Adaptive modulation for cooperative coded systems

Abstract

A system, method, and apparatus are provided for a source to choose a partner/relay from at least one candidate to transmit at least part of a message from the source to destination. The choice depends on the channel conditions of the source and when the source is experiencing poor channel quality to the destination, the source selects the candidate and the modulation modes of the source and partner/relay such that the frame error rate (FER) of the source is lowered. Otherwise, a modulation mode is selected for each of the source and the partner/relay that most improves the gain in throughput of the source. The present invention applies to modes consisting of binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), and 16-state quadrature amplitude modulation (16-QAM).

Description

The present invention relates to a system and method for a coded cooperative wireless communication system in which users can adapt their modulation mode based on their channel qualities to maximize data throughput.

The destructive addition of time-varying multipaths and interference from other users causes severe attenuation of a transmitted signal at a receiver side. Diversity techniques provide the receiver several independent (at least uncorrelated) replicas of the same information signal such that the probability is considerably reduced that all the signal components are simultaneously faded. In a wireless network, collaboration among mobile stations at the physical layer has been shown to be an efficient way to introduce diversity. Such a wireless system has independent nodes that communicate with a common destination, such as an access point (AP) in a Wireless LAN system and a base station in a cellular system. For low mobility nodes, it is difficult to exploit temporal diversity through interleaving. Also, spatial diversity through multiple antennas placed on a single device may be limited due to size constraints of the node. Cooperative wireless communication enables nodes to use each other's antenna to obtain an effective form of spatial diversity. A partnering node processes signals overheard from an original source and then transmit them to a destination, such as an AP. The destination, e.g., AP, combines signals received from the original node and the partner, thus creating an efficient form of spatial diversity.

Conventionally, partners are chosen in advance and analyses have shown that cooperation provides full diversity while improving overall performance in terms of outage probability or frame error rate (FER), see, e.g., A. Sendonaris, et al, “User Cooperation Diversity-Part I: System Description,”, IEEE Trans. Commun. Vol. 51, No. 11, pp. 1927-1938, November 2003, and “User Cooperation Diversity-Part II: Implementation Aspects and Performance Analysis,” IEEE Trans. Commun. Vol. 51, No. 11, pp. 1939-1948, November 2003, the entire contents of which are hereby incorporated by reference. Further in Lin et al., the condition is derived under which cooperation improves the original user's FER performance when a coded cooperative algorithm of Stefanov et al. is used, see Z. Lin et al., “An Asymptotic Analysis On the Performance of Coded Cooperation Systems,” Proc. IEEE Vehicular Technology Conference, Los Angeles, Fall 2004 and A. Stefanov et al., “Cooperative Coding For Wireless Networks,” IEEE Trans. Commun., vol. 52, no. 9, pp. 1470-1476, September 2004, the entire contents of which are hereby incorporated by reference. Further, the cited references show that high channel quality of a partner guarantees that a user gets benefits from cooperative coding. However, in all the above cited works as well as current research, partnering users are assumed to be using a fixed and common modulation mode.

In wireless services, higher data rate is one of the main design considerations. Further, in some wireless systems, e.g., IEEE 802.11, nodes are able to transmit their data at multiple rates and are allowed to adapt their data rates to match their channel conditions such that the throughput for their given channel conditions is maximized, see, respectively, IEEE 802.11, “Wireless LAN MAC and PHY Specifications, Standard, August 1999 and G. Holland, et al. “A Rate-Adaptive MAC Protocol for Multi-Hop Wireless Networks,” Proc of the 7^th Annual International Conference on Mobile Computing and Networking, pp 236-251, Rome, Italy, 2001, the entire contents of both of which are hereby incorporated by reference.

Thus, a way for cooperating partners to adjust their data rates to their prevailing channel conditions is needed such that an original user's (source's) data throughput is maximized. The present invention provides an apparatus and method for cooperating partners of coded cooperative systems to select their modulation modes based on their channel qualities to an access point (AP) in order to optimize the data throughput of the original user to the AP.

In the present invention not only a partner's channel quality but also the source's channel quality is taken into consideration in the selection of the partner's modulation rate. Further, in the present invention the source also takes into consideration its partner's channel quality as well when selecting the source's modulation rate.

The present invention defines a system and method for cooperating partners of a coded cooperative system to determine

• • 1. for a pair of cooperating users, the best modulation rate pair used by the pair of cooperating users in different signal-to-noise ratio regimes such that the throughput gain due to cooperation is maximized; and
  • 2. for multiple candidate partners, how to choose a partner such that the throughput gain for the source is maximized.

FIG. 1 is a flowchart of cooperating partners;

FIG. 2A illustrates user-cooperation for two sources and one common destination;

FIG. 2B illustrates time-division channel allocation using orthogonal direct transmission;

FIG. 2C illustrates time-division channel allocation using orthogonal cooperative diversity transmission;

FIG. 2D illustrates three transmission schemes: direct, multi-hop and cooperative;

FIG. 3 illustrates data throughput gain for S₁ when both the source and the partner use BPSK modulation mode;

FIG. 4 illustrates data throughput gain for S₁ when S₁ uses BPSK modulation mode and S₂ switches to QPSK modulation mode;

FIG. 5 illustrates data throughput gain for S₁ when S₁ uses BPSK modulation mode and S₁ switches to 16 QAM modulation mode;

FIG. 6 illustrates for γ₁=−10 dB, a comparison of the data throughput gain for S₁ when S₁ uses BPSK modulation mode and S₂ uses BPSK, QPSK and 16 QAM, respectively;

FIG. 7 illustrates γ₁=−5 dB, a comparison of the data throughput gain for S₁ when S₁ uses BPSK modulation mode and S₂ uses BPSK, QPSK and 16 QAM, respectively;

FIG. 8 illustrates γ₁=0 dB, a comparison of the data throughput gain for S₁ when S₁ uses BPSK modulation mode and S₂ uses BPSK, QPSK and 16 QAM, respectively;

FIG. 9 illustrates γ₁=5 dB, a comparison of the data throughput gain for S₁ when S₁ uses BPSK modulation mode and S₂ uses BPSK, QPSK and 16 QAM, respectively;

FIG. 10 illustrates γ₁=10 dB, a comparison of the data throughput gain for S₁ when S₁ uses BPSK modulation mode and S₂ uses BPSK, QPSK and 16 QAM, respectively;

FIG. 11 illustrates γ₁=15 dB, a comparison of the data throughput gain for S₁ when S₁ uses BPSK modulation mode and S₂ uses BPSK, QPSK and 16 QAM, respectively;

FIG. 12 illustrates γ₁=−5 dB, threshold comparison and P_f,1^BF when S₁ uses BPSK and S₂ uses QBSK and 16QAM;

FIG. 13 illustrates an example of choice of partner in a network where two choices are possible;

FIG. 14 illustrates throughput gain comparison for γ₁=−5 dB, D₁=1 and the angle between D₁ and D₂ is π/6 when the partner uses BPSK, QPKS AND 16QAM respectively;

FIG. 15 illustrates throughput gain comparison for γ₁=0 dB, D₁=1 and the angle between D₁ and D₂ is π/6 when the partner uses BPSK, QPKS AND 16QAM respectively;

FIG. 16 illustrates throughput of direct transmission;

FIG. 17 illustrates throughput of direct transmission, multi-hop and coded cooperation with adaptive modulation for D₁=1.0, D₂=0.6 and D_in=0.57;

FIG. 18 illustrates throughput of direct transmission, multi-hop and coded cooperation with adaptive modulation for D₁=1.0, D₂=0.2 and D_in=0.69; and

FIG. 19 illustrates an apparatus for selecting a partner and source/partner modulation modes according to the present invention.

