US9270398B2 - Methods and devices for transmission of signals in a telecommunication system - Google Patents
Methods and devices for transmission of signals in a telecommunication system Download PDFInfo
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- US9270398B2 US9270398B2 US13/880,989 US201313880989A US9270398B2 US 9270398 B2 US9270398 B2 US 9270398B2 US 201313880989 A US201313880989 A US 201313880989A US 9270398 B2 US9270398 B2 US 9270398B2
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2602—Signal structure
- H04L27/261—Details of reference signals
- H04L27/2613—Structure of the reference signals
- H04L27/26132—Structure of the reference signals using repetition
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04J—MULTIPLEX COMMUNICATION
- H04J3/00—Time-division multiplex systems
- H04J3/16—Time-division multiplex systems in which the time allocation to individual channels within a transmission cycle is variable, e.g. to accommodate varying complexity of signals, to vary number of channels transmitted
- H04J3/1694—Allocation of channels in TDM/TDMA networks, e.g. distributed multiplexers
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2602—Signal structure
- H04L27/261—Details of reference signals
- H04L27/2613—Structure of the reference signals
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L25/00—Baseband systems
- H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
- H04L25/0202—Channel estimation
- H04L25/0238—Channel estimation using blind estimation
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2602—Signal structure
- H04L27/2605—Symbol extensions, e.g. Zero Tail, Unique Word [UW]
- H04L27/2607—Cyclic extensions
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W72/00—Local resource management
- H04W72/04—Wireless resource allocation
- H04W72/044—Wireless resource allocation based on the type of the allocated resource
- H04W72/0446—Resources in time domain, e.g. slots or frames
Definitions
- the present invention relates to methods and devices for transmission of signals in a telecommunication system.
- a fast feedback channel for Voice over IP is proposed, see GERAN#44 GP-091988 “Fast Feedback Channel” (v1).
- IP Voice over internet protocol
- VAMOS Voice services over Adaptive Multi-user channels on One Slot
- VAMOS is a standardized 3GPP/GSM feature.
- the receiver requires complex multi-user detection since the two user signals are ordinary Gaussian minimum shift keying (GMSK) co-channel interferers.
- GMSK Gaussian minimum shift keying
- Multi-user multiplexing has been standardized in GSM.
- the users transmit using GMSK modulation, and become ordinary co-channel interferers.
- AQPSK modulation is used, see 3GPP Technical Specification TS 45.004 v9.1.0. Note that neither the uplink nor the downlink transmission modes in VAMOS create truly orthogonal sub-channels.
- the users are ordinary co-channel interferers, and the signals are separated in the receiver with the help of their training sequences. The training sequences are such that they have low cross correlation.
- FIGS. 1-3 the Cramer-Rao lower bound of the estimation error for a 5-tap channel estimate is shown in FIGS. 1-3 , assuming a single antenna receiver.
- This is a theoretical bound on the variance of the estimation error, see Steven M. Kay, “Fundamentals of Statistical Signal Processing, Estimation Theory”, Prentice Hall 1993. Theorem 4.1. Training sequences 0, 3 of set 1 and set 2, in 3GPP TS 45.002 v9.5.0, see tables 5.2.3a and 5.2.3b, have been chosen due to their good cross correlation properties.
- FIG. 2 shows the power of the estimation error for one user, normalized so that the maximum error has 0 dB variance.
- FIG. 2 shows the variance of the estimation error for two users.
- FIG. 3 shows the variance of the estimation error for 4 users.
- a straightforward extension of the VAMOS technique in the uplink to more than two users seems unfeasible. There is therefore a need for a technique that can handle more than two users in one slot in the uplink of a radio communication system.
- GMSK Gaussian Minimum Shift Keying
- MU-MIMO Multi Input Multiple Output
- CPM Continuous Phase Modulation
- a block of bits is processed by, first, block repetition, second, bit flipping and third, frequency shift.
- a modulated signal in particular a GMSK modulated signal
- the training bit patterns By a judicious choice of the training bit patterns and by appropriately shifting in frequency the continuous time modulated signals, it is possible to ensure that all the user's signals are truly orthogonal in the frequency domain.
- truly orthogonal is used to emphasize that unlike the VAMOS training sequences as described in 3GPP TS 45.002 v9.5.0, perfect orthogonality (in the frequency domain) can be achieved for two or more users.
- a training sequence for a user sharing the same slot with other users is formed, where multiple users are multiplexed in the same time slot.
