WO1999065166A2

WO1999065166A2 - Wireless telecommunications system employing a coherently modulated time slot having differentially modulated pilot symbols

Info

Publication number: WO1999065166A2
Application number: PCT/EP1999/003954
Authority: WO
Inventors: Risto Paatelma
Original assignee: Nokia Mobile Phones Limited
Priority date: 1998-06-08
Filing date: 1999-06-08
Publication date: 1999-12-16
Also published as: WO1999065166A3; AU5279799A

Abstract

A method operates a wireless mobile or user station to receive a time slot sent through a radio channel from a transmitter, the time slot being coherently modulated except for a portion that contains a predetermined symbol sequence that is differentially modulated. A next step differentially detects the portion of the received time slot that contains the predetermined symbol sequence. A further step of the method operates to perform a channel estimation for a coherent detector that is used to detect other portions of the time slot. By example, the predetermined symbol sequence is modulated in accordance with one of Differential Quadrature Phase Shift Keying (DQPSK), Differential 8 Phase Shift Keying (D8PSK), π/4-shifted DQPSK, or 16 level DQPSK. In a preferred embodiment of the invention the portion of the time slot that is differentially modulated is a CDVCC/PILOT field, while the data fields and other fields are modulated using 8PSK.

Description

Wireless Telecommunications System Employing a Coherently Modulated Time Slot Having a Differentially Modulated Field (CDVCC) Providing Pilot Symbols Useful for Channel Estimation

This invention relates generally to radiotelephones and, in particular, to radiotelephones or mobile stations such as those capable of operation with a digital wireless communication network.

Fig. 1 A, 1 B and 1 C show the frame and time slot formats for an exemplary prior art digital Time Division Multiple Access (TDMA) cellular air interface known in the art as IS-136 (see, for example, IS-136.1, Rev. A, 3/21/96 and IS-136.2, Rev. A, 2/12/1996, as well as later revisions).

Fig. 1A shows that a 40 millisecond frame consists of six time slots. Slots 1-3 and 4-6 each comprise one TDMA Block. In the forward direction from a base station to a mobile station the frames are continuously transmitted. A given mobile station is assigned to receive in one time slot per frame for a half data rate case, and is assigned to receive in two time slots for a full data rate case. Fig. 1B illustrates the format of one slot in the direction from the mobile station to the base station, while Fig. 1C illustrates the format of one slot in the direction from the base station to the mobile station. The base station forms a part of a Base Station/Mobile Switching Center/I nterworking function (BMI).

Of particular interest herein is the Coded Digital Verification Color Code (CDVCC) field shown in Figs. 1B and 1C. The CDVCC is a 12-bit data field containing an 8-bit Digital Verification Color Code (DVCC) and four protection bits. The DVCC is sent from the base station to the mobile station. The CDVCC is sent in each digital traffic channel (DTC) time slot sent to and received from the mobile station. It is used to indicate that the correct rather than co-channel data is being decoded.

As defined in Section 2.4.3 of IS-136.1 , a DVCC of 8-bits is coded with a (15,11 ) Hamming code shortened to (12,8), giving a CDVCC of 12-bits. In the mobile station the CDVCC is decoded and compared to a previously received and stored DVCC.

In conventional IS-136 operation the modulation scheme uses the π/4-shifted DQPSK constellation shown in Fig. 1D. Gray code is used in the mapping; i.e., two di-bit symbols corresponding to adjacent signal phases differ only in a single bit. Since most probable errors due to noise result in the erroneous selection of an adjacent phase, most di-bit symbol errors contain only a single bit error. Note the rotation by τr/4 of the basic QPSK constellation for odd (denoted θ) and even (denoted <8>) symbols.

In this modulation technique the information is differentially encoded and symbols are transmitted as changes in phase rather than absolute phases. For a corresponding differential encoder the binary data stream (b enters the modulator and is converted by a serial-to-parallel converter into two separate binary streams (X_k) and (Y_k). Starting from bit 1 in time of stream b_m, all odd numbered bits form stream X_k and all even numbered bits form stream Y_k.

