Patent Application
20120127914
Traditionally, a relay may be used to extend communication range when the range of single-hop wireless communication is limited by distance or harsh radio propagation conditions. Such relays build a multi-hop path between an out-of range node and an access point. The frames are forwarded hop-by-hop from the source to the destination. However, to avoid self-interference, transmission on different hops cannot be activated simultaneously when the nodes are within direct communication range. Consequently, the decrease in throughput over multi-hop paths is inversely proportional to the number of hops between the sender and the destination. For example, in a two-hop path with a single relay, throughput from the sender to the destination is only half of the data rate compared to a single hop wireless link.
For example when a source sends four frames to a destination with a traditional multi-hop relay system using two relays, the source sends each packet to one of the relays and waits for the relay to forward the packet to the destination since the two relays can interfere with each other. Thus, typically, to send four frames requires eight time slots.
Described herein are techniques and corresponding systems implementing techniques that improve throughput between a pair of nodes by using two multi-hop paths of one-way flows regardless of the one-way flows interfering with each other. These techniques enable nearly full-rate data flow through frame transmissions, even though these frame transmissions can interfere with substantially concurrent relay transmissions. In some implementations, relays on the two paths forward mixed frame signals to the next hop without trying to decode the mixed frame signals of interfered frames. The destination successfully recovers the useful information from the mixed frame signals by canceling out interference based on previously received frames.
This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key or essential features of the claimed subject matter; nor is it to be used for determining or limiting the scope of the claimed subject matter.
The detailed description is described with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the drawings to reference like features and components.
The disclosure describes techniques utilizing two multi-hop paths of one-way flows regardless of the paths interfering with one another. The techniques improve throughput between a pair of nodes by leveraging interference when it occurs. For instance, a source node can transmit frames, also called source frames, substantially continuously to achieve nearly full data rate even though the transmissions can interfere with substantially concurrent relay transmissions. Relays on the two paths forward interfered frame signals as a mixed frame to a next hop without decoding the interfered frame signals. When the receiver receives such a mixed frame, useful information may be recovered by canceling out interference based on previously received frame signals. Herein such an approach may be called FULPLEX.
In some implementations, to ensure effective interference cancelation at the receiver, wireless channel information is precisely tracked. For example, when relaying along multi-hop paths, the frame will travel on multiple different wireless channels, and each channel randomly attenuates and distorts the frame signal. The techniques discussed herein leverage a preamble scheme called “onion preambles.” Onion preambles are tagged at each hop to record channel states. The destination can effectively use these preambles to reconstruct channel information when performing interference cancelation.
In addition, in some implementations the protocol for a cooperative relay in a wireless network as described herein ensures substantially concurrent transmissions are mixed in such a way that the destination can estimate wireless channel attenuation and reliably perform interference cancelation. Thus, the protocol effectively coordinates the transmissions among the source and the relays to control the interference pattern, leading to a substantially full-rate cooperative relay scheme. The techniques discussed herein represent a cooperative medium access control (MAC) protocol called “chain access.” Chain access schedules a chain of co-operative relays to transmit substantially simultaneously. Chain access is a distributed protocol, meaning that different source-destination pairs are complete and schedule their transmissions independently.
Several implementations of FULPLEX, a cooperative relay in a wireless network described herein, make use of a programmable software-defined radio platform operated over a network conforming to the IEEE 802.11b standard. The techniques and such a platform are designed to be compatible with the set of IEEE 802.11 standards such as 802.11a, 802.11g, and 802.11n. However the techniques are not limited to implementations conforming to these standards, and other embodiments including a cooperative relay architecture to improve throughput between other multi-hop connected nodes are contemplated.
In one specific example of many, nearly full-rate throughput is achieved via the cooperative relay in a wireless network by using two potentially mutually interfering paths. The FULPLEX techniques leverage onion preambles and chain access to enable a destination to reliably cancel interference and reliably decode frames from mutually interfering paths.
