Patent Application
20080316955
1. Field
This disclosure relates generally to wireless networking systems, and more specifically but not exclusively to technologies for enabling spatial division multiple access (SDMA) in a wireless network.
2. Description
Wireless communication systems are generally composed of one or more local central sites that serve a local area wherein a number of wireless users, fixed or mobile, are located. The local central sites are commonly referred to as base stations (BS) or access points (AP). In what follows, the term base station is used to describe the local central site. Base stations are equipped with transmitters and receivers through which wireless users with transmitter and receivers gain access to larger networks such as the public switching telephone network (PSTN) or the Internet. One of functions performed by a BS is to relay messages to and from wireless users all over the network. For multiple users to access the same AP or BS, traditional wireless systems use Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), or any combinations of the three. FDMA works by allocating different frequencies to multiple users accessing a base station at the same time while TDMA works by allocating different time slots to multiple users accessing a base station at the same frequency. CDMA works by assigning multiple users accessing the same base station with unique time-frequency waveforms.
Spatial Division Multiple Access (SDMA) is a system access technology that allows a BS to provide multiple communication channels to multiple users by dividing the radio coverage into non-overlapping areas in the spatial domain. Each area may be assigned to one user so that the same frequency and time resource may be used by multiple users. In wireless communication systems that do not have the problem of multi-paths, such as satellite communications, SDMA is usually achieved through the use of directional beam pattern antennas. In wireless communications systems where multi-path is prevalent, one commonly used method to enable SDMA is to use an adaptive antenna system (AAS), or smart antenna system. AAS may include an antenna array that is capable of combining, constructively or destructively, multiple copies of the same signal received at each antenna. Multiple copies of the same desired signal received at each of the antenna may be combined constructively to enhance the desired signal while multiple copies of the same undesired signal received at each of the antenna may be combined destructively. The end result is an increase in signal-to-interference-noise ratio (SINR). In an SDMA system, each antenna receives multiple user signals. Each user is de-multiplexed from other users by treating other user signals as undesired signals through the aforementioned array processing, or by jointly detecting users on the same SDMA channel. SDMA may be regarded as a fourth type of multiple access method. Multiple accesses may thus be achieved in four domains: frequency, time, code, and space. In addition, SDMA may be combined with any or all of the other three types of multiple access method.
Because SDMA increases the capacity of a wireless system as the same frequency, time, and code resources may be reused for multiple users, SDMA has become very popular in today's broadband wireless systems, especially with the increasing demand for data throughput.
To enable SDMA, a wireless system usually requires special protocols and features related to SDMA that are not required in a conventional non-SDMA system. Examples of SDMA protocols and features may include:
Due to the large volume of user terminals and consumer's sensitivity to the price of user terminals, it is highly desirable to keep the user terminal simple and low cost. The special SDMA protocols and features are usually complex and are not needed in a system that does not support SDMA. In addition, they may be required to be implemented not only on the access network side but also the user terminal side.
One example is IEEE802.16 standard. IEEE 208.16, commonly known as World Interoperability for Microwave Access (WiMAX), is a broadband wireless standard described in “Air Interface for Fixed Broadband Wireless Access Systems,” IEEE STD 802.16-2004, October, 2004, and “Air Interface for Fixed and Mobile Broadband Wireless Access Systems,” IEEE P802.16e/D12, February, 2005. SDMA is one of access methods supported by the WiMAX standard. To support and enable SDMA, the IEEE 802.16 standard specifies a number of features and protocols related to MS and SDMA, such as MS zone, SDMA pilots, Downlink Information element for SDMA (MS_SDMA_DL_IE), and Uplink information element for SDMA (MS_SDMA_UL_IE). MS zone includes channels that have a special structure including MS preambles and SDMA pilots. AAS_SDMA_DL_IE are control messages that a base station uses to allocate SDMA channels to users in the downlink. MS_SDMA_UL_IE are control messages that a base station uses to allocate SDMA channels to users in the uplink. A WiMAX network with some or all of these features and protocols enabled on both base stations and user terminals will be able to support SDMA.