In an enhanced version of IS-136 (TIA IS-136, Rev. C) that has been proposed a total of four coherently modulated (8PSK) pilot symbol sequences are defined per time slot. Reference in this regard can be had to Fig. 1 E, which illustrates a currently proposed slot format. Each pilot symbol sequence consists of three symbols. In this enhanced TDMA system there is a capability for double rate data as well as triple rate data in various combinations, which are useful in transmitting packet data. In the conventional IS-136 π/4-shifted DQPSK slot format that was described above the CDVCC field can be used to determine the channel quality. However, for the proposed 8PSK slot format these symbols were removed at least on part to increase the gross or maximum bit rate, and because of a channel estimation and detection problem that arose. Since the CDVCC symbols are not reserved in the proposed slot format, this enables the transmission of data bits instead of the CDVCC bits.

The channel estimation/detection problem is as follows. The coherent (e.g., 8PSK) receiver requires that a channel estimation be performed. However, if the predetermined CDVCC symbols are first used to make the channel estimation, and the CDVCC sequence is then detected using this channel estimation, the result is highly biased. More generally, if in the channel estimation the coherent receiver assumes that sequence d was transmitted, and if then the decision is made based on this channel estimation, the decision will more likely yield the sequence d than some other sequence. In any event, reliable information regarding the amount of interference in the channel is not obtained.

It is thus a first object and advantage of this invention to provide a technique to overcome the aforementioned problems by differentially coding the CDVCC for use as pilot symbols, thereby enabling the use of the CDVCC to obtain an accurate channel interference estimation that is independent of a channel quality estimation.

It is a further object and advantage of this invention to provide a technique for detecting a time slot symbol sequence, wherein a phase of one symbol is known beforehand, and wherein a receiver has knowledge of the sequence, such as the CDVCC, such that the receiver can reconstruct the transmitted signal, thereby enabling the sequence to be used as a pilot sequence.

The foregoing and other problems are overcome and the objects and advantages are realized by methods and apparatus in accordance with embodiments of this invention.

A method of this invention is disclosed for operating a wireless mobile or user station to receive a time slot sent through a radio channel from a transmitter, the time slot being coherently modulated except for a portion that contains a predetermined symbol sequence that is differentially modulated. A next step differentially detects the portion of the received time slot that is comprised of the predetermined symbol sequence. A further step of the method operates to perform a channel estimation for a coherent detector used to detect the other portions of the time slot.

By example, the predetermined symbol sequence is modulated in accordance with a differential phase shift technique, such as Differential Quadrature Phase Shift Keying (DQPSK), Differential 8 Phase Shift Keying (D8PSK), π/4-shifted DQPSK, or 16 level DQPSK. By example, in one embodiment first and fourth occurring symbol sequences within a time slot are differentially modulated using DQPSK, while the second and third occurring symbol sequences are differentially modulated using D8PSK. In another embodiment all of the symbol sequence occurrences are differentially modulated using D8PSK. In a preferred embodiment of the invention the portion of the time slot that is differentially modulated is a CDVCC/PILOT field, while the data fields and any other fields are modulated using 8PSK. The above set forth and other features of the invention are made more apparent in the ensuing Detailed Description of an example of the Invention when read in conjunction with the attached Drawings, wherein:

Figs. 1A, 1 B and 1C depict the frame and time slot formats for an exemplary prior art TDMA air interface;

Fig. 1D illustrates the ττ/4 shifted DQPSK phase constellation used by the exemplary prior art TDMA air interface;

Fig. 1E depicts a proposed forward link slot format for use in a coherently modulated (8PSK) TDMA air interface;

Fig. 2 is a block diagram of a mobile station that is constructed and operated in accordance with this invention;

