WO2008149304A2

WO2008149304A2 - Apparatus and method for performing transmission rate adaptation in wireless systems

Info

Publication number: WO2008149304A2
Application number: PCT/IB2008/052193
Authority: WO
Inventors: Hongqiang Zhai; Chun-Ting Chou; Richard Chen
Original assignee: Koninklijke Philips Electronics, N.V.
Priority date: 2007-06-05
Filing date: 2008-06-04
Publication date: 2008-12-11
Also published as: WO2008149304A3

Abstract

A method and apparatus improve throughput of a forward communications link between a source device and a reception device. The method includes receiving a beacon frame from the reception device, determining a link quality associated with the forward communications link based on information obtained from the received beacon frame, and determining an optimum transmission rate based on the signal strength. A signal is transmitted from the source device to the reception device using the optimum transmission rate.

Description

APPARATUS AND METHOD FOR PERFORMING LINK ADAPTATION

IN WIRELESS SYSTEMS

A claim of priority is made to U.S. Provisional Application No. 60/941,960, filed June 5, 2007, the subject matter of which is hereby incorporated by reference.

Advancements continue to be made in wireless communications technology. For example, wireless local area networks (WLANs) and wireless personal area networks (WPANs) are becoming more common in homes and businesses. Such networks may include a variety of independent wireless electronic devices or terminals, which wirelessly communicate with one another. WLANs and WPANs may operate according to a number of different available standards, including the WiMedia Alliance Ultra- Wideband (UWB) standard.

FIG. 1 is a block diagram showing a conventional wireless network 100, including multiple terminals configured to communicate with one another over exemplary WPAN 125. The wireless terminals may include any electronic devices or nodes configured to communicate with one another. For example, FIG.1 depicts a home network in which the electronic devices include a personal computer 120, a digital television set 121, a digital camera 122 and a personal digital assistant (PDA) 123. The network 100 may also include an interface to other networks, such as modem 130, to provide connectivity of all or some of the wireless devices 120-123 to the Internet 140, for example. Of course, there are many other types of wireless networks in which electronic devices communicate with one anther, including networks in manufacturing plants, medical facilities, security systems, and the like.

Transmitters and receivers typically exchange information, such as control information, using beacons. Beacons may be included in periodic superframes of respective transmissions, and are usually broadcast so that all devices in the transmission range of the beaconing device can receive the beacons. For example, an access point periodically sends out beacons so that wireless devices around the access point can associate with the access point and communicate. Beacons (or beacon frames) may include information such as the medium access control (MAC) address of the beaconing device, data rates, signal strengths and the like.

In wireless networks, link quality changes with time and distance between transmitters and receivers based on various conditions, such as movement of the transmitters/receivers, obstructions, multipath fading, and the like. Therefore, different transmission rates may be appropriate over the same transmission link and various times, depending on the corresponding link quality. For example, in order to accommodate various network conditions, wireless UWB systems support multiple different transmission rates. The WiMedia standard for UWB systems, for example, defines eight data rates, shown in Table 1 :

TABLE 1

Generally, a high transmission data rate should be used when link quality is good, otherwise a low transmission data rate should be used. For example, when a transmitter moves toward its intended receiver from a relatively far distance, thus improving the link quality, the transmitter should gradually increase its transmission rate to achieve higher throughput. Accordingly, it would be desirable to link adaptation in a wireless system that reliably and efficiently controls adjustments to transmission rates and/or other parameters to adapt to variations in link quality.

According to one embodiment of the invention, a method is provided for improving throughput of a forward communications link between a source device and a reception device. The method includes receiving a beacon frame from the reception device; determining a link quality associated with the forward communications link based on information obtained from the received beacon frame; determining an optimum transmission rate based on the link quality; and transmitting a signal to the reception device using the optimum transmission rate. According to another embodiment of the invention, an apparatus is provided for improving throughput of a forward communications link with a reception device. The apparatus includes a receiver, a processor and a transmitter. The receiver is configured to receive a beacon frame from the reception device. The processor is configured to determine a signal strength associated with the forward communications link based on information obtained from the received beacon frame, and to determine an optimum transmission rate based on the signal strength. The transmitter is configured to transmit a signal to the reception device using the optimum transmission rate.

According to still another embodiment of the invention, a method is provided for improving throughput. The method includes sending an information element request from a source device to a reception device in a first transmission over a wireless communications link, and subsequently receiving a beacon frame from the reception device, the beacon frame having a link feedback information element in response to the information element request, the link feedback information element indicating a feedback rate corresponding to the first transmission over the communications link. An optimum transmission rate associated the communications link is determined based on at least the link feedback information from the received beacon frame. A second transmission is sent over the wireless communications link from the source device to the reception device using the determined optimum transmission rate.

FIG. 1 is a block diagram of a conventional wireless communications network. FIG. 2 is a functional block diagram of wireless devices communicating over a wireless network according to one embodiment of the invention.

FIG. 3 is a diagram of sample frames for communication by unsynchronized wireless devices in a wireless network according to an embodiment of the invention.

FIG. 4 is a flow chart of a basic adaptation process according to various embodiments.

FIG. 5 is a flow chart of an enhanced adaptation process according to various embodiments.

FIG. 6 is a flow chart of an enhanced adaptation process according to various embodiments. FIG. 7 is a flow chart of an enhanced adaptation process according to various embodiments.

FIG. 8A-8D show graphs showing simulation results according to various embodiments.