IEEE 802.16 is a standard that has many options. To facilitate inter-operability between products from different vendors, the WiMAX Forum, which is a consortium of members consisting of companies and parties who are interested in promoting WiMAX, has devised a list of the features (called mobile system profile) that the members have agreed to implement. The most recent system profile is described in WiMAX Forum Mobile System Profile: Release 1.0 Approved Specification, WiMax Forum, Apr. 12, 2007. To keep the user terminals simple, the advanced AAS and SDMA related features, such as AAS zone and SDMA pilots, which are listed under Section AAS, are described as features not required to be implemented in this profile,. As a result, the majority of the user terminals produced for WiMAX will not support AAS and SDMA features. This will make it very difficult to use SDMA in a WiMAX network. Even if the AAS and SDMA features are implemented in base stations of a WiMAX network, it may still not be possible to use SDMA as specified in the IEEE 802.16 standard since there will not be enough user terminals to support the AAS and SDMA features.
The features and advantages of the disclosed subject matter will become apparent from the following detailed description of the subject matter in which:
The wireless network 100 may also include one or multiple access service networks (ASN) 114 and one or multiple core service networks (CSN) 124. The ASN 114 may have different names in different wireless systems. One such commonly used alternative name is radio access network (RAN). It provides network functions needed to enable a wireless user terminal with radio access. It includes functions such as connectivity with user terminals, radio resource management, relay functions, etc. ASN 114 may comprise one or more base stations, such as BS's 116A and 116B, and one or more ASN gateway(s) such as ASN gateway 120. The ASN gateway 120 may have different names in different wireless systems. One such commonly used name is base station controller (BSC). An ASN gateway aggregates BS traffic and interfaces with CSN. CSN may have different names such as core network (CN) in different wireless standard. In addition, the ASN gateway may provide radio resource control and management as well as mobility management functions. A BS is a generalized equipment for providing connectivity, management, and control of user terminals. A BS is a network element providing an air interface between user terminals (e.g., terminal 110A, 110B, and 110C) and an access service network (ASN) (e.g., ASN 114). A BS may have a single sector or multiple sectors. In a single sector BS, it usually uses omni-directional antenna to cover its coverage area. In a multi-sector BS, the BS's coverage area is divided into radial sectors with directional antenna covering each sector. For example, three 120 degree antennas installed at a BS forms a 3-sector BS. A BS may connect to an ASN gateway. In
CSN may perform a set of network functions that provide network connectivity services to a wireless subscriber. CSN may comprise network elements such as routers, authentication, authorization, and accounting (AAA) proxy/servers, user databases, etc.
A user terminal may be coupled to one or several BSs through wireless air link 112. According to one embodiment shown in
At least one BS (e.g., BS 116A and/or BS 116B) may implement SDMA according to an embodiment of the subject matter disclosed in this application. A BS that implements SDMA according to an embodiment of the subject matter disclosed in this application will be referred to as SDMA BS hereinafter. A BS that does not have SDMA capability, or a BS that implements SDMA other than an embodiment of the subject matter disclosed in this application, will be referred to as conventional BS hereinafter. A BS without SDMA capability will be referred to as non-SDMA BS hereinafter. A conventional BS may thus include a non-SDMA BS, or a BS that implements SDMA through special SDMA protocols and features.
Both SDMA BS's and conventional BS's including non-SDMA BS's may include BS's that have multiple-input-multiple-output (MIMO) capability. MIMO systems use both multiple antennas at the transmitter and receiver. Performance of wireless communication may be impaired by multi-path fading. Multi-path occurs when the transmitted signal arrives at an intended receiver through different paths. A MIMO system exploits the multi-path signals in the spatial domain in addition to the time and frequency domains which are domains usually used in a single antenna system. A MIMO system increases the spatial diversity up to M×N times, where M and N are the number of transmit and receive antennas respectively. The increase in spatial diversity may be used to increase the coverage and/or data throughput of a wireless system.
For wireless network 100 as shown in
Although the user terminals might not be aware of the SDMA nature of their access to a wireless network, SDMA BS 210 is fully aware of SDMA and may need to coordinate multiple access among emulated non-SDMA BS's. Within each of the emulated non-SDMA BS's, multiple access is supported without using SDMA. An emulated non-SDMA BS may provide multiple access by user terminals through one or a combination of FDMA, TDMA, and CDMA methods. In
Additionally, service areas of emulated BS's 250, 252, and 254 may also overlap. In overlapped coverage areas of the emulated BS's, the SDMA BS 210 may use spatial multiplexing and de-multiplexing technologies to separate their supported communication channels in spatial domain. Channel separating, or de-multiplexing may be achieved by equipping an SDMA BS with an adaptive antenna system that includes an array of antennas with multiple receivers and transmitters, and by adding an SDMA module capable of spatial domain processing. The capability of antenna arrays to separate signals in the spatial domain is well known in the signal processing community. One such an adaptive antenna system is described in “Digital Beamforming in Wireless Communications,” by John Litva, published by Artech House in 1996.
