Abstract:
A method of wirelessly communicating between units comprises using at least two repeaters between a transmit unit and a receive unit. The repeaters enable a diversity scheme to be emulated.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention is related to a method of creating spatial diversity in wireless communication networks.  
           [0002]    Wireless communication systems have exploded in popularity in recent years. Cellular phones, pagers, and more recently Bluetooth devices all take advantage of these wireless communication systems. A common obstacle to wireless communications is a phenomenon known as fading, where messages transmitted between two units in the system are lost or garbled.  
           [0003]    One technique to combat fading is using diversity schemes such as a spatial transmit diversity scheme. Transmit diversity schemes send at least two signals from distinct antennas or antenna elements. The signals contain parts or all of the same message. The goal of transmit diversity it to alleviate the effects of fading. The signals are transmitted in such a way that they do not cancel each other at the receiver. The receiver can process the signals to exploit diversity and improve performance. Transmit diversity can be viewed as a dual of traditional receive diversity, where signals from multiple receive antennas are processed by the receiver. Transmit and receive diversity can also be used together to obtain further improvements. Transmit diversity methods include delay diversity, the Alamouti code, the Lindskog-Paulraj technique, space-time codes, BLAST, and other similar methods. Some transmit diversity schemes require multiple transmit antennas, which may be difficult to implement in small, inexpensive wireless devices. For example, Bluetooth devices typically have a single transmit antenna operative in the Bluetooth frequency band.  
         BRIEF SUMMARY OF THE INVENTION  
         [0004]    The present invention may be used in ad-hoc networks such as are created in Bluetooth systems. In particular, a transmit unit, using a single transmit antenna may send a message to at least two intermediate units that act as repeaters to send the message to a receive unit. By routing the message through the two repeaters, spatial transmit diversity is created.  
           [0005]    To route the message through the two repeaters, the message may be encoded into two distinct signals, each of which is sent to a different repeater. The encoding process may use any number of coding schemes such as an Alamouti code, a Linskogg-Paulraj code, or the like.  
           [0006]    Further, the present method of using repeaters may be used to fine tune power control feedback loops. The receiver may control the repeaters and the repeaters may control the originating station such that power is conserved. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a block diagram of a wireless communication system according to one embodiment of the present invention;  
         [0008]    [0008]FIG. 2 is a flow diagram illustrating one embodiment of a power control method associated with the present invention;  
         [0009]    [0009]FIG. 3 is a flow diagram illustrating a second embodiment of a power control method associated with the present invention; and  
         [0010]    [0010]FIG. 4 is a functional block diagram of a mobile terminal such as may be used in conjunction with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]    The present invention relates to diversify techniques for wireless communication network. In particular, the present invention provides a technique of creating special diversity between a transmit unit using a single antenna and a receive unit using a single antenna. Other forms of diversity may also be created. Spatial diversity, as used herein, is defined to be those situations where diversity is achieved by antenna separation. This is conventionally achieved by using a plurality of antennas at either the transmitter or receiver. According to the present invention, partial diversity is achieved by using a plurality of repeaters, interposed between a transmitter and a receiver. This invention is primarily directed at ad hoc networks, such as a Bluetooth network, but is also applicable to WCDMA-TDD systems and other types of networks.  
         [0012]    [0012]FIG. 1 illustrates the basic concept of the present invention. A wireless communication network  10  comprises a transmit unit  20 , a first intermediate unit  30 , a second intermediate unit  40 , and a receive unit  50 . An obstacle  60  may be present within the network  10 . Obstacles include geographic or man-made features that inhibit wireless communication as well as environmental factors that inhibit wireless communication. While obstacles  60  may encourage the use of the present invention, they need not be present to justify using the present methodology. Transmit unit  20  uses a single transmit antenna  22  within an operative frequency band. Likewise, receive unit  50  uses a single receive antenna  52  within the operative frequency band.  
         [0013]    In one embodiment, the network  10  is an ad hoc network, such as envisioned by the Bluetooth standard. The Bluetooth standard enables seamless communication of data and voice over short-range wireless links between both mobile devices and fixed devices. The Bluetooth standard permits ad hoc networking of devices equipped with a Bluetooth interface. Different Bluetooth devices can automatically connect and link up with one another when they come into range to form an ad hoc network, generally referred to as a piconet. The Bluetooth standard specifies how mobile devices, such as phones, personal digital assistants (PDAs), and wireless information devices (WIDS), can interconnect with one another and with stationary devices, such as desktop computers, printers, scanners, and stationary phones.  
