GB2360676A - Extension of midamble codes in a UTRA combined TDD, TDMA and CDMA system - Google Patents

Abstract

It is known to transmit eight signals in a single time slot, each signal having a different spreading code. Each signal consists of a first data burst, a midamble code and a second data burst. The midamble code structure for such signals comprises a base code (12), a repetition thereof (14), and eight midamble codes (16, 18, 20, 22, 24, 26, 28, 30), one for each signal which is to be transmitted in a single time slot. Each midamble code (16, 18, 20, 22, 24, 26, 28, 30) is a copy of the portion of the base code (12) plus repetition (14) which is directly above it. Described herein is a method of increasing the extension for the midamble code at the front thereof. The second midamble code (18) is removed and first midamble code (16') is extended to include the code elements associated with the missing midamble code (18). Extending the first midamble code (16') allows optimal channel estimation for all the signals in that time slot.

Description

IMPROVEMENTS IN OR RELATING TO MOBILE TELECOMMUNICATIONS SYSTEMS The

present invention relates to improvements in or relating to mobile 5 telecommunications systems.

The UMTS terrestrial radio access (UTRA) - time division duplex (TDD) system is based on a combination of code division multiple access (CDNIA) and hybrid time division multiple access (TDMA) and TDD. (UMT'S is an acronym for universal mobile telecommunication system as understood by persons skilled in the art) UTRA TDD operates a combined TDI)/TDMA and CDMA structure. Within any given time slot several CDMA codes can be transmitted contemporaneously on the same frequency. These codes may be transmitted to/from one mobile station or to/from several mobile stations or any combination thereof In the emerging 3 GPP standard, there are fifteen TDI)/TMDA time slots per frame. One of these time slots is designated as a physical random access channel (PRACH). This is the normal position for the transmission by user equipment (UE) to a base station (referred to in 3 GPP as the Node B) of random access transmission which may contend for access in this time slot.

Within the TDMA/TDI) structure of UTRA TDD, the transmission of each time slot consists of three components, namely, a first data burst, a midamble code which serves as a training sequence, and a second data burst.

It should be appreciated that the UTRA TDD structure also contains a CDMA component wherein several spread spectrum modulated signals can be made contemporaneously in any given time slot. Thus, for example, eight signals, each using a different spreading code and having its own unique data bursts and midamble code, may be transmitted in a given time slot. Thus, the first data bursts for each transmission signal are transmitted contemporaneously followed by the midamble codes associated with each transmission signal, followed by the second data bursts. In the downlink direction, that is, from base station to mobile station, all signals are added together before being transmitted from the base station site. In the uplink direction, that is, from mobile station to base station, one or more mobile stations may each transmit one or more signals in a given time slot.

UMTS TDD uses a highly optimised structure for the midamble codes. This structure has been arranged to allow a channel estimate to be obtained from a given midamble code in such a way that interference from other midamble codes transmitted in the same time slot is substantially eliminated - see "Optimum and Suboptimum Channel Estimation for the Uplink of CDMA Mobile Radio Systems with Joint Detection" by Bernd Steiner and Peter Jung, ETT, Vol. 5, No. 1, Jan/Feb 1994, pages 39 to 50.

The principles described in the paper can be summarised as, for a given base station, the same base code is used for all midamble codes. The different midamble codes for the different signal transmissions are obtained as cyclic shifts of an extension to the base code where each midamble code is a copy of the portion of the base code plus repetition directly above it.

In a conventional receiver, a channel estimate would be formed from the midamble code by correlating thereagainst. However, if this is done, there will be substantial interference from the other midamble codes due to their non-ideal cross-correlation properties. As described in the paper referenced above, a cyclic correlation is performed against the midamble code with the reference for the cyclic correlation being the 'inverse' of the midamble base code.

This 'inverse' can be obtained either by forming the inverse of a Toeplitz matrix formed from the base code and reversing the order of the first row or by forming the discrete Fourier transform of the base code, taking the reciprocal of each value and then performing the inverse discrete Fourier transform and reversing the order. Correlation against the reverse of a waveform is equivalent to convolution. Convolution of a code with its 5 inverse essentially creates an impulse.

The base code(s) is(are) selected by computer search to have a reasonably flat spectrum and, in particular, to avoid deep troughs in the magnitude which would lead to peaks in the inverse spectrum.

The shifts in the base code which provide the midamble codes are selected to exceed the maximum delay spread plus delay uncertainties for the anticipated radio channel. Thus, the output of the cyclic correlators will be a set of impulse responses for each of the signals, separated by the shifts. A time slot comprises 2560 chips with the midamble code typically comprising 512 chips. The 512 chips comprises a base code of 456 chips and an extension of 56 chips which accommodates a delay spread of up to 57 chips.

In order to maximise the useful period for multipath resolution, time advance is applied to the UE's transmitted signals. It is necessary for the UE to synchronise to the Node B's transmission and to make a transmission which is received by the Node B which notes the time of arrival in order to implement the timing advance. The Node B uses the information to signal to the UE the amount of time by which it must advance its timing. The initial UE transmission, which is usually made in the PRACH time slot, by definition does not have a timing advance applied. Because of this, it can be substantially delayed. For this reason, the alternate midamble numbers are not used in the random access channel (RACH) slot. This doubles the available delay before ambiguous detection of the channel sounding. By only using alternate midamble codes, the delay spread which can be accommodated is increased from 57 chips to 114 chips.

However, when performing a handover from GSM, the network can tell the UE which time slot to use over the existing GSM link. This means that a dedicated RACH time slot can be provided. This has the disadvantage that no timing advance can be used when making this transmission as there is no two-way signalling, and the dedicated RACH transmission will be misaligned by the round trip delay.