In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth in order to provide a thorough understanding of the invention according to the present teachings. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known devices and methods may be omitted so as to not obscure the description of the example embodiments. Such methods and devices are clearly within the scope of the present teachings. FIG. 2 is a functional block diagram of representative wireless communication device 210 (referred to as source device 210) configured to communicate with other wireless communication devices, such as representative wireless communication device 220 (referred to as reception device 220), over a wireless network 230 (e.g., WLAN, WPAN, etc.), according to various embodiments of the invention. For example, wireless network 230 may be a UWB network and wireless devices 210 and 220 may be adapted to operate using a UWB protocol in accordance with WiMedia specifications. The wireless network 230 may represent a communications link between the wireless devices 210 and 220.

The wireless devices 210 and 220 may be adapted to operate using other communications protocols, although the protocols must provide for appropriate information elements to accommodate certain embodiments, discussed below. The WiMedia UWB protocol, for example, provides a link feedback information element (IE), which can be used to send link quality information to a requesting device to achieve better performance of link adaptation. As will be appreciated by those skilled in the art, one or more of the various "parts" shown in FIG. 2 may be physically implemented using a software-controlled microprocessor, hard-wired logic circuits, or a combination thereof. Also, while the parts are functionally segregated in FIG. 2 for explanation purposes, they may be combined variously in any physical implementation. Further, the names of source device 210 and reception device 220 are intended to be descriptive for purposes of discussion, and thus it is understood that both the source device 210 and reception device 220 are capable of transmitting information to and receiving information from one another, as well as other devices within the wireless network 230.

Source device 210 includes transceiver 214, processor 216, memory 218, and antenna system 212. Transceiver 214 includes a receiver 213 and a transmitter 215, and provides functionality for source device 210 to communicate with other wireless devices, such as reception device 220, over wireless communication network 230 according to the appropriate standard protocols. Processor 216 is configured to execute one or more software algorithms, including the link adaptation process algorithms of the embodiments described herein, in conjunction with memory 218 to provide the functionality of source device 210. For example, the enhanced link adaptation processes may be software implemented in the MAC layer. Processor 216 may include its own memory (e.g., nonvolatile memory) for storing executable software code that allows it to perform the various functions of source device 210, discussed herein. Alternatively, the executable code may be stored in designated memory locations within memory 218.

In FIG. 2, in one embodiment, antenna system 212 may include a directional antenna system that provides a capability for the source device 210 to select from multiple antenna beams for communicating with other wireless devices in multiple directions. For example, antenna system 212 may include multiple antennas, each corresponding to one antenna beam, or antenna system 212 may include a steerable antenna that can combine multiple different antenna elements to form a beam in different directions. Alternatively, antenna system 212 may include a single antenna, and/or a non-directional or omnidirectional antenna system.

Generally, reception device 220 may be substantially the same as source device 210, for ease of explanation. Accordingly, reception device 220 includes transceiver 224, processor 226, memory 228 and antenna system 222. Transceiver 224 includes a receiver 223 and a transmitter 225, and provides functionality for reception device 220 to communicate with other wireless devices, such as source device 210, over wireless communication network 230, according to the appropriate standard protocols.

Processor 226 is configured to execute one or more software algorithms in conjunction with memory 228 to provide the functionality of reception device 220. Processor 226 may include its own memory (e.g., nonvolatile memory) for storing executable software code that allows it to perform the various functions of wireless device 220, discussed herein. Alternatively, the executable code may be stored in designated memory locations within memory 228.

Antenna system 222 may include a directional antenna system and provide a capability for the reception device 220 to select from multiple antenna beams for communicating with other wireless devices (e.g., source device 210) in multiple directions. Alternatively, antenna system 222 may be a non-directional or omnidirectional antenna system. When both antenna system 212 of source device 210 and antenna system 222 of reception device 220 are directional antenna systems, the antennas must be aligned in order for the devices to discover and communicate with one another.

In the discussion to follow, exemplary embodiments are described for methods of enabling wireless devices, such as representative source and reception devices 210, 220, to maximize communication links by basic and enhanced link adaptation processes. Link adaptation adapts transmission rates to changing link quality to improve throughput performance. Link adaptation is thus intended to maximize the throughput seen by the networking layer, which is above the medium access control (MAC) and physical (PHY) layers. The embodiments enable the source and reception devices 210, 220 to maintain and optimize communication quality through varying network conditions, for example, by optimizing rate and/or frame payload sizes based on actual or estimated signal to noise ratios (SNRs) of transmission links. Examples are provided for illustration purposes and are not to be construed as limiting the scope of the teachings of this specification, or the claims to follow.

FIG. 3 shows timing diagrams corresponding to examples of different connections between wireless devices, according to UWB specifications. Each frame transaction includes an optional ready to send (RTS) frame/clear to send (CTS) frame exchange, a single frame, and an associated acknowledgement (ACK) frame, if requested by the ACK policy.

The exemplary embodiments are discussed using a number of notations, a partial list of which is provided in Table 2A below for convenience of explanation:

TABLE 2A

The payload in each MAC frame may consist of multiple trunks of six OFDM symbols, for example. Different rates, such as those indicated by Table 2A above, have different information bits N₁ in each trunk of six OFDM symbols, as shown in Table 2B:

TABLE 2B

Therefore, for a given payload size /, the number of trunks of six OFDM symbols U_1J may be determined as follows, where function |^~x] is the smallest integer larger than or equal to x :

J_ n,,ι = N

In each frame transaction, the MAC payload can be transmitted at any rate supported by both the source device 210 (transmitter) and the reception device 220 (receiver). Other portions of the transaction, such as RTS, CTS, ACK, PHY and MAC headers, are transmitted at the lowest rate supported by the PHY layer. Therefore, throughput s(ηj) for various rates and frame payload sizes may be determined as follows:

taver + n.j ^X tov.r.