- the training sequence for a user is formed by repeating a (original) bit sequence and adding a cyclic prefix and a cyclic postfix to the repeated bit sequence.
- the repeated bit sequence will be repeated a number of times corresponding to the number of users in the same time slot.
- the bits in some sub-blocks of the repeated bit sequence can have the bits flipped.
- the sub-blocks can correspond to the original bit sequence. The flipping in different sub-blocks can be user specific.
- a transmitter for a user transmitting signals and sharing the same slot with other users, where multiple users are multiplexed in the same time slot is provided.
- a first, original, block of training symbols and the number of users sharing the same slot is obtained.
- a block comprising repeated blocks of the first, original, block of training symbols and having a cyclic prefix and a cyclic postfix is formed.
- the block comprising repeated blocks of the first, original, block of training symbols and having a cyclic prefix and a cyclic postfix is the training sequence for the user.
- a burst to be transmitted is formatted by adding other bits such as tail bits, guard bits, user specific payload bits in a predetermined order to the training sequence for the user.
- GSM guard bits are added at the beginning, followed by tail bits, followed by the first half of the payload, then training sequence bits for the user are added, followed by the second half of the payload bits. Finally more tail bits and guard bits are added.
- the formatted burst is fed to a modulator.
- the modulator can be a Continuous Phase Modulator (CPM) modulator.
- the modulator is typically a GMSK modulator.
- the output from the modulator is a baseband signal.
- a user specific rotation angle is applied to the baseband signal.
- the (rotated) baseband signal can be fed to a Radio Frequency (RF) modulator.
- RF Radio Frequency
- methods for receiver processing are provided.
- received samples and a hypothesized synchronization position are obtained.
- a Discrete Fourier Transform (DFT) is applied to a block of samples corresponding to the training sequence of the received samples.
- a set of user specific samples is zeroed by means of multiplying the set of user specific samples by zero.
- An Inverse DFT (IDFT) is applied to the resulting block of samples.
- a processed received signal can be used in channel estimation. Also, such a processed received signal with a channel estimate can be fed to a demodulator.
- a received signal is examined and it is determined if training sequences of the received signal are block-repeated. In accordance with one embodiment it is determined if the received training sequence is bit flipped. Depending on the outcome, the received signals are determined to be single layer EGPRS/EGPRS2 or multilayer (MIMO/MU-MIMO).
- the invention also extends to a receivers and transmitters arranged to perform the methods as described herein.
- the receivers and transmitters can be provided with a controller/controller circuitry for performing the above methods.
- the controller(s) can be implemented using suitable hardware and or software.
- the hardware can comprise one or many processors that can be arranged to execute software stored in a readable storage media.
- the processor(s) can be implemented by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared or distributed.
- a processor may include, without limitation, digital signal processor (DSP) hardware, ASIC hardware, read only memory (ROM), random access memory (RAM), and/or other storage media.
- DSP digital signal processor
- ASIC application specific integrated circuitry
- ROM read only memory
- RAM random access memory
- FIGS. 1-3 illustrate Cramer-Rao lower bound of the estimation error for a 5-tap channel estimate
- FIG. 4 is a flow chart illustrating steps performed when generating training bits for a user.
- FIG. 5 is a flow chart illustrating steps performed when generating training bits for a user
- FIG. 6 is a flowchart illustrating processing steps performed in a transmitter
- FIG. 7 is a flowchart illustrating receiver processing for one user
- FIG. 8 is a flowchart illustrating a procedure for blind detection
- FIGS. 9 and 10 illustrate an example with 4 users
- FIG. 11 illustrate the effect of the frequency shift on a signal's spectra
- FIG. 12 is a general view of a transmitter
- FIG. 13 is a general view of a receiver.
- the modulating data values ⁇ i as represented by Dirac pulses excite a linear filter with impulse response defined by:
- the phase of the modulated signal is:
- ⁇ ⁇ ( t ′ ; d -> ) ⁇ i ⁇ ⁇ i ⁇ ⁇ ⁇ ⁇ h ⁇ ⁇ - ⁇ t ′ - iT ⁇ g ⁇ ( u ) ⁇ d u
- i ⁇ b -> b -> ⁇ i ⁇ [ 1 - i 2 ⁇ b 1 , 1 - i 2 ⁇ b 2 , ... ⁇ , 1 - i 2 ⁇ b N ] .
- bit flipping the operation ⁇ 1 ⁇ right arrow over (b) ⁇ can referred to as bit flipping. In other words in bit flipping 1's are mapped to 0's and vice versa.