The discussion below begins with a section entitled “Example Computing Architecture,” which describes one non-limiting environment that may implement the described techniques. Next, a section entitled “Example Data Flow” presents several examples of data flow for cooperative relay. A third section, entitled “Example Preambles” describes several examples of preambles implemented for cooperative relay. A fourth section, entitled “Example Processes” presents several example processes for cooperative relay. A brief conclusion ends the discussion.
This brief introduction, including section titles and corresponding summaries, is provided for the reader's convenience and is not intended to limit the scope of the claims, nor the proceeding sections.
While
The applications 118(1)-(N) may comprise desktop applications, web applications provided over a network such as network 102, and/or any other type of application capable of running on the computing device 104. The network 102, meanwhile, may represent a combination of multiple different types of networks, interconnected with each other and functioning as a single large network (e.g., the Internet or an intranet). The network 102 may include wire-based network components (e.g., cable) and wireless network components (e.g., cellular, satellite, etc.).
In several FULPLEX implementations, for communication via wireless networks, the source computing device 104 uses physical layer (PHY) coding and modulation to modulate frames into wireless signals, e.g., IEEE 802.11 signals. In at least one instance, the source computing device 104 may transmit frames in a batch. When the source computing device 104 accesses a wireless channel, the source computing device 104 continues to transmit frames in the batch using the “chain access” MAC protocol, and to wait for an acknowledgement (ACK) from the destination of the frames, which may be an end destination 104, a relay 106, or a wireless access point 108. The ACKs are sent when the destination receives the entirety of the batch of frames, and since ACKs are small, may be transmitted using a traditional multi-hop relay scheme. When the source computing device 104 receives the acknowledgement, it will create a new batch, containing both retransmissions for the last batch (if any) and new frames, thus effecting continuous transmission.
Relays, e.g. relays 106, which in various embodiments may be an access point, router, or computing device such as a laptop, personal digital assistant (PDA), smart phone, cell phone, etc., receive frames and forward them out to the next hop. Unlike conventional multi-hop forwarding, where only successfully decoded frames are remodulated and forwarded, the cooperative relay techniques described herein facilitate a FULPLEX programmed relay 106 forwarding raw signals that the relay 106 received in the previous time slot. Each relay 106 employs the sample buffer 124 to hold the received signal. When a FULPLEX relay 106 detects a frame transmission, it records the raw signal to the sample buffer 124. The relay 106 decodes a FULPLEX header, e.g., a small leading header, to identify the frame. The relay 106 does not decode the whole frame. Although the relay may also be configured to decode the whole frame in some embodiments. Before forwarding, the relay 106 amplifies the stored signals and attaches a new preamble. The new preamble enables the FULPLEX system to track the wireless channel the frame will traverse. For example, the new preamble enables a FULPLEX programmed destination 104 to track wireless channel distortion.
A destination device 104, such as destination device 104C, receives signals, the destination device 104 allocates an array of signal buffers 114 to store the raw signals received in the same batch. When a new frame is received, the destination device 104 cancels self-interference using previously received signals. This enables decoding the new frame. After a batch of frames has been received, the destination 104 generates an acknowledgement to the source 104 using conventional multi-hop relaying to report the successfully received batch.
Compared to the number of time slots, m, that are needed to send a number of frames, n, in traditional multi-hop relay systems, FULPLEX techniques significantly reduce the number of time slots, m, needed to send the same number of frames, n.
As shown in
As shown in the example illustrated in
A wireless signal is distorted and attenuated when the signal passes a wireless channel, which presents a challenge. This is particularly true in a multi-hop environment, such as that shown at 300, where the same wireless signal may pass multiple different wireless channels and be distorted many times. In the examples described herein, in order to cancel the interfering signals, the destination 202 learns the wireless channels to compensate for their effects. Thus, FULPLEX employs a new distributed preamble scheme developed to track the complex channel structure in multi-hop relay paths.
A second challenge is that in a conventional wireless medium access control (MAC) protocol, e.g. IEEE 802.11, each node independently and randomly contends for transmission. A new cooperative path access control protocol, “chain access” MAC, enables FULPLEX, where the relays along the two multi-hop paths cooperatively control the interference pattern. The FULPLEX protocol leverages existing wireless local-area network (WLAN) coordination mechanisms such as carrier-sensing and broadcasting to distributively coordinate transmissions.