As used in the foregoing equation, t_over is the frame transaction time excluding the payload transmission time, / is the frame payload size in bits, t_sym is the transmission time of one OFDM symbol, and T₁ is the transmission rate in bits per second (bps). For a given type of frame transaction, t_over may be fixed. Therefore, the relative protocol overhead becomes larger, as the frame payload size / becomes smaller. When choosing different rates, the protocol overhead must be taken into consideration.

Effective throughput S(T₁₁I), which is the throughput seen by the networking layer, excludes packets having errors. The effective throughput S(T₁J) may be determined as follows, where p_r is the packet error rate:

P₁)

The packet error rate P₁ may be determined by the signal to noise ratio (SNR) and the payload size, for example.

Assuming the error rate of each trunk of OFDM symbols, e.g., consisting of six OFDM symbols, is independent of other trunks, the corresponding error rate of each may be denoted by pβ,i. The packet error rate may then be determined as follows, where H_{1 1} is the number of trunks of six OFDM symbols for transmission rate T₁ and payload size /.

A = I - (I - A,)""

Notably, p_t accounts only for the error of the payload. It is assumed that the error rate for other portions of the frame transaction may be neglected because they are transmitted at the lowest PHY rate, as stated above. Thus, pβ,i may be determined given a rate T₁ and SNR. According to the above equation, the larger the payload size, the larger the packet error rate is. A transmission rate may be optimized by maximizing the throughout, discussed above. Optimizing the transmission rate is determined differently, depending on whether the frame payload size is fixed or adjustable.

When the frame payload size is fixed, the throughput s(η,l) is determined by the following equation, discussed above, where the fixed payload size is indicated by / and the transmission rate is indicated by r{.

_S(r_ι,l) = s(r_ι,l) x (l - p_ι) = ^l—— x (1 - p₆ ,)"^■'

Rate r is the transmission rate corresponding to maximized throughput based on a fixed payload size, determined according to Equation (1):

r^* = arg max [?(>,, /)}= arg max j x (l -ft,)"" f (1)

L_^ + n V, , x 6t s_ym

When the frame payload size and/or n_{t t} can be adjusted, a higher throughput may be obtained, where the frame transaction transmits n_t , trunks of six OFDM symbols. The largest information bits or payload size / transmitted in the transaction is H_{1 1} ^ N₁ , and therefore the throughput s may be determined by the following equation:

» = t_over "+"^"_l"X^■βt^ (i -_Λ) = t_over "+-_n' _i ^x _l ^Λxl6t_sym 0-ft.r

The derivative of s with respect to n_{t t} is allowed to be equal to zero. ds

= 0 dⁿ,J

[(I - P₆, T ' + n_tJ (1 - P₆, )"^■ ' In(I - A, )](t_over + n_tJ x 6t_sym ) - n_tJ (1 - A, )"^■ ' x 6^_ffl = 0 6^_ffl In(I - P₆, ) x nj + t_over In(I - p_{6 l} ) x «_M + t_over = 0

T"⁷ over

R ^~ .

Therefore, given transmission rate r_t and error rate p₆₁ , the optimal payload size

K l_opptt iiss ddeetermined as follows:

The optimum s^* throughput for rate η is determined as follows:

Therefore the optimal rate r^* corresponding to maximized throughput for adjustable payload size is determined according to Equation (2):

r = arg max{s_: ^* (2)

Link adaptation processes implemented by the source device 210 (and/or reception device 220) according to various embodiments of the invention determine the optimal transmission rate, maximizing throughput according to the link quality, based on Equation (1) (e.g., when the frame payload size is fixed) or Equation (2) (e.g., when the frame payload size is adjustable).

According to both Equation (1) and Equation (2), the error rate of trunks of six OFDM symbols must be provided given a link quality in order to perform the link adaptation processes. This can be accomplished through estimations and/or simulations. Accordingly, an analysis of how to obtain accurate estimates of throughput and error rate of trunks of six OFDM symbols pβi through simulations is provided below.

Simulations may be conducted to estimate packet error rate p_t given a packet length or payload size I₁ for each rate r_u Basically, simulations are run for N _p packets. If N_e error packets are found, then the estimated packet error rate p_lX' is determined as follows:

and --£

The packet error rate for payload size I₂ isp_l2 , which may be determined as follows:

P_i2'=l-(l-_P6i')"^ =l-(l-_Pil')"^^

For each rate r_t and given payload length /, the number of packets N _p effectively determines the accuracy of the estimated p_t and p₆₁. Since the intent is to maximize throughput, an accurate estimate of the throughput is necessary. Here, throughput accuracy u and corresponding confidence interval are defined by probability \- a , such thatPr{s(ζ,/)(l-M) ≤ s(η,iy≤

+ M)} = \-a , where s(r_t,l)' is the estimated value of throughput s(η , /) . For a given confidence interval and its probability, the following lemma and theorem gives the number of packets N _p :

Theorem 1: N = and

« = 1 " » gi^ven P^{acket error} rate/?,, u, and I- a

confidence interval s(r_t , /)(1 -u)< s(r_t , I)' < s(r_t , /)(1 + u) .