- N t denote the number of training bits, to be placed as a midamble in a burst to be transmitted.
- the training bits can be constructed as follows.
- FIG. 4 a flow chart illustrating some processing steps performed when generating training bits for a user is depicted.
- FIG. 4 illustrates the generation of training bits for one user. Each user will repeat the same procedure.
- a basic block of training bits is chosen. It may or may not be user specific. It does not matter. In order to be economic it can be assumed that all users employ the same basic block.
- step S 2 the basic block of bits is repeated.
- the block can be repeated as many times as there are users. For example if four users share the same time slot, then the basic block is repeated 4 times. This generates a repeated block consisting of a number of sub-blocks, where each sub-block is identical to the basic block.
- bit flipping is applied per sub-block. For example, if the second sub-block is to be bit flipped, the all the bits in that sub-block are flipped. (1's are mapped to 0's and vice versa.)
- step S 4 the basic block that has been repeated and flipped sequence of bits (i.e. the output from step S 3 ) is extended periodically.
- a cyclic prefix and a cyclic postfix are appended at the beginning and at the end.
- N t denote the number of training bits, to be placed as a midamble in a burst to be transmitted.
- the training bits can be constructed as follows.
- FIG. 5 a flow chart illustrating some processing steps performed when generating training bits for a user is depicted.
- FIG. 5 illustrates the generation of training bits for one user. Each user will repeat the same procedure.
- a basic block of training bits is chosen. It may or may not be user specific. It does not matter. In order to be economic it can be assumed that all users use the same basic block.
- the basic block of bits is repeated to form a repeated block.
- the block can be repeated as many times as there are users For example if four users share the same time slot, then the repeated block can consist of the basic block repeated 4 times.
- step S 13 the repeated block repeated sequence is extended periodically.
- a cyclic prefix and a cyclic postfix are appended at the beginning and at the end.
- the output from the training sequence generation process will be a repeated bit sequence having a cyclic prefix and a cyclic postfix.
- the repeated bit sequence will be repeated a number of times corresponding to the number of users in a slot.
- the bits in some sub-blocks of the repeated bit sequence will have the bits flipped. The flipping in different sub-blocks can be user specific.
- the transmitters can be used for transmitting the training sequences described above.
- a GMSK modulator starts by differentially encoding the input bits, as explained above. If this assumption does not hold, then it will be necessary to change the training sequence generation procedures as described above. However, since it well known that the lack of differential encoding degrades the wireless link performance, the details are omitted here.
- FIG. 6 some processing steps performed in a transmitter are depicted.
- FIG. 6 depicts the transmitter for one user. In a deployment scenario it is assumed that all users apply the same transmission method.
- step 21 a basic block of training symbols and the number of users is obtained.
- step 21 It a training sequence having the properties described above is generated.
- a repeated bit sequence having a cyclic prefix and a cyclic postfix is generated. This can for example be achieved by following the processing steps in accordance with FIG. 4 or FIG. 5 .
- the output from step S 21 is the training sequence for the user.
- a burst is formatted using the training sequence from step S 21 .
- an addition of other bits such as tail bits, guard bits, user specific payload bits is performed in a predetermined order.
- tail bits For example in GSM a normal burst normal is formatted as follows, see 3GPP TS 45.002 v9.5.0.
- guard bits are added at the beginning, followed by tail bits, followed by the first half of the payload, and then training sequence bits are added, followed by the second half of the payload bits. Finally more tail bits and guard bits are added.
- the formatted burst is fed to a modulator.
- the modulator can be a CPM modulator.
- the modulator is typically a GMSK modulator (see above).
- the output is the modulated baseband signal.
- a rotation angle in particular a user specific rotation angle, is applied to the baseband signal. This step may not be performed for all types of signals.
- step S 25 the (rotated) baseband signal is fed to a Radio Frequency (RF) modulator.
- RF Radio Frequency
- ⁇ u 2 ⁇ ⁇ ⁇ u N u ⁇ N s ⁇ T + 2 ⁇ ⁇ ⁇ ⁇ 0 T ,
- the training sequence generation method in accordance with the examples given in the first version, used together with the transmitter given in the first version, generates signals that are completely orthogonal in the frequency domain. Specifically, block repetition and user specific bit flipping, followed addition of cyclic prefix and suffix, create GMSK signals that are orthogonal in the frequency domain, provided the signals are restricted to the training sequence. Orthogonality is preserved even if there is some jitter which causes some signals to arrive earlier or later than the others.
