Patent Application
20100142436
This invention relates in general to communication systems, and more particularly to a relay-amble in a communication frame.
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.
a and 7b illustrate a number of applications for relay stations. For fixed infrastructure, the coverage provided by a relay station may be “in-fill” to allow access to the communication network for mobile stations which may otherwise be in the shadow of other objects or otherwise unable to receive a signal of sufficient strength from the base station despite being within the normal range of the base station. “Range extension” is also shown, in which a relay station allows access when a mobile station is outside the normal data transmission range of a base station. One example of in-fill shown at the top right of
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 IEEE 802.16 standard incorporates both an FDD and TDD mode.
As an example,
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 transmission method for use in a multi-hop wireless communication system is provided. The system includes a source apparatus, a destination apparatus and one or more intermediate apparatuses. The source apparatus is operable to transmit information along a series of links forming a communication path extending from the source apparatus to the destination apparatus via the intermediate apparatus. The intermediate apparatus is operable to receive information from a previous apparatus along the path and to transmit the received information to a subsequent apparatus along the path. The system has access to at least one predetermined transmission introduction sequence and also has access to a time-frequency format for use in assigning available transmission frequency bandwidth during a discrete transmission interval, said format defining a plurality of transmission windows within such an interval. Each window occupies a different part of that interval and has a frequency bandwidth profile within said available transmission frequency bandwidth over its part of that interval. Each said window being assignable for such a transmission interval to at least one of said apparatuses for use in transmission. The method includes, when transmitting a message with a preamble in a particular transmission interval, transmitting the preamble in a first transmission window of that transmission interval. Furthermore, the method includes transmitting the transmission introduction sequence in a second transmission window of that transmission interval other than the first transmission window preferably as control information for use by at least one said intermediate apparatus or the destination apparatus.
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:
a and 7b show applications of relay stations; and
The process of modulation, transmission, reception and demodulation of an information signal, as performed in a communication system, will cause typically the original signal to experience some distortion. These distortions may include delay, frequency offset and phase rotation and can result in the reception of multiple independently distorted replicas of the original signal. In order to correct for these distortions in the receiver, it is common for a communication system to make use of special training sequences, transmitting them through the channel so that they undergo the same distortion as the information signal. As these training sequences are known in the receiver, it is possible to estimate the distortion introduced by the transmission process and then correct the received information signal so that the distortion is minimized or completely removed. Thus such a training signal can be used in both the synchronization (time & frequency) and channel estimation and equalization stages of the receiver.
It is possible to form a set of a number of known training sequences for transmission within the communication system. Each sequence in the set is distinct from all other sequences such that it is possible at the receiver to distinguish the identity of a transmitter in a communication network where multiple transmitters exist. This allows the receiver to ascertain certain properties possessed by the transmitter as well as estimate the transmitter and channel induced distortion that will be experienced on a signal that is received from that particular transmitter.
In single hop communication systems (e.g. IEEE 802.16e-2005) one such transmission signal that can be used for the purposes of identification and training and synchronization is the preamble sequence. As its name suggests, it is transmitted at the start of every frame prior to the transmission of data. A 802.16e-2005 single hop subscriber or mobile station (SS or MS) will utilize the preamble to perform a number of tasks, including transmitter identification to determine the IDCell parameter and segment number. It will also use it to synchronize (i.e. correct timing and frequency offsets) to the transmitter.
Thus to support legacy MS or SS, a relay station may be required to transmit a preamble to enable the MS or SS to identify, synchronize and communication with it. As all the preamble transmissions from all of the transmitters (BS & RS) should be time synchronized in a cellular style network, such a requirement precludes an active RS from being able to receive the preamble sequence from a BS or another RS due to the physical limitation that it cannot transmit and receive on the same transmission resource at the same time.
When operating a TDD network it is also desirable for all transmitters to be synchronized in both time and frequency. This enables a MS that is synchronized with one transmitter to be automatically synchronized with all other transmitters in the network and hence exploit this to realize fast handover between transmitters (as re-synchronization is not required) and also perform macro-diversity like operations, such as multi-BS MBS as described in IEEE 802.16e-2005 standard and receive control and data information from two different sources.