For large N -_Pp . ,,» Qv ^(wVPβ),) _*~~^% —,iββ «_* ^«~ QnQ-^ι1G^<a)^* . To determine the optimum rate, the packet error rate p_r for a given SNR must be determined. A large range of SNRs requires many simulations, although critical loss ratio may be utilized to reduce the considered SNR interval in the simulations, as indicated by the following theorem:

Eb

Theorem 2: For — > M₁ or p_l+l < q_t , s(η , l_optι ) ≤ s(η , /_max ) < s(r_i+l , /_max ) ≤ s(r_i+l , l_optι+ι )

Eb

From Theorem 2, it is known that, when > M , rate r , has a higher

No throughput than rate η if using an appropriate payload sizes. Therefore, obtaining packet

Eb error rate for interval <M is sufficient for rater, as indicated in the following

No corollary:

Eb Corollary 1: For each rater, only p for interval <M needs to be considered,

No or SNR <M^, where 0 < i ≤ 7 , W = 528MHz, and M₁ = M₆ is defined.

Simulations may be run for any allowed payload size, although this approach would require a significant amount of time. As an alternative, simulations were run for one payload size and the packet error rates for other payload size were derived, as discussed above. The following lemmas and theorem are provided to indicate how large estimation error is if throughput is calculated for any packet size based on the result for a given size:

Theorem 3: U₁ ≤ HIaX(I-(I-M^ ^⁶"" "' ,(\ + u_r)^N6sy" "" -1} . It can be approximated as U₁ ~ U₁, ^N6sym'^lJ when (1) /</* and _Ul,«\ ; or (2) />/* and ^βsym'¹'¹ U₁, «1. The

N 6sym,ι,l* ^ βsym, i, I* approximation error e ( e = U₁-U₁ ) satisfies: (1) e< au_r, ~ U₁ ; (2)

2(1-U₁,) 2(1-U₁,)

COl₁, U₁ ^N6sym,,,l

< ^■ — CCu₁, ~ — — U₁ , where a = ^L^^J—

X-Ou₁, X-U₁ N_6symιJ,

N Corollary 2. U₁, ~ u_lmΑX ^6sym'^lJ* given u_lmΑX « X and /* < /_max .

6sym,i,lτaax Theorem 3 illustrates that the change of u is independent of packet error rates and only decided by the change of number of trunks of six OFDM symbols. The foregoing discussion addresses how to obtain accurate estimations of throughput based on simulation results of packet error rate. The throughput accuracy is represented by the confidence interval and its probability.

It is known, generally, that when SNR is small or when the packet error rate is large for rate η , a smaller transmission rate should be used. Therefore, there should be a lower bound (SNR^) for the appropriate SNR interval for each transmission rate. The upper bound (SNR^) of each SNR interval for the transmission rates may be determined using Theorem 2, as follows:

SNIL_J = M₁ -^-(O ≤ i ≤ 7)

Preliminary simulation results may be used to determine the lower bound. First, the simulation was run for a small number of packets (setting of this number is discussed below) to obtain the SNR values SNR₁, _t (1 < i < 7) , at which the rate changes to maximize the throughput. The lower bound for each rate is set as the obtained SNR value minus a constant C_L , as follows:

SNR₁₁ = SNR₁₁ - C_L,(l ≤ i ≤ 7), SNR₁₀ = SNR₁₁

A second upper bound for each rate is set as the obtained SNR value plus a constant C_υ , as follows:

+ Q,,(0 < i < 6), SNR_{1 0} = SNR_{1 1}

Then, the upper bound is the minimum value of SNR_{σ t} and SNR_{σ t} ' :

SNR^₁ = min [SNR_{17 1} , SNR_n^1 ' } In the simulations, the following procedures may be used. First, set

N = of is

M₁ . Second, the turning point of SNR values SNR_{T l}(l ≤ i ≤ 7) is calculated for rate changes according to the previously described throughput analysis to maximize throughput and determine the SNR intervals, e.g., [SNR₁₁, SNR^₁](I < i < 7) . Third, during the SNR interval, N ^' is set according to the estimated p_t for each SNR value found in the first step and the simulations are run. Fourth, recalculate the turning points according to the simulation results.

FIGs. 8A-8D show results of a sample simulation, in which the packet size was 1024 bytes. FIGs. 8A-8D depict packet error rate, throughput, optimum packet size and optimum rate index, respectively. The turning points of SNR values for rate changes at optimum size are indicated in FIG. 8D, and summarized in Table 3, below:

TABLE 3

The corresponding optimum frame payload sizes (in bytes) for rate indexes 1 through 8 (from Table 1) are shown in Table 4. The asterisk (*) indicates the rate index and payload size that are the optimum at each SNR value, thus maximizing throughput. The illustrative optimum frame payload sizes and rates shown in Table 4 coincide with the simulation results depicted in FIGs. 8C and 8D, respectively.