         [0014]    Bluetooth devices operate in the Industrial-Scientific-Medical (ISM) frequency band at approximately 2.45 GHz. The ISM band is an unlicensed frequency band. The Bluetooth standard employs spread spectrum techniques that provide a high degree of interference immunity and multiple access. In particular, the Bluetooth standard employs a spread spectrum technique called frequency hopping to spread a narrowband signal over a wide spectrum of frequencies. This technique spreads a narrowband signal by “hopping” from one frequency to another in a defined sequence in accordance with a pseudo-random code and at a defined hop rate.  
         [0015]    Frequency hop systems divide the frequency band into a plurality of hop carriers or frequencies. Each hop channel comprises a defined sequence of frequency hops. The Bluetooth standard defines seventy-nine hop frequencies with one MHz spacing. A hop channel comprises a particular sequence of frequency hops. A hop channel is divided into 625 microsecond intervals—called slots—each corresponding to a different hop frequency. Thus, the Bluetooth device hops from one hop frequency to another, remaining on each hop frequency for a period of 625 microseconds, giving a nominal hop rate of 1,600 hops per second. One packet can be transmitted per slot or hop. Slots within a hop channel are alternately used for transmitting and receiving, which results in a time division duplex (TDD) scheme.  
         [0016]    Each hop channel is determined by the hop sequence (the order in which the hop frequencies are visited) and by the phase of the hop sequence. Two or more units sharing the same hop channel form a piconet, where one unit acts as a master—controlling traffic on the piconet—and the remaining units act as slaves. Under the Bluetooth standard, the hop sequence is determined by the master unit&#39;s system clock. The slaves use the master identity to select the same hop sequence and add time offsets to their respective native clocks to synchronize to the master unit.  
         [0017]    The last bit of the Bluetooth puzzle that may be helpful in understanding the basics of the standard is how the piconets are formed. Every Bluetooth device is identified by a unique address called the Bluetooth device address. A first device obtains this address from a second device through a procedure called an “inquiry.” When the first device invokes the inquiry procedure, all listening devices in range of the first device will respond to this inquiry by returning a response that includes inter alia the Bluetooth device address of the responding device. The standard has provisions for preventing the responding devices from responding all at the same time. Thus, after the inquiry procedure, the first device has the Bluetooth device address for all Bluetooth devices within range of the first device. The first device may now establish a connection to form a piconet. The procedure for establishing this connection is called “paging.” A page is always directed towards one device, typically, one of the devices for which a response to an inquiry was received. However, the paging device may already have the Bluetooth device address of the paged device without the need for sending an Inquiry message to obtain the address. When the first device initiates a page to the second device, the second device answers the page and synchronizes itself to the first device&#39;s hop channel, while also offsetting its internal clock. Thus, for that piconet, the paged device becomes a slave unit with respect to the paging device. There may be an optional authentication step in this process if needed or desired.  
         [0018]    In network  10 , any of the units  20 ,  30 ,  40 , or  50  may be the master unit. However, for the purpose of further description, it is assumed that the transmit unit  20  is the master and connects to the receive unit  50 . While establishing that connection, the transmit unit  20  and the receive unit  50  may negotiate a protocol to specify how further communications between the two will be conducted. This negotiation may cover whether diversity is to be employed and who will be the master. From this initial negotiation, the two units  20 ,  50  may recruit the intermediate units  30 ,  40  to serve as repeaters as described below. This negotiation step may involve an extension or modification of the existing Bluetooth standard to implement the present invention. If transmit unit  20  and receive unit  50  elect to implement diversity in further communications, intermediate units  30  and  40  are notified, typically by the transmit unit  20 to serve as repeaters. Intermediate units  30  and  40  relay messages from transmit unit  20  to receive unit  50 , as hereinafter described, in a fashion that creates a spatial diversity despite the fact that the transmit unit  20  and the receive unit  50  only use a single antenna  22 ,  52  respectively, at the operative frequency band to transmit and receive the message.  