The midamble with also be misaligned with the following effects:- a) there is a potential for ambiguity due to interpreting the RACH midamble code as the midamble code of another signal because of the shifted structure of the midamble codes; b) there will be a breakdown in the orthogonalisation resulting ftom correlating against the 'inverse' code if the misalignment is greater than the overlap region at the end of the code.

It is therefore an object of the present invention to provide a method of optimising channel estimation for all signals within a time slot thereby overcoming the problems discussed above.

In accordance with one aspect of the present invention, there is provided a method of eliminating deconvolution errors in a mobile telecommunications cell comprising a base station and a plurality of mobile stations, each mobile station transmitting a signal burst including a midamble code, the method comprising extending the midamble code at the front thereof to accommodate delay spread.

By increasing the extension of the midamble code, more chips are available for the delay spread thereby accommodating the uncertainty in the delay of the received signal.

For a better understanding of the present invention, reference will now be made. by way of example only, to the accompanying drawings in which:Figure 1 illustrates a basic midamble code structure; Figure 2 illustrates midamble correlations; Figure 3 illustrates a midamble code structure with the second midamble code removed to increase the delay spread which can be accommodated; and Figure 4 is similar to the midamble code structure of Figure 3 with the first midamble code extended in accordance with the present invention.

It is to be noted that Figures 1 to 4 are not timing diagrams and only serve to illustrate how the midamble codes are contructed from a base code.

Figure 1 illustrates a basic midainble structure 10 which comprises a base code 12 and a repetition thereof 14. The base code 12 and the repetition 14 can be through of as each comprising eight code elements, say 1, 2, 3, 4, 5, 6, 7, 8. The structure 10 also comprises eight midamble codes 16, 18, 20, 225 24, 26, 28, 30, one for each signal which is to be transmitted in a single time slot. As discussed above, each midamble code 16, 18, 20, 22, 24, 26, 28, 30 is a copy of the portion of the base code 12 plus repetition 14 which is directly above it. For example, midamble code 16, designated as k = 1, comprises all of base code 12 and one code element 1 of the repetition 14; midamble code 18, designated as k = 2, comprises seven code elements of base code 12, code elements 2, 3, 4, 5, 6, 7, 8 and two code elements 15 2 of repetition 14; midamble code 20, designated as k = 3, comprises six code elements 3, 4, 5, 6, 7, 8 of base code 12 and three code elements 1, 25 3 of repetition 14; midamble code 22, designated as k = 4, comprises five code elements 4, 55 6, 7, 8 of base code 12 and four code elements 1, 2, 3, 4 of repetition 14; midamble code 24, designated as k = 5, comprises four code elements 5, 6, 7, 8 of base code 12 and five code elements 1, 251 351 4, 5 of repetition 14; midamble code 26, designated as k = 6, comprises three code elements 6, 7, 8 of base code 12 and six code elements 1. 2, 35 4, 5, 6 of repetition 14; midamble code 28, designated as k = 7, comprises two code elements 7, 8 of base code 12 and seven code elements 1, 2, 35. 4515, 65 7 of repetition 14; and midamble code 30, designated as k = 8, comprises one code element 8 of base code 12 and eight code elements 1, 2, 3, 4, 5, 6, 7, 8 5 of repetition 14.

From Figure 1 and the example above, it will readily be appreciated that each midamble code 16, 185 20, 22, 24, 26, 28, 30 effectively comprises a discrete code consisting of the entire base code 12 (albeit not in the same order) and one extra discrete portion thereof which is not present in any of the other midamble codes. As discussed above,, each transmitted midamble code comprises 512 chips. However, the receiver only receives 456 chips and the remaining 56 chips allow for the delay spread. It is to be noted that the whole code is required for deconvolution so that deconvolution errors are eliminated.

Figure 2 shows a set of midamble correlation estimates for an uplink connection with eight codes although code 8 is not transmitted. The correlation estimates are complex values and only the magnitudes are shown in Figure 2 for clarity. It will be appreciated that the region of delay or multipath delay spread which can be unambiguously resolved is restricted to the time shift between adjacent midamble codes, that is, the number of code elements shifted multiplied by the chip period.

Figure 3 illustrates a midamble code structure in which the second midamble code is removed. This corresponds to introducing a timing advance into the RACH time slot. As discussed above, the fourth, sixth and eighth midambles could also be removed and not used in the RACH time slot to double the available delay spread before ambiguous detection is obtained.

In accordance with the present invention, Figure 4 illustrates a method of increasing the extension for the midamble code at the front thereof. As shown in Figure 4, midamble code 18 is removed and midamble code 16' is extended to include the code elements associated with the missing midamble code 18. This increase is required only for the RACH burst so that the timing advance can be determined for the UE transmitting the signal having that midamble code. The misalignment will degrade the performance of the joint detector but this effect is not exacerbated by the extended midamble code 16'. Extending the midamble code 16' allows optimal channel estimation for all the signals in that time slot by providing a greater delay spread than that provided by the normal unextended midamble code.

The extension to the midamble code 16' may comprise any number of chips within the range of 1 to 56 chips. This means that the midamble code 16' can comprise 513 to 568 chips.

Claims (2)

CLAIMS:

1. A method of eliminating deconvolution errors in a mobile telecommunications cell comprising a base station and a plurality of mobile stations, each mobile station transmitting a signal burst including a midamble code, the method comprising extending the midamble code at the front thereof to accommodate delay spread.

2. A method of eliminating deconvolution errors in a mobile telecommunications cell substantially as hereinbefore described with reference to the accompanying drawings.