The present invention relates to radio communications devices with adaptive combination.
The demand for wireless communication systems has grown steadily over recent decades, and has included several technological jumps over this time, particularly in the area of cellular and wireless local area network (WLAN) communication systems. Analogue cellular phones have been replaced with digital handsets using for example GSM and CDMA technologies, and so called third generation systems such as UMTS are now being introduced. Similarly WLAN technologies such as HyperLan and IEEE 802.11b are also being introduced. The number of users continues to increase and data traffic is now becoming an important part of the wireless network. Both of these factors mean that it is important for operators to look for methods of increasing the capacity of their networks to meet future demands.
As well as the need to increase capacity there is a general requirement to keep costs down whilst providing good performance. For example, costs for basestation and user terminal equipment should be reduced where possible whilst still enabling satisfactory wireless services to be provided.
One performance related problem relates to multipath fading. Typically basestations and user terminals are located in “cluttered” environments. This means that communications signals arrive at such basestations or user terminals via many paths because of scattering due to reflections and diffractions from buildings, furniture or other objects in the environment. Incoming scattered signals can add constructively or destructively depending on the relative amplitude and phase of the different components. This means that the received signal at the basestation or user terminal varies considerably in magnitude depending on the relative location of the basestation, user terminal and other objects in the environment. This effect is known as multipath fading.
Previously, one way of addressing multipath fading has been to use transmit or receive antenna diversity. Receive antenna diversity involves transmitting from one transmit antenna whilst providing two or more diverse receive antennas (e.g. with spatial or polarisation diversity). By using diverse antennas uncorrelated signals are received at those antennas. When one of those signals is in a fade the other is typically unfaded. In the case of switched antenna diversity, one of the receive antennas is selected for reception at any one time. Alternatively, adaptive combination is used in conjunction with all the receive antennas to produce one channel output. Thus in the ideal situation, the receive antennas can always be used to obtain an unfaded signal.
A similar situation occurs for transmit diversity. Here two or more diverse transmit antennas are used in conjunction with one receive antenna. Feedback about receive performance is used to either select one of the transmit antennas to use at a particular time, or to adjust adaptive combination of the transmit antennas to create one channel output. The present invention seeks to provide improved capacity and performance as compared with such known transmit and receive diversity antenna arrangements.
Another known approach for increasing capacity involves using multiple-input multiple-output (MIMO) communications systems to increase data rates. A MIMO wireless communications system (see
MIMO wireless communication systems are advantageous in that they enable the capacity of the wireless link between the transmitter and receiver to be improved compared with previous systems in the respect that higher data rates can be obtained. The multipath rich environment enables multiple orthogonal channels to be generated between the transmitter and receiver. Data for a single user can then be transmitted over the air in parallel over those channels, simultaneously and using the same bandwidth. Consequently, higher spectral efficiencies are achieved than with non-MIMO systems.
However, one problem with known MIMO arrangements is that they are relatively expensive in the respect that multiple antennas are required together with multiple transmit and receive chains. One receive antenna is used for each MIMO channel. Thus, for example, a receive MIMO antenna arrangement can comprise four antennas together with four receive chains one for each of those antennas. Receive chains are relatively expensive, bulky and power must be provided to each of those receive chains. This is particularly disadvantageous for user terminals that are required to be compact and also for basestations which need to be unobtrusive. Similar problems occur for transmit chains.
An object of the present invention is to provide a radio communications device which overcomes or at least mitigates one or more of the problems noted above.
Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.
According to an aspect of the present invention there is provided a radio communications device comprising
This provides the advantage that directional antenna beams are used in a communications system which, for a plurality of such devices in a network, reduces overall carrier to interference levels and hence increases capacity. The device may be either a basestation or a user terminal. For example, the user terminal can be a mobile telephone, a personal digital assistant, a personal computer, a subscriber premises installation or any other suitable type of terminal. Also, the combiner preferably comprises at least one beamformer or the antenna elements are provided as a phased array. Preferably, the communications system is a multiple-input multiple-output communications system. In this case the advantages of increased data rates are achieved in addition to improvements in carrier to interference levels.
Preferably the antenna beams are diverse as a result of any of polarisation diversity, angle diversity and space diversity.
In a preferred embodiment a pair of antenna beams is provided with substantially orthogonal polarisations and at substantially similar directions. MIMO communications can then be provided over the two antenna beams to create a high data rate link.