TABLE 4

SNR(dB) rate 1 rate 2 rate 3 rate 4 rate 5 rate 6 rate 6 rate 8

-9 204* 0 0 0 0 0 0 0

-8 220* 228 0 0 0 0 0 0

-7 275* 238 0 0 0 0 0 0

-6 331* 269 254 0 0 0 0 0

-5 416* 338 292 271 0 0 0 0

-4 571* 414 367 284 261 0 0 0

-3 833* 536 457 315 285 0 0 0 -2 1415* 754 605 395 304 282 0 0

-1 2533 1172* 860 498 355 311 249 0

0 4095 2273 1411* 652 446 331 292 247

1 4095 4095 2654* 924 575 386 318 289

2 4095 4095 4095* 1446 774 497 347 328

3 4095 4095 4095 2660* 1144 641 418 357

4 4095 4095 4095 4095* 1862 865 537 401

5 4095 4095 4095 4095* 3441 1281 690 488

6 4095 4095 4095 4095 4095* 2188 943 618

7 4095 4095 4095 4095 4095 4095* 1426 807

8 4095 4095 4095 4095 4095 4095* 2523 1122

9 4095 4095 4095 4095 4095 4095* 4095 1728

10 4095 4095 4095 4095 4095 4095 4095* 2978

11 4095 4095 4095 4095 4095 4095 4095* 4095

12+ 4095 4095 4095 4095 4095 4095 4095 4095*

In one embodiment of the invention, when the transmitting device does not want to or is unable to change the frame payload size, Table 5 may be used instead of Table 4 to obtain the optimum rate for a given SNR, based on the fixed frame payload size. More particularly, Table 5 provides the turning points (dB) for rate changes at different packet sizes.

TABLE 5

Bytes SNR₇._>λ SNR_Tfi SNR_T3 SNR_TA SNR_T5 SNR_T6 SNR_TJ

128 -8.0 -1.0 0.4 3.0 5.4 infinite infinite

256 -5.4 -2.2 0.4 4.2 5.4 8.0 infinite

512 -2.8 -2.0 1.4 3.8 5.6 8.8 10.6

1024 -1.8 -0.8 2.0 4.4 5.8 8.6 11.4

1536 -1.2 0 2.2 4.6 6.2 8.8 11.0

2048 -1.0 0.2 2.6 4.8 6.4 8.8 11.4

2560 -0.8 0.4 2.6 5.0 6.4 9.2 11.2

3072 -0.6 0.6 2.8 5.2 6.6 9.2 11.4

3584 -0.4 0.8 2.8 5.2 6.6 9.4 11.4

4095 -0.4 0.8 3.0 5.4 6.8 9.4 11.4

To obtain the optimum rate from Table 3 and 5, SNRτ,o may be set to equal -∞ and SNRτ.8 is set to equal +∞. When SNR₁₁ ≤ SNR < SNR_1^1+1 , where (0 ≤ i < 7) , the optimum rate index is i. The link may then be adapted accordingly. The link adaptation process runs once in the beacon period of each super frame, for example. The link adaptation process is robust, and reduces complexity and implementation cost. Each device (e.g., source device 210 and reception device 220) maintains a neighbor table, which records the link quality information for corresponding links between the device and each neighboring device. The neighbor table may be arranged in columns and rows, such that each row corresponds to a different neighbor device. The neighbor devices may be tracked, for example, based on the corresponding MAC address or other identifying information contained in beacon frames.

In various embodiments, the link adaptation process may include a basic link adaptation process, discussed with respect to FIG. 4, and an enhanced link adaptation process, discussed with respect to FIGs. 5-7. Generally, the basic link adaptation process enables a transmitting device to adapt its transmission rate to measured SNR of a received beacon frame. No additional information is required to be exchanged between the transmitter (e.g., source device 210) and the receiver (e.g., reception device 220). The source device 210 utilizes the physical layer information of an incoming beacon frame, including the link quality indication (LQI) or measured SNR, to obtain the link quality of the reverse link, which is the transmission link from the reception device 220 to the source device 210. The source device 210 then regards the reverse link quality information as that of its forward link, which is the transmission link from the source device 210 to the reception device 220. Accordingly, the source device chooses the best rate and frame payload size based on this link quality information.

FIG. 4 is a flow diagram showing the basic link adaptation process, according to one embodiment of the invention. The depicted process may be a program stored in the memory 218 and executed by the processor 216 of the source device 210, for example. More particularly, the source device 210 receives an incoming frame, for example, from the reception device 220 at S410. It is assumed for purposes of explanation that the reception device 220 is a neighboring device, so that the source device 210 already has a neighbor table (e.g., Table 6, below) corresponding to the reception device 220. In one embodiment of the invention, the received information may include a PHY-RX- START. confirm signal, for example, which includes information such as data rate, length, received signal strength indication (RSSI), and the like. The source device 210 analyzes the information in the incoming frame, such as RXVECTOR, at S412. Based on the analysis, the source device 210 determines whether the incoming frame is a beacon frame at S414. When the incoming frame is not a beacon frame (S414: NO), the process ends, and the source device 210 awaits another frame. When the incoming frame is a beacon frame (S414: YES), the source device 210 obtains the SNR of the reverse link, for example, from the link quality indication (LQI) field of the beacon frame at S416 and obtains the address of the reception device 220, for example, from the MAC Header field, at S418. The source device 210 is thus able to identify the appropriate set of entries in neighbor table, Table 6, based on the address.

At S420, the source device 210 obtains the optimum rate corresponding to the SNR of the reverse link. The optimum rate may be obtained from Table 4 or Table 5, both of which may be stored in the memory 218 of the source device 210. When the frame payload size is to be determined along with the rate index, Table 4 is consulted. For example, if the SNR of the reverse link is 3 dB, a rate index of 4 (e.g., corresponding to 160.0 Mbps) and a payload size of 2660 bytes are selected for subsequent transmission. The optimized combinations of rate and payload size may be determined as discussed above with respect to the simulation results, e.g., as shown in FIGs. 8C and 8D. If the SNR of the reverse link falls between two values listed in Table 4, the lower SNR entry may be used, or the SNR may be rounded to the nearest SNR entry.