It is to be understood by persons of ordinary skill in the art that the following descriptions are provided for purposes of illustration and not for limitation. An artisan understands that there are many variations that lie within the spirit of the invention and the scope of the appended claims. Unnecessary detail of known functions and operations may be omitted from the current description so as not to obscure the present invention.

Without loss of generality, assume there are two nodes (S₁ and S₂) communicating with the same destination, e.g., AP, as in FIG. 2A. Let γ₁ and γ₂ denote the average received SNR at the destination 203 from S₁ 201 and S₂ 202, respectively. Assume that the channel between partnering users S₁ 201 and S₂ 202 is symmetric. Denote the average received SNR for the inter-user channel by γ_in. Assume cooperation is via time division multiplexing as illustrated in FIG. 2B and assume an underlying convolutional code. Further, assume non-cooperating users have separate time slots consisting of N coded symbols and that when they decide to cooperate at step 101 each user divides its own slot in two, as illustrated at step 101 in the flow chart of FIG. 1. As illustrated at 231 of FIG. 2C, in the first N/2 channel uses 231, the source transmits half of its coded bits, as indicated at step 103 of FIG. 1. This transmission is received by both the partner and the destination at step 104. The partner attempts to decode the information bits of the source at step 105. An error detection mechanism such as cyclic Redundancy Check (CRC) indicates whether the decoded copy at the partner is identical to the original. If decoding is successful, the partner re-encodes the information bits using different parity bits and transmits them in the second N/2 channel uses 232, as indicated at step 107. If decoding is not successful, the source continues the transmission by itself at step 106. The destination combines signals received in all N channel uses and decodes the combined signal. In the next frame, the roles 233234 of the source and partner are reversed. Since multi-hop mitigates the path loss effect, multi-hop embodiments are provided in the system and method of the present invention. It is assumed that the transmitted energy per symbol is fixed as ε and that S₁ is the source node and S₂ acts as a pure relay. The packet is first transmitted from S₁ to S₂. Upon successful reception, the relay S₂ forwards the packet to the destination. In multi-hop, all the packets are transmitted through two hops. Therefore, for fair comparison, assume that S₁ and S₂ use half transmit energy individually, that is ε/2, in multi-hop. In direct transmission (non-cooperative case) and coded cooperation, each packet takes one time slot to transmit. Therefore, in direct transmission, S_i (i=1, 2) still uses ε to transmit the whole packet m and in coded cooperation, S_i uses ε to transmit half of the whole packet cooperatively. These three schemes are illustrated in FIG. 2D.

Assume that the noise is additive white Gaussian with zero mean and power spectral density

$\frac{N_0}{2}.$ For simplicity, ignore the processing power at the partner. Consider a complex Gaussian, flat fading channel with zero mean and unit variance. For a low mobility environment, assume that during the course of transmission or for each time slot, each user observes only one fading level towards the destination. Due to the spatial separation between users, these fades are independent. Hence, the user-to-destination channel is quasi-static and the cooperative transmission results in a block fading environment. The inter-user channel is also assumed to be quasi-static and independent of user-to-destination links. This cooperative scheme and channel model is described in A. Stefanov et al., “Cooperative coding for Wireless Networks,” IEEE Transactions on Communications, vol. 52, no. 9, pp. 1470-1476, September 2004., the entire contents of which are hereby incorporated by reference.

For adaptive modulation used in multi-hop and cooperative systems and methods according to the present invention, assume that both partnering nodes can select their own modulation modes from candidates based on the averaged received SNRs in the destination, e.g., AP. The candidates include, but are not limited to, binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), and 16 state quadrature amplitude modulation (16-QAM). Let M₁ and M₂ denote the number of bits per symbol sent by S₁ 201 and S₂ 202, respectively, when the cooperation protocol according to the present invention is performed and N₁ and N₂ be the number of bits per symbol transmitted by S₁ 201 and S₂ 202, respectively, when communication directly with the destination 203. Let K₁ and K₂ denote the number of bits per symbol sent by S₁ and S₂ respectively in multi-hop. Hence, for BPSK, QPSK, 16-QAM, M₁, M₂, N₁, N₂, K₁, K₂ ε {1, 2, 4}. Without loss of generality assume N₁=N₂.

There are three ways to employ the adaptive modulation scheme of the present invention in a coded cooperative system:

• • 1. the partner changes its modulation mode but the source keeps its own rate constant, that is M₁=N₁;
  • 2. the source adapts its modulation rate and the partner fixes its modulation mode, that is M₂=N₂; and
  • 3. both the source and the partner change their modulation rates simultaneously.Only the corresponding throughput performance of each of the first two cases is analyzed below. The third case is readily obtained by extension from the results for the first two.

In the following sections, first, the throughput performance of a direct transmission, multi-hop and coded cooperative system are analyzed, then throughput gain due to cooperation is defined and finally conditions under which cooperation results in throughput gain for the source are derived. Assume:

• • the destination, e.g., AP, uses CRCs to detect all errors in each packet and the probability of an undetected transmission error is negligible;
  • there is no error in the transmission of acknowledgments from the receiver to the transmitter and this transmission is instantaneous;
  • the symbol transmission rate for each user is fixed as R_s and each user uses the same convolutional code with rate R;
  • each data packet contains B data bits, and overhead bits are ignored so that the length of each frame/packet is equal to N bits, where N=B/R.; and
  • the throughput is defined as the number of payload bits per second received correctly.Direct transmission: The FER for non-cooperative (direct) case is P_f,i^no-coop={circumflex over (P)}_f,i^QS. In this case, the throughput of the coded non-cooperative (direct transmission) system for the data of user i is written as:

$\begin{array}{cc}{\Gamma }_{\mathrm{no}-\mathrm{coop},i}=\frac{B\left(1-P_{f,i}^{\mathrm{no}-\mathrm{coop}}\right)}{\frac{B}{N_i{R}_{S}R}}=N_i{R}_{S}R\left(1-P_{f,i}^{\mathrm{no}-\mathrm{coop}}\right)=N_i{R}_{s}R\left(1-{\hat{P}}_{f,i}^{\mathrm{QS}}\right)& \left(1\right)\end{array}$ Note that {circumflex over (P)}_f,i^QS is a function of N₁ and γ₁. For higher order modulation, {circumflex over (P)}_f,i^QS increases, but so does the multiplicative factor in Γ_no-coop,i. Hence, there exists an optimal modulation scheme N₁ which depends on the average received SNR, γ₁ from S₁ to the destination.