The combiner is preferably arranged to electronically steer the directional antenna beams. This provides the advantage that problems associated with spatial fading are addressed by steering the beam. Also, because the direction of the antenna beams can be adjusted, pointing losses are reduced as compared with systems that use fixed directional beams.
The invention also encompasses a method of operating a radio communications device comprising the steps of:
A corresponding method for transmitting is also provided. Preferably, the signals are space-time coded and a multiple-input multiple-output radio communications device is used.
The invention is directed to a method by which the described apparatus operates and including method steps for carrying out every function of the apparatus.
The invention also provides for a system for the purposes of digital signal processing which comprises one or more instances of apparatus embodying the present invention, together with other additional apparatus.
The preferred features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any of the aspects of the invention.
In order to show how the invention may be carried into effect, embodiments of the invention are now described below by way of example only and with reference to the accompanying figures in which:
a is a schematic diagram of a prior art receive diversity antenna arrangement;
b is a schematic diagram of an embodiment of the present invention using transmit or receive diversity in either a MIMO or a non-MIMO system;
Embodiments of the present invention are described below by way of example only. These examples represent the best ways of putting the invention into practice that are currently known to the Applicant although they are not the only ways in which this could be achieved.
The term “receive chain” is used to refer to any part of apparatus which processes radio frequency signals received at a receiver by downconverting those signals to baseband frequency. This involves many stages of filtering, demodulation and downconversion as known in the art. Thus the term “receive chain” is used herein to refer to either all the apparatus needed for this conversion process or only some of that apparatus.
The term “transmit chain” is used to refer to any part of apparatus which processes baseband signals and converts those to radio frequency signals for transmission at a transmitter. This involves many stages of upconverting, modulation and power amplification as known in the art. Thus the term “transmit chain” is used herein to refer to either all the apparatus needed for this conversion process or only some of that apparatus.
As mentioned above, previous MIMO systems have used as many MIMO channels as antennas. One receive chain (also referred to as a radio frequency receiver) per channel is required and this poses a practical restriction because the number of receive chains that can be provided in practice is limited by cost, complexity and power consumption in a basestation, user terminal or other communications device. This same limitation applies in the case of transmit chains. A similar practical restriction applies to non-MIMO radio communications devices such as those using transmit or receive diversity.
The present invention recognises this problem and enables the number of receive or transmit chains to be reduced whilst still increasing the number of antennas. That is, more receive antennas are used than receive chains (or more transmit antennas than transmit chains). For both MIMO and non-MIMO arrangements many benefits are achieved in this way including increases in capacity, improved carrier to interference levels, reduced costs and improved ability to cope with multipath fading. It is acknowledged that prior art transmit or receive diversity antenna arrangements are known with more receive antennas than receive chains for example. However, these have involved using a plurality of diverse antennas and producing a single channel output from those diverse antennas, by switched selection. The present invention recognises that additional benefits can be achieved by producing two or more channels of output from those diverse antennas. In this case, three or more diverse antennas must be used. These benefits include an improved ability to cope with multipath fading and improved receive gain.
A prior art receive diversity antenna arrangement is illustrated in
An embodiment of the present invention is illustrated in
Although
As mentioned above the present invention is applicable to MIMO communications systems as well as to non-MIMO arrangements such as those illustrated in
Previously, MIMO arrangements have used the same number of antennas as MIMO channels and thus the same number of receive chains as receive antennas (or transmit chains as transmit antennas). The present invention recognises that advantages are obtained by using fewer receive chains than receive antennas as illustrated in
We have shown that by using more receive antennas than receive chains in a MIMO system, capacity increases are obtained as compared with reducing the number of antennas to match the number of receive chains. This is the case both for nomadic user terminals which are moved from place to place, but are generally static when in use and also for mobile terminals which are moveable during use. The situation involving nomadic user terminals is now discussed.
Nomadic User Terminals
Nomadic user terminals are typically located indoors in environments where scattering occurs. Spatial fading of the multipath in the indoor environment has an envelope that is Rayleigh distributed, and this results in a MIMO link capacity which is dependent on the spatial location of the nomadic terminal. For a given area of constant local mean one obtains a capacity distribution for the MIMO link when that terminal is moved through spatial fading. Thus it is possible for any given user terminal to be placed in a “bad” location where the capacity is at the bottom end of the capacity distribution.
