FOR TV WHITE SPACE

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for unlicensed radio transmitters to operate in unoccupied portions of the digital broadcast television spectrum.

BACKGROUND OF THE INVENTION

In multicast and broadcast applications, data are transmitted from a server to multiple receivers over wired and/or wireless networks. A multicast system as used herein is a system in which a server transmits the same data to multiple receivers simultaneously, where the multiple receivers form a subset of all the receivers up to and including all of the receivers. A broadcast system is a system in which a server transmits the same data to all of the receivers simultaneously. That is, a multicast system by definition can include a broadcast system.

The use of unoccupied digital TV spectrum by unlicensed radio transmitters has not before been addressed since the transition of TV broadcasting from analog to digital was only recently completed.

SUMMARY OF THE INVENTION

When multiple TV white space (TVWS) devices want to access the same TV channel, a resource sharing mechanism is required. This is known as a coexistence problem of heterogeneous systems in TVWS. A coexistence mechanism utilizing an over-the-air common control channel is described herein. In addition, the design of the common control channel is also described.

A common physical (PHY) channel or common control channel (CCH) design is described. The common control channel is used to resolve the coexistence problem when heterogeneous systems use the same channel. Coexistence mechanisms based on a common control channel are also described. As used herein a node includes (but is not limited to) a station (STA), a mobile device, a mobile terminal, a dual mode smart phone, a computer, a laptop computer or any other equivalent device capable of operating in TVWS.

A method and apparatus are described including receiving a request-to-send message and transmitting a clear-to-send message responsive to the received request-to- send message, wherein the request-to-send message includes available channel list information, and wherein the clear-to-send message includes, channel selection information. Also described area a method and apparatus including allocating a control channel, listening to the allocated control channel to determine whether there is a transmitted signal, adjusting a time slot count responsive to the listening when there is no transmitted signal detected, determining if the time slot count has reached a threshold, competing for use of a next time slot responsive to the first determination, determining if the competition is successful, transmitting a request-to-send message to a destination node responsive to the second determination receiving a clear-to-send message from a destination node responsive to the request-to-send message and transmitting data to the destination node responsive to the received clear-to-send message, wherein the request- to-send message includes available channel list information, and wherein the clear-to- send message includes, channel selection information.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is best understood from the following detailed description when read in conjunction with the accompanying drawings. The drawings include the following figures briefly described below:

Fig. 1 shows time synchronization TVWS devices.

Fig. 2 shows cognitive networks.

Fig. 3 shows the frame structures of the exemplary control signals in accordance with the principles of the present invention.

Fig. 4A is a flowchart of the allocation scheme selection process.

Fig. 4B is a flowchart of an exemplary embodiment of the present invention from the perspective of a destination node of a communication pair. Fig. 4C is a flowchart of an exemplary embodiment of the present invention from the perspective of a source node of a communication pair.

Fig. 5 is a block diagram of an exemplary embodiment of a station (STA) of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The transition of TV broadcast signals from analog to digital in North America was completed June 12, 2009. At the same time, unlicensed radio transmitters were able to operate in the broadcast television spectrum at locations where that spectrum was not being used by licensed services (this unused TV spectrum is often termed "white space") under regulations specified by FCC R&O. Any system can use TV white space (TVWS) as long as it follows regulations specified by FCC R&O. Thus, it can be expected that there will be heterogeneous systems operating in TVWS. When the number of unused TV channels is less than the number of channels requested by TVWS devices, multiple systems have to share the spectrum resource, i.e., multiple systems need to coexist in the same channel. Coexistence is the main issue in TVWS usage. Spread spectrum (SS) and ultra wide band (UWB) techniques are two modulation methods which allow coexistence of different systems in the same frequency band without cooperation. Potential TVWS users include IEEE 802.11, IEEE 802.15, IEEE 802.16, IEEE 802.22 devices and non- IEEE 802 systems such as cellular devices. Neither spread spectrum nor ultra wide band technique is a common modulation method used by these systems. Thus, a coexistence mechanism which provides a bridge to cooperate in TVWS for heterogeneous systems is required. There are two possible ways to provide a connection between TVWS devices for coexistence. One way is through a back haul link connecting to the Internet. The other is through an over-the-air channel. In the present invention, the structure of an over- the-air common control channel is described.