Multi-hop: S₁ re-transmits the coded packet to S₂ until the packet is to successfully received by S₂. Then S₂ relays the packet to the destination. If there is an error in the received packet at the destination, S₂ re-transmits. We let P^QS_m,in and P^Qs_m,2 denote the FER of the channel code for the quasi-static channel from S₁-to-S₂ and from S₂-to-destination in multi-hop respectively. For the multi-hop scheme, it takes an average

$\frac{1}{\left(1-P_{m,\mathrm{in}}^{\mathrm{QS}}\right)}$ transmissions in the first hop (from S₁ to S₂) and

$\frac{1}{\left(1-P_{m,2}^{\mathrm{QS}}\right)}$ transmissions in the second hop (from S₂ to the destination) to get one packet through. Hence, on the average, the first hop transmission takes

$\frac{B}{\left(1-P_{m,\mathrm{in}}^{\mathrm{QS}}\right)K_1{R}_{s}R}$ seconds and the second hop takes

$\frac{B}{\left(1-P_{m,2}^{\mathrm{QS}}\right)K_2{R}_{s}R}$ seconds. Summing these up from the source to the destination, it takes

$\frac{B}{\left(1-P_{m,\mathrm{in}}^{\mathrm{QS}}\right)K_1{R}_{s}R}+\frac{B}{\left(1-P_{m,2}^{\mathrm{QS}}\right)K_2{R}_{s}R}$ seconds in total for each packet to get through successfully. Then the data throughput of S₁ in multi-hop is:

${\Gamma }_{\mathrm{mhop}}=\frac{B}{\frac{B}{\left(1-P_{m,\mathrm{in}}^{\mathrm{QS}}\right)K_1{R}_{s}R}+\frac{B}{\left(1-P_{m,2}^{\mathrm{QS}}\right)K_2{R}_{s}R}}=\frac{R_{s}R}{\frac{1}{\left(1-P_{m,\mathrm{in}}^{\mathrm{QS}}\right)K_1}+\frac{1}{\left(1-P_{m,2}^{\mathrm{QS}}\right)K_2}}$ We find from the above equation that as P_m,in^QS and P_m,2^QS depend on the channel quality from S₁ to S₂ and from S₂ to the destination independently, S₁ and S₂ adapt their modulation rates K₁ and K₂ based on channel qualities of S₁-to-S₂ and S₂-to-destination independently.

For coded cooperative transmission, as illustrated in FIG. 2D, in the first transmission of coded cooperation, S_i transmits half of the coded bits to the destination and S_j, where i≠j, i, jε{1,2}. If S_i decodes information bits sent by S_j correctly, S_j sends the other half of the coded bits to the destination. If there is an error at the destination, all successive packets are transmitted cooperatively by S_i and S_j. If, on the other hand, S_j cannot decode S_i's information, S_i continues transmitting the remaining coded bits. In this case, all re-transmissions will come directly from the source. This allows us to free up the partner's sources quickly while still enjoying the benefits of cooperation. Let P_f,i^QS denote the FER for the quasi-static S_i-to-destination channel, P_f,iⁱⁿ denote the FER of the first half channel code for the quasistatic channel of S_i-to-S_j and P_f,i^BF denote the FER for the cooperative block fading channel when destination receives half of the packet from S_i and the remaining half from S_j. When S_i and S_j transmit cooperatively, which happens with probability (1−P_f,iⁱⁿ), an average of

$\frac{1}{\left(1-P_{f,i}^{\mathrm{BF}}\right)}$ retransmissions are needed, with each transmission of a packet taking

$\frac{B}{2M_1{R}_{S}R}+\frac{B}{2M_2{R}_{S}R}$ seconds. When S_i transmits by itself, which happens with the probability of P_f,iⁱⁿ, we need an average of

$\frac{1}{\left(1-P_{f,i}^{\mathrm{QS}}\right)}$ retransmissions, with each transmission taking

$\frac{B}{M_i{R}_{S}R}$ seconds. Then,

$\begin{array}{cc}\begin{array}{c}{\Gamma }_{\mathrm{coop},i}=\frac{B\left(1-P_{f,i}^{\mathrm{in}}\right)\left(1-P_{f,i}^{\mathrm{BF}}\right)}{\frac{B}{2M_1{R}_{S}R}+\frac{B}{2M_2{R}_{S}R}}+\frac{{\mathrm{BP}}_{f,i}^{\mathrm{in}}\left(1-P_{f,i}^{\mathrm{QS}}\right)}{\frac{B}{2M_i{R}_{S}R}}\\ =\frac{R_{S}R\left(1-P_{f,i}^{\mathrm{in}}\right)\left(1-P_{f,i}^{\mathrm{BF}}\right)}{\frac{1}{2M_1}+\frac{1}{2M_2}}+\frac{R_S{\mathrm{RP}}_{f,i}^{\mathrm{in}}\left(1-P_{f,i}^{\mathrm{QS}}\right)}{\frac{1}{2M_i}}\end{array}& \left(2\right)\end{array}$ Since S₁ and S₂ may use different modulation modes, P_f,1ⁱⁿ may not be equal to P_f,2ⁱⁿ

Comparing the direct transmission and cooperative transmission schemes, P_f,i^QS is not necessarily equal to {circumflex over (P)}_f,i^QS as S_i may have a different modulation scheme for non-cooperative (direct) transmission and cooperative scheme. Note that in multi-hop, S₁ transmits all the coded bits to S₂ with transmit energy ε/2 per symbol, but in coded cooperation, S₁ sends only half of the coded bits to S₂ with transmit energy ε per symbol. Hence, P_m,in^QS is different from P_f,1ⁱⁿ. It can be observed from Eqn. (2) that the throughput of S_i in coded cooperative system depends on P_f,i^BF, P_f,i^QS and P_f,iⁱⁿ, these FER probabilities depend on all three link SNRs, γ₁, γ₂ and γ₁₂. Therefore, to optimize Γ_coop, S₁ and S₂ should base their modulation not only on their own channel quality to the destination but on all these links.

It has been shown that cooperation benefits the user under certain conditions when

$\left(\frac{P_{f,i}^{\mathrm{BF}}}{P_{f,i}^{\mathrm{QS}}}<1\right),$ see Zinan Lin, et al. “An Asymptotic Analysis On the Performance of Coded Cooperation Systems,” Proc. IEEE Vehicular Technology Conference, Los Angeles, Fall 2004, the entire contents of which are hereby incorporated by reference.

The following sections define the user throughput gain in order to measure the throughput improvement obtained from cooperation. Assume cooperating users S₁ and S₂ adapt their modulation modes based on the quality of their channel to the destination when in cooperative communication. For the cooperation between them using a fixed channel code, the data throughput gain for S_i due to cooperation is defined as

$\begin{array}{cc}{G}_{\Gamma ,i}=\frac{{\Gamma }_{\mathrm{coop},i}}{{\Gamma }_{\mathrm{no}-\mathrm{coop},i}}=\frac{2\left(1-P_{f,i}^{\mathrm{in}}\right)\left(1-P_{f,i}^{\mathrm{BF}}\right)}{N_i\left(\frac{1}{M_i}+\frac{1}{M_j}\right)\left(1-{\hat{P}}_{f,i}^{\mathrm{QS}}\right)}+P_{f,i}^{\mathrm{in}}\frac{M_i}{N_i}\frac{1-P_{f,i}^{\mathrm{QS}}}{1-{\hat{P}}_{f,i}^{\mathrm{QS}}}& \left(3\right)\end{array}$ where N_i,M_i,M_jε{1,2,4,6}, i≠j, i and i, jε{1,2}. Based on this definition, that when G_Γ,i>1, cooperation improves the data throughput for S_i and the overall data rate for S_i is increased. Please note if both partnering users use the same modulation rate and fix it, i.e., M_i=N_i=M_j, then throughput gain due to cooperation is equivalent to FER gain due to cooperation. All the results on how the channel qualities affect the FER gain is valid in this context.

Proposition 1a: A user benefits from coded cooperation in terms of throughput, that is G_Γ,i>1, if and only if

${\theta }_{f,i}=\frac{P_{f,i}^{\mathrm{BF}}}{P_{f,i}^{\mathrm{QS}}}<1.$

Proposition 2a: If

${\theta }_{f,i}=\frac{P_{f,i}^{\mathrm{BF}}}{P_{f,i}^{\mathrm{QS}}}<1,$ then G_Γ,i is an increasing function of γ_in (or decreasing function of P_f,1ⁱⁿ, that is, the cooperation gain increases as the inter-user channel quality improves.