When RS are introduced into a synchronous network it is furthermore desirable for there transmissions to be synchronized with those of the existing BS, so that the MS can continue to benefit from the associated advantages of a synchronous network. Therefore, RS must start its transmission at the same time as a BS and they must both transmit there synchronization signal for the purposes of MS transmitter identification and synchronization at the same time instant. This then makes it impractical for the RS, once transmitting the synchronization signal in a single frequency TDD network to receive the BS synchronization signal simultaneously. Hence there is no reference which the RS can use to maintain synchronization with the BS whilst operational (i.e. transmitting its own synchronization and identification signal).
Particular embodiments involve devising a new signal for transmission by the BS or RS which can be received by the RS to enable it to both transmit a standard preamble sequence and receive the new signal to enable it to perform transmitter identification, synchronization and channel estimation. One solution is to transmit a special BS-RS (or RS-RS in the case of more than two-hop) synchronization signal. The signal could also be an RS-MS signal if appropriate. However, the signal should preferably have the properties that it cannot be accidentally detected as a false frame start point by an MS who is not aware of the fact that BS or RS may transmit this “relay midamble”.
As an example,
Particular embodiments introduce a new signal that is transmitted in another region of the DL sub-frame (other than the region where the preamble is located). This signal could be in the middle of the DL sub-frame, thus forming a mid-amble or at the end of the sub-frame, thus forming a post-amble. From here on, for the sake of generality, the new signal is referred to as the relay-amble (RA) or relay midamble (RM).
The requirements for the RA, similar to those of the preamble, are that it can be used by the receiver to identify and distinguish the transmitter from potentially a number of other transmitters in the communication network. It must also enable the receiver to estimate, or update an existing estimate, of the transmitter and channel induced distortion. It must not be accidentally identified by an MS as a normal preamble sequence, as this may confuse a legacy MS that is not aware of the existence of relay-ambles.
In order to meet these requirements, it is possible to envisage that a number of different well-known mathematical sequences could be used to generate the relay-amble or set of relay-ambles used in a communication network.
In general, the properties of the transmitted RA signal may therefore be:
Based on these requirements it could be possible to use either: PN (pseudo-noise) sequences as used in the IEEE 802.16 standard; Golay sequences [4] [5]; or CAZAC sequences (Constant Amplitude & Zero Auto Correlation) (see [3] for more information on use of CAZAC sequences for training) such as Chu [2] and Frank-Zadoff [1] sequences to construct the relay-amble. All of these sequences are known to exhibit some or all of the required properties and hence have been previously proposed for use in forming such training or identification sequences.
However, depending on the sequence types used for the normal preamble and the ability to provide a set of sequences with the properties listed above, it might not be possible to consider use of any of the sequence types. For example, if PN sequences are used for the normal preamble, then it could be found that it is not possible to generate a sufficient number of further PN sequences with the properties listed (for example low PAPR) for the relay-amble set. In which case, it would be more appropriate to use a set of sequences of a different type, ensuring that the selected set of relay-ambles maintain the required properties of low correlation with all the normal preamble sequences.
The BS or RS that is transmitting an RA will first decide on the location of the RA transmission within the downlink sub-frame. As mentioned earlier, the transmission could be located anywhere within the frame. However, it is possible to envisage that certain formal frame structures may be required to support relaying that limit the flexibility afforded to the transmitter in placement of the RA.
Once the location of the RA within the frame is determined, the transmitter then determines the amount of transmission resource that will be allocated to the RA. Various factors will have an effect on this decision including: the effective frequency reuse to be achieved at a multi-sector transmitter; the requirement to reduce interference; the amount of transmission resource that will be utilized by the BS to RS or RS to RS data transmission; the method used for separating different transmitters operating on the same frequency in a cellular network; and also the type of sequence used to form the RA.
One solution is to form an RA zone in the downlink sub-frame, as shown in
The former is appropriate if the whole band is available for BS to RS or RS to RS data transmission, whilst the latter could be adopted to minimize the amount of transmission resource required if a full symbol is not required as could be the case if the set of RAs is small or the BS to RS or RS to RS data transmission is only utilizing a part of the total frequency transmission resource (i.e. a sub-band).
