This invention relates in general to communication systems, and more particularly to estimating the mobility of radio devices.
Overview
Currently there exists interest in the use of multihop techniques in packet based radio and other communication systems, where it is purported that such techniques will enable both extension in coverage range and increase in system capacity (throughput).
In a multi-hop communication system, communication signals are sent in a communication direction along a communication path (C) from a source apparatus to a destination apparatus via one or more intermediate apparatuses.
Simple analogue repeaters or digital repeaters have been used as relays to improve or provide coverage in dead spots. They can either operate in a different transmission frequency band from the source station to prevent interference between the source transmission and the repeater transmission, or they can operate at a time when there is no transmission from the source station.
Other applications are nomadic relay stations which are brought into effect for temporary cover, providing access during events or emergencies/disasters. A final application shown in the bottom right of
Relays may also be used in conjunction with advanced transmission techniques to enhance gain of the communications system as explained below.
It is known that the occurrence of propagation loss, or “pathloss”, due to the scattering or absorption of a radio communication as it travels through space, causes the strength of a signal to diminish. Factors which influence the pathloss between a transmitter and a receiver include: transmitter antenna height, receiver antenna height, carrier frequency, clutter type (urban, sub-urban, rural), details of morphology such as height, density, separation, terrain type (hilly, flat). The pathloss L (dB) between a transmitter and a receiver can be modeled by:
L=b+10n log d (A)
Where d (meters) is the transmitter-receiver separation, b(db) and n are the pathloss parameters and the absolute pathloss is given by l=10(L/10).
The sum of the absolute path losses experienced over the indirect link SI+ID may be less than the pathloss experienced over the direct link SD. In other words it is possible for:
L(SI)+L(ID)<L(SD) (B)
Splitting a single transmission link into two shorter transmission segments therefore exploits the non-linear relationship between pathloss verses distance. From a simple theoretical analysis of the pathloss using equation (A), it can be appreciated that a reduction in the overall pathloss (and therefore an improvement, or gain, in signal strength and thus data throughput) can be achieved if a signal is sent from a source apparatus to a destination apparatus via an intermediate apparatus (e.g. relay node), rather than being sent directly from the source apparatus to the destination apparatus. If implemented appropriately, multi-hop communication systems can allow for a reduction in the transmit power of transmitters which facilitate wireless transmissions, leading to a reduction in interference levels as well as decreasing exposure to electromagnetic emissions. Alternatively, the reduction in overall pathloss can be exploited to improve the received signal quality at the receiver without an increase in the overall radiated transmission power required to convey the signal.
Multi-hop systems are suitable for use with multi-carrier transmission. In a multi-carrier transmission system, such as FDM (frequency division multiplex), OFDM (orthogonal frequency division multiplex) or DMT (discrete multi-tone), a single data stream is modulated onto N parallel sub-carriers, each sub-carrier signal having its own frequency range. This allows the total bandwidth (i.e. the amount of data to be sent in a given time interval) to be divided over a plurality of sub-carriers thereby increasing the duration of each data symbol. Since each sub-carrier has a lower information rate, multi-carrier systems benefit from enhanced immunity to channel induced distortion compared with single carrier systems. This is made possible by ensuring that the transmission rate and hence bandwidth of each subcarrier is less than the coherence bandwidth of the channel. As a result, the channel distortion experienced on a signal subcarrier is frequency independent and can hence be corrected by a simple phase and amplitude correction factor. Thus the channel distortion correction entity within a multicarrier receiver can be of significantly lower complexity of its counterpart within a single carrier receiver when the system bandwidth is in excess of the coherence bandwidth of the channel.
Orthogonal frequency division multiplexing (OFDM) is a modulation technique that is based on FDM. An OFDM system uses a plurality of sub-carrier frequencies which are orthogonal in a mathematical sense so that the sub-carriers' spectra may overlap without interference due to the fact they are mutually independent. The orthogonality of OFDM systems removes the need for guard band frequencies and thereby increases the spectral efficiency of the system. OFDM has been proposed and adopted for many wireless systems. It is currently used in Asymmetric Digital Subscriber Line (ADSL) connections, in some wireless LAN applications (such as WiFi devices based on the IEEE 802.11a/g standard), and in wireless MAN applications such as WiMAX (based on the IEEE 802.16 standard). OFDM is often used in conjunction with channel coding, an error correction technique, to create coded orthogonal FDM or COFDM. COFDM is now widely used in digital telecommunications systems to improve the performance of an OFDM based system in a multipath environment where variations in the channel distortion can be seen across both subcarriers in the frequency domain and symbols in the time domain. The system has found use in video and audio broadcasting, such as DVB and DAB, as well as certain types of computer networking technology.