Without loss of generality, only S₁ is considered in the flowing propositions:

Proposition 3a (Partner has good link quality): Assuming fixed received SNR for S₁ at the destination and for the inter-user channel, that is, γ₁ and γ_in are fixed, the cooperation gain for S₁, G_Γ,1>1, is an increasing function of γ₂. As γ₂∞, G_Γ,1>1, that is cooperation benefits S₁, irrespective of γ₁ and γ_in. Hence, it is always beneficial to cooperate with a good user in terms of throughput.

Proposition 4a (User has good link quality): Suppose γ₂ and γ_in are fixed. Then, G_Γ,1 a decreasing function of γ₁. As γ₁∞, G_Γ,i if and only if γ₂≧γ*₂, where the threshold γ*₂ only depends on the channel code used. Hence irrespective of the inter-user channel quality, cooperation benefits the good user only when the partner has a received SNR above a certain threshold.

Proposition 5a (Symmetric users with good link qualities): Consider coded cooperation among users S₁ and S₂, both of which have similar channel qualities to the destination, that is γ₁≈γ₂=γ. We assume γ_in is fixed. Then cooperation gain for each user, G_Γ,1 or G_Γ,2 is an increasing function of γ. As γ∞, G_Γ,i>1 irrespective of γ_in. Hence, cooperation among two good users always benefits both of them.

However, in order to improve the throughput of the system more efficiently, users select their modulation rates based on their different channel qualities, that is, M_i is not necessarily identical to M_j or N_i. Therefore, the equivalence between FER gain and throughput gain due to cooperation does not hold any more. However, how the partner's received SNR affects the throughput gain and FER gain are similar.

Without loss of generality, S₁ is the focus in all the following discussions.

Proposition 1: For fixed γ₁ and γ_in (P_f,1ⁱⁿ) and selected N₁, M₁ and M₂, G_Γ,1 increases with γ₂.

Proof: For fixed γ₁ and γ_in (P_f,1ⁱⁿ) and selected N₁, M₁ and M₂, all the terms in (3) are fixed except P_f,1^BF. With the increasing of γ₂, 1−P_f,1^BF increases as well. Therefore, the throughput gain of S₁, G_Γ,1 is improved by increasing γ₂.

Proposition 1 shows that when a partner is in a better situation, the throughput gain increases. In other words, cooperating with a “better” partner brings more benefits to the original user, where better means a better quality channel to the destination.

In the following sections, it is assumed that only a partner adapts its modulation rate to its channel conditions and a source keeps its modulation mode unchanged, that is M₁=N₁.

Proposition 2: When users S₁ 201 and S₂ 202 use a modulation rate with M₁ bits/symbol and M₂ bits/symbol, respectively, during cooperation, user 1 obtains a throughput gain from cooperation, i.e., G_Γ,1>1 if and only if

${\Theta }_{M_1}\frac{1-P_{f,1}^{\mathrm{BF}}}{1-P_{f,1}^{\mathrm{QS}}}$ $\mathrm{where}$ $\begin{array}{cc}{\Theta }_{M_1}=\frac{2}{\frac{M_1}{M_2}+1}& \left(4\right)\end{array}$

• • Proof: The source fixes its modulation mode and hence, P_f,1^QS={circumflex over (P)}_f,1^QS. Using the definition of G_Γ,1, we can easily obtain G_Γ,1

$1\iff {\Theta }_{M_1}\begin{array}{c}<\\ >\end{array}\frac{1-P_{f,1}^{\mathrm{BF}}}{1-P_{f,1}^{\mathrm{QS}}}1.$ Based on Proposition 1, when the source does not change its modulation rate during cooperation, the throughput gain due to cooperation depends only on

${\Theta }_{M_1}\frac{1-P_{f,1}^{\mathrm{BF}}}{1-P_{f,1}^{\mathrm{QS}}}$ and the quality of the inter-user channel (which results in different values of P_f,1ⁱⁿ) does not determine whether or not the source gets throughput benefits from cooperation. This is consistent with the result given in Zinan Lin, et al. “An Asymptotic Analysis On the Performance of Coded Cooperation Systems,” Proc. IEEE Vehicular Technology Conference, Los Angeles, Fall 2004, that whether the FER of user i due to cooperation is improved or not depends only on

${\Theta }_f=\frac{P_{f,i}^{\mathrm{BF}}}{P_{f,i}^{\mathrm{QS}}}$ and is not related to the inter-user channel quality. However, how much throughput benefits can be obtained through cooperation is determined by the channel quality of the inter-user channel.

Using the condition under which the user gets benefits from cooperation as defined in Zinan Lin, et al. “An Asymptotic Analysis On the Performance of Coded Cooperation Systems,” Proc. IEEE Vehicular Technology Conference, Los Angeles, Fall 2004, (θ_f<1) in terms of FER and Proposition 2, the following relationship between throughput gain and FER gain due to cooperation:

Proposition 3: Consider the coded cooperation between S₁ and S₂, in which S₁ fixes its modulation mode and S₂ changes its modulation mode based on channel qualities. When S₂ uses a higher modulation rate than S₁, i.e., M₂<M₁, the FER improvement for S₁ due to cooperation guarantees that S₁ has a higher throughput resulting from cooperation, i.e., P_f,1^coop<P_f,1^no-coopG_Γ,1>1. However, when S₂ uses lower modulation rate than S₁, i.e., M₂<M₁, if cooperation brings S₁ a higher throughput, then cooperation must improve S₁'s FER performance as well, i.e., G_Γ,1>1P_f,1^coop<P_f,1^no-coop.

Proof: It is shown in Zinan Lin, et al. “An Asymptotic Analysis On the Performance of Coded Cooperation Systems,” Proc. IEEE Vehicular Technology Conference, Fall 2003 that for coded cooperation as described in FIG. 1, P_f,1^coop<P_f,1^no-coop if and only if P_f,1^BF<P_f,1^OS.

For the case of M₂>M₁ and θ_M1>1, when

$P_{f,1}^{\mathrm{coop}}<P_{f,1}^{\mathrm{no}-\mathrm{coop}},\frac{1-P_{f,i}^{\mathrm{BF}}}{1-P_{f,1}^{\mathrm{QS}}}>1$ and using Proposition 2,

${\Theta }_{M_1}\frac{1-P_{f,1}^{\mathrm{BF}}}{1-P_{f,1}^{\mathrm{QS}}}>1\iff G_{\Gamma ,1}>1).$ For the case of M₂<M₁ and θ_M1<1, if G_Γ,1>1 then

${\Theta }_{M_1}\frac{1-P_{f,i}^{\mathrm{BF}}}{1-P_{f,i}^{\mathrm{QS}}}>1$ and hence,

$\frac{1-P_{f,i}^{\mathrm{BF}}}{1-P_{f,i}^{\mathrm{QS}}}$ must be greater than 1 as θ_M1<1. Therefore, for the case of M₁>M₂ having G_Γ,1>1 guarantees

$\frac{P_{f,i}^{\mathrm{BF}}}{P_{f,i}^{\mathrm{QS}}}<1\mathrm{and}{P}_{f,1}^{\mathrm{coop}}<P_{f,1}^{\mathrm{no}-\mathrm{coop}}.$

The following sections show how the received SNR's of the original user affects its throughput gain when it keeps the same modulation mode in cooperation as the one in its individual communication with the destination:

Proposition 4: For fixed γ₂ and γ_in (P_f,2ⁱⁿ) and selected M₁ and M₂, if cooperation leads to less FER for the source, i.e., P_f,1^coop<P_f,1^no-coop, then the data throughput gain of S₁, which is G_Γ,1, decreases as γ₁ increases.