PHY 310 receives and transmits signals. It includes a receive section which comprises a multi-channel receiver 320, a spatial de-multiplexer 322, and a signal demodulator and decoder 324. Multi-channel receiver 320 receives multiple user signals from an antenna array, conducts receive processing of single antenna signal such as amplification, filtering, down conversion, analog-to-digital conversion, etc. The spatial de-multiplexer 322 separates multiple user signals in the spatial domain through methods such adaptive array processing whenever needed. With the separation capability of spatial de-multiplexer 322, multi-channel receiver 320 will be able to receive signals from multiple user signals using a single antenna array. Signal demodulator and decoder 324 removes signal modulation and coding on each user's signal to recover the original data transmitted by user terminals.
PHY 310 also includes a transmit section which comprises a multi-channel transmitter 328, a spatial multiplexer 330, and a signal modulator and encoder 332. The signal modulator and encoder 332 encodes each user's data and modulates the data to make it suitable for conversion into radio frequency (RF). The spatial multiplexer 330 multiplexes each user signal on to the input of the multi-channel transmitter 328. It may also conduct spatial processing such as adaptive array transmit processing to enable easy reception of each user's signal by a user terminal after they are transmitted from the antenna array. Spatial multiplexer 330 enables multiple emulated non-SDMA BS's signals to be transmitted through a single antenna array. The multi-channel transmitter 328 carries out transmit functions such as digital-to-analog-conversion, filtering, up-conversion, signal amplification, etc. After performing these transmit functions, the multi-channel transmitter 328 transmits RF signals through an antenna array to user terminals. The multiple-channel receiver 320 and the multiple-channel transmitter 328 may share the same antenna array.
PHY 310 may also include an SDMA PHY control unit 326 which conducts computing and signal processing needed for controlling spatial multiplexing and de-multiplexing. SDMA PHY control unit 326 may obtain control information for spatial multiplexer 322 from signals received from multi-channel receiver 320.
MAC 312 may include a receive MAC processing unit 334 and a transmit MAC processing unit 338 which carry out MAC message processing for the receive and transmit sides, respectively. MAC 312 may also include an SDMA MAC control unit 336 which control other components in the MAC layer and make decisions such as user scheduling in the MAC layer. The SDMA MAC control unit 336 may work in conjunction with the SDMA PHY control unit 326 to jointly make decisions related to SDMA control such as user scheduling.
In the example SDMA BS 300 shown in
SDMA module 316 may also conduct the analysis of the suitability of performing SDMA. Depending on the capability of SDMA module 316 and factors such as the number of antennas used, there may be situations in which separation of one user signal from others in spatial domain is not reliable. One example is when one user's signal power is so high that it completely drowns out other user signals. In this case, it is desired that such a high power user is not allocated on SDMA with other users, or is allocated on SDMA with other users that have similarly high signal powers. SDMA module 316 may conduct the analysis of the suitability of performing SDMA, scheduling SDMA users through SDMA MAC control unit 336 only when the conditions are favorable to SDMA.
In wireless communications, user terminals communicate with base stations through wireless channels. Communication between a user terminal and a base station can be generally characterized into three types: unicast, broadcast, and multicast. In a unicast communication, there is one-to-one relationship between the transmitter and the receiver. In broadcast communication, the transmitter sends signals/messages directed to everyone. Multicast communication is similar to broadcast communication, the difference is that signals/messages are sent to a set of selected receivers rather than everyone. Information and messages related to network parameters, user terminal synchronization, such as pilots, synchronization channels, and user terminal paging channels, are usually broadcast or multicast while user traffic are usually unicast.