FULPLEX adopts an event-trigger approach to schedule transmission along two paths in the chain access MAC.
As shown in
FULPLEX trigger scheduling is effective with little or no more than coarse-grained, packet-level synchronization among neighboring relays. Even high-performance software radios, such as Sora, for example, take several μs processing time to decode the triggering preamble. However, compared to the packet length, the latency of μs processing time represents negligible overhead. In addition, raw samples are scaled and forwarded directly, which keeps sample-processing delay at the relay nodes to a minimum. Moreover, the source node can modulate new packets in parallel with transmission.
In various embodiments, FULPLEX uses the following rules to control cooperative forwarding among relays:
In the example shown at 300, odd numbered relays 106(O) include darker cross-hatching, while even numbered relays 106(E) include lighter cross-hatching. At each hop, in each pair of relays (e.g., 106(O) and 106(E)), when the odd-numbered relay 106(O) is receiving the even-numbered relay 106(E) is forwarding. Or, the even-numbered relay 106(E) is receiving and the odd-numbered relay 106(O) is forwarding.
FULPLEX takes advantage of the source 104 transmitting a new frame at each time slot in order to accomplish full-rate forwarding. However, these transmissions interfere with other forwarding signals transmitted by neighboring relays 106. Such substantially concurrently transmitted signals are superposed at the receiving relays 106 at the next hop. Superposition means that the signals are layered whether partially or fully overlapping, resulting in a mixed signal. Then, these mixed signals are further recorded and forwarded in the next time slot and further superposed with other forwarding signals. This creates a complex interference pattern at the destination 202, where the useful information is deeply mixed with multiple interfering signals. At the destination 202, the interference signals are removed using a recursive algorithm, and the leftover signal will contain one residual frame. As described herein, FULPLEX employs such a recursive algorithm to remove interference signals. In at least one embodiment, the residual frame is decoded using decoder 118(2). In at least one other embodiment, the residual frame is decoded using a conventional decoder.
Before describing the decoding algorithm at the destination 202, consider the signal received at the destination 202 after FULPLEX forwarding, using the two-hop path scenario from
A signal forwarded by relay Ri,j at time t, is denoted si,j(t), and the signal received by Ri,j at time t is denoted ri,j(t). In accordance with the FULPLEX protocol, relays 106 do not decode the received signal; instead the signal is amplified and forwarded. Thus, with αi,j(t) as a scaling factor, Equation 1 is obtained. However, the scaling factor may be omitted from the calculations for ease of understanding.
s
i,j(t)=αi,j(t)ri,j(t−1) (1)
The signal received by the destination at time t is denoted d(t). Thus, Equation 2 follows.
At time slot t=2m−1, relay R0,0, receives a superposition of signals transmitted simultaneously by relay R0,1 and the source. Equation 3 represents the superposed signal that is received.
r
0,0(2m−1)=hs,0 f(2m−1)+h0,1s0,1(2m−1) (3)
In addition, equation 4 is derived using equation 2.
Equation 4 suggests that in two-hop paths, the signal currently received at the destination 202 is a sum of a new frame f(2m−1) and the signal the destination 202 received in the last time slot, d(2m−1). Thus, with a knowledge of the channel parameters, d(2m−1) is canceled from the currently received signal d(2m). A residual signal containing only f(2m−1) remains. Accordingly, destination 202 may pass f(2m−1) to a decoder 118(2), or to a conventional decoder to decode information bits from frame f(2m−1). Similar results are obtained when t=2m.
The operation of canceling signal x from y is denoted “y˜x,” and “yx” denotes when x and y differ by a channel factor. Thus, Equation 5 represents a two-hop path such as the path illustrated in
f(t−1)d(t)˜d(t−1) (5)
FULPLEX equations representing three-hop paths follow as shown in equation 6, below.