Proof: We have

$\begin{array}{cc}\frac{\partial G_{\Gamma ,1}}{\partial {\gamma }_1}={\Theta }_{M_1}(\hspace{1em}1-P_{f,1}^{\mathrm{in}}\left[\frac{\frac{1}{{\left(P_{f,1}^{\mathrm{QS}}\right)}^2}\frac{\partial P_{f,1}^{\mathrm{QS}}}{\partial {\gamma }_1}\left(1-{\Theta }_{f,1}\right)+\frac{\partial {\Theta }_{f,1}}{\partial {\gamma }_1}\left(1-\frac{1}{P_{f,1}^{\mathrm{QS}}}\right)}{{\left(\frac{1}{P_{f,1}^{\mathrm{QS}}}-1\right)}^2}\right]& \left(5\right)\end{array}$ where

${\Theta }_{f,1}=\frac{P_{f,1}^{\mathrm{BF}}}{P_{f,1}^{\mathrm{QS}}}.$ It has been shown in Zinan Lin, et al. “An Asymptotic Analysis On the Performance of Coded Cooperation Systems,” Proc. IEEE Vehicular Technology Conference, Fall 2003, that for fixed γ_in and γ₂ that θ_f,1 is an increasing function of γ₁. Therefore

$\frac{\partial G_{\Gamma ,1}}{\partial {\gamma }_1}>0.$ From the fact that P_f,1^QS<1 and the equivalence of P_f,1^coop<P_f,1^no-coopθ_f,1<1, it follows that

$\frac{\partial G_{\Gamma ,1}}{\partial {\gamma }_i}<0.$ Therefore, G_Γ,1 decreases with γ₁.

Based on Proposition 4, if the cooperative coding benefits the original user in terms of FER, then the throughput gain due to cooperation decreases with the improvement of the source's channel quality. However, if cooperation does not bring benefits to the source in terms of FER (θ_f,1>1), it is difficult to determine how the throughput gain of the source changes when its channel quality improves.

In the following sections, the partner keeps the same modulation mode as the one when it communicates with the destination individually but the source changes its modulation mode dynamically. Since the source may use different modulation modes from the one in its individual communication with the destination, M₁ is not necessarily same as N₁ then P_f,1^no-coop is not always equal to P_f,1^qs. Based on (3), it follows that if the source adapts its modulation mode, the throughput gain of the source due to cooperation is dependent on the FER of the inter-user channel, which is different from the case that the source fixes its own modulation mode. Therefore, whether the cooperation improves the source's throughput or not depends on the inter-user channel quality if the source adapts its modulation rate during cooperation.

The following is an analysis of the case where the source fixes its modulation mode and the partner changes its modulation mode that investigates the optimal modulation modes that can be used by the partner in different ranges of SNRs such that the data throughput for the source is maximized.

In Zinan Lin, et al. “An Asymptotic Analysis On the Performance of Coded Cooperation Systems,” Proc. IEEE Vehicular Technology Conference, Fall 2003, it has been shown that cooperating with a user having good channel quality to the destination always benefits the source. In such a situation, the partner may take advantage of its good channel quality and may choose a higher modulation mode such that the overall data rate to the destination can be increased.

Next, it is determined whether or not a higher modulation rate alone used by the partner increases throughput of the system or whether the selection of the modulation rate of the partner also depends on the source's SNR.

Without loss of generality, the case where M₁≦M₂ is considered first. The following sections investigate (a) the partner selects its modulation mode based only on its own channel quality, (b) the original user's channel quality also affects the partner's modulation rate choice and (c) the partner selects the best modulation rate depending on different ranges of SNRs. Finally, an analysis is presented of combining these three cases with the case of M₁>M₂ to determine the best modulation rate pair for the partnering users at the different ranges of SNR pairs for these two users such that the choice maximizes the throughput gain for the source. As M₁, M₂ε{1, 2, 4} and M₁≦M₂, we have six possible values of θ_M1. In the coded cooperative algorithm, the first half of the coded bits is sent by the source and is received by both the partner and the destination. If the coded bits are successfully decoded by the partner, the partner helps the original user transmit the other half of the coded bits. Otherwise, the source continues its own transmission. Therefore, in order to improve the successful decoding probability of the first half coded bits sent by the source, assume that the source's modulation mode is fixed to BPSK, i.e., M₁=1. Please note that the source may use another modulation rate if its channel quality to the destination is good. Also, when M₁=1, M₂ has more choices for its value under the condition M₁≦M₂. Using (4), when M₂=1, 2, 4 respectively results in

${\Theta }_{M_1}^1=\frac{1}{2},{\Theta }_{M_1}^2=\frac{2}{3},{\Theta }_{M_1}^3=\frac{4}{5}$ and we refer to G_Γⁿ as the corresponding throughput gain for the respective case with the value of θ_M1ⁿ, where n=1, 2, 3 corresponds to M₂=1, 2, 4, respectively. Now the problem becomes that for M₁=1 and given γ₁ and γ₂, how to select the best modulation rate, M₂, with the goal of choosing the largest value of G_Γⁿ. For comparing values of G_Γⁿ at given γ₁ and γ₂, since the channel qualities of the inter-user channel and the source to the destination channel are the same, P_f,1ⁱⁿ and P_f,1^QS for the source are unchanged. However, the value of P_f,1^BF,n is different due to the different value of M₂ where P_f,1^BF,n is the FER for the cooperative block fading channel with different value of M₂ and n=1, 2, 3 represents M₂=1, 2, 4, respectively. Therefore,

$\begin{array}{cc}{G}_{\Gamma }^n=2\frac{\left(1-P_f^{\mathrm{in}}\right)}{\left(1-P_{f,1}^{\mathrm{QS}}\right)}{\Theta }_{M_1}^n\left(1-P_{f,1}^{\mathrm{BF},n}\right)+P_{f,1}^{\mathrm{in}}& \left(6\right)\end{array}$ Comparing values of G_Γⁿ by using (6), the following conditions hold:

$\begin{array}{cc}1)G_T^1\underset{<}{>}{G}_T^2\iff P_{f,1}^{\mathrm{BF},2}\underset{<}{>}{\Lambda }_{12}\mathrm{where}{\Lambda }_{12}=\frac{1}{4}+\frac{1}{2}{P}_{f,1}^{\mathrm{BF},1}& \left(7\right)\\ 2)G_T^1\underset{<}{>}{G}_T^3\iff P_{f,1}^{\mathrm{BF},3}\underset{<}{>}{\Lambda }_{13}\mathrm{where}{\Lambda }_{13}=\frac{3}{8}+\frac{5}{8}{P}_{f,1}^{\mathrm{BF},1}& \left(8\right)\\ 3)G_T^2\underset{<}{>}{G}_T^3\iff P_{f,1}^{\mathrm{BF},3}\underset{<}{>}{\Lambda }_{23}\mathrm{where}{\Lambda }_{23}=\frac{1}{6}+\frac{5}{6}{P}_{f,1}^{\mathrm{BF},2}& \left(9\right)\end{array}$ For any given γ₁ and γ₂, comparing Λ₁₂, Λ₁₃, and Λ₂₃ we haveΛ₁₂<Λ₁₃  (10)It is shown in J. Proakis, Digital Communications, 4^th Edition, McGraw-Hill, New York, 2001 pp. 264-272, the entire contents of which is hereby included by reference, that higher modulation rate leads to higher error rate. Therefore,P_f,1^BF,1<P_f,2^BF,2<P_f,1^BF,3  (11)Combining inequalities (10) to (11) and conditions 1 to 3, we obtain the following results under the assumption that the original user uses BP SK modulation:1) If P_f,1^BF,3>Λ₁₃ and P_f,1^BF,2>Λ₁₂, G_T¹ is the largest, that is, throughput is maximized if the

partner uses BPSK modulation mode.