There are also training sequences or pilots 418, 420, and 422 in most of the channels as illustrated in
Since an SDMA BS emulates multiple co-located non-SDMA BS's, signals to and from these emulated BSs are likely to collide, or interfere with each other. Although user terminals accessing a SDMA base station may not be aware of the nature of SDMA, the SDMA base station is fully aware of SDMA. The SDMA base station may use spatial processing techniques such as those disclosed in the present application to separate them. In many situations, it is also desirable for the SDMA BS to coordinate among its emulated non-SDMA BS's to facilitate the use of additional frequency domain, time domain, or code domain processing techniques. These techniques may be used alone, combined together, or used together with the spatial domain techniques, to make multiple access channels of the emulated non-SDMA BS's work more reliably. For example, the aforementioned adaptive array processing may be less effective for downlink broadcast or multicast channels when the array need to focus energy on multiple user terminals simultaneously. To make the downlink broadcast and multicast channels more reliable, the SDMA BS may choose to place the broadcast and multicast channels of each of the emulated BS's on different frequency band so they do not overlap in frequency domain. User terminals associated with different emulated non-SDMA BS's can detect their desired downlink broadcast and multicast channels through frequency domain filtering. Even in the situation of unicast channels, the SDMA BS may coordinate among its emulated BS's to facilitate additional signal processing in frequency, time, or code domains to make communication more reliable. In what follows, several embodiments of the subject matter disclosed in the present application using these techniques are described.
In one embodiment, channels that carry information conveyed through a set of codes or a data sequence may be so chosen that they are different in one or multiple of the emulated non-SDMA BSs of an SDMA BS. For example, in the example WiMAX channels 400 in
In another embodiment of the subject matter disclosed in the present application, channels may be allocated so that they are on different frequency bands for one or multiple of the emulated non-SDMA BSs of an SDMA BS. For example, in the example WiMAX channels 400 in
In another embodiment of the subject matter disclosed in the present application, channels may be allocated so that they are not overlapping in time in one or multiple of the emulated non-SDMA BSs of an SDMA BS. For example, in the example WiMAX channels 400 in
Yet In another embodiment of the subject matter disclosed in the present application, one or multiple of pilots, which are predefined training sequences, my be chosen to be different for channels in one or multiple of the emulated non-SDMA BS's of an SDMA BS. This makes it easier for a user terminal or a base station to estimate its channel when demodulating and decoding the signals on SDMA channels.
Although example embodiments of the disclosed subject matter are described above using examples of preamble, downlink control channel, uplink control channel, and pilots, persons of ordinary skill in the art will readily appreciate that many other methods of implementing the disclosed subject matter may alternatively be used. In addition, the methods of implementing the disclosed subject matter may be changed, or combined. For example, the preambles may be chosen to be placed on different frequency bands rather than using different data sequences, or the preambles may be chosen to be placed on different frequencies as well as using different data sequences.
Process 500 starts with operation 510 during which a wireless system powers up and makes itself ready for operation. Also at this operation, a BS is allocated to support SDMA when communicating with a plurality of user terminals. Subsequently, the SDMA BS performs operation 512 in which it emulates multiple co-located non-SDMA BSs, or multiple non-SDMA sectors if it is a sectorized SDMA BS. To a user terminal, the channels from the SDMA BS are just as if they were received from multiple non-SDMA BSs located at the same cell site. Each user terminal communicates with the SDMA BS through one of its emulated non-SDMA BS. Whenever needed, the SDMA BS performs operation 514 in which it allocates SDMA channels among one or multiple emulated BS's. The SDMA base station may use spatial processing techniques such as those disclosed in the present application to separate them. Additionally, the SDMA BS may coordinate among its emulated non-SDMA BS's to facilitate the use of additional frequency domain, time domain, or code domain processing techniques. After operation 514, process 500 returns to operation operation 512.
Process 500 also applies to BSs with MIMO capability. To support MIMO communication, among the multiple emulated non-SDMA BSs, some or all of them may be base stations that support MIMO communication.