- the signals generated in the second version of training sequence generation and the second version of a transmitter is orthogonal in the frequency domain. However, in this method it is typically necessary to shift the whole signal for the second user in the frequency domain in order to achieve orthogonality between the two users.
- the training bits can be constructed as follows.
- the training bits can be constructed as follows.
- the training sequence generation method in accordance with the examples given in the third version, used together with the transmitter given in the third version, generates signals that are completely orthogonal in the frequency domain. Specifically, block repetition and user specific bit flipping, followed addition of cyclic prefix and suffix, create GMSK signals that are orthogonal in the frequency domain, provided the signals are restricted to the training sequence. Orthogonality is preserved even if there is some jitter which causes some signals to arrive earlier or later than the others.
- the signals generated in the fourth version of training sequence generation and the fourth version of a transmitter is orthogonal in the frequency domain. However, in this method it is typically necessary to shift the signals in the frequency domain in order to achieve orthogonality among all users.
- BPSK Binary phase-shift keying
- n 0 be the hypothesized synchronization position
- TABLE 1 Index for user u according to Tx Transmitter version u ⁇ (u) number 1 2 1 2 1 1 1 1 1 2 2 2 2 1 4 3 2 1 3 3 2 3 4 3 3 1 1 4 2 2 4 3 3 4 4 4 4
- the periodicity of the training sequences implies that the received signal can be obtained by circular convolution of the channel and the transmitted Binary Phase Shift Keying (BPSK) symbols in the Laurent linear approximation of GMSK.
- BPSK Binary Phase Shift Keying
- C u is a circulant matrix whose non-zero entries are exactly the channel coefficients h u (0), . . . , h u (L ⁇ 1), the noise power has been reduced by 10 log 10 (N u )dB, the color of the noise is preserved, and the contribution to the received signal of all users except user u has been eliminated.
- This provides a single user linear model from which h u (0), . . . , h u (L ⁇ 1) can be estimated using any algorithm chosen among the plethora of linear estimation algorithms, such as least squares or Minimum mean-square error (MMSE).
- MMSE Minimum mean-square error
- FIG. 7 A flow chart illustrating some receiver processing steps is shown in FIG. 7 .
- FIG. 7 illustrates receiver processing for one user.
- the receiver can be adapted to perform the same process for every user.
- a step S 31 received samples and a hypothesized synchronization position are obtained. Then a DFT is applied to a block of samples corresponding to the training sequence. These frequency domain samples are output to the next step S 32 .
- step S 32 a set of user specific samples is zeroed.
- a sample means to multiply it by zero.
- the frequency domain block with some samples zeroed is output to a subsequent step S 33 .
- step S 33 an IDFT is applied. This gives a time domain signal where the energy from all other users (over the training sequence only) has been eliminated. Moreover application of the IDFT also increases the Signal to Noise Ratio (SNR) since the noise contribution from the zeroed samples is eliminated.
- SNR Signal to Noise Ratio
- step S 34 the signal from step S 33 , i.e. a single user time domain signal, is processed.
- a channel estimation is performed. Any algorithm for channel estimation can be applied. There are many well known algorithms in the art, such as Least Squares or Minimum Mean Square Error estimators.
- the signal is fed to a demodulator.
- a demodulator In particular any ordinary (single user) GMSK demodulator can be used. Demodulation is outside of the scope of the description provided herein.
- a rotation by a user specific rotation angle ⁇ u introduces a shift in the power spectrum of the transmitted signal.
- the arbitrary rotation angle ⁇ 0 has been introduced in order to minimize the total spread of the transmitted signals around the nominal center of frequency.
- FIG. 11 shows the effect of the frequency shift on the signal's spectra.
- the GSM transmission mask for the Mobile Station (MS) is also shown.
- GMSK modulated signals shifted by 4.84 kHz are well within the GSM spectrum mask defined in 3GPP TS 45.005 for GMSK modulated signals, as illustrated in FIG. 11 .
- FIG. 9 shows the received signal (over training sequence) in the frequency domain with a Typical Urban (TU) TU3 propagation model.
- FIG. 10 shows the received signal (over the training sequence) in the frequency domain, TU3 propagation. The contribution from each user is shown separately.
- the received signal is the superposition of all individual signals.
- the signals have been generated and modulated according to the methods described in conjunction with the third version of training sequence generation and the third version of a transmitter.
- the signals undergo independent Rayleigh fading according to a Typical Urban 3 km/h propagation model.