Fig. 3 is an elevational view of the mobile station shown in Fig. 2, and which further illustrates a cellular communication system to which the mobile station is bidirectionally coupled through wireless RF links;

Figs. 4A and 4B each depict an embodiment of a pilot symbol structure in accordance with this invention, wherein Fig. 4A depicts a DQPSK CDVCC/PILOT for the first and last pilot sequences shown in Fig. 1 E, and Fig. 4B depicts a D8PSK CDVCC/PILOT, or the middle pilot sequences if the DQPSK technique of Fig. 4A is employed; and

Fig. 5 is a diagram that is useful in explaining how the pilot symbols are modulated in accordance with one exemplary embodiment of this invention, wherein a pilot block includes three symbols, where a first symbol is a phase reference, and the second and third symbols encode two bits each using DQPSK.

Reference is made to Figs. 2 and 3 for illustrating a wireless user terminal or mobile station 10, such as but not limited to a cellular radiotelephone or a personal communicator, that is suitable for practicing this invention. The mobile station 10 includes an antenna 12 for transmitting signals to and for receiving signals from a base site or base station 30. The base station 30 is a part of a cellular network comprising the Base Station/Mobile Switching Center/I nterworking function (BMI) 32 that includes a mobile switching center (MSC) 34. The MSC 34 provides a connection to landline trunks when the mobile station 10 is involved in a call.

The mobile station includes a modulator (MOD) 14A, a transmitter 14, a receiver 16, a demodulator (DEMOD) 16A, and a controller 18 that provides signals to and receives signals from the transmitter 14 and receiver 16, respectively. These signals include signalling information in accordance with the air interface standard of the applicable cellular system, and also user speech and/or user generated data. The air interface standard is assumed for this invention to include a physical and logical frame and time slot structure, although the teaching of this invention is not intended to be limited for use only in TDMA type systems.

It is understood that the controller 18 also includes the circuitry required for implementing the audio and logic functions of the mobile station. By example, the controller 18 may be comprised of a digital signal processor device, a microprocessor device, and various analog to digital converters, digital to analog converters, and other support circuits. The control and signal processing functions of the mobile station are allocated between these devices according to their respective capabilities. For the purposes of this description the controller 18 is assumed to contain or implement a differential CDVCC/PILOT detector 18a, as described in further detail below, as well as a coherent (e.g., 8PSK) detector 18b for detecting the data and other fields of received time slots. The coherent detector 18b requires that a channel estimation be performed.

More particularly, and referring also to Fig. 1 E, differentially modulated CDVCC/PILOT sequences are detected by the differential detector 18a, and an approximate estimation of channel quality is obtained. Since the CDVCC/PILOT sequences are known a priori, the coherent detector 18b can reconstruct the transmitted signal and thereby estimate the channel. The data fields are then detected by the coherent detector, using the obtained channel estimation. The problem of generating a biased estimation of the channel is thus overcome.

A user interface includes a conventional earphone or speaker 17, a conventional microphone 19, a display 20, and a user input device, typically a keypad 22, all of which are coupled to the controller 18. The keypad 22 includes the conventional numeric (0-9) and related keys (#,*) 22a, and other keys 22b used for operating the mobile station 10. The mobile station 10 also includes a battery 26 for powering the various circuits that are required to operate the mobile station.

The mobile station 10 also includes various memories, shown collectively as the memory 24, wherein are stored a plurality of constants and variables that are used by the controller 18 during the operation of the mobile station.

It should be understood that the mobile station 10 can be a vehicle mounted or a handheld device. It should further be appreciated that the mobile station 10 can be capable of operating with one or more air interface standards, modulation types, and access types.