The first problem to solve is where to allocate a common control channel. According to FCC R&O, fixed devices may operate on any unoccupied channel between 2 and 51, except channels 3, 4 and 37. Personal portable devices may operate on any unoccupied channel between 21 and 51, except channel 37. Based on the rules above the following common PHY channel allocation method is proposed and described:

1) If two common control channels are used. A common control channel is allocated between channel 2 and 20, e.g., channel 11, and another is allocated between channel 21 and 51, e.g., channel 35. The first common channel is used for coexistence of fixed devices between channel 2 and 20. The second common channel is used for coexistence of fixed or personal portable devices between channel 21 and 51. This method has an advantage that for fixed devices operating between channel 2 and 20, the maximum transmit power of all devices is the same. The coverage of these devices is comparable. Thus, the situation that a low-power device can hear a high- power device but a high-power device cannot hear a low-power device will not occur. The disadvantage is that more bandwidth is used.

2) Only one common PHY channels is allocated between channel 21 and 51 for the coexistence mechanism used by all TVWS devices.

3) A common PHY channel in each unoccupied channel. This method has an advantage that TVWS devices do not have to listen to two channels simultaneously and hence only one RF receiver chain is needed.

4) Another alternative embodiment of the present invention is that the common control channel is allocated in the Industrial Scientific and Medical (ISM) band. In this way, advantage may be taken of the mature technologies developed in ISM band, such as WLAN technology.

The second problem to be solved is what construction method is to be used for a common control channel. There are three main possibilities.

1) FDMA method:

The data rate of the common control signal is much smaller than that of the data signal. Thus, a narrow band, e.g., 100 kHz frequency band at the edge of a 6 MHz TV channel can be used to transmit common control signals.

2) Spread spectrum method: Instead of using a narrow band for a common PHY channel, spread spectrum techniques can be used to modulate control signals. In this way, even if the common control channel and the data channel are overlap, the control signals of the common control channel look like background noise to the data channel.

3) TDMA method:

If heterogeneous TVWS systems are time- synchronized, common control signals can be sent in specific time slots and controlled by a spectrum manager.

Based on the above, it is assumed that TVWS devices are at least roughly time synchronized so it is further assumed that TDMA will be used. The next problem to be solved is the design of coexistence mechanisms for use on the common control channel.

Assume that TVWS devices are roughly time- synchronized so that time slot structure exists as shown in Fig. 1. Note that here a time slot may be as large as 100 ms. In an embodiment of this invention, a time slot is further partitioned into multiple sub- slots. For example, each sub-slot will have duration of aSlotTime as defined IEEE 802.11. The aSlotTime is the time unit of the random back-off counter and its unit is /s. Note also that due to the requirement of performing spectrum sensing for incumbent signals, common quiet periods (QP) have to be scheduled which means that the TVWS devices have a time synchronization mechanism. Thus, the assumption of synchronization in time simply follows the requirement of QP scheduling. In addition, with the help of a common control channel, time synchronization can be achieved by sending time- synchronization beacons. Based on the time slot structure, a next- time- slot competition coexistence scheme is described. During time slot n, all TVWS devices compete to use time slot n+1 of available channels. The competitions occur in the common control channel.

Fig. 2 shows a cognitive network with two access points (APs) and their corresponding stations. Each station forms a communication pair with its AP. Each communication pair has to compete for time slots of the available channels in the CCH. The competition can be accomplished by carrier sense multiple access with collision avoidance (CSMA/CA) with binary exponential back-off, as used in IEEE 802.11.