- the digital signal has been downsampled to one sample per bit period.
- the frequency domain characteristics of the digital received signals are shown separately for each user.
- the actual received signal is the superposition of the 4 user's signals. It can be seen that
- the receiver described above is based upon a linear model of the received signal.
- This linear model can be based on the Laurent decomposition of GMSK with bandwidth-time product BT ⁇ 0.3.
- the training sequence generation and modulation techniques described herein do not rely in any way on the approximate linearity of GMSK, which is valid when BT ⁇ 0.3.
- the same techniques can be applied to highly non-linear variants of GMSK modulation with small bandwidth-time product BT ⁇ 0.3.
- the user's signals are still orthogonal in the frequency domain, when restricted to the training sequence.
- EGPRS/EGPRS2 EDGE and GPRS, General Packet Radio Services
- the modulation type is unknown at the receiver. However, it is implicitly signaled by applying different rotations to the training symbols.
- Each modulation e.g. 8PSK, 16QAM
- Each modulation has its own, unique rotation angle.
- the process of discovering the modulation type of the signal is known as blind detection.
- the blind detection of EGPRS/EGPRS2 can be configured to include also signals whose training sequence has been created using the block repetition technique described herein. This is useful because it allows users to adaptively switch between single layer EGPRS/EGPRS2 and MIMO/MU-MIMO modes depending on the radio channel conditions or the signaling needs.
- the training sequences described herein have the following property that is not shared by the EGPRS/EGPRS2 training sequences.
- This property is enough to blindly detect the modulation type at the receiver.
- a procedure for blind detection is depicted in FIG. 8 .
- FIG. 8 a blind detection procedure for signals modulated according to the principles described herein is illustrated.
- a received signal is examined and it is determined if the training sequences are block-repeated. In accordance with one embodiment it is also determined if the received training sequence is bit flipped.
- the EGPRS training sequences do not have (any) of these properties.
- the signals are determined to be single layer EGPRS/EGPRS2 or multilayer (MIMO/MU-MIMO).
- MIMO/MU-MIMO multilayer
- the signals are determined to be multilayer in a step S 42 . If the training sequences are not block-repeated the signals are determined to be single layer EGPRS/EGPRS2.
- FIG. 12 depicts a transmitter 701 for generating and transmitting signals as described herein.
- the transmitter can be implemented in a mobile station.
- the transmitter 701 comprises controller circuitry 703 for performing the various steps required when forming a signal for transmission in accordance with the principles described herein.
- the controller circuitry can be implemented using suitable hardware and or software.
- the hardware can comprise one or many processors that can be arranged to execute software stored in a readable storage media.
- the processor(s) can be implemented by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared or distributed.
- a processor or may include, without limitation, digital signal processor (DSP) hardware, ASIC hardware, read only memory (ROM), random access memory (RAM), and/or other storage media.
- DSP digital signal processor
- ASIC application specific integrated circuit
- ROM read only memory
- RAM random access memory
- FIG. 13 depicts a receiver 701 for receiving and processing received signals as described herein.
- the receiver can be implemented in a radio base station.
- the receiver 301 comprises controller circuitry 703 for performing the various steps required when receiving signals in accordance with the principles described herein.
- the controller circuitry can be implemented using suitable hardware and or software.
- the hardware can comprise one or many processors that can be arranged to execute software stored in a readable storage media.
- the processor(s) can be implemented by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared or distributed.
- a processor or may include, without limitation, digital signal processor (DSP) hardware, ASIC hardware, read only memory (ROM), random access memory (RAM), and/or other storage media.
- DSP digital signal processor
- ASIC application specific integrated circuit
- ROM read only memory
- RAM random access memory
- Using the transmission methods and devices as described herein provides advantages over existing transmission methods. For example in accordance with some embodiments It is possible to perfectly separate, detect, synchronize and estimate the channels for multiple users user, even with a single antenna receiver.
- Some embodiments will pose modest computational requirements at the receiver side. Joint detection is not necessary.
- Some embodiments are possible to apply to highly non-linear modulations such as GMSK with small bandwidth-time product or other forms of CPM.
- the training sequence design described herein can be useful in GSM/EDGE for MIMO or MU-MIMO scenarios, since it allows simple and accurate multi-user detection of GMSK modulated signals.