The theoretical Shannon capacity for a MIMO link is dependent on the spatially averaged carrier to interference levels and on the instantaneous received voltages on each MIMO path (when the mean power on each path has been normalised to unity). A capacity distribution then arises, because of the spatial fading of the MIMO paths. For a static terminal there is still some fading on the paths which is caused by the movement of objects in the environment. As a result the temporal fading tends to be Ricean with a high K-factor. Consequently, if a user terminal is placed in a ‘bad’ spot it tends to remain bad for the duration of the link. This is particularly bad if the delay spread is low such that there is only one resolvable tap in the time domain (no time diversity in CDMA systems), and a flat channel in the frequency domain (no frequency diversity in OFDM systems). This is likely to be the case, for example, in residential suburban environments (e.g. small office, home office SOHO applications).
In order to circumvent this problem we have equipped the user terminal with more antennas than it has receive chains. We show that a type of eigenvalue selection diversity can be employed which improves the lower end of the MIMO Shannon capacity distribution by about 20%. This in turn increases the overall throughput. The method is now described.
In a preferred embodiment a MIMO configuration is provided whereby there are two antennas at the basestation (also referred to as Node B), four antennas at the user terminal (also referred to as user equipment, UE), but only two receive chains at the UE. The situation is illustrated in
In the embodiment shown in
As an example, each of the MIMO paths can be represented as a random Gaussian process, having a mean power of unity. The paths are independent (uncorrelated) and power imbalance between the paths is not considered. Each path is represented by the following equation:—
is a random number having a normal distribution with a mean of zero and a standard deviation of 1/√{square root over (2)}. Thus, each matrix element consists of a complex voltage where the I and Q components are normally distributed. These result in an envelope which is Rayleigh distributed with a mean power of 2σ2=1.
Using this expression for each MIMO path (with different samples of the random variable) the six instantaneous 2:2 channel matrices can be constructed. Then, for each instance the eigenvalues of the channel product matrix can be determined, where the channel product matrix is given by the following:—
The eigenvalues of this matrix represent the power gains of the eigenmodes, of which there will be two.
The upper Shannon bound on capacity for a 2:2 MIMO configuration with independent Rayleigh fading channels can be determined using the following formula:—
where,
N=Number of transmit antennas
λi=ith eigenvalue
ρ=Signal to noise ratio
The capacity is the sum of the Shannon capacities of N orthogonal channels, where the power gains of the channels are given by the eigenvalues of the channel product matrix. In this sum the available transmit power is equally distributed between the two channels.
In order to implement an eigenvalue selection diversity scheme a metric is required to enable selection between the six possible UE antenna combinations. Two different metrics are considered:—
Sum of eigenvalues;
In the case that the selection metric is the maximum instantaneous link capacity we investigated the capacity distributions. The eigenvalue selection diversity improved the lower end of the capacity distribution, although in this case the improvement is greater than for the case where the maximum sum of eigenvalues is used as the selection metric. The capacity distributions for the two forms of eigenvalue selection diversity were compared. The difference between the two schemes is only significant at high SNR's.
For the ‘sum of eigenvalues’ scheme the six 2:2 channel matrices are estimated and the eigenvalues computed. This is preferably done on an average basis to average out any effects of temporal fading. For the maximum capacity scheme the eigenvalues are estimated as well as the signal to noise ratio (SNR) or carrier to interference level (C/I). Estimates of the instantaneous capacity are then made for the six possible channel matrices. Some averaging is used to eliminate any effects of temporal fading.
We also compared the capacity distribution for a standard 2:4 MIMO configuration to the capacity distribution for the ‘maximum capacity’ eigenvalue selection diversity scheme. The 2:4 MIMO system achieves a higher capacity, but this would require four receive chains at the UE rather than two. The ‘maximum capacity’ eigenvalue selection diversity curves were also compared to the capacity curves for 2:2 and 2:4 MIMO configurations. It was found that the eigenvalue selection diversity scheme achieved a significant part of the extra capacity gain that could be obtained from a 2:4 MIMO system with four receive chains.
In the examples described immediately above with respect to nomadic terminals, two selection metrics were considered, one being the sum of the eigenvalues and the other being the instantaneous link capacity. It is not essential to use these particular selection metrics and as discussed below other types of metric can be used. In addition, the results described above with respect to nomadic terminals also apply to mobile terminal situations at least to some extent.
Switching Mechanism
Consider an example of a MIMO switching receiver architecture as illustrated in
Mobile Terminals—High Speed Mobility
We have also found that MIMO configurations in which there are fewer receive chains than receive antennas can advantageously be used for high speed mobility applications where the user terminal is moving at speeds of up to about 100 km/hour. In one such embodiment using two transmit antennas, and selecting two from four receive antennas for use with two receive chains, we found a 3 dB gain as compared with standard 2:2 MIMO.
