a mix of medium and low mobility cells), use CL1 171 irrespective of whether or not transmission is in HSDPA mode.

If at step 140 the MS's active set only includes cells where CL2 is preferred (i.e. low mobility cells) and transmission is in a non-HSDPA mode, use CL2 172, otherwise use CL1 171.

The above decisions may be performed sequentially, or may be performed independently according to the composition of the MS's active set.

The above decisions result in the selection of the diversity technique best suited to the highest indicated mobility in the active set (higher mobility being preferred), unless a combination with an enhancement technique is known to provide better overall performance.

In short, an embodiment of the present invention assumes the availability of cell mobility designation information.

It also assumes knowledge of DTDT constraints due to combination with other enhancements, the mixture of DTDT CE12924EP/GBRI/DRE/Molkdar modes within the MS active set and the predefined characteristics and capabilities of the mobile and base stations.

The decision logic then: i. Determines the mobility designations within the MS's active set; ii. Determines what enhancements are active for the call; iii. Determines the set of valid DTDTs given the information determined in steps i) and ii) and the active enhancements; and iv. Selects the DTDT mode that gives the best expected performance for the highest mobility type identified in step i).

The above decision logic may clearly be applied to preferences determined by other link channel characteristics than speed, such as delay spread or interference (particularly on the uplink for closed DTDTs).

Such characteristics may be measured or in some cases be related to a predefined characteristic of the cell.

The above decision logic may also clearly be applied to other enhancements of the air interface configuration than HSDPA, such as coding scheme or downlink shared channel (DSCH), or any differing capabilities of mobile stations.

Optionally, the above decision logic may also incorporate the additional constraint in step iii. that a given base station is in fact incapable of performing a given DTDT.

CE12924EP/GBRI/DRE/Molkdar In another embodiment of the present invention, the preferences informing the above decisions at steps 100 to are determined in conjunction with or solely by reference to any or all of the measurements directly or indirectly indicating MS speed listed previously.

Using direct or indirect measurements, the four-point decision logic substitutes the mobility designations for the measurement or measurements made. Decisions analogous to steps 100, 110, 120, 130 and 140 are then applied, assuming a uniform DTDT preference according to the measured speed, or if multiple measurements differ, a mix of DTDT preferences as appropriate.

Thus, for example, an analogous decision 130 would be: If at step 130 the MS's speed measurements only includes a mix of speeds where CL1 and CL2 would be preferred (i.e. a mix of medium and low speeds), use CL1 171 irrespective of whether or not transmission is in HSDPA mode.

Again, it will be clear to a person skilled in the art that measurements indicative of other link channel characteristics may be used in the same manner.

Thus more generally, in an embodiment of the present invention, at least a first estimated value of a link characteristic is determined either by measurement or in relation to a predetermined cell characteristic.

Then, a set of DTDTs is selected that is compatible with the predetermined characteristic or the measured link characteristic value (or at least one of a set of values determined from an MS's active set) and the active CE12924EP/GBRI/DRE/Molkdar enhancements. Finally, the DTDT mode is selected that gives the best expected performance for the link characteristic value (or the preferred value of a set).

In the above embodiments, the chosen diversity technique may be periodically re-evaluated, or re-evaluated in response to an event (e.g. a handover, or a change in a link channel characteristic exceeding a given threshold).

At such a re-evaluation, the technique may be replaced by a preferable alternative in accordance with the process described above; for UMTS, higher layer signalling exists that allows the transmit diversity mode to be changed mid- call.

In an embodiment of the present invention, the selection of a downlink transmit diversity technique is conducted by a radio network controller (RNC) or similar, such as a base station controller. Typically the RNC will comprise decision means arranged in operation to carry out the decision logic described herein, the decision means typically being a processor.

It will be understood that the method of selecting a downlink transmit diversity technique (DTDT) from amongst a plurality of DTDTs for communication with a mobile station as described above, provides the advantages of an overall increase in throughput/capacity for the network.

Claims

CE12924EP/GBRI/DRE/Molkdar Claims 1. A method of selecting a downlink

transmit diversity technique (DTDT) from amongst a plurality of DTDTs for communication with a mobile station (MS), characterized by the step of determining at least a first estimated value of a link characteristic.
2. A method according to claim 1 wherein the estimated value of a link characteristic is determined by measurement.
3. A method according to claim 1 wherein the estimated value of a link characteristic is determined in relation to the value of a predefined cell characteristic.
4. A method according to claim 3 wherein a set of estimated link characteristic values is determined respectively from a predefined cell characteristic of each cell in the MS's active set.
5. A method according to any one of the preceding claims further comprising the step of determining what if any enhancement techniques are active for the communication.
6. A method according to any one of the preceding claims further comprising the step of selecting a set of one or more DTDTs, hereinafter an SOT, compatible with at least one estimated value of the link characteristic and any enhancement technique active for the communication.
7. A method according to claim 6 wherein if the link characteristic value is determined by measurement, the DTDT CE12924EP/GBRI/DRE/Molkdar expected to give the best performance for the measured link characteristic value is selected from the set SDT.
8. A method according to claim 6 wherein if a set of link characteristic values is determined respectively from a predefined cell characteristic of each cell in the MS's active set, the DTDT expected to give the best performance for the preferred link characteristic value of the set of link characteristic values is selected from the set SDT.
9. A method according to any one of the preceding claims wherein the link characteristic is MS speed.
10. A method according to claim 9 wherein MS speed is determined by measurement of any or all of the following: i. MS active set dynamics; ii. idle mode activities (cell selection reselection dynamics); iii. layer change dynamics in the hierarchical cell structure; iv. closed loop power control; and v. MS positioning.
11. A method according to claim 9 wherein a set of cell mobility designations (SCMD) is generated by determining the cell mobility designation of each cell in the MS's active set.
12. A method according to claim 11 further comprising the step of selecting from the set SDT the DTDT expected to give the best performance for the highest mobility designation in the set SCMD.

CE12924EP/GBRI/DRE/Molkdar
13. A method according to claim 4 wherein for a UMTS communication system, if the active set only includes non- diversity (ND) cells, then use an ND technique irrespective of whether or not transmission is in HSDPA mode.
14. A method according to claim 4 wherein for a UMTS communication system, if the active set only includes non- diversity cells and cells where an open loop diversity technique (OL) would be preferable, use OL.
15. A method according to claim 4 wherein for a UMTS communication system, if the active set includes at least a first cell where OL is preferable and at least a first cell where a closed loop diversity technique (CL) is preferable, use CL1 if using high speed downlink packet access (HSDPA), and OL otherwise.
16. A method according to claim 4 wherein for a UMTS communication system, if the active set includes at least a first cell where a closed loop mode 1 diversity technique (CL1) is preferable and at least a first cell where a closed loop mode 2 diversity technique (CL2) is preferable, use CL1.
17. A method according to claim 4 wherein for a UMTS communication system, if the active set only includes cells where CL2 is preferable, use CL1 if using HSDPA and CL2 otherwise.
18. A radio network controller (RNC) arranged in operation to carry out the method as claimed in any one of the preceding claims, and comprising a decision means.

CE12924EP/GBRI/DRE/Molkdar
19. A method of selecting a downlink transmit diversity technique substantially as hereinbefore described with reference to the accompanying drawing.