In the proposed enhanced IS-136 air interface of most interest to this invention, which uses coherent 8PSK modulation, there is no requirement for an equalizer, and a so-called single shot receiver can be used. In the case of differentially coded modulation the traditional differential detector is very practical, but its performance is poorer than that exhibited by a coherent receiver. However, one benefit of the differential detector is that it does not require that any channel estimation be performed, since the previously received symbol is used as the phase reference for the current symbol. Hence, if the CDVCC sequence within the time slot is differentially modulated, the CDVCC symbols can be detected with a differential detector without channel estimation.

The inventor has realized that the symbol (bit) error probability of the CDVCC bits is independent of the channel estimation if these bits are differentially detected. The coherent detector that is used for the detection of the data and other fields of the time slot can, however, calculate the phase of these symbols, and it can use these symbols as a pilot sequence. While coherent modulation requires the use of pilot tones or symbols, a differentially encoded CDVCC, when used as a pilot sequence, does not adversely affect the bit rate (the number of data symbols).

The transmitter of the base station 30 is assumed to be capable of transmitting most of the time slot modulated using the 8PSK constellation, and for transmitting the CDVCC portion modulated using some type of differential phase shift constellation, such as a DQPSK or a D8PSK constellation. The modulation of the CDVCC could also be, by example, π/4-shifted DQPSK, as shown in Fig. 1D, or even 16 level differential phase shift modulation. The method in accordance with this invention thus enables one to obtain the benefits of the current CDVCC technique, wherein a bit error rate measurement can be derived from the CDVCC sequences, for use in making the channel quality estimation. Since the pilot symbols are being employed for this purpose, which must be present in any case for use by the coherent modulator/demodulator, the maximum data transfer rate of the channel is not adversely affected.

Presently preferred pilot symbol structures are shown in Figs. 4A and 4B. More specifically, Figs. 4A and 4B each depict an embodiment of a pilot symbol structure in accordance with this invention, wherein Fig. 4A depicts a DQPSK CDVCC/PILOT for the first and last pilot sequences (i.e. PILOT-1 and PILOT-4) shown in Fig. 1E, and Fig. 4B depicts a D8PSK CDVCC/PILOT that can be used for all four pilot sequences, or for the middle two pilot sequences (i.e., PILOT-2 and PILOT-3) if the DQPSK technique of Fig. 4A is employed for the first and last sequences. The phase reference for the differentially coded CDVCC/PILOT field is the first pilot symbol, which is set to a value of, by example, 1+j0 (phase 0°).

The following discussion presents but one example of how this invention can be applied, and is not to be read or construed in a limiting sense upon the scope and practice of this invention.

More particularly, in the IS-136, Rev. C embodiment used herein there are 12 CDVCC bits and 12 pilot symbols. The DQPSK modulation encodes 2- bits/symbol, while the 8PSK modulation encodes 3-bits/symbol. In the pilot sequence or tone there must be one coherent symbol whose phase the receiver knows. Since the receiver also has knowledge of the CDVCC sequence (having previously received the sequence from the base station), it is enabled to calculate the modulated pilot symbols. The transmitted signal (slot) has the format:

Where

P_n is the Pilot/CDVCC symbol sequence modulated from bits p. These sequences are differentially modulated, the bits p are beforehand known in the receiver, D_n is the coherently modulated data sequence, and at least one symbol of each pilot sequence is beforehand known in the receiver.

In the receiver the processing is as follows.

(1) First the receiver differentially detects the Pilot/CDVCC sequences. As a result the symbol decision ρ_n or bit decisions p can be made. The channel quality estimation is made based on these decisions, such as comparing decided bits p to transmitted bits p. It should be noted that for this detection the receiver does not utilize knowledge of the bits p.

(2) Second, the data sequences D_n must be coherently detected due to the coherent modulation used in these sequences. This implies that the receiver inherently requires a channel estimation c. While the phase of at least one symbol in each Pilot/CDVCC sequences P_n is predetermined, the receiver can exactly calculate the transmitted symbols of each Pilot/CDVCC sequences P_n (which are differentially modulated). With the aid of the exactly known symbols P_n the receiver is able to calculate the channel estimation c, and with the aid of the calculated estimation the data sequences D_π can be coherently detected.