Selection Metrics
Two selection metrics were mentioned above, one being related to the link capacity and the other being the sum of the eigenvalues. Other selection metrics or methods can be used and some examples are given below:
Consider the situation in which the number of receive chains L is less than the number of receiving antennas M. In that case there are a total of K=M!/L!(M−L)! possible switching configurations where the symbol ! indicates “factorial”. It is beneficial to perform a full search of all possible switching configurations at the expense of high computational complexity. However, this is not essential as explained below. In
The best switch rules depicted in
Alternatively, in the case of two receive-chains, we could keep one of the currently selected antennas (the one with the better channel quality) and switch the other currently selected antenna. In
The switching criteria comprising RSSI, Shannon Capacity and eigenvalues (as mentioned above) can be implemented at base-band signal processing at the MIMO channel estimation output 103 (see
Using a simulation we have compared the performance of the various selection methods and metrics mentioned above and the results are shown in
The best performance was found when using the minimum BER as the selection metric (see line 112 in
Thus in a preferred embodiment of the present invention the selection metric is related to CRC. Also we have found that by using CRC based metrics the number of times at which switching is deemed to be required is far fewer than when using the eigenvalue based metrics. Thus when eigenvalue based metrics are used additional criteria such as some type of threshold mechanism may be required to prevent too frequent switching between antennas.
Results of our further simulations are shown in
where i,j=1, 2, 3, 4. In order to obtain the upper bound of the selection performance, we simulated the situation using three selection criteria (RSSI, Shannon Capacity, and Eigenvalues) using genie aided calculations; that is assuming perfect apriori channel knowledge or perfection channel prediction. All six possible selections could be selected. The CRC based criterion was simulated with real world receiver operation. The switch criteria used in the simulations are described below in more detail where λ1,λ2 are the eigen values of matrix Hi,j.
Although the selection metrics discussed above have been described with reference to selection from receive antennas these metrics can also be used for selection between transmit antennas by using any type of feedback mechanism from the receive apparatus to the transmit apparatus.
Use of Directional Antennas
When we consider situations involving a plurality of user terminals interference can occur as a result of signals from one terminal reaching another terminal and interfering with signals from a basestation to that terminal. By using directional antennas at the user terminals, for example, as previously implemented in fixed wireless access arrangements, it is possible to reduce such interference as compared with a reference situation using omnidirectional antennas at the user terminals. However we have found that by using directional antennas at the user terminal in a MIMO system, and in addition, using switched antenna selection at the user terminal, significant advantages are found. Even though switched selection effectively reduces the number of antenna elements which would be expected to reduce the resulting MIMO performance, we have found that benefits can be achieved.
This is explained in more detail below with respect to several embodiments concerning a MIMO antenna arrangement in a stand-alone unit for use with a user terminal such as a personal computer. In these embodiments an antenna arrangement for a user terminal is provided such that the user terminal can use a high data rate MIMO wireless link. Preferably the MIMO link comprises four orthogonal channels with similar signal levels in order to facilitate increase in capacity as a result of the use of MIMO communications.
The antenna arrangement is supported in a box, cube, or other stand alone structure which can be connected to a personal computer, laptop computer, personal digital assistant, or any other type of user terminal. This provides the advantage that the antenna arrangement is easily placed on a desk, table or other surface and can be used in conjunction with any suitable type of user terminal rather than being permanently integrated into one user terminal. The size of the antenna arrangement is minimised in order that the stand alone unit is compact and portable.
We compared the performance of the embodiments illustrated in
We found that configuration 1 (
In the examples described above with reference to
Using a computer simulation we found that for the arrangement of
Our simulation assumed that each nomadic user terminal (as in
We also found a 3 dB improvement in carrier to interference when the best combination of 2 antenna elements are selected from the 8 available as opposed to using any combination of 2 of the 8 antennas.
Thus we have found that by using switched selection between directional antennas at the user terminal in a MIMO arrangement improvements in carrier to interference levels are found and in addition situations involving spatial fading can more effectively be dealt with.
Switched selection between directional antennas at a MIMO user equipment is also particularly advantageous for small user equipment. In such cases the structure of the user equipment itself often blocks antenna patterns in particular directions for antennas mounted on the user equipment. A nominally omnidirectional azimuth pattern is then difficult to realise. For example, in a particular embodiment we provide 4 antennas for MIMO communications in a personal digital assistant (PDA).