- the methods and devices as described herein can thus be applied to two (or more) mobile stations, each having one transmit antenna, and it can equally well be applied to one mobile station having two (or more) transmit antennas, when the one mobile station is transmitting in single user MIMO mode. In both these scenarios there will be two different data streams, two training sequences and two antennas (in the case of two users in the same transmission slot).
- one mobile station with 4 transmit antennas, 2 mobile stations each with 2 transmit antennas, 4 mobile stations each with one transmit antenna, and so on.
- the generation of 4 training sequences is identical in all cases: In the case of 4 transmitting antennas there are 4 orthogonal training sequences.
- one layer or data stream can be sent via multiple transmit antennas.
- the number of training sequences corresponds to the total number of layers or data streams transmitted in the same transmission slot.
- two layers or data streams and two orthogonal training sequences can be sent through 4 transmit antennas, but sending exactly the same data stream through two Tx antennas.
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Abstract
Description
{circumflex over (d)} 1 =d 1
{circumflex over (d)} i =d i ⊕d i−1
-
- Where ⊕ denotes modulo 2 addition.
αi=1−2{circumflex over (d)} i(αiε{−1,+1})
-
- where the function rect(x) is defined by:
-
- and * means convolution. T is the bit period (=48/13 us in GSM), and h(t) is defined by:
-
- where the modulating index h is ½ (maximum phase change in radians is π/2 per data interval).
x(t′;{right arrow over (d)})=exp(j(φ)t′;{right arrow over (d)})+φ0)
-
- where φ0 is a random phase and is constant during one burst
-
- 1. Select Ns training bits {right arrow over (b)}=[b(1), . . . , b(Ns)], with 2Ns<Nt.
- 2. Form a block repeated sequence {right arrow over (b)}rep=└{right arrow over (b)}, {right arrow over (b)}┘.
- 3. Form user specific bit flipped sequences. For
user 1, {right arrow over (b)}1 flip=[{right arrow over (b)}, {right arrow over (b)}], and foruser 2, {right arrow over (b)}2 flip=[{right arrow over (b)}, −1·{right arrow over (b)}] - 4. For each user u=1, 2 extend the rotated repeated sequences periodically, by adding a cyclic prefix and a cyclic postfix. Choose two integers Lpre and Lpost such that Lpre+Lpost+2Ns=Nt. Here Lpre is the length of a cyclic prefix and Lpost is the length of a cyclic postfix. If L denotes the discrete channel length then Lpre
≧L− 1. Lpost should be chosen to be at least as large as the expected time offset between the two signals. The periodically extended training sequences are
-
- 1. Select Ns training bits {right arrow over (b)}=[b(1), . . . , b(Ns)], with 2Ns<Nt.
- 2. Form a block repeated sequence {right arrow over (b)}rep=└{right arrow over (b)}, {right arrow over (b)}┘,
- 3. Extend the rotated repeated sequences periodically, by adding a cyclic prefix and a cyclic postfix. Choose two integers Lpre and Lpost such that Lpre+Lpost+2Ns=Nt. Here Lpre is the length of a cyclic prefix and Lpost is the length of a cyclic postfix. If L denotes the discrete channel length then Lpre
≧L− 1. Lpost should be chosen to be at least as large as the expected time offset between the two signals. The periodically extended training sequences are
-
- Note that both training sequences are identical.
-
- 1. For each user u=1, 2, feed the training sequence {right arrow over (v)}u, to the burst formatter. The burst formatter takes the tail bits, guard bits, user specific data bits and training sequence bits and formats the burst according to a predefined format. For example in GSM tail bits are followed by data bits, the training sequence bits are placed in the middle, followed by more data bits and finally more tail bits. The output is a formatted burst {right arrow over (d)}u containing the training sequence {right arrow over (v)}u as a midamble.
- 2. For each user u=1, 2, the bit stream {right arrow over (d)}u is fed to a GMSK modulator, such as the GMSK modulator described above. The output of the GMSK modulator is a continuous time baseband signal xu(t′; {right arrow over (d)}u)=exp(j(φ(t′;{right arrow over (d)}u)+φ0)).
- 3. The baseband signal is fed to an RF modulator and transmitted.
-
- 1. For each user u=1, 2, feed the training sequence {right arrow over (v)}u to the burst formatter. The burst formatter takes the tail bits, guard bits, user specific data bits and training sequence bits and formats the burst according to a predefined format. For example in GSM tail bits are followed by data bits, the training sequence bits are placed in the middle, followed by more data bits and finally more tail bits. The output is a formatted burst {right arrow over (d)}u containing the training sequence {right arrow over (v)}u as a midamble.