When the frame payload size is to be fixed, the source device 210 accesses Table 5 instead of Table 4 to obtain the optimum rate. For example, if the frame payload size is fixed at 4095 bytes, then the optimum rate is determined to be the rate corresponding to the next SNR higher than the SNR of the reverse link (indicating the next turning point) recorded in Table 5. Therefore, if the SNR of the reverse link is 6 dB, a rate of 200.0 Mbps (corresponding to rate index 5) is selected for subsequent transmission, because column SNRr, j indicates the highest rate (corresponding to 6.8 dB for a 4095 byte payload) at which a 6 dB signal is reliably transmitted. In other words, referring to Table 5, the 200.0

Mbps rate is the optimum rate for a 4095 byte payload when the measured SNR is greater than 5.4 dB and less than or equal to 6.8 dB. Once the SNR exceeds 6.8 dB, the optimum rate increases to 320.0 Mbps (corresponding to rate index 6) for 4095 byte frame payload.

Once the optimum rate and/or frame payload size have been determined, the source device 210 updates the entries in its (basic) neighbor table corresponding to the reception device 220, accordingly, at S422. An example of a neighbor table for storing entries according to the basic link adaptation process of FIG. 4 is shown in Table 6, below: TABLE 6

The reception device 220 is identified by its MAC address, which is previously stored in the first column of Table 6. If this is the first beacon frame received from the reception device 220, a new row may be entered in Table 6 corresponding to the reception device 220. The measured SNR is entered in the second column of Table 6, which is the SNR of the reverse link of the frame received by the source device 210 from the reception device 220, as discussed above. The optimum rate, as determined by referencing Table 4 or Table 5 in S420, is entered in the third column, and the optimum frame payload size, as determined by referencing Table 4 or which has been fixed to a particular value, is entered in the fourth column of Table 6. To the extent that the frame payload size is fixed, the optimum size entry may remain unchanged throughout communications with the reception device 220.

Once Table 6 has been completed with respect to the reception device 220, the source device 210 may transmit frames to the reception device 220 on its forward link, using the optimum rate and payload size retrieved from Table 6, which had been previously entered based on the reverse link information, according to the process depicted in FIG. 4. Each time the source device 210 receives beacon frames from the reception device 220, Table 6 may be updated, enabling the source device 210 to adjust its transmissions in accordance with current network conditions. Further, in an embodiment of the invention, the reception device 220 likewise executes the link adaptation process of FIG. 4, and thus maintains its own version of Table 6, which has a set of entries corresponding to transmissions to the source device 210 based on reverse link information received in beacon frames from the source device 210. Referring to FIGs. 5-7, a transmitting device may alternatively adapt its transmission rate to actual SNR of its forward link based on previous transmission(s) using the enhanced link adaptation process. Notably, the link quality of the reverse link, discussed above, may not always exactly represent that of the forward link over which a data frame is transmitted for a number of reasons. For example, interference and hence SNR are location sensitive. Therefore, obtaining link quality of the forward link itself typically achieves better performance of link adaptation. The WiMedia MAC standard defines a link feedback IE, which may be used to perform this task.

The link quality of the reverse link is already recorded according to the basic link adaptation process of FIG. 4. Therefore, to perform the enhanced link adaptation process, the transmitting device (e.g., source device 210) simply requests a link feedback IE according to the link quality information in Table 6, inserts the IE in outgoing beacon frame(s), and receives and analyzes a responsive Link Feedback IE constructed by the receiving device (e.g., reception device 220).

FIG. 5 is a flow diagram showing an example of a receiving device receiving an IE in a beacon frame transmitted by a transmitting source device, enabling the enhanced link adaptation process, according to an embodiment of the invention. The depicted process may be a program stored in the memory 228 and executed by the processor 226 of the reception device 220, for example. At S510, the reception device 220 receives an incoming beacon frame from the source device 210. The beacon includes an IE for providing link feedback in subsequent beacon frames transmitted from the reception device 220. The reception device 220 identifies the source device 210, for example, based on a MAC address in the received beacon frame, at S512. At S514, the reception device 220 determines whether its Table 4 includes a valid entry for the source device 210.

When there is no valid entry (S514: No), the reception device 220 transmits its beacon without regard for the IE of the beacon received from the source device 210. When there is a valid entry (S514: Yes), the reception device 220 constructs a link feedback IE according to an optimum rate at S516. For example, the reception device 220 is able to obtain information on the reverse link (i.e., from the source device 210 to the reception device 220), such as the SNR of the reverse link LQI field of the received frame. This information may be used to construct the link feedback IE, which is inserted in a subsequent beacon frame at S518 and transmitted at S520.