In an OFDM system, a block of N modulated parallel data source signals is mapped to N orthogonal parallel sub-carriers by using an Inverse Discrete or Fast Fourier Transform algorithm (IDFT/IFFT) to form a signal known as an “OFDM symbol” in the time domain at the transmitter. Thus, an “OFDM symbol” is the composite signal of all N sub-carrier signals. An OFDM symbol can be represented mathematically as:
where Δf is the sub-carrier separation in Hz, Ts=1/Δf is symbol time interval in seconds, and cn are the modulated source signals. The sub-carrier vector in (1) onto which each of the source signals is modulated cεCn, c=(c0, C1 . . . CN−1) is a vector of N constellation symbols from a finite constellation. At the receiver, the received time-domain signal is transformed back to frequency domain by applying Discrete Fourier Transform (DFT) or Fast Fourier Transform (FFT) algorithm.
OFDMA (Orthogonal Frequency Division Multiple Access) is a multiple access variant of OFDM. It works by assigning a subset of sub-carriers, to an individual user. This allows simultaneous transmission from several users leading to better spectral efficiency. However, there is still the issue of allowing bi-directional communication, that is, in the uplink and download directions, without interference.
In order to enable bi-directional communication between two nodes, two well known different approaches exist for duplexing the two (forward or download and reverse or uplink) communication links to overcome the physical limitation that a device cannot simultaneously transmit and receive on the same resource medium. The first, frequency division duplexing (FDD), involves operating the two links simultaneously but on different frequency bands by subdividing the transmission medium into two distinct bands, one for forward link and the other for reverse link communications. The second, time division duplexing (TDD), involves operating the two links on the same frequency band, but subdividing the access to the medium in time so that only the forward or the reverse link will be utilizing the medium at any one point in time. Both approaches (TDD & FDD) have their relative merits and are both well used techniques for single hop wired and wireless communication systems. For example the IEEE802.16 standard incorporates both an FDD and TDD mode.
As an example,
The FCH contains the DL Frame Prefix (DLFP) to specify the burst profile and the length of the DL-MAP. The DLFP is a data structure transmitted at the beginning of each frame and contains information regarding the current frame; it is mapped to the FCH.
Simultaneous DL allocations can be broadcast, multicast and unicast and they can also include an allocation for another BS rather than a serving BS. Simultaneous ULs can be data allocations and ranging or bandwidth requests.
In accordance with one embodiment of the present invention, a wireless transmission method for use in a wireless communication system is provided. The system includes at least two communication apparatuses which may experience relative movement therebetween. One such communication apparatus is a source apparatus that is operable to transmit information to the destination apparatus, which is the other communication apparatus. The wireless transmission method includes, at two or more time instants, obtaining a measure of channel characteristics between the source apparatus and the destination apparatus. Furthermore, the method includes calculating a correlation factor between the at least two time-consecutive measures over time, and employing the correlation factor or a value derived therefrom as an estimate of relative movement between the source apparatus and the destination apparatus in determining a transmission parameter. Moreover, the method includes transmitting a transmission signal from the source apparatus to the destination apparatus using the determined parameter.
For a more complete understanding of the present invention and its advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:
The mobility (or relative movement between a transmitting apparatus and the corresponding receiving apparatus) is an important characteristic of mobile stations (MSs), relay stations (RSs), and other wireless communication devices. Mainly, it determines the radio channel characteristics, such as radio channel coherent time. Therefore, radio devices will rely on the mobility estimation to adapt to dynamically changing radio channel conditions.
For example, in OFDMA-based WiMAX, in light of the mobility of MS, the base station (BS) will arrange service connections to a corresponding permutation zone having a certain subcarrier allocation mode. For instance, low mobility MSs will be arranged to communicate within AMC permutation zone. Moreover, based on the knowledge of mobility, a BS also can adjust the dimension of the burst to optimize the diversity gain in both time dimension and in frequency dimension.
Therefore, the mobility estimation is important to optimize the performance of radio systems. Particular embodiments of the invention provide approaches to accurately estimate mobility for wireless communications.
Previously, the standard deviation of CINR (Carrier-Interference-Noise Ratio) has been used to estimate the mobility. The CINR measurement can be defined by:
where rk,n is the nth received sample within the message measured in the kth frame, and sk,n is the corresponding preamble or pilot samples of the transmitter of interest with channel state weighting applied.
Estimation based on CINR is strongly impacted by noise and interference in communication systems. For example, in multi-cell systems, the interference will be affected by the users in neighboring cells.
The method below is based on the frequency domain channel-gain-vector correlation factor (CGVCF, Channel-Gain-Vector Correlation Factor), which has three main steps.