If a station wants to transmit control signals over the CCH in a time slot, it first listens to the CCH. The station (node) selects a random number, T _b , uniformly between [0, CW], where CW denotes for the length of the contention window. Note that 7 _b is sub- slot. The station (node) waits for T _b sub-slot during which the media is idle before transmission of its control signals. This is called random back-off. For example, when the medium continues to be idle for one more sub-slot than To, the back-off counter decreases by 1 to T _b -1 sub-slots. When the medium becomes occupied, the back-off counter defers the count-down process until the medium becomes idle again. This process continues until the back-off counter reaches the 0. If the back-off counter reaches the 0, the station starts transmission of its control signals. If a collision occurs during the transmission over the CCH, the station doubles the contention window CW to reduce the probability for future collisions and competes again for the channel access using the process described above. This is called the binary exponential back-off algorithm. In IEEE 802.11, the size of the CW doubles with every collision, until a maximum size is reached. Since only control signals are transmitted over the CCH, it may be unnecessary to double the size of the CW when collisions occur, or a small value for the maximum CW can be used.

When a communication pair competes for resources, a pair of control signals, i.e., request-to- send (RTS) and clear-to-send (CTS) modified from RTS and CTS in IEEE 802.11, are used. The structure of modified RTS and CTS signals are shown in Fig. 3. The RTS signal (message) contains the available channel list. When a dedicated (destination) station receives a RTS signal (message), it replies with a CTS signal (message) indicating the selected channels to use to the station which sent the RTS signal (message).

If only APs compete for resources, the AP sends a CTS signal to each of its corresponding stations indicating the selection of channels once the AP has successfully competed for the next time slot (transmission opportunity). Once a station decides to schedule a quiet period for a channel, it can compete for the next time slot of that channel. When a station obtains the right to use a channel and schedule a QP, it could control the channel for several consecutive time slots needed for a QP.

As shown in Fig. 3, a frame format similar to that of IEEE 802.11 is used. The difference is that first, available channel list information is added to the RTS signal (message) and channel selection information is added to the CTS signal (message). Second, the address information may contain multiple addresses.

The physical layer characteristics of the control signal (message) are as follows:

1) Error Correction Code: reuse the convolution code with coding rate R = ½, as defined in IEEE 802.11. The Binary Convolutional Code that is reused is a 64-state, rate ½ code. The generator matrix for the code is given as

G = [D ⁶ + D ⁴ + D ³ + D + 1, D ⁶ + D ⁵ + D ⁴ + D ³ + D ² + 1] or in octal notation, it is given by G = [133, 175].

2) Symbol Mapping: Reuse the Differential BPSK specified in IEEE 802.11b since it is the most robust symbol mapping method and it has a simple decoding method.

Direct Sequence Spread Spectrum (DSSS) is used because using spread spectrum techniques can alleviate the multipath channel effect. Also for the same transmit power, spread spectrum techniques can increase SNR by decreasing data rate. Thus, the control signals become more robust and the coverage of the CCH increases. For example, barker code of length 11 (+1 +1 +1 -1 -1 -1 +1 -1 -1 +1 -1) which is used in IEEE 802.11b can be reused. Longer spreading code could be used to have larger coverage.

Referring to Fig 4A, which is a flowchart of the allocation scheme selection process. This process is executed (performed, accomplished) at the system level by a system administrator. Information regarding the allocated channel is provided to the stations in order for them to allocate/use the allocated/selected control channel(s). At 405 a determination is made if allocation scheme 1 is to be used. If allocation scheme 1 is to be used then at 410 a determination is made if the TVWS device is a fixed device. If the TVWS device is a fixed device then at 420, the device uses for example, channels 11 and 35. If the TVWS device is not a fixed device (personal portable device) then at 420, the device is configured to use for example, channel 35. If allocation scheme 1 is not to be used then at 425 a determination is made if allocation scheme 2 is to be used. If allocation scheme 2 is to be used then at 430 configure the device to use a single channel, for example, channel 35. If allocation scheme 2 is not to be used then at 435 a determination is made if allocation scheme 3 is to be used. If allocation scheme 3 is to be used then at 445 the device is configured to use a common PHY channel in each unoccupied channel space. If allocation scheme 3 is not to be used then at 440 the device is configured to use a common channel in the ISM Band. Especially with personal portable devices, it may be necessary to configure them to search for the common channel from among a small set of possible common channels.