FIG. 6 is a flow diagram showing the enhanced link adaptation process, according to an illustrative embodiment, performed by the source device 210 upon receiving the beacon frame, containing the link feedback IE, transmitted from the reception device 220 at S520 of FIG. 5. The depicted process may be a program stored in the memory 218 and executed by the processor 216 of the source device 210, for example. More particularly, the source device 210 receives an incoming frame, for example, from the reception device 220 at S610 of FIG. 6. In an embodiment of the invention, the received information may include a PHY-RX-START. confirm signal, for example, which includes information such as data rate, length, RSSI and the like. The source device 210 analyzes the information in the incoming frame at S612. Based on the analysis, the source device 210 determines whether the incoming frame is a beacon frame at S614. When the incoming frame is not a beacon frame (S614: NO), the process ends, and the source device 210 awaits another frame. When the incoming frame is a beacon frame (S614: YES), the source device 210 determines whether the incoming beacon frame includes a link feedback IE at S616. When the incoming beacon frame does not include the link feedback IE (S616: NO), the process ends, and the source device 210 awaits another frame. Alternatively, when there is no link feedback IE, which prevents implementation of the enhanced link adaptation process, the source device 210 may proceed with the basic link adaptation process using only reverse link quality information, as discussed with respect to FIG. 4.

When the incoming beacon frame includes the link feedback IE (S616: YES), the source device 210 obtains the feedback rate index and/or the corresponding feedback rate from the link feedback IE at S618, which is determined by the reception device 220 as the measured SNR of its reverse link (which coincides with the SNR of the previous forward link of the source device 210). The address of the reception device 220 is obtained at S620, for example, from the MAC Header field of the beacon frame. At S622, the source device 210 updates the entries in its (enhanced) neighbor table corresponding to the reception device 220. An example of a neighbor table for storing entries according to the enhanced link adaptation process of FIG. 6 is shown in Table 7, below:

TABLE 7

The reception device 220 may be identified by its MAC address, which is previously stored in the first column of Table 7. The feedback rate retrieved from the link feedback IE (or corresponding to the feedback rate index retrieved from the link feedback IE) is entered in the second column of Table 7, which is based on the SNR of the reverse link of the reception device 220. At S624, the source device 210 calls a link feedback process in order to determine the optimum rate and the optimum payload size (when not fixed) to be entered in the third and forth columns of Table 7, respectively. The link feedback process is explained below with reference to FIG. 7. Once the optimum rate and/or payload size have been determined, the source device 210 updates the entries in its (enhanced) neighbor table, Table 7, corresponding to the reception device 220 accordingly at S626. Subsequent transmission(s) are performed using the updated entries.

FIG. 7 is a flow diagram showing the link feedback process called in S624 of FIG. 6, according to an embodiment of the invention. The depicted process may be stored as a program in the memory 218 and executed by the processor 216 of the source device 210, for example. As indicated in S710, initial SNR values corresponding to rate indexes are set to indicate turning points for rate changes at optimum frame sizes, as shown in Table 3, with respect to SNRr, _t, where 1 < i < l. Also, SNRr, o is set to -∞ and SNRr, s is set to +oo. It is understood that these initial SNR values need not be set each time the link feedback process is performed or executed. Rather, the initial values may be set once upon initiating the source device 210, or each time the source device 210 is powered on, etc., and stored, for example, in the memory 218.

At S712, the measured SNR corresponding to the address of the reception device 220 is obtained from Table 6, which is the reverse link SNR (indicated as SNR*) between source device 210 and reception device 220. At S714, the feedback rate from the link feedback IE corresponding to the address of the reception device 220 is obtained from Table 7. The measured SNR* is compared to the feedback rates in S716.

In particular, when SNRr_,, < SNR* < SNRr_,,₊; (S716: YES), the optimum rate (or corresponding optimum rate index) and the frame payload size for transmissions to the reception device 220 are determined to be the same as those provided in Table 6 at S718. The process then returns to S626 of FIG. 6, where Table 7 is modified by effectively copying the optimum rate and the frame payload size from Table 6 to Table 7.

However, when SNR* is less than SNRr, _z or greater than or equal to SNRy₂₊; (S716: NO), the SNR is set to the following at S720:

SNR_{1 1} + SNR₁^

SNR = At S722, the source device 210 obtains the optimum rate (or corresponding optimum rate index) corresponding to the calculated SNR. The optimum rate may be obtained from Table 4 or Table 5, each of which may be stored in the memory 218 of the source device 210. When the frame payload size is to be determined along with the rate index, Table 4 is consulted. Alternatively, when the frame payload size is to be fixed, the source device 210 accesses Table 5 instead of Table 4 to obtain the optimum rate index. The process then returns to S626 of FIG. 6, where Table 7 is modified by entering the rate and frame payload size corresponding to the reception device 220 into the third and fourth columns of Table 7, respectively. To the extent that the payload size is fixed and the optimum rate has been determined by reference to Table 5, the optimum size entry may remain unchanged throughout communications with the reception device 220.

For example, if the measured SNR* obtained from Table 6 at S712 is 3 dB and the rate index obtained from Table 7 at S714 is rate index 3 (corresponding to 106.7 Mbps), then SNR* (3 dB) is compared to SNR₇-,* (2.8 dB) and SNR^ (5.4 dB) at S716. Because 2.8 dB < 3 dB < 5.4 dB, the optimum rate index and frame payload size of Table 7 is set to equal the optimum rate index and frame payload size of Table 6 with respect to the reception device 220. However, if the rate index obtained from Table 7 at S714 is rate 4 (corresponding to 160 Mbps), then SNR* (3 dB) is compared to SNR^ (5.4 dB) and SNR_Γ,_J (6.8 dB) at S716. Because 3 dB < 5.4 dB (and 6.8 dB), the optimum rate index and frame payload size are determined using an SNR calculated at S720:

SNR₁. , + SNR₁. _t .