Step One: Obtaining the Frequency Domain Instantaneous Channel Gain Vectors, HkT
HkT
HkT
where Ts is the sampling period in time domain, fn is the sampling point in frequency domain. HkT
In normal OFDM or OFDMA based system. kTs can be equivalent to integral times of symbol durations. fn can be the subcarrier frequency of pre-decided sequences in frequency domain, such as pilots, and preamble. For example, frequency domain instantaneous channel gain vector HkT
where PRX
After this step, a sequence of frequency domain channel gain vector, H, can be obtained as below:
H={HT
where N is the length of the sequence.
Step two: Computing CGVCF
A CGVCF between HkT
where E{·} is the expectation, σ{·} is the standard deviation.
Let ρm be the correlation factor between two frequency domain channel gain vectors with the time difference of mT. By applying (5) to the sequence (4), a sequence of correlation factors can be obtained as below:
ρ={ρ1 ρ2 . . . ρm . . . ρN−1} (6)
Averaging methods can be used for calculating ρm to decrease the effect of noise. For example, ρm can be calculated by:
where i, and j are the all possible numbers between 1 and N, and satisfying the condition of j−i=m.
Also, averaging methods can be used for calculating ρ to decrease the effect of noise. For example, if ρa, and ρb are calculated from Ha, and Hb respectively, a sequence of ρ can be:
ρ=a·ρa+(1−α)·ρb 1>α>0 (8)
Step Three: Estimating Mobility.
The mobility can be estimated by the following optional methods:
Normally, in low noise environment, the higher values of correlation factor means lower mobility. From the Figure, we can find that the relationship between the correlation factor and velocity has been entirely damaged by the noise. By the same reason, we can expect that the CINR cannot accurately estimate mobility in noise environment.
However, the mobility still can be easily identified by the grads of the correlation factors. Therefore, the method, estimating mobility by the grads of ρ, will have a good performance in low SNR environment.
Particular embodiments give approaches to estimate the mobility for wireless communication devices. The benefits from one or more embodiments may be as follows:
1. The proposed method gives a higher accuracy in low SNR environment to estimate the mobility of radio devices.
2. Since the mobility is an essential parameter with wide range of applications, such as radio resource management algorithms, routing, and channel estimation etc, the proposed methods can be used for enhancing these algorithms in wireless communication devices.
3. By implementing mobility estimation algorithms presented in this proposal, a wireless communication device can adapt to dynamically changing radio channel conditions, thus improving the performance.
Embodiments of the present invention may be implemented in hardware, or as software modules running on one or more processors, or on a combination thereof. That is, those skilled in the art will appreciate that a microprocessor or digital signal processor (DSP) may be used in practice to implement some or all of the functionality of a transmitter embodying the present invention. The invention may also be embodied as one or more device or apparatus programs (e.g. computer programs and computer program products) for carrying out part or all of any of the methods described herein. Such programs embodying the present invention may be stored on computer-readable media, or could, for example, be in the form of one or more signals. Such signals may be data signals downloadable from an Internet website, or provided on a carrier signal, or in any other form.
Although the present invention has been described with several embodiments, a myriad of changes, variations, alterations, transformations, and modifications may be suggested to one skilled in the art, and it is intended that the present invention encompass such changes, variations, alterations, transformations, and modifications as fall within the scope of the appended claims.
Number | Date | Country | Kind |
---|---|---|---|
GB 0616472.7 | Aug 2006 | GB | national |
This application claims foreign priority benefits under 35 U.S.C. § 119 of United Kingdom Application No. GB 0616472.7, filed on Aug. 18, 2006, entitled “Communication Systems”. This application relates to the following applications, each of which is incorporated herein by reference: COMMUNICATION SYSTEMS, Attorney Docket 017071.0125, application Ser. No. ______, filed Aug. 17, 2007 and currently pending; COMMUNICATION SYSTEMS, Attorney Docket 017071.0127, application Ser. No. ______, filed Aug. 17, 2007 and currently pending; COMMUNICATION SYSTEMS, Attorney Docket 017071.0128, application Ser. No. ______, filed Aug. 17, 2007 and currently pending; COMMUNICATION SYSTEMS, Attorney Docket 017071.0129, application Ser. No. ______, filed Aug. 17, 2007 and currently pending; COMMUNICATION SYSTEMS, Attorney Docket 017071.0130, application Ser. No. ______, filed Aug. 17, 2007 and currently pending; COMMUNICATION SYSTEMS, Attorney Docket 017071.0131, application Ser. No. ______, filed Aug. 17, 2007 and currently pending; COMMUNICATION SYSTEMS, United Kingdom Application No. GB 0616478.4, filed on Aug. 18, 2006; COMMUNICATION SYSTEMS, United Kingdom Application No. GB 0616475.0, filed on Aug. 18, 2006; and COMMUNICATION SYSTEMS, United Kingdom Application No. GB 0616476.8, filed on Aug. 18, 2006.