2) If P_f,1^BF,3>Λ₁₃ and P_f,1^BF,2<Λ₁₂, G_T² is the largest, that is, throughput is maximized if the

partner uses QPSK modulation mode.

3) If P_f,1^BF,3<Λ₁₃ and P_f,1^BF,2>Λ₁₂, G_T³ is the largest, that is, throughput is maximized if the

partner uses 16-QAM modulation mode.

4) If P_f,1^BF,3<Λ₁₃ and P_f,1^BF,2<Λ₁₂, G_T² is the largest, that is, throughput is maximized if the

partner uses QPSK modulation mode; otherwise G_T³ is the largest, that is, throughput is

maximized if the partner uses 16-QAM modulation mode.

The above results provide the means for determining the proper modulation rate used by the partner such that the data throughput of the source is optimized. They show that not only the partner's channel quality but also the source's channel quality affect the selection of the partner modulation rate. They also require that the source consider its partner's channel quality as well when the source selects its modulation rate, with the goal of maximizing its data throughput.

One skilled in the art will readily be able to extend the foregoing results to any modulation mode used by the original user. For example, if the source uses QPSK, result 4 can be applied, that is if P_f,1^BF,3>Λ₂₃, G_Γ² is the largest and the throughput is maximized if the partner uses QPSK modulation mode; otherwise, G_Γ³ is the largest and 16-QAM modulation mode selected by the partner brings the largest throughput to the original user. On the other hand, if the source has a higher modulation rate than the partner, the values of θ_M1ⁿ change. But, the above algorithm is used to determine the conditions under which the selected modulation rate used by the partner is the best, with the goal of maximizing the source's data throughput.

In this section, the discussion focuses on the situation when the source fixes its modulation mode as QPSK or 16-QAM and how a partner adapts its modulation mode such that the source's throughput is maximized. Similar to the case when the source uses BPSK, when the source uses QPSK, Λ₁₂, Λ₁₃, and Λ₂₃ become

${\Lambda }_{12}=\frac{1}{3}+\frac{2}{3}{P}_{f,1}^{\mathrm{BF},1},{\Lambda }_{13}=\frac{1}{2}+\frac{1}{2}{P}_{f,1}^{\mathrm{BF},1}\mathrm{and}{\Lambda }_{23}=\frac{1}{4}+\frac{3}{4}{P}_{f,1}^{\mathrm{BF},2}.$ For any given γ₁ and γ₂, we haveΛ₁₂<Λ₁₃  (12)andP_f,1^BF,1<P_f,1^BF,2<P_f,1^BF,3  (13)

Using inequalities (12) to (13) we obtain the following results under the assumption that the original user uses QPSK modulation:

1) If P_f,1^BF,3>Λ₁₃ and P_f,1^BF,2>Λ₁₂, G_Γ¹ is the largest, that is, throughput is maximized if the

partner uses BPSK modulation mode.

2) If P_f,1^BF,3>Λ₁₃ and P_f,1^BF,2<Λ₁₂, G_Γ² is the largest, that is, throughput is maximized if the

partner uses QPSK modulation mode.

3) If P_f,1^BF,3<Λ₁₃ and P_f,1^BF,2>Λ₁₂, G_Γ³ is the largest, that is, throughput is maximized if the

partner uses 16-QAM modulation mode.

4) when P_f,1^BF,3<Λ₁₃ and P_f,1^BF,2<Λ₁₂, if P_f,1^BF,3>Λ₂₃, G_Γ² is the largest, that is throughput is maximized if the partner uses QPSK modulation mode; otherwise G_Γ³ is the largest, that is 16-Qam modulation mode selected by the partner brings the largest throughput to the source.

When the source uses 16-QAM, Λ₁₂, Λ₁₃, and Λ₂₃ become

${\Lambda }_{12}=\frac{2}{5}+\frac{3}{5}{P}_{f,1}^{\mathrm{BF},1},{\Lambda }_{13}=\frac{3}{5}+\frac{2}{5}{P}_{f,1}^{\mathrm{BF},1}$ $\mathrm{and}$ ${\Lambda }_{23}=\frac{1}{3}+\frac{2}{3}{P}_{f,1}^{\mathrm{BF},2}.$ For any given γ₁ and γ₂, we still haveΛ₁₂<Λ₁₃  (14)Using inequalities (13) and (14) we obtain similar results when the source uses 16-QAM:1) If P_f,1^BF,3>Λ₁₃ and P_f,1^BF,2>Λ₁₂, G_Γ¹ is the largest, that is, throughput is maximized if the

partner uses BPSK modulation mode.

2) If P_f,1^BF,3>Λ₁₃ and P_f,1^BF,2<Λ₁₂, G_Γ² is the largest, that is, throughput is maximized if the

partner uses QPSK modulation mode.

3) If P_f,1^BF,3<Λ₁₃ and P_f,1^BF,2>Λ₁₂, G_Γ³ is the largest, that is, throughput is maximized if the

partner uses 16-QAM modulation mode.

4) when P_f,1^BF,3<Λ₁₃ and P_f,1^BF,2>Λ₁₂, if P_f,1^BF,3>Λ₂₃, G_Γ² is the largest, that is, throughput is maximized if the partner uses QPSK modulation mode; otherwise G_Γ³ is the largest, that is 16-QAM modulation mode selected by the partner brings the largest throughput to the source.

In this section two cases are discussed: the first case is when the partner fixes its modulation rate and how the source adapts its modulation mode, and the second case is both partnering users adapt their modulation modes simultaneously.

In these two cases, as the source changes its modulation mode P_fⁱⁿ, P_f,1^QS and P_f,1^BF are all changed. It is hard to just base on the throughput gain expression, G_Γ, to tell which modulation pairs used by the partnering users bring the highest throughput, . . . . However, we can proceed as follows: we can simulate the FERs for the cases when the partnering users use different modulation modes for any fixed γ₁ and γ₂ and then compute the throughput for these FERs. Then we compare the throughput values when the partnering users use different modulation modes and figure out what the modulation modes used by the users are such that the throughput is maximized for different γ₁ and γ₂. In the practical situation, the partnering users may base on the results obtained from simulation to choose the modulation modes to maximize the throughput for different received SNRs (γ₁ and γ₂).

In the following sections, numerical results for throughput gain are presented that illustrate how the source's channel qualities affect the data throughput gain, how much throughput gain can be obtained through cooperation, and what further improvement can be obtained by using adaptive modulation according to the system and method of the present invention. In order to simplify the presentation, in all the following simulation examples, only BPSK, QPSK and 16QAM modulation modes are considered and a [5, 7, 5, 7] convolutional code is used as the channel code.