- 2. For each user u=1, 2, the bit streams are fed to a GMSK modulator, described above. The output of the GMSK modulator is a continuous time baseband signal xu(t′; {right arrow over (d)}u)=exp(j(φ(t′; {right arrow over (d)}u)+φ0)).
- 3. The continuous time baseband signal is rotated by a user specific angle. Let T be the bit duration (=48/13 us in GSM). Then the rotation angle is of the form
-
-
- where ψ0 is a fixed but otherwise arbitrary angle.
- The rotated baseband signal is
x u rot(t′;{right arrow over (d)} u)=exp(j(φ(t′;{right arrow over (d)} u)+φ0))·exp(jω u t′). - 4. The rotated baseband signal is fed to an RF modulator and transmitted.
-
-
- 1. Select Ns training bits {right arrow over (b)}=[b(1), . . . , b(Ns)], with Nu·Ns<Nt.
- 2. Form a block repeated sequence {right arrow over (b)}rep=[{right arrow over (b)}, {right arrow over (b)}, {right arrow over (b)}, {right arrow over (b)}].
- 3. Form user specific bit flipped sequences. These are denoted {right arrow over (b)}u flip, u=1, . . . , 4. They are defined as follows.
{right arrow over (b)} 1 flip =[{right arrow over (b)},{right arrow over (b)},{right arrow over (b)},{right arrow over (b)}],
{right arrow over (b)} 2 flip =[{right arrow over (b)},{right arrow over (b)},{right arrow over (b)},{right arrow over (b)}],
{right arrow over (b)} 3 flip =[{right arrow over (b)},−1·{right arrow over (b)},{right arrow over (b)},−1{right arrow over (b)}],
{right arrow over (b)} 4 flip =[{right arrow over (b)},−1·{right arrow over (b)},{right arrow over (b)},−1·{right arrow over (b)}]. - 4. For each user u=1, . . . ,4 extend the rotated repeated sequences periodically, by adding a cyclic prefix and a cyclic postfix. Choose two integers Lpre and Lpost such that
- Lpre+Lpost+NuNs−Nt. Here Lpre is the length of a cyclic prefix and Lpost is the length of a cyclic postfix. If L denotes the discrete channel length then Lpre
≧L− 1. Lpost can be chosen to be at least as large as the expected time offset between the two signals. The periodically extended training sequences are
- Lpre+Lpost+NuNs−Nt. Here Lpre is the length of a cyclic prefix and Lpost is the length of a cyclic postfix. If L denotes the discrete channel length then Lpre
-
- 1. Select Ns training bits
- {right arrow over (b)}=[b(1), . . . , b(Ns)], with Nu·Ns<Nt.
- 2. Form a block repeated
- sequence {right arrow over (b)}rep=└{right arrow over (b)}, {right arrow over (b)}, {right arrow over (b)}, {right arrow over (b)},
- 3. Extend the rotated repeated sequences periodically, by adding a cyclic prefix and a cyclic postfix. Choose two integers Lpre and Lpost such that Lpre+Lpost+NuNs=Nt. Here Lpre is the length of a cyclic prefix and Lpost is the length of a cyclic postfix. If L denotes the discrete channel length then Lpre
≧L− 1. Lpost should be chosen to be at least as large as the expected time offset between the two signals. The periodically extended training sequences are
- 1. Select Ns training bits
-
- Note that all training sequences are identical.
-
- 1. For each user u=1, . . . , 4, feed the training sequence {right arrow over (v)}u to the burst formatter. The burst formatter takes the tail bits, guard bits, user specific data bits and training sequence bits and formats the burst according to a predefined format. For example in GSM tail bits are followed by data bits, the training sequence bits are placed in the middle, followed by more data bits and finally more tail bits. The output is a formatted burst {right arrow over (d)}u containing the training sequence {right arrow over (v)}u as a midamble.
- 2. For each user u=1, . . . , 4, the bit stream {right arrow over (d)}u is fed to a modulator such as a GMSK modulator, described above. The output of a GMSK modulator is a continuous time baseband
- signal xu(t′;{right arrow over (d)}u)=exp(j(φ(t′;{right arrow over (d)}u)+φ0)).
- 3. The continuous time baseband signal is rotated by a user specific angle. Let T be the bit duration (=48/13 us in GSM). Then the rotation angle is of the form
-
- where ψ0 is a fixed but otherwise arbitrary angle.
