WO2012064173A1

WO2012064173A1 - A method of detecting signals

Info

Publication number: WO2012064173A1
Application number: PCT/MY2011/000086
Authority: WO
Inventors: Wahidah Hashim; Azmi Yaacob; Nordin Ramli
Original assignee: Mimos Berhad
Priority date: 2010-11-09
Filing date: 2011-06-08
Publication date: 2012-05-18
Also published as: MY168829A

Abstract

A method of detecting signals transmitted using differential cyclic delay diversity is provided, the method including the steps of: computing a plurality of matrices using received sequences of observation symbols; determining possible transmitted symbol based on the set of possible information symbols; and computing transmitted symbol using maximum likelihood detection.

Description

A METHOD OF DETECTING SIGNALS

FIELD OF INVENTION The present invention relates to a method of detecting signals through a sequence of differential cyclic delay diversity.

BACKGROUND OF INVENTION In wireless communication systems, a detection method called coherent detection uses pilot symbols to detect transmitted signals. Because of time-varying characteristics of a channel, it can be difficult to extract channel state information (CSI) with reasonable complexity and accuracy for a coherent detection scheme. Although having pilot symbols have a potential to provide a near optimum Bit Error Rate (BER) performance at certain Signal-to-Noise Ratio (SNR), however, these same pilot symbols cause a reduction of data throughput and bandwidth efficiency, since some parts of data capacity must be sacrificed for pilot symbols. This is not apparent in a single transmit antenna but it is evident for multiple transmit antennas such as Multiple Input Multiple Output (MIMO) systems. Such MIMO system are favourable in Orthogonal Frequency Division Multiplexing (OFDM) modulation, which represents a key air interface in several current telecommunication technologies such as Worldwide Interoperability for Microwave Access (WiMAX) and Long Term Evolution (LTE). In each OFDM symbol, a requirement for pilot spacing in frequency should be less than the reciprocal of a subcarrier spacing and maximum delay spread.

According to IEEE 802.16e specification, a transmission bandwidth of 20 MHz with total number of subcarriers, N = 128 for an OFDM symbol yields a subcarrier spacing of 156.25 kHz. For Non-Line of Sight (NLOS) environment with T_m = 400 nanoseconds, the maximum pilot spacing required is 8 subcarriers, which means that pilot symbols are to be inserted in every 8^th subcarriers or less. This will result in 16 out of 128 total subcarriers that need to be allocated for pilot symbols. When such a number of subcarriers are transmitted for each antenna, the total numbers of subcarners that need to be allocated for pilot symbols (N_p) in N, transmit antennas are linearly increased

Besides reduction in data throughput and inefficiency of the bandwidth, the increasing number of parameters for channel estimation increases the complexity of the coherent detection system. On the other hand, a transmit diversity technique suffers from throughput reduction due to rapid channel variation environment. This required high numbers of pilot symbols to estimate channel values should the system engage in coherent detection.

Therefore, there is a need for a solution that detects signals in a coherent manner that is not affected by channel characteristics.

SUMMARY OF INVENTION

Accordingly, there is provided a method of detecting signals through a sequence of differential cyclic delay diversity, the method includes the steps of computing a plurality of fading coefficient matrices, determining possible transmitted symbols based on M-ary communication data, computing transmitted symbol based on an algorithm and determining maximum value of estimated symbol sequence.

The present invention consists of several novel features and a combination of parts hereinafter fully described and illustrated in the accompanying description and drawings, it being understood that various changes in the details may be made without departing from the scope of the invention or sacrificing any of the advantages of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, wherein:

Figure 1 shows a block diagram of architecture of a transmitter performing a method of detecting signals through a sequence of differential cyclic delay diversity in the preferred embodiment of the invention;

Figure 2 shows a block diagram of architecture of a transmitter performing a method of detecting signals through a sequence of differential cyclic delay diversity in the preferred embodiment of the invention;

Figure 3 shows a flowchart that illustrates a method of detecting signals through a sequence of differential cyclic delay diversity in the preferred embodiment of the invention; and Figure 4 shows an example of application of the method in wireless communication.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates to a method of detecting signals through a sequence of differential cyclic delay diversity. More particularly, the method is a method of incorporating multiple performance metrics into a shortest routing protocol in wireless mesh networks. Hereinafter, this specification will describe the present invention according to the preferred embodiment of the present invention. However, it is to be understood that limiting the description to the preferred embodiment of the invention is merely to facilitate discussion of the present invention and it is envisioned that those skilled in the art may devise various modifications and equivalents without departing from the scope of the appended claims.

The following detailed description of the preferred embodiment will now be described in accordance with the attached drawings, either individually or in combination.

Figure 3 is a flowchart that illustrates a method of detecting signals through a sequence of differential cyclic delay diversity. The method includes the steps of computing a plurality of fading coefficient matrices, determining possible transmitted symbols based on M-ary communication data, computing transmitted symbol based on an algorithm and determining maximum value of estimated symbol sequence. The method is a Multiple Symbol Differential Detection (MSDD) for wireless communication.