The three-hop path equations result in equations 7 and 8, below.
r1,0(2m−2)d(2m)˜d(2m−1) (7)
r1,1(2m−3)d(2m−1)˜d(2m−2), (8)
and,
f(2m−2)r1,0(2m−2)˜r0,0(2m−3)˜r1,1(2m−3)
f(2m−3)r1,1(2m−3)˜r0,1(2m−4)˜r1,0(2m−4).
Alternately, three-hop paths may be written as equation 9, where j ∈ {0, 1} and δ=1−j. Hence, Ri,δ denotes the relay that has the same hop identity i with Ri,j, but on the opposite path.
f(t−2)r1,0(t−1)˜r0,j(t−2)˜r1,δ(t−2) (9)
Similar recursive equations represent general (k+2)-hop paths such as the five-hop paths illustrated in
r0,j(t−1)d(t)
r1,j(t−2)r0,j(t−1)˜r0,δ(t−2)
ri,j(t−i−1)ri-1,j(t−i)˜ri-2j(t−i−1)
˜ri-1,δ(t−i−1), ∀k:2≦i≦k
f(t−k−1)rk,j(t−k)˜rk-1,j(t−k−1)˜rk,δ(t−k−1) (10)
Note the signals ri-1,δ(t−i−1) are from the intermediate decoding results of previous slot, which are generated by the concurrent transmission of nodes with a different odevity.
Equation 10 directly implies the decoding algorithm for FULPLEX in a k-hop network. For each received signal, the destination 202 only needs to successively perform 2(k+1)−1=2k+1 times signal cancelation. Then, the residual signal contains only f(t−k−1), so that the residual signal is decodable using a wireless decoder 118(2), which in some instances represents a conventional decoder.
Table 1, below, shows pseudo-code for implementing the FULPLEX decoding algorithm at a destination 202.
For (k+2)-hop paths, the destination 202 allocates a signal buffer array in a signal buffer 114 that stores the intermediate decoding results of the previous two time slots, i.e., the signals ri,j(t−i−1) and ri,j(t−i−2). When a new signal is received from relays 106, the destination 202 cancels the signals stored in its buffer 114 as shown in lines 5-9 of Table 1. In at least one embodiment, the residual signal is passed to frame decoder 118(2), which in some instances represents a conventional frame decoder.
When a signal passes a wireless channel, it is distorted and attenuated. Different wireless channels will distort and attenuate the same signal differently. Thus, two received versions of the same signal cannot be canceled directly. To better understand this issue, consider the two-hop path discussed above with regard to
One naïve solution is to collect parameters for each individual wireless channel, i.e. hi,j, and report them to the destination 202. Then, the destination 202 may reverse the channel effects on the signal one-by-one before cancelation. However, such a solution is impractical because wireless channels are time-varying, making it impractical for a destination 202 to track each individual wireless channel. FULPLEX does not track each wireless channel individually. Instead, FULPLEX exploits a preamble scheme to track the overall distortions when a signal passes multiple channels. Since the relays 106 in FULPLEX keep the originally received signals and forward them with only linear amplification, when the signal passes a wireless channel, the channel effects are “remembered” in the preambles. Such “memories” in the preambles may be used to align different signal versions for cancelation.
However, FULPLEX contains a complex self-interference pattern, and the interference on these preambles will eventually destroy the “memories” of the wireless channel without taking further action. Thus, FULPLEX was designed with a distributed preamble scheme to ensure interference-free preambles for use in successive signal cancelation. An example of such a preamble scheme, called onion preambles, is presented in the next section.
There is one more issue for the onion preamble scheme. As shown in
As shown in
As mentioned earlier, an equalizer is a linear filter c={c−L, . . . , cL} that reverses the impact of multi-path wireless channel. Mathematically, an optimal equalizer minimizes the difference between the recovered signals and the signal that is transmitted, as:
c=arg minc∥x−c*y∥,
where x is the transmitted signal, y is the received signal, ∥.∥ represents the 2-norm, and “*” is the convolution operation. To obtain each component value of c, the destination uses the known preamble to train the equalizer with an adaptive searching algorithm. FULPLEX uses a fractional spaced equalizer (FSE) that works on oversampling rate, i.e., four times oversampling rate.