Referring also to Fig. 5, and for example, the pilot symbols are modulated with π/4-shifted DQPSK with the following phase shifts: 00 π/4

01 -π/4

10 3π/4

11 -3π/4

Assume now that the encoder transmits the CDVCC bits in the pilot sequence consisting of 3 symbols, wherein the first symbol is defined to always be 1 +0j.

As a result: symbol_0 = 1+0j symboM = bits 00 cause a phase shift of π/4, thus symboM = 0.7 + 0.7] symbol_2 = bits 01 cause a phase shift of -π/4, thus symbol_2 = 1 +0j.

Since the first symbol (or any other symbol) was determined beforehand, the exact phase values can be calculated. Hence the CDVCC sequence can be used for coherent channel estimation, which is a desired result.

Although described in the context of preferred embodiments, it should be realized that a number of modifications to these teachings may occur to one skilled in the art. By example, various other types of coherent and differential modulation techniques may be employed other than those specifically referred to above, as can other time slot formats, other numbers of symbols in particular time slot fields, and so forth. It should thus be clear that the teachings of this invention are limited for use with any particular air interface standard or protocol, such as IS-136, Rev. C.

Furthermore, and while described above in the context of a wireless telecommunications system wherein the differential phase shift modulated PILOT/CDVCC fields are sent on the forward link from the base station to the mobile station, it is within the scope of this invention to transmit the same or similar information on the reverse link from the mobile station 10 to the base station 30. In this case it can be appreciated that the base station 30 will also include a suitable differential demodulator, and the mobile station 10 a suitable differential modulator.

Thus, while the invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the scope and spirit of the invention.

Claims

1. A method for operating a wireless user station, comprising steps of:

receiving a time slot sent through a radio channel from a transmitter, the time slot being coherently modulated except for a portion that contains a predetermined symbol sequence that is modulated using differential modulation; and

differentially detecting the portion of the received time slot that is comprised of the predetermined symbol sequence.

2. A method as in claim 1 , and further comprising a step of performing a channel estimation for a coherent detector.

3. A method as in claim 1 or 2, wherein the predetermined symbol sequence is modulated using differential phase shift modulation.

4. A method as in claim 1 ; 2 or 3, wherein the predetermined symbol sequence is differentially modulated.

5. A method as in any preceding claim, wherein the portion of the time slot is a CDVCC/PILOT field.

6. A method as in claim 2, wherein the coherent detector is used to detect the remainder of the time slot, the remainder being coherently modulated.

7. A receiver unit for use in a digital wireless communication system, comprising: a receiver for receiving a time slot sent through a radio channel from a transmitter, the time slot being coherently modulated except for a portion that contains a predetermined symbol sequence that is modulated using differential phase shift modulation;

a detector having an input coupled to an output of said receiver for detecting the portion of the received time slot that is comprised of the predetermined symbol sequence; and

a channel estimator for performing a channel estimation.

8. A receiver unit as in claim 7, wherein the predetermined symbol sequence is differentially modulated.

9. A receiver unit as in claim 7 or 8, wherein the portion of the time slot is a CDVCC/PILOT field.

10. A receiver unit as in claim 7, 8 or 9 and further comprising a coherent detector for detecting at least one other portion of the time slot, the at least one other portion being coherently modulated.

11. A receiver unit as in any of claims 7 to 10, wherein said receiver unit is located within a mobile station.

12. A receiver unit as in any of claims 7 to 11 , wherein said receiver unit is located within a base station.

13. A receiver unit as in claim 8, wherein the predetermined symbol sequence is differentially modulated in accordance with one of Differential Quadrature Phase Shift Keying (DQPSK), Differential 8 Phase Shift Keying (D8PSK), ╧Ç/4-shifted DQPSK, or 16-DQPSK.