Once a zone or region is defined within the transmitter, the transmitter then determines the usage of the transmission resource within the zone or region. Numerous usage scenarios can be envisaged, including: all tones are allocated for RA transmission; the total number of tones are decimated so that the RA is allocated to every second, third, fourth, etc, tone; a contiguous sub-band of tones is allocated. Each of these mechanisms is illustrated in
The benefit of the first approach is that it enables accurate channel estimation as each tone is illuminated with a known transmission enabling distortion to be determined on each individual subcarrier. The benefit of the second approach is that in a frequency reuse 1 scenario, by decimating the tones and using different offsets of decimated sequences at different transmitters it is possible to achieve an effective frequency reuse of greater than 1. An example could be a three sector site, where a decimation factor of three is employed at each sector using an incrementing offset of the starting subcarrier number on each sector (i.e. sector 1 uses subcarriers {0, 3, 6, etc}, sector 2 uses {1, 4, 7, etc} and sector 3 uses {2, 5, 8, etc}. The benefit of the third approach is that similar to the case above, it is possible to achieve an effective frequency reuse of greater than 1 by assigning different sub-bands to different sectors.
Now that the number and location of the available tones for the transmitter is decided, the final stage is to generate the training and identification sequence to be transmitted on the identified tones. As discussed previously, it is possible to make use of a number of different well known sequences for this purpose.
It is worth noting that in a synchronous cellular network, it is likely that the zone or region allocation will be performed in some network management entity (this could be located within the core network or within one of the transmitters). Also the same situation may exist for the case of allocation of a particular sequence to a transmitter, especially if the sequence is conveying inherent identification parameters. This network management entity will then ensure that the location of the zone or region across all transmitters in the cellular network is harmonized. This then prevents interference between RA transmissions from one transmitter and data transmissions from another, which could be significant especially if the RA transmission power is boosted due to its lower PAPR properties. It will also ensure that the allocation of identification parameters ensures that from a receiver point of view, it will never experience receiving the same identification from two visible transmitters (i.e. there is sufficient spatial separation between the reuse of the same identification sequence).
Finally, the transmitter (RS/BS) may include some signaling information in the broadcast message to indicate the existence and location of the RA zone or region to the RS, alternatively it may also include signaling information in a multicast or unicast message specifically directed towards the RS to inform it of the RA existence.
In summary,
Further to the above discussion on relay midamble (RM) design an alternative embodiment is proposed where the same set of sequences used for the normal preamble are utilised. The benefit is that the optimal choice for a system can be used for both the preamble and the RM (it is not required to further extended the set of preambles and thus result in suboptimal sequences that mean a higher PAPR or worse correlation properties). The simple way to differentiate the preamble and the RM is to transmit them at different degrees of boosting over the normal data transmission (or even not to boost RM transmission).
For example in the IEEE 802.16 standard the preamble power shall be boosted by 9 dB over the average data power. Once such solution is to then set the RM power at 3 dB below the preamble power. An MS or RS then scanning the spectrum for a preamble, may see a RM. However, the preamble will always appear with a stronger correlation peak compared to the RM, due to the fact that the preamble/RM pairs from a BS or active RS will experience the same pathloss. Therefore, when deciding on a target, the MS (or RS that is entering the network) will always lock to the preamble transmission rather than the RM transmission.
It is also possible to further increase the robustness of this technique by changing the rate of RM transmission to greater than one frame. An MS that is expecting a certain period between preambles will also correctly detect the preamble rather than the RM as the frame start point.
The RM position can be controlled dynamically through signaling messages contained in the normal (access-link) transmission period (i.e. BS or RS to MS) or in the dedicated RS (relay link) transmission period.
Note that it is not required to always transmit a RM. Two mechanisms can be defined for deciding whether to transmit an RM.
In summary, the benefits of particular embodiments may include:
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.
Particular embodiments involve a relay midamble that can optionally be transmitted by a MR-BS or RS in the R-DL interval. This midamble can received by an RS instead of the preamble transmitted in the access link when the RS is transmitting its own preamble.