At 704, the sender 104 encodes and sends a first frame transmission, F1, toward destination 202 and a first identified relay 106 such as 106A. Such a frame transmission includes a source frame transmission.
At 706, the sender 104 encodes and sends a second frame transmission, F2, toward destination 202 and a second identified relay, e.g., 106B.
At 708, the sender 104 repeats encoding and sending remaining source frames F3 . . . FN as at 704 and 706, alternating between the first identified relay 106A and the second identified relay 106E until each frame from a batch has been sent toward destination 202.
At 710, meanwhile, substantially simultaneous with 706, the first identified relay, e.g., 106A, forwards the first frame F1 on to a next hop toward the destination 202 without decoding the first frame F1.
At 712, meanwhile, substantially simultaneous with 708, the second identified relay, e.g., 106B, forwards the second frame F2 on to a next hop toward the destination 202 without decoding the second frame F2.
At 714, meanwhile, substantially simultaneous with the sender 104 repeating encoding and sending the remaining source frames as at 704 and 706, the first and second relays, e.g., 106A and 106E repeat forwarding frames as at 710 and 712. Consistent with 710 and 712, the frames are forwarded without decoding, with the forwarding alternating between the first identified relay 106A and the second identified relay 106B, until each frame from the batch has been sent toward the destination 202.
At 804, relay 106A adds a new preamble at the first preamble position of frame F1. In various embodiments the new preamble is an interference-free full-power preamble.
At 806, relay 106A amplifies the signal of the first frame F1 without decoding the first frame F1.
At 808, relay 106A forwards the amplified first frame F1 with two preambles on to a next hop toward the destination 202.
Operations 802-808 are repeated by alternating relays (e.g., the first relay 106A and the second relay 106B) on the pair of relay paths until no frames remain in the batch from the source 104.
At 904, the destination 202 recovers useful information from the received frame signals by canceling out interference from the frame signals based on previously received frame signals.
The process 1000, includes at 1002 the destination 202 receiving a current frame signal such as the frame signal forwarded at 808.
At 1004, the destination 202 identifies the current received frame signal as a cooperative relay signal.
At 1006, the destination 202 determines whether the current received frame signal is a mixed frame signal.
At 1008, when the current frame signal is not a mixed frame signal, the destination 202 reads the preamble from the first preamble position to track wireless channel distortion.
At 1010, the destination 202 decodes the frame signal and preamble.
At 1012, when the current frame signal is a mixed frame signal, the destination 202 aligns the preamble from the first position of the current frame signal with the preamble from the first position of the previously most recently received frame signal.
At 1014, the destination 202 cancels the previously most recently received frame signal from the current frame signal to obtain a residual frame signal with a single preamble.
At 1016, the destination 202 decodes the residual frame signal and single preamble. Thus, 1016 results in a decoded frame signal consistent with the result at 1010.
At 1018, the destination 202 saves decoded frame signals to align with later received frame signals.
The process 1100, includes at 1102 the destination 202 initializing a signal buffer such as signal buffer 114. For example, the destination 202, allocates a signal buffer array in a signal buffer 114.
At 1104, the destination 202 decodes a new signal by recursively canceling the signal using a cancelation delta stored in the signal buffer 114. For example, when a new signal is received such as from relays 106, the destination 202 cancels the signals stored in its buffer 114.
At 1106, the destination 202 passes the residual signal to a frame decoder 118(2). For example, the residual signal is passed to frame decoder 118(2), which in some instances represents a conventional frame decoder.
At 1108, the destination 202 stores the residual signal in the signal buffer 114. For example, destination 202 stores the residual signal in a signal buffer array in signal buffer 114 that, at least in some instances, stores the intermediate decoding results of the previous two time slots.
At 1110, the destination 202 updates the cancelation delta. For example, different wireless channels will distort and attenuate the same signal differently. Thus, the cancelation delta is updated rather than canceling two received versions of the same signal directly.
Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as exemplary forms of implementing the claims.