Antennas 231 and 234 are preferably co-planar umbrella monopoles. Antenna 232 is preferably provided in the form of a slot mounted in the flap whilst antenna 234 is preferably a monopole mounted on the PDA body. We found that the profile of the PDA limited the amount of omnidirectionality of the antenna patterns because of blocking. This is addressed by using complementary directive patterns designed with the body of the PDA in mind. In a preferred example the four antennas are arranged to provide the directional antenna patterns of
In order to achieve the complementary directional patterns as described above any suitable method is used as known in the art.
It is noted that pointing losses are typically experienced by antenna switching systems. Such pointing losses limit the improvement in carrier to interference levels found for directional antenna systems with switched selection. One way to overcome this is to use steered beam systems, either with mechanical beam steering or adaptive combination techniques.
In the examples described above antenna selection is used to enable the number of transmit or receive claims to be reduced. A related advantage is achieved by using adaptive combination techniques which involve combining the effects of a plurality of antenna elements to produce directional antenna beams. An example is described in our earlier U.S. patent application Ser. No. 09/975,653 which is also assigned to Nortel Networks. In that document we describe a basestation antenna array with six columns of dual polarised antenna elements. The six columns have a spacing of half a wavelength in azimuth. Two fixed multiple beamformers, which do not allow beam steering, are used in conjunction with this array to form three directional antenna beams at each of the two polarisations. This basestation antenna array functions over a limited sector and as such is not suitable for user terminals which are nomadic or mobile and which may be placed in any orientation with respect to a basestation in use.
According to another aspect of the present invention we use an adaptive combination technique, in combination with a MIMO antenna arrangement. By using an adaptive combination technique to create directional antenna beams, carrier to interference levels are improved and capacity thus increased. Advantageously, the adaptive combination method can be electronically controlled in order to change the direction of the antenna beams produced.
For example, in the embodiment of
In order that a MIMO system can be provided the antenna beams are arranged to be diverse. For example, in the embodiment of
Any suitable type of adaptive combination may be used. For example, using beamformers or by using phased combination.
Any range or device value given herein may be extended or altered without losing the effect sought, as will be apparent to the skilled person for an understanding of the teachings herein.
Suppose that all receive antennas are independent, then by picking any two receive antennas, the multi-path components of these two channels are uncorrelated, which means for a stationary process, the frequency diversity gain is unchanged comparing to the fixed antenna case. This appendix analyses antenna switching by exploiting temporal up fades. Note that this is the minimal we can achieve, as antenna switching based on both temporal up fades and frequency diversity is not concerned in this appendix. However, note that the most significant contributing factor in our system is temporal fading rather than frequency selective fading. The reason is that we can effectively mitigate the effects of frequency selective fading with a channel interleaver, but we can only effectively mitigate temporal fading with space diversity (e.g., antenna switching). This is especially true when Doppler is low, e.g., with a vehicle moving at a speed of 30 kmph.
Let P1>P2>P3>P4 be the received powers from four receive antennas, then, with antenna switching, the average power from the two active antennas is:
PAS=E(P1+P2) (10)
With fixed antenna, the average power received by two antennas is:
Therefore, the gain we can get from antenna switching is:
This means that the maximum gain we can get from antenna switching is 3 dB, and this happens when the signals received from the two weak antennas are negligible compared to the signals received from the two strong antennas. Note that when P1=P2=P3=P4, the gain calculated from Equation (30) is 0 dB, which means no gain can be achieved by selecting antennas from equal strong antenna pools.
With Rayleigh fading,
can be fairly small. Suppose that
then the gain would be 2.2 dB.
The above results can be extended to N>4 receive antennas. With P1>P2> . . . >PN, the gain obtained from antenna switching can be updated to
which means the gain is bounded by
If N=6, then the up-bound for the gain is 4.77 dB. Notice that this bound is achieved with
With N increases, the up-bound 10 log(N/2) (dB) becomes looser and looser, as the condition of
is unlikely to be true.
Equation (0) indicates that when N→∞, the two selected antennas can always catch the up-fades, hence the gain from antenna switching equals to the peak-to-average ratio of the fade, which is channel dependent.
Number | Date | Country | |
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Parent | 10083094 | Feb 2002 | US |
Child | 11363359 | Feb 2006 | US |