         [0019]    In one exemplary embodiment of the invention, transmit unit  20  processes a message to be transmitted to produce two distinct signals s 1  and s 2 . A message, as used herein, comprises the information that the transmit unit  20  wishes to convey to the receive unit  50 . It may comprise voice, data, or some other information as needed or desired. Transmit unit  20  may use an Alamouti coder to process the message into the two distinct signals. Other codes are also contemplated, such as the Linskogg-Paulraj code. Transmit unit  20  sends signal s 1  to a first intermediate unit  30  and signal s 2  to a second intermediate unit  40 . The two signals may be transmitted simultaneously on different frequencies, or sequentially on the same frequency. In a CDMA system, such as WCDMA-TDD, the two signals can be transmitted simultaneously on different spreading codes, which provide orthogonality or quasi-orthogonality, enabling the receiver to distinguish between them. More generally, each of the two signals can be transmitted on a number of spreading codes. In either case, intermediate units  30 ,  40  retransmit their respective received signals s 1  and s 2  on the same frequency and at the same time to the receive unit  50 . The signal r received at receive unit  50  can thus be modeled as:  
           r=a   1   s   1   +a   2   s   2   +n   Eq. (1)  
         [0020]    where a 1  and a 2  represent the cumulative fading channels between transmit unit  20  and receive unit  50  through intermediate units  30 ,  40 , respectively, and n represents the cumulative noise and interference from transmit unit  20  to receive unit  50 . Receive unit  50  processes the received signal r to recover the original message. Such processing is well known in the art. To summarize briefly, receive unit  50  generates channel estimates of a 1  and a 2  and then uses these channel estimates to generate estimates of signals s 1  and s 2 . The estimates of s 1  and s 2  are then provided to a decoder, which process the estimates of s 1  and s 2  to generate an estimate of the original message. For example, if Alamouti encoding is used, receive unit  50  would include an Alamouti decoder to combine or decode signals s 1  and s 2 . Of course, other diversity combining techniques could be employed by receive unit  50 .  
         [0021]    Equation (1) assumes flat fading channels with a single tap, but the extension to multi-tap channels is well within the skill of those proficient in the art. If intermediate units  30  and  40  are separated enough in space, then the cumulative fades are uncorrelated. In this case, equation (1) corresponds with the received signal in a system with two antenna transmit diversity. To this extent, the network  10  has created a spatial transmit diversity scheme. The present invention may be particularly useful in a Bluetooth network, spatial diversity is a helpful feature, as most units  20 ,  30 ,  40 , and  50 , will only use a single antenna at the ISM frequencies.  
         [0022]    This concept is easily extended. For example, intermediate units  30 ,  40  may retransmit their received signals to receive unit  50  twice. In one variation, the retransmitted signals s 1 , s 2  may be sent at a first time on a first common frequency and at a second time on a different common frequency. In a second variation, the retransmitted signals s 1 , s 2  may be transmitted simultaneously at a first time, and again at a second time. The signals r 1  and r 2  received at receive unit  50  may be expressed as:  
           r   1   =a   1   s   1   +a   2   s   2   +n   1   Eq. (2)  
         [0023]    and  
           r   2   =a   1   ′s   1   +a   2   ′s   2   +n   2   Eq. (3)  
         [0024]    Equations (2) and (3) correspond with the received signals for a system with two transmit antennas and two receive antennas. Receive unit  50  processes received signals r 1  and r 2  as previously described to recover the original message. Thus, not only is transmit diversity created, but also receive diversity is created.  
         [0025]    In another variation of the invention, intermediate units  30 ,  40  may retransmit their respective received signals s 1 , s 2  on different frequencies or at different times to the receive unit  50 . The signals r 1  and r 2  received at receive unit  50  may be expressed as:  
           r   1   =a   1   s   1   +n   1   Eq. (4)  
         [0026]    and  
           r   2   =a   2   s   2   +n   2   Eq. (5)  
         [0027]    This variation may simplify processing of the message at the receive unit  50  as there is no crosstalk between the signals s 1  and s 2  that needs to be separated. It should be noted that even in networks  10  that require intermediate units  30 ,  40  to retransmit their respective received signals s 1 , s 2  on adjacent frequencies and/or times, spatial separation of intermediate units  30 ,  40  still provides some form of diversity.  
         [0028]    In all the variations, it is expected that the transmit unit  20  instructs the intermediate units  30 ,  40  on when and how to retransmit the signals, s 1 , s 2  but it is possible, especially when the receive unit  50  is the master unit, that the receive unit  50  is providing these instructions.  
         [0029]    Units  20 ,  30 ,  40 , and  50  may further include power control feedback loops therebetween. In many systems there is a mechanism for fast feedback, allowing a transmitting unit ( 20 ,  30 , or  40 ) to receive information from the receiving unit ( 30 ,  40 , or  50 ) on a reverse channel regarding the quality of the received signal. This signal quality is reflected in large part by estimates of a 1  and a 2  and the noise and interference level, where a 1  and a 2  represent the fading of the channels as previously defined. The feedback is useful to the transmitting unit  20 , which can adjust its transmit power accordingly, using one of several known strategies.  