When the BS and RSs operate in a frame time synchronous manner [1], it is not practical for the RS to receive preamble transmissions in a TDD system due to the fact that they are also required to transmit preambles to support connection of SS as defined in IEEE 802.16. Consequently, the proposal is to define a new relay midamble that can be transmitted by a BS or RS during the R-Link transmission interval for reception by an RS in place of reception of the preamble in the access link interval.
The midamble is designed to have properties very similar to the normal preamble to minimize the impact on the existing standard and also enable reuse of existing technology defined for MS receiver at the RS receiver.
The properties of the relay midamble are summarized in Table 1.
In summary, the sequence used for the relay midamble is the same as the set of sequences used for the preamble. The two differences are that the power of each tone is boosted by +6 dB over unboosted data subcarrier power and the location of the RM is flexible [1]. This prevents a simple correlation function at the SS from selecting the RM over the preamble as the candidate point for frame start and downlink channel selection during network entry.
Table 2 compares the power boosting difference between the various different data and pilot tone modulation types.
The existence of the RM is controlled by the BS. The option for the RS to request transmission of an RM is left FFS. However, two mechanisms are envisaged. The first is static request during network entry through a SBC message indicating RM is required for operation. The second is dynamic request through an unsolicited MAC management message from the RS to the BS.
The following are proposed amendments to the IEEE 802.16 standard for conformity with particular embodiments of the present invention:
Insert a new subclause at the end of Section 8.4.6.1.1 as indicated:
The BS or RS may also transmit the RM in the R-DL transmission interval to facilitate RS synchronization and identification of the BS or RS by other RSs.
The subcarrier sets and the series used to modulate the RM pilots shall be the same as that defined for the preamble in 8.4.6.1.1. The modulation used for the RM pilots is boosted BPSK as defined in 8.4.9.4.3.3.
Insert new subclause 8.4.9.4.3.3:
The pilots in the RM on the R-DL shall follow the instructions in 8.4.6.1.1.3, and shall be modulated according to Equation (137a):
Insert a new subclause at the end of Section 8.4.10.1 as indicated:
For TDD and FDD realizations, it is recommended that RSs be time synchronized to a common timing signal that is also used for BS synchronization, as described in Section 8.4.10.1.1. The timing signal shall be a 1 pps timing pulse and a 10 MHz frequency reference. These signals are typically provided by a GPS receiver. In the event the reference is not available from a common reference, the RS may utilize an RM transmission from a BS or other RS, as described in Section 8.4.6.1.1.3, to maintain synchronization. In the event of loss of a network timing signal that is not provided by a BS or RS, the RS shall continue to operate. The RS shall automatically resynchronize to the network timing signal when it becomes available.
For both FDD and TDD realizations, frequency references derived from the timing reference may be used to control the frequency accuracy of RSs provided that they meet the accuracy requirements of 8.4.14. This applies during normal operation and during loss of timing reference.
Insert the following text at the end of Section 8.4.14.1:
At the RS, both transmitter center frequency and the sampling frequency shall be derived from the same reference oscillator. The reference frequency accuracy at the RS shall be better than ±2 ppm and the RS uplink transmission shall be locked to the BS, so that its center frequency shall deviate no more than 2% of the subcarrier spacing compared to the BS center frequency. The RS downlink transmission shall be locked to the BS, so that its center frequency shall deviate no more than 2% of the subcarrier spacing compared to the BS center frequency.
| Number | Date | Country | Kind |
|---|---|---|---|
| GB 0616474.3 | Aug 2006 | GB | national |
| GB 0622124.6 | Nov 2007 | GB | national |
This application is a continuation application of pending U.S. patent application Ser. No. 11/840,669 filed Aug. 17, 2007; which claims foreign priority benefits under 35 U.S.C. §119 of United Kingdom Application No. GB 0616474.3, filed on Aug. 18, 2006, entitled “Communication Systems” and United Kingdom Application No. GB 0622124.6, filed on Nov. 6, 2006, entitled “Communication Systems”.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11840669 | Aug 2007 | US |
| Child | 12649096 | US |