Fixed Source Modulation Mode:

We assume perfect inter-user channel, that is P_fⁱⁿ=0. In all the examples, we fix γ₁ as −10 dB, −5 dB, 0 dB, 10 dB, 15 dB respectively and change γ₂. S₁ uses BPSK modulation mode for the non-cooperative communication. We assume perfect inter-user channel. FIGS. 3, 4, 5 show the throughput gain for S₁ when S₁ fixes its modulation mode as BPSK and S₂ uses BPSK, QPSK and 16-QAM modulation mode, respectively. We can determine from these three figures that with the increasing of the partner's SNR (γ₂), S₁'s throughput gain improves more and more for any fixed γ₁. This is consistent with Proposition 1. When S₁ has poor channel quality (low SNR regime), the throughput gain due to cooperation is the largest for any fixed γ₂. When S₁ uses BPSK and S₂ uses BPSK and QPSK, S₁ get FER benefits from cooperation in the examples provided in FIGS. 3, 4, and 5. We observe from FIGS. 3 and 4 that for any fixed γ₂, the throughput gain for S₁ is decreasing with γ₁. For the case that S₁ uses BPSK and S₂ uses 16-QAM, since 16-QAM leads to higher FER compared with BPSK and QPSK modulation modes, the original user only gets benefits from cooperation in terms of FED, i.e. P^coop<P^no-coop, when is high enough (>7.5 dB in this example. Therefore, we can observer from FIG. 5 that when γ₂>7.5 dB, which results in P^coop<P^no-coop, the throughput gain for S₁ is decreasing with γ₁. This matches Proposition 4. Again, we can observe from FIG. 5 that even when γ₂<7.5 dB, cooperating with S₂ still can improve S₁ a lot in terms of throughput when S₁ is in a low SNR situation (for example, γ₁=−10 dB, −5 dB). This illustrates the non-equivalence between FER gain and throughput gain when partnering users have different rates. In short, we conclude from these figures that cooperation improves the original user's throughput much more when

S₁ in a low SNR situation; if the partner uses higher modulation mode that the original user, the original user still can get benefits from cooperation in terms of throughput even when its FER is not improved by cooperation.

FIGS. 6-11 compare the data throughput gains of S₂ due to cooperation when S₂ uses BPSK, QPSK and 16-QAM, respectively, for the cases where γ₁=−10 dB, −5 dB, 0 dB, 5 dB, 10 dB and 15 dB. Observing these figures, we can find than when γ₁ is in a very low SNR situation, i.e., γ₁=−10 dB or −5 dB, for the low γ₂ values, BPSK modulation mode used by S₂ can lead to higher throughput gain of S₁. This is because that if both users are experiencing low SNR, lower modulation mode results in lower FER values compared with higher modulation mode. Again, for cases where γ₁=−10 dB or −5 dB, with the increasing of γ₂, cooperating with S₂ using QPSK modulation will bring the largest throughput gain to S₁; when γ₂ is high enough (i.e., γ₂>14 dB for γ₁=−10 dB and γ₂>13.5 dB for γ₁=−5 dB). 16-QAM used by S₂ gives S₁ the highest throughput gain. For the case of γ₁>0 dB, when γ₂=−11 dB, cooperating with S₂ using QPSK modulation bring the original user the highest throughput; when γ₂=11 dB, S₁ gets the largest throughput benefits from cooperation if S₂ uses 16-QAM. This is because although higher modulation mode can transmit more bits every time, when the original user has poor channel quality, the resulting FER from cooperating with the partner having higher modulation mode is very high, which results in lower successful transmission rate for every information bit. When γ₁ is large, for example γ₁=5 dB, 10 dB, 15 dB, we can observe from FIGS. 9-11 that 16-QAM used by the partner always brings the largest throughput gain to S₁ as long as γ₂ is not very low. The reason is that when partnering users have good enough channel qualities, the resulting FER doe to higher modulation mode is not very high anymore and hence, higher modulation mode used by the partner gives the original user higher throughput gain. As shown in these figures, we can find that with the goal of maximizing the throughput of the original user, if the partner adapts its modulation rate based on channel qualities, the throughput gain due to cooperation can be increased more and the selection of modulation mode by the partner depends not only on its own channel quality but also the original user's channel quality. Because cooperation involves two users and not only one, when the original user has very low SNR, higher modulation mode used by the partner cannot bring higher throughput gain to the original user; only when the original user has good enough channel quality higher modulation mode used by the partner can make cooperation result in higher throughput gain to the original user.

For the selection of modulation mode by the partner we may either base on the direct calculation of throughput gain or use the criteria of Results 1-4. For example, γ₁=−5 dB, when γ₂<3 dB, S₂ choosing BPSK, gives S₁ the largest throughput gain as shown in FIG. 7. FIG. 12 shows the values of P_f,1^BF for the cases that S₁ uses BPSK and S₁ uses QPSK and 16-QAM, respectively, and the threshold values of Λ₁₂, Λ₁₃, and Λ₂₃. For γ₂<3 dB, P_f,1^BF,2 is larger than Λ₁₂ and P^BF,3 is larger than Λ₂₃. Based on Result 1 BPSK used by S₂ brings the largest benefits to S₁ in terms of throughput. For 3 dB<γ₂<10.5 dB, P_f,1^BF,2<Λ₁₂ and P^BF,3>Λ₁₃, using Result 2 we obtain that S₁ chooses S₂ with QPSK to get the largest throughput gain. For 10.5 dB<<13.5 dB, P_f,1^BF,2<Λ₁₂, P^BF,3<Λ₁₃ and P^BF,3>Λ₂₃. Therefore, using Result 4 we find that S₂ should select QPSK to maximize the throughput gain for S₁. When γ₂>13.5 dB, Λ₁₃>P_f,1^BF,3,Λ₁₂>P_f,1^BF,2, and Λ₂₃>P_f,1^BF,3, using Result 4, S₂ selects 16-QAM to maximize the throughput for S₁. All these thresholds of γ₂ match the results shown in FIG. 7.

The following sections address the choice of partner by a source, i.e., how to choose a best partner among a list of candidates such that the data throughput of the source can be improved most by cooperating with the partner. Also presented in the following sections is an illustration of how the source's channel quality affects the partner choice. Without loss of generality, consider a scenario where the possible partners are classified into two groups, one group has very good channel quality to the destination, but low inter-user SNR, the other group has a very good inter-user channel quality, but the channel to the destination does not have good quality. Such a scenario is depicted in FIG. 13.

Assume that partners already use the best modulation such that the throughput gain is the largest when the individual partner and the source cooperate. Here S₂ represents the partner with good quality inter-user channel (e.g. S₂ could be close to S1) and similar channel quality to the destination as the source, S₁, and S₃ represent the partner with good channel to the destination (e.g. S₃ is close to the destination). Note that cooperation with S₂ results in two level diversity and cooperation with S₃ always helps the source improve the throughput significantly, as illustrated in the foregoing numerical examples. Therefore, it is of interest to find which effect dominates and whether the source's channel quality affects its partner's selection of modulation rate. The following numerical example illustrates the partner choice problem.