         [0030]    Exemplary flow charts illustrating this power control method are presented in FIGS. 2 and 3. In FIG. 2, the transmit unit  20  transmits signals s 1  and s 2  to the intermediate units  30 ,  40  (block  100 ). Intermediate units  30 ,  40  evaluate the respective received signals s 1  and s 2  (block  102 ). This evaluation may include an estimation of fading on the channels that exist between the transmit unit  20  and the intermediate units  30 ,  40 . Intermediate units  30 ,  40  may also make an estimate of the fading for the complete channels a 1  and a 2  based on information received from receive unit  50  on the reverse channels. Intermediate units  30 ,  40  may send information, possibly including channel estimates, as well as other well known parameters, to the transmit unit  20  on an appropriate reverse channel (block  104 ). Transmit unit  20  compensates for fading in the propagation channel if needed (block  106 ). Compensation may be according to any known scheme, such as boosting transmitted power to equalize received power or reducing power to faded channels and boosting power to unfaded channels. Other techniques may also be used.  
         [0031]    A similar feedback loop exists between the intermediate units  30 ,  40  and the receive unit  50 . This is illustrated in FIG. 3. Intermediate units  30 ,  40  transmit their respective signals s 1  and s 2  to the receive unit  50  (block  110 ). Receive unit  50  evaluates the received signals (block  112 ). Again this evaluation may include estimating fading on the channels. Receive unit  50  then sends information about the received signals to the intermediate units  30 ,  40  on an appropriate reverse channel (block  114 ). Intermediate channels  30 ,  40  may compensate for fading if needed (block  116 ). This two-stage power control ability enables the network  10  to do a finer form of power control than contemplated in other systems.  
         [0032]    As another variation on the present invention, it is possible that the intermediate units  30 ,  40  are the units that separate a message into s 1  and s 2 . This would require the space-time encoder to be present at the intermediate units  30 ,  40 , but such is within the scope of the present invention.  
         [0033]    Note that the present invention may function with more than two repeaters in a single stage or multiple stages of repeaters. This allows the network  10  to extend the range of the connection or to circumvent obstacles  60  as needed or desired. Other reasons for implementing the present invention in a network  10  are also contemplated.  
         [0034]    Further note that the present invention is not limited to ad hoc networks  10 , but also may be used in networks with fixed elements, for example, fixed intermediate units  30 ,  40 . In essence, this method will work in almost any TDD system.  
         [0035]    The present invention, as previously discussed, is particularly well suited for use in Bluetooth networks. While Bluetooth networks may include many different types of devices, a typical device for a Bluetooth network is a mobile terminal. An exemplary mobile terminal  200  is shown in FIG. 4. Mobile terminal  200  comprises a main control unit  220  for controlling the operation of the mobile terminal  200  and memory  240  for storing control programs and data used by the mobile terminal  200  during operation. Memory  240  may be contained in a removable smart card if desired. Input/output circuits  260  interface the control unit  220  with a keypad  280 , display  300 , audio processing circuits  320 , receiver  380 , and transmitter  400 . The keypad  280  allows the operator to dial numbers, enter commands, and select options. The display  300  allows the operator to see dialed digits, stored information, and call status information. The audio processing circuits  320  provide basic analog audio outputs to a speaker  340  and accept analog audio inputs from a microphone  360 . The receiver  380  and transmitter  400  receive and transmit signals using shared antenna  440 . The mobile terminal  200  further includes a Bluetooth module  410  operating as previously described and having a single antenna  412  operating in the ISM band.  
         [0036]    It should be noted that, as used herein, the term “mobile terminal”  200  may include a cellular radiotelephone with or without a multi-line display; a Personal Communications System (PCS) terminal that may combine a cellular radiotelephone with data processing, facsimile and data communications capabilities; a Personal Digital Assistant (PDA) may include a radiotelephone, pager, Internet/intranet access, Web browser, organizer, calendar and/or a global positioning system (GPS) receiver; and a conventional laptop and/or palmtop receiver or other appliance that includes a radiotelephone transceiver. Mobile terminals  200  may also be referred to as “pervasive computing” devices.  
         [0037]    The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope and the essential characteristics of the invention. The present embodiments are therefore to be construed in all aspects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.