SNR = ,:+l

s_{m =} 5A_± 6Λ _{= 6Λ άB}

Accordingly, using Table 4 (assuming the frame payload size is not fixed), the optimum rate index is rate 4 and the optimum frame payload size is 4095 bytes. Table 7 is updated with these values with respect to the reception device 220, accordingly.

Once Table 7 has been completed with respect to the reception device 220, the source device 210 may transmit frames to the reception device 220 on its forward link, using the optimum rate and payload size retrieved from Table 7. Each time the source device 210 receives beacon frames from the reception device 220, Table 7 may be updated, enabling the source device 210 to adjust its transmissions in accordance with current network conditions. Further, in an embodiment, the reception device 220 likewise executes the enhanced link adaptation process of FIGs. 6 and 7, and thus maintains its own version of Table 7, which has a set of entries corresponding to transmissions to the source device 210 based on link feedback IEs received in beacon frames from the source device 210.

According to the exemplary embodiments, neighboring wireless devices, such as representative wireless devices 210 and 220 are able to communicate with one another over links adapted for current network conditions based on reverse link information and/or link feedback IEs exchanged in beacon frames. Examples are provided herein for illustration purposes and are not to be construed as limiting the scope of the teachings of this specification, or the claims to follow.

While preferred embodiments are disclosed herein, many variations are possible which remain within the concept and scope of the invention. Such variations would become clear to one of ordinary skill in the art after inspection of the specification, drawings and claims herein. The invention therefore is not to be restricted except within the spirit and scope of the appended claims.

Claims

CLAIMSWhat is claimed is:

1. A method of improving throughput of a forward communications link between a source device and a reception device, comprising: receiving a beacon frame from the reception device; determining a link quality associated with the forward communications link based on information obtained from the received beacon frame; determining an optimum transmission rate based on the link quality; and transmitting a signal to the reception device using the optimum transmission rate.

2. The method of claim 1, wherein the link quality comprises signal strength indicated by a signal to noise ratio.

3. The method of claim 2, further comprising: determining an optimum frame payload size based on the signal strength; and transmitting the signal to the reception device using the optimum frame payload size in addition to the optimum transmission rate.

4. The method of claim 3, further comprising: identifying the reception device based on an address in the received beacon frame; and storing the optimum transmission rate and the optimum payload size in association with the reception device address.

5. The method of claim 1, wherein determining the link quality associated with the forward communications link include obtaining a quality of a reverse link from the received beacon frame and using the reverse link quality as the link quality associated with the forward communications link.

6. The method of claim 1 , wherein determining the link quality associated with the forward communications link includes: retrieving feedback information from the received beacon frame indicating a previous link quality associated with the forward communications link from a previous transmission from the source device to the reception device; and using the previous link quality as the link quality of the forward communications link.

7. The method of claim 6, wherein the feedback information comprises a link feedback information element.

8. The method of claim 7, wherein the previous link quality associated with the forward communications link comprises a reverse link quality detected by the reception device from the previous transmission.

9. The method of claim 2, wherein determining the optimum transmission rate comprises: identifying a fixed frame payload size; comparing the signal strength to a plurality of predetermined signal strength ranges for the fixed payload size, each predetermined signal strength range corresponding to a predetermined transmission rate; and identifying one predetermined signal strength range of the plurality of predetermined signal strength ranges containing the signal strength, based on the comparing; wherein the optimum transmission rate is determined to be the predetermined transmission rate corresponding to the identified one predetermined signal strength range.

10. The method of claim 3, wherein determining the optimum transmission rate and the optimum frame packet size comprises: comparing the signal strength to a plurality of predetermined combinations of transmission rates and frame packet sizes, at least one predetermined combination comprising a predetermined optimum combination corresponding to the signal strength; and selecting the predetermined optimum combination based on the comparison; wherein the optimum transmission rate and the optimum frame packet size are determined to be the transmission rate and frame packet size corresponding to the selected predetermined optimum combination.

11. An apparatus for improving throughput of a forward communications link with a reception device, the apparatus comprising: a receiver configured to receive a beacon frame from the reception device; a processor configured to determine a signal strength associated with the forward communications link based on information obtained from the received beacon frame, and to determine an optimum transmission rate based on the signal strength; and a transmitter configured to transmit a signal to the reception device using the optimum transmission rate.

12. The apparatus of claim 11, wherein the information obtained from the received beacon frame comprises a signal strength of a reverse link from the reception device, the determined signal strength being set to the reverse link signal strength.

13. The method of claim 11, wherein the information obtained from the received beacon frame comprises a link feedback information element indicating a previous signal strength associated with the forward communications link from a previous transmission to the reception device, the determined signal strength being set to the previous signal strength.

14. A method of improving throughput, comprising: sending an information element request from a source device to a reception device in a first transmission over a wireless communications link; subsequently receiving a beacon frame from the reception device, the beacon frame comprising a link feedback information element in response to the information element request, the link feedback information element indicating a feedback rate corresponding to the first transmission over the communications link; determining an optimum transmission rate associated the communications link based on at least the link feedback information from the received beacon frame; and sending a second transmission over the wireless communications link from the source device to the reception device using the determined optimum transmission rate.

15. The method of claim 14, wherein the link feedback information comprises one of a feedback transmission rate or a feedback transmission rate index of a plurality of predetermined transmission rate indexes corresponding to the feedback transmission rate.