The method is employed into Differential Cyclic Delay Diversity (DCDD) modulation by incorporating Orthogonal Frequency Division Multiplexing (OFDM). The method which is termed as MSDD-DCDD, considers transmitted symbols from multiple transmission antennas and received on one antenna. This configuration is referred to as Multiple-Input Single-Output (MISO), which is one of a Multiple-Input and Multiple-Output (MIMO) classifications. Most importantly, the method does not need to consider channel information either at transmitter level or at receiver level during transmission. An example of application in wireless communication is depicted in Figure 4. However, the fading coefficient matrices are calculated at beginning of signaling in the method. By eliminating the channel information, pilot symbols are no longer needed. Therefore, the method also increases bandwidth efficiency and data capacity of communication means wherein subcarriers in OFDM symbols are fully used in data transmission. Further, the method improves probability of Bit Error Rate (BER) and Signal-to-Noise Ratio (SNR) compared to conventional differential detection with similar system parameters. Receiving data is processed with a low complexity for a low number of OFDM symbols under observation. A number of symbols under observation may be realized for any number by which me number of transmitted symbols detected by the method is always one unit less than that of the number of symbols observed.

Figure 1 shows a block diagram of architecture of a transmitter performing the method. Information bits are passed to a conventional channel encoder. After serial to parallel conversion, encoded data is then Grey mapped onto M-ary Phase Shift Keying (MPSK).

S ) e |ex ^"2jz7« !M\m ~ 0,..., M - 1}

Output is defined as and S_v(n) denotes information symbol prior to differential modulation, which is assigned as wth subcarrier of vth OFDM symbol with N subcarriers. M represents a number of constellation points.

Differentially modulated or encoded symbol X_v(n) is thus expressed as a product of previously encoded symbol and current unmodulated symbols.

These symbols are transmitted on a dedicated subcamer of a specific OFDM symbol. A complex symbol sequence is then processed by an N-point inverse Fast Fourier Transform (IFFT) to yield N time-domain samples corresponding to the OFDM symbol.

A cyclic delay is introduced to each branch of a transmitter. A choice of cyclic delay is investigated and optimum selection of cyclic delay for each antenna set is achieved by taking into account number of transmitting antennas, a delay spread and number of subcarriers per OFDM symbol. After cyclic shifting, a guard interval (GI) is inserted to avoid intersymbol interference (1SI) and to maintain subcarrier orthogonal over a multipath channel. Power per symbol transmitted from each antenna is normalized. This is based on a derivation that differential decoding technique requires no channel estimation in operation. The equation involvin differentially encoded data and current transmitted symbol is developed such that method is systematic.

Figure 2 shows architecture of a receiver that performs the method. After removing the GI, received signals are passed into a Fast Fourier Transform (FFT) block wherein expression of received symbols are understood to be a mixture of Mi MO channel values, noise and transmitted symbols.

In the method, received sequences of N_m observation symbols are expressed using matrices. This involves transpose and Hermitian transpose.

Conditions for slow fading channel are assumed to be generally constant over a time interval. This implies that a channel is fixed during several observations interval. Based on a theory of multivariate Gaussian conditional probability density function (PDF), for a block of Nm observations and given that a message symbol matrix S (n) is transmitted through differential encoding of X_v(n), the received vector Y_v(n) is then computed. Covariance matrices of received symbols are taken into account. A resultant covariance matrix is simplified into expected value terms.

Assuming both channel matrix and noise matrix are samples of complex values with statistically independent and identically distributed variables that own variance σ² = 1, the covariance matrices of the channel, ΦΗΗ and noise, Φζζ can easily be identified as an identity matrix, Amusing unitary property of identity matrix, it can be deduced that matrix determinant is independent of transmitted symbols. The matrix can further be expanded to show a relationship with message symbols for detection purposes. The differentially encoded message S^(n) is then detected from

Syin = argmax

This is used in the method which is a MSDD-DCDD detection matrix for observation interval of any N_m blocks. The method as described above has a low complexity as fewer parameters are to be estimated especially in multiple antenna systems. This is because all subcarriers in OFDM symbols are fully utilized for data transmission since there are no subcarriers being used as pilot symbols, in contrast with coherent counterparts. Since no pilot symbols are used, the method is a bandwidth efficient method. Significant performance improvement is obtained compared to conventional differential detection.

This invention is adapted for use for Multiple Symbol Differential Detection (MSDD) for wireless communication. The disclosed invention is suitable, but not restricted to, for use in signal detection in OFDM signals.

Claims

1. A method of detecting signals through a sequence of differential cyclic delay diversity, the method includes the steps of:

i. computing a plurality of fading coefficient matrices;

ii. determining possible transmitted symbols based on communication data; iii. computing transmitted symbol based on an algorithm; and

iv. determining maximum value of estimated symbol sequence.

2. The method as claimed in claim 1, wherein the transmitted symbols are based on communication data such as M-ary communication data.

3. The method as claimed in claim 1, wherein the method is a Multiple Symbol Differential Detection (MSDD) for wireless communication. 4. The method as claimed in claim 1, wherein the method is employed into Differential Cyclic Delay Diversity (DCDD) modulation by incorporating Orthogonal Frequency Division Multiplexing (OFDM).

The method as claimed in claim 1, wherein the method considers transmitted symbols from multiple transmission antennas and received on one antenna.

The method as claimed in claim 1, wherein the method does not need to consider channel information either at transmitter level or at receiver level.

7. The method as claimed in claim 1, wherein the fading coefficient matrices are calculated at beginning of signaling.

8. The method as claimed in claim 1, wherein the method increases bandwidth efficiency and data capacity of communication means.

9. The method as claimed in claim 1, wherein the method improves probability of Bit Error Rate (BER) and Signal-to-Noise Ratio (SNR) compared to conventional differential detection.