Path loss effect is incorporated with flat Rayleigh fading in the following example. As illustrated in FIGS. 14 and 15, where is D1 fixed at 1.0 and the angle between S₁ and S₂ as

$\frac{\pi }{6}\mathrm{and}\frac{\varepsilon }{N_0}$ is assumed to be 0 dB and 5 dB, respectively we obtain the throughput gains for the different distances between the partner and the destination. When the distance between the partner and the destination is smaller, which means the partner is close to the destination but further away from the source, the inter-user channel between the two partnering users is worse and hence, P_fⁱⁿ is higher. Assume that D2=0.7 and D3=0.1. As shown FIGS. 14 and 15, for

$\frac{\varepsilon }{N_0}=0\mathrm{dB},$ 16QAM is the best modulation rate for use by S₃ to achieve the maximum throughput gain for S₁ when S₁ cooperates with S₃. QPSK is the best modulation rate used by S₂ such that the throughput gain for S₁ due to cooperation between S₁ and S₂ is maximized. In this situation, S₁ chooses S₃ rather than S₂ to achieve greater cooperation gain as the better user can help S₁ more when S₁ is not experiencing poor channel quality to the source. However, for

$\frac{\varepsilon }{N_0}=-5\mathrm{dB},$ which implies that the source has very bad link quality to the destination, the source prefers S₂ because S₂ uses a lower modulation rate than S₃ during cooperation. When the source is already in experiencing poor channel quality to the destination, the source chooses the partner that can help the source achieve a lower FER value rather than the partner that has a higher data rate. From these two examples it follows that when there is a list of candidates that use different modulation modes to maximize the throughput gain for the source, the source's channel quality affecting its best partner selection such that its throughput gain can be improved most.

The system and method of the present invention provide adaptive modulation for at cooperating users in coded cooperative systems to optimize the throughput for a source. The throughput gain due to cooperation has been defined in terms of the conditions under which cooperation improves the data throughput of the source. Channel qualities have been demonstrated to affect the throughput gain due to cooperation. For the case of fixed source modulation mode and variable partner modulation mode, a method has been provided for selecting a partner's modulation rate based on two operating users' channel qualities conditions 1-4. Cooperation improves the data throughput for the source and when the adaptive modulation of the present invention is used by the cooperating users, throughput can be further increased.

The present invention also provides a way for a source to select a partner among a plurality of available partners, by having the selected partner relaying information for the source such that the throughput gain of the source due to cooperation with the selected partner is the highest achievable of that which could be achieved by partnering with each of the available partners.

In the following examples, the path loss effect in each link is considered and numerical results of throughput performance are presented for direct transmission, multi-hop and coded cooperation. The examples illustrate how the users channel quality affects the data throughput gain due to cooperation and multi-hop and how much throughput gain can be obtained. We denote D₁ and D₂ as the distances between S₁ and the destination, S₂ and the destination respectively, and let D_in be the distance between S₁ and S₂.

The path loss component, α is 4. Assume that the normalized distance D₁=1.0, D₂=0.6 and D_in=0.57. Hence, for the direct transmission and cooperative transmission, the received SNR at the destination from S₁ is

${\gamma }_1=\frac{\varepsilon }{N_0}+10{\log }_{10}\left(D_1^{-\alpha }\right)=\frac{\varepsilon }{N_0}$ and the received SNR from S₂ at the destination is

${\gamma }_2=\frac{\varepsilon }{N_0}+10{\log }_{10}\left(D_2^{-\alpha }\right)=\frac{\varepsilon }{N_0}+8.87\left(\mathrm{dB}\right).$ However, for the multi-hop, the transmitter uses only half transmitted energy, the received SNR at S₂ from S₁ is

${\gamma }_{m,in}=\frac{\varepsilon }{N_0}+10{\log }_{10}\left(0.5\right)+10{\log }_{10}\left(D_{in}^{-\alpha }\right)=\frac{\varepsilon }{N_0}+6.87\left(\mathrm{dB}\right)$ and the received SNR atthe destination from S₂ is

${\gamma }_{m,i2}=\frac{\varepsilon }{N_0}+10{\log }_{10}\left(0.5\right)+10{\log }_{10}\left(D_{in}^{-\alpha }\right)=\frac{\varepsilon }{N_0}+5.86\left(\mathrm{dB}\right).$

FIG. 16 illustrates how the throughput of direct transmission changes as a function of SNR and modulation mode. As illustrated in FIG. 16, among BPSK, QPSK and 16QAM, when the received SNR is below 0 dB, direct transmission with BPSK modulation has the highest throughput. When the received SNR is 0 dB-6 dB, QPSK is the preferred modulation preferred. Finally, when the received SNR is higher than 6 dB, 16QAM modulation is the best choice.

FIG. 17 illustrates the normalized throughput (with respect to R_s) of direct transmission, multihop and cooperative scheme. FIG. 4 shows that coded cooperation with adaptive modulation leads to much higher throughput than either multi-hop transmission with adaptive modulation or direct transmission with adaptive modulation. The comparison of the throughput value of cooperation with that of direct transmission for this example leads to the result that cooperation provides at least a 20% gain of cooperation over direct transmission. When SNR is below 4 dB, the gain is as much as 100%. The sources of gain include diversity at the receiver, cooperative channel coding and multi-hop.

The gain solely due to multi-hop is also illustrated in FIG. 17. While multi-hop transmission with adaptive modulation is inferior to cooperation it provides superior throughput over direct transmission with adaptive modulation for low to medium SNR. But, when SNR is sufficiently high (about 6 dB in this example), the throughput of direct transmission is higher than that of multi-hop transmission. There are two main reasons for this behavior. First, the highest modulation mode is 16 QAM, and, therefore, even when the received SNR in every hop is high enough, no higher order modulation can be used. Second, when SNR increases, the FER of direct transmission also decreases, resulting in a smaller difference in FER between direct transmission and each multiphop transmission. In other words, the path loss does not have a significant effect in FER performance when SNR is high enough. As a result, multi-hop transmission does not have any advantage over direct transmission in terms of throughput for high SNR.

From FIG. 4, it follows that the optimum modulation rate per hop is determined by the channel qualities of each hop, i.e., S₁-to-S₂ and S₂-to-destination separately. When γ_m,12 is from −4+6.86=2.86 dB to −2+6.86=4.86 dB and γ_m,2 is from −4+5.86=1.86 dB to −2+5.86=3.86 dB, S₁ and S₂ both select QPSK to maximize throughput. As SNR increases, i.e., for

$\frac{\varepsilon }{N_0}>0\mathrm{dB},$ 16 QAM becomes the best modulation choice. Unlike multi-hop, in coded cooperation, the throughput is maximized when S₁ and S₂ jointly adapt their modulation modes. For example, when γ₁=−2 dB and γ₂=6.87 dB, if S₁ and S₂ just base their modulation choices on their respective channel qualities to the destination, they would choose BPSK and 16 QAM. However, as illustrated in FIG. 4, both choosing QPSK provides the highest throughput. When γ₁=4 dB and γ₂=12.87 dB, S₁ and S₂ respectively choosing 16QAM and BPSK provides the highest throughput.

Results for D₁=1.0, D₂=0.2 and D_in=0.69 are illustrated in FIG. 18, where the throughput gains due to cooperation are comparable to those of FIG. 17, even though the inter-user channel between S₁ and S₂ is worse. Note, however, that since the relay is close to the destination it always uses the highest order modulation, 16QAM, in both cooperative and multi-hop modes.

FIG. 19 illustrates an apparatus 1900 for selecting a partner and adapting modes of the source and the partner comprising an antenna 1901 connected to a transmitter 1902 and a receiver 1903 for a source to send and receive messages, respectively, to and from candidate partners/relays. The messages received by the receiver 1903 are processed by a partner selection and modulation mode adaptation module 1904 which determines the quality of the channels and the improvements in throughput gain possible by partnering with candidate partners. The modulation mode adaptation module determines the modulation modes for