Referring to Fig. 4B, which is a flowchart of an exemplary embodiment of the present invention from the perspective of a destination node of a communication pair. In this case the communication pair is competing for use of a next time slot. At 453 a test is performed to determine if the destination node has received a RTS message (signal) from a source node (node wishing to transmit data to the destination node). If the destination node has not received a RTS message (signal) from a source node then processing returns to 453. If the destination node has received a RTS message (signal) from a source node, after waiting for a short time duration, for example, a short inter- frame space (SIFS) defined in IEEE 802.11, at 473 the destination node sends a CTS signal (message) with channel selection information to the source node (station). Then data transmission starts at the selected channel. All other communication pairs also listen to the CTS messages and will not choose the selected channel in the next time slot.

Referring to Fig. 4C, which is a flowchart of an exemplary embodiment of the present invention from the perspective of a source node wishing to transmit data to a destination node. At 474 the source node uses (follows) the CSMA/CA with binary exponential back-off method. In accordance with the binary exponential back-off method the back-off counter is decremented during media idle sub- slots. A test is performed at 475 to determine if the back-off counter has reached 0. If the back-off counter has reached 0 then at 477 the source node sends a RTS signal (message) to the destination node including its available channel list. At 483 a test is performed to determine if the source node (station) has received a CTS signal (message) from the destination node indicating the selected common channel. Once the source node (station) has received the CTS signal from the destination node then at 485 the source node (station) transmits its data in the selected channel.

Referring to Fig. 5, which is a block diagram of an exemplary embodiment of a STA of the present invention. The STA has a control logic module which executes the CSMA/CA with binary exponential back-off method in the CCH and a transceiver module for data communications over a data communication channel. There may be multiple antennas so that the data transmission and next time slot competition can progress simultaneously.

Specifically, when behaving as a destination node of a communications pair (of nodes), the transceiver of the station includes means for receiving a request-to-send message, and means for transmitting a clear-to-send message responsive to the received request-to-send message, wherein the request-to-send message includes available channel list information, and wherein the clear-to-send message includes, channel selection information. The control logic of the station includes means for waiting a predetermined time interval.

Specifically, when behaving as a source node of a communications pair (of nodes), the control logic includes means for allocating a control channel, means for listening to the allocated control channel to determine whether there is a transmitted signal, means for adjusting a time slot count responsive to the listening when there is no transmitted signal detected, means for determining if the time slot count has reached a threshold, means for competing for use of a next time slot responsive to the first determination and means for determining if the competition is successful. The transceiver of the stations includes means for transmitting a request-to-send message to a destination node of a pair of communication nodes responsive to the second determination, means for receiving a clear-to-send message from a destination node of a pair of communication nodes responsive to the request-to-send message and means for transmitting data to the destination node of the pair of communication nodes responsive to the received clear-to- send message, wherein the request-to-send message includes available channel list information, and wherein the clear-to-send message includes, channel selection information.

It is to be understood that the present invention may be implemented in various forms of hardware, software, firmware, special purpose processors, or a combination thereof. Preferably, the present invention is implemented as a combination of hardware and software. Moreover, the software is preferably implemented as an application program tangibly embodied on a program storage device. The application program may be uploaded to, and executed by, a machine comprising any suitable architecture. Preferably, the machine is implemented on a computer platform having hardware such as one or more central processing units (CPU), a random access memory (RAM), and input/output (I/O) interface(s). The computer platform also includes an operating system and microinstruction code. The various processes and functions described herein may either be part of the microinstruction code or part of the application program (or a combination thereof), which is executed via the operating system. In addition, various other peripheral devices may be connected to the computer platform such as an additional data storage device and a printing device.

It is to be further understood that, because some of the constituent system components and method steps depicted in the accompanying figures are preferably implemented in software, the actual connections between the system components (or the process steps) may differ depending upon the manner in which the present invention is programmed. Given the teachings herein, one of ordinary skill in the related art will be able to contemplate these and similar implementations or configurations of the present invention.