Process 600 starts with operation 610 during which a WiMAX SDMA base station powers up and makes itself ready for operation. Subsequently, the SDMA BS performs operation 612 in which it sends out multiple preambles. The preambles may be allocated on different frequency segments or bands to facilitate reception by user terminals. The SDMA BS then performs operation 614 in which it sends out multiple sets of broadcast channels including downlink control channels. The downlink control channels may be on different frequency bands to facilitate reception by user terminals. The SDMA BS has thus emulated multiple non-SDMA BSs' preambles and downlink broadcast channels. User terminals in the SDMA base station coverage area synchronize to the emulated BSs' downlink and complete network entry in operation 616. A user terminal registers and associates with a BS through the network entry process. In operation 618, the SDMA base station decides if there are data transfer(s) required. It goes back to operation 612 if there is no data transfer needed, otherwise it informs the user terminals the data allocations in operation 620 so the user terminals understand where to transmit and/or receive their corresponding data messages. Operation 622 multiplex/de-multiplex the user signals. The allocation in operation 620 may be multiple access without SDMA, in which case frequency domain, time domain, or code domain multiple access processing is need in operation 622 to multiplex and de-multiplex user signals. If SDMA allocation is used, additional spatial domain processing may be need in addition to the time/frequency/code domain processing in operation 622. Process 600 also applies to BSs with MIMO capability. To support MIMO communication, data allocation in operation 620 may be on MIMO channels.
Although example embodiments of the disclosed subject matter are described with reference to block and flow diagrams in
In the preceding description, various aspects of the disclosed subject matter have been described. For purposes of explanation, specific numbers, systems and configurations were set forth in order to provide a thorough understanding of the subject matter. However, it is apparent to one skilled in the art having the benefit of this disclosure that the subject matter may be practiced without the specific details. In other instances, well-known features, components, or modules were omitted, simplified, combined, or split in order not to obscure the disclosed subject matter.
Various embodiments of the disclosed subject matter may be implemented in hardware, firmware, software, or combination thereof, and may be described by reference to or in conjunction with program code, such as instructions, functions, procedures, data structures, logic, application programs, design representations or formats for simulation, emulation, and fabrication of a design, which when accessed by a machine results in the machine performing tasks, defining abstract data types or low-level hardware contexts, or producing a result.
For simulations, program code may represent hardware using a hardware description language or another functional description language which essentially provides a model of how designed hardware is expected to perform. Program code may be assembly or machine language, or data that may be compiled and/or interpreted. Furthermore, it is common in the art to speak of software, in one form or another as taking an action or causing a result. Such expressions are merely a shorthand way of stating execution of program code by a processing system which causes a processor to perform an action or produce a result.
Program code may be stored in, for example, volatile and/or non-volatile memory, such as storage devices and/or an associated machine readable or machine accessible medium including solid-state memory, hard-drives, floppy-disks, optical storage, tapes, flash memory, memory sticks, digital video disks, digital versatile discs (DVDs), etc., as well as more exotic mediums such as machine-accessible biological state preserving storage. A machine readable medium may include any mechanism for storing, transmitting, or receiving information in a form readable by a machine, and the medium may include a tangible medium through which electrical, optical, acoustical or other form of propagated signals or carrier wave encoding the program code may pass, such as antennas, optical fibers, communications interfaces, etc. Program code may be transmitted in the form of packets, serial data, parallel data, propagated signals, etc., and may be used in a compressed or encrypted format.
Program code may be implemented in programs executing on programmable machines such as mobile or stationary computers, personal digital assistants, set top boxes, cellular telephones and pagers, and other electronic devices, each including a processor, volatile and/or non-volatile memory readable by the processor, at least one input device and/or one or more output devices. Program code may be applied to the data entered using the input device to perform the described embodiments and to generate output information. The output information may be applied to one or more output devices. One of ordinary skill in the art may appreciate that embodiments of the disclosed subject matter can be practiced with various computer system configurations, including multiprocessor or multiple-core processor systems, minicomputers, mainframe computers, as well as pervasive or miniature computers or processors that may be embedded into virtually any device. Embodiments of the disclosed subject matter can also be practiced in distributed computing environments where tasks may be performed by remote processing devices that are linked through a communications network.
Although operations may be described as a sequential process, some of the operations may in fact be performed in parallel, concurrently, and/or in a distributed environment, and with program code stored locally and/or remotely for access by single or multi-processor machines. In addition, in some embodiments the order of operations may be rearranged without departing from the spirit of the disclosed subject matter. Program code may be used by or in conjunction with embedded controllers.
While the disclosed subject matter has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the subject matter, which are apparent to persons skilled in the art to which the disclosed subject matter pertains are deemed to lie within the scope of the disclosed subject matter.