- The rotated baseband signal is
x u rot(t′,{right arrow over (d)} u)=exp(j(φ(t′;{right arrow over (d)} u)+φ0))·exp(jω u t′).
- The rotated baseband signal is
- 4. The baseband signal is fed to an RF modulator and transmitted.
The transmitter processing can be performed in accordance with the steps described in conjunction withFIG. 6 .
- where ψ0 is a fixed but otherwise arbitrary angle.
for all n, where w(n) denotes the noise, hu(k) denotes the channel to or from user, depending on whether it is for uplink or downlink communication, Lu denotes the number of taps in hu(k), su(n) are Binary phase-shift keying (BPSK) symbols corresponding to the modulating bits for user u, and mu is the relative timing offset among the users. Without loss of generality it can be assumed that 0≦mu≦Lpost and Lu=L. Assume that w(n)˜N(0,σw 2), and that all hu(k) remain unchanged over each burst. The received signal model applies to both uplink and downlink. The training sequence bits are mapped to BPSK symbols through the linearization of GMSK: tu(n)=1−2vu(n).
r(n)={tilde over (r)}(n)·exp(−j2π·nψ 0).
-
- be the vector of received samples over the training sequences. The matrix F shall denote the Discrete Fourier Transform (DFT) matrix. Compute the discrete Fourier transform Zn
0 =F·R(n0). In order to obtain the channel estimate for user u, zero all the entries of Zn0 except those with indices
ρ(u),ρ(u)+N u,ρ(u)+2N u, . . . ,ρ(u)+(N s−1)N u, - where ρ(u) is an integer index defined in Table 1.
- be the vector of received samples over the training sequences. The matrix F shall denote the Discrete Fourier Transform (DFT) matrix. Compute the discrete Fourier transform Zn
TABLE 1 |
Index for user u, according to Tx |
Transmitter version | ||
u | ρ(u) | |
1 | 2 | 1 |
2 | 1 | 1 |
1 | 1 | 2 |
2 | 2 | 2 |
1 | 4 | 3 |
2 | 1 | 3 |
3 | 2 | 3 |
4 | 3 | 3 |
1 | 1 | 4 |
2 | 2 | 4 |
3 | 3 | 4 |
4 | 4 | 4 |
-
- In other words, set
Z n0 u≡[0 . . . 0Z n0 (ρ(u))0 . . . 0Z n0 (ρ(u)+N u)0 . . . 0Z n0 (ρ(u)+2N u)0 . . . ]T
- In other words, set
W≡DFT{[w(n 0 +L), . . . ,w(n 0 +L+N u N s−1)]}
-
- and apply the Inverse Discrete Fourier Transform (IDFT) to obtain
{right arrow over (z)} u ≡F H ·Z n0 u =C u ·[t u(L pre+1+m u), . . . ,t u(L pre +N u N s +m u)]T+IDFT{[0 . . . 0W(ρ(u))0 . . . 0W(ρ(u)+N u)0 . . . 0W(ρ(u)+N s N u)0 . . . ]}T.
- and apply the Inverse Discrete Fourier Transform (IDFT) to obtain
By choosing
it is ensured that two of the carriers will have their spectra shifted −4.84 kHz with respect to the center of frequency, while the other two carriers will have their spectra shifted +4.84 kHz from the center of frequency. Note that GMSK modulated signals shifted by 4.84 kHz are well within the GSM spectrum mask defined in 3GPP TS 45.005 for GMSK modulated signals, as illustrated in
-
- The energy of each signal is concentrated on a subset of the sub-carriers in the frequency domain.
- For practical purposes, the sub-carriers corresponding to different users do not overlap.
Therefore, each user can be completely separated by applying the Discrete Fourier Transform, zeroing the sub-carriers where the energy of the other users is concentrated, and getting back to the time domain via the Inverse Discrete Fourier Transform.
Generalizations
-
- The number of users Nu can be chosen arbitrarily.
- The basic block of bits {right arrow over (b)} may be chosen to be user specific. Block repetition, user specific bit flipping and user specific rotation of the baseband signal will ensure that the received signals have orthogonal spectra over the training sequence. Here the word orthogonality is used in the sense that the energy in the DFT of the signals is concentrated on non-overlapping frequencies. Therefore, over the training sequence, the users are separated in the frequency domain.
- Other type of Continous Phase Modulation (CPM) may be used in lieu of GMSK.
-
- The new training sequences consist of block repeated bits, possibly bit flipped.
Claims (22)
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WO2013158014A2 (en) | 2013-10-24 |
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