Code division multiple access system having improved pilot channels

In a CDMA network, each mobile is arranged to combined pilot symbols from its dedicated pilot channel with pilot symbols from at least one common channel as an input to its channel impulse response sensing means. The common channel may be a BCH, a FACH or a PCH, and the pilot symbols on all such channels may be combined. The pilot energy required on each dedicated pilot channel can thereby be reduced. By communicating the quality of the received pilot symbols to a base station, the base station can reduce the energy on the dedicated pilot channel, providing a yet further saving of energy.

FIELD OF THE INVENTION

This invention relates to a code division multiple access (CDMA) system, especially a wide band or direct sequence (DS) CDMA system, and relates particularly to the arrangements for providing pilot channels.

BACKGROUND OF THE INVENTION

For effective use of direct sequence CDMA systems for digital mobile cellular telephone and personal communication network applications, a detection technique must be used which performs well at low signal to interference ratios. Coherent detection is preferred to non-coherent detection because it has better performance in the slow fading environments which typify personal communication channels. To apply coherent detection, the channel impulse response at a receiver must be known, and this can be achieved by transmitting pilot symbols.

Pilot symbols can be transmitted in two ways; a) a dedicated pilot channel, i.e. one pilot channel for each user, in which pilot symbols are embedded periodically (time- or code-multiplexed) in the same channel as the data symbols, or b) a common pilot channel, i.e. one pilot channel for all users, in which pilot symbols are continuously sent on a separate channel in parallel with data channels.

An advantage of dedicated pilot channels is that power can be varied, so that a mobile at a boundary of a cell can ramp up the power of its received symbols to overcome channel propagation as well as fast fading; however the system relies on goodstatistical multiplexingof users to ensure that there is always spare transmitter capacity to meet a sudden demand from a mobile for increased power, which can create instability.

The well known differences between the two arrangements will now be described with reference toFIGS. 1-6.

FIG. 1ashows the sector coverage angle alpha (e.g. 30° to 40°) over which a small base station transmits.FIG. 1bindicates by the enclosed area the energy Edrequired to transmit data, and this is assumed to be constant.FIG. 1cindicates by the shaded area the energy Eprequired to transmit pilot symbols in either a dedicated pilot channel or a common pilot channel.

FIG. 2illustrates energy requirements in a common pilot channel arrangement, and is effectively a merger ofFIGS. 1band1c; a single continuous pilot channel is broadcast to all users.

FIG. 3illustrates energy requirements in a dedicated pilot channel arrangement; each of the N users (where N±5) has a different energy requirement E1to E5, shown by the shaded and crosshatched areas. The total energy requirement for the pilot channels is N*Ep. This arrangement assumes there is no power control facility to vary power transmission.

FIG. 4shows a variation ofFIG. 3including a power control facility. The power supplied to each pilot channel can be controlled individually, as indicated by the different areas of the shaded and crosshatched bands E6to E10. At certain times, in theory, the pilot in a channel can even be switched off completely, saving energy, and allowing other data or control information to be transported by that channel. Pilot energy requirement is ΣEp,i·βiwhere βiis the scaling factor for each user, dependant on power control and time multiplexing. β is between 1 and 0, i.e. it is small when a mobile is close to its base station.

However comparison withFIG. 1cshows that the total power used is unchanged.

FIG. 5indicates energy requirements where spatially adaptive antennas are used. Data energy is transmitted in much narrower sections α1to α4within the sector angle α, i.e. a beam forming technique is used. The narrow sectors α1to α4are directed towards active mobiles, and the pilot energy required for each narrow sector is also varied in accordance with need, as indicated by the shaded areas. The total energy requirement is greatly reduced. The pilot energy requirement is

∑iN⁢⁢βi⁢Ep⁢GA
where GAis the gain of the directed antennas.

FIG. 6shows that, in addition to the directed channels ofFIG. 5, some common channel facility is required across the whole sector angle α, e.g. for mobiles attempting to make a call, and the data power for this is indicated at Edc, between the directed sectors with the associated pilot energy indicated by the cross-hatched areas Epc. Pilot energy requirements are

∑iN⁢βi⁢Ep⁢GA+∑iC⁢βi⁢Ep,
where C is the number of common channels.

BRIEF SUMMARY OF THE INVENTION

It is the object of the invention to provide a pilot channel arrangement having reduced energy requirements.

According to the invention in a code division multiple access mobile radio telecommunications network, a method of providing pilot symbols comprises providing a first set of pilot symbols through a plurality of pilot channels, each such channel being dedicated to one mobile user; providing a second set of pilot symbols through at least one common control channel; and in a mobile combining the first and second sets of pilot symbols and providing said combination to channel impulse response sensing means.

In effect the common pilot energy

∑iC⁢βi⁢Ep
is use by a mobile in addition to pilot energy provided on its dedicated pilot channel.

The common control downlink channel may be a broadcast channel or a forward access channel or a paging channel.

Also according to the invention a code division multiple access mobile radio telecommunications network comprising a plurality of mobiles each having a dedicated pilot channel; a plurality of base stations; first pilot symbol generation means arranged to supply pilot symbols to each dedicated pilot channel; second pilot symbol generation means arranged to supply to at least one common control channel dedicated pilot symbols embedded between data symbols broadcast by the common control channel; and in each mobile receiving means arranged to receive pilot symbols in the dedicated pilot channel and the common control channel, combining means to combine the received pilot symbols, channel estimation means to process the combined pilot symbols, and coherent detection means arranged to vary at least one property of the mobile in accordance with the output of the channel estimation means.

The invention will now be described by way of example with reference toFIGS. 7 to 10in which:—

DETAILED DESCRIPTION

InFIG. 7a wireless telecommunication system10comprises a number of mobile stations (MS)11,12,13,14and a number of base transceiver stations N node B15,16,17,18connected through a radio network controller (RNC)19,20(all in the radio access network RAN21) to a core network (CN)22. The CN is connected to the public switched telephone network PSTN23.

InFIG. 7, each mobile12is provided with a dedicated pilot channel which carries pilot symbols to the mobile; the mobile uses these symbols to determine the extent of some of the key radio channel effects on desired transmitted signal to the mobile.

Examples of these radio channel effects are:i) Offset in frequency due to the well known mobile radio channel Doppler effect;ii) Offset in timing synchronization due to multipath propagation;iii) Energy loss in the transmitted signal due to propagation loss and fast-fading induced by multipath propagation.
With a knowledge of the extent of key radio channel phenomena the mobile can configure the various functions/processes/schemes that demodulate the received signal, such as the timing and tracking synchronization and channel estimation, to minimize the distortion caused by radio channel phenomena. In addition, the mobile can provide feedback to the network, suggesting means to vary certain characteristics of its downlink reception and demodulation.

In the system illustrated inFIG. 7, as is well known, there are a number of common control channels in the downlink which are commonly broadcast by the network; such channels typically include a broadcast channel BCH, a forward access channel FACH and a paging channel PCH.

The BCH is used to provide cell-specific information, such as the cell identity and the available short and long codes for random access channel RACH transmission; information about neighbouring cells can also be provided. In the majority of cell scenarios the information carried by the BCH can be assumed to be static for the duration of most telephone calls.

The FACH is primarily used to carry initial call set-up control information to a mobile when the system knows the location cell of the mobile. The FACH can also carry short intermittent packet information.

The PCH is used to carry information primarily to initiate network originated calls, eg from a landline telephone, to a mobile station when the system does not know the location cell of the mobile. The PCH may have asleep modewhen traffic is low.

These and other common control channels are separated from one another by channelisation codes, and possibly also be fixed time-offsets.

In the arrangement there is no common pilot channel, therefore each channel in addition has its own dedicated pilot symbols which are embedded at regular intervals between the transmitted data symbols.

In the present invention a mobile12utilises the pilot symbols in existing common downlink channels in addition to the pilot symbols provided by its own dedicated pilot channel or channels. By use of such a combination decreased energy is required in the dedicated pilot channel of the mobile.

FIG. 8shows the energy flows. The pilot channel of the mobile12is referred to as a hybridised common pilot channel HCPC30, and pilot information from it flows through a first rake finger32to a channel estimator in two stages34,36. Pilot information from at least one of the downlink channels, indicated at38as a dedicated traffic channel, passes through a second rake finger40to the channel estimators34,36. (Although two sets of rake fingers are shown, in practice only one may be needed). The output of the estimators34,36passes to a conjugate multiplication stage42which also receives input directly from the dedicated traffic channel, and then to a maximum ratio combining stage44. The output of the combining stage, connected to the processor of the mobile, indicates channel impulse response of the mobile's receiver, and permits use within the mobile of coherent detection techniques.

The channel estimation functional units34,36, are shown in two stages to indicate the possibilities of:i) Combining in stage34the de-spread pilot symbol energy from the dedicated traffic channel and Hybridised Common Pilot Channel sources to create effectively a single pilot source which is then used to estimate the extent of channel distortion in stage36;ii) Independently calculating channel estimates from the two despread pilot symbol sources in stage34and then combining the two resultant sets of channel estimates in stage36.
The channel estimates produced by the channel estimation functional units34,36are used by the conjugate multiplication stage42to mitigate the effects of channel distortion on the desired de-spread signal using coherent detection.

The use of information from at least one broadcast channel allows lower pilot energy input through the hybrid channel, i.e. inFIG. 2, Epcan be reduced.

Usually there will exist an almost continuous stream of pilot symbols from the available downlink several common control channels such as the BCH, FACH, and PCH. Since all the channels experience the same channel conditions, the mobile can obtain accurate and robust multi-path tracking information, as well as channel estimates.

The energy flow arrangement illustrated inFIG. 8applies when there is a non-zero offset between the two sets of pilots; the parallel receiver structure allows the two sets of pilot symbols to be demodulated in parallel.

If there is no time offset between the two sets of pilot symbols, parallel pilot symbol energy flows such as illustrated inFIG. 8will not be necessary, and a sub-set of existing rake fingers can be allocated to demodulate just the multipath components which are deemed to require extra robustchannel estimation; this requires additional rake finger management.

A typical mobile12is shown inFIG. 9. It has a RF transceiver50connected to a baseband demodulator52which passes control data to a control signal processor54and data signals to a decoder connected to a user data processor58.

A further reduction in pilot energy can be achieved by incorporation of base station (BTS) intervention. The mobile12is arranged to provide feedback signals to the BTS about the quality of its HCPCH, ie the noise or power or phase rotation of the pilot symbols. The BTS can then reduce the power of the pilot symbols in the mobile's dedicated pilot channel in comparison with the power of the data symbols. A further advantage of such a power variation is that, depending on the cell scenario, it will reduce the overall power transmitted by a BTS on the downlink, which for a multi-user CDMA system (FIG. 7) improves the downlink capacity.

To provide BTS intervention, additional parts of the mobile12are affected; referring toFIG. 9, the control data processor provides the required information to the BTS by a loop to the base band demodulator60in the mobile, its output being connected to the RF transceiver50.

A yet further improvement is provided by adapting one of the common channels, using the BTS intervention arrangement described above. The channel and tracking estimation stages,34,36inFIG. 8, are arranged to estimate the performance gains from the HCPCH; this performance gain can be improved by adapting the time-offsets between the pilot symbols belonging to the common channel(s) being used to generate the HCPC and the pilot symbols transmitted on the dedicated traffic channel(s) to suit channel conditions. In most circumstances it will be preferable to have the HCPCH pilot aligned in time with the DTCH to simplify the channel estimation combining process represented by34,36. However, in some circumstances, e.g. when the channel is varying very quickly, it will be preferable to have the HCPCH pilots occurring half-way between DTCH pilots; this can significantly improve multi-path tracking performance.

Such an arrangement affects a base station BTS; a typical arrangement is shown inFIG. 10in which a mobile70is connected to a BTS72which is controlled by a RNC74. The RNC74controls the timing of the pilot symbols in the common channels which provide inputs as dedicated traffic channel energy inFIG. 8, and can therefore vary the timing of the symbols with respect to the dedicated pilot channel symbols, as required.

Instead of shifting the timing of the pilot symbols in the common channels, in another variation the RNC74can be arranged to substitute pilot symbols for data symbols on common channels such as the FACH or PCH to create Extended HCPC (EHCPC) channels. Doing this trades-off common channel capacity (eg the number of calls that can be set up or acknowledged in a unit of time) for improved channel estimation at a mobile.

Alternatively without the need for RNC intervention, the mobile can use the effectively static data symbols on a common channel such as the BCH as pilot symbols to create EHCPC. A mobile must always listen to such a channel for new calls, or for a paging service message, and such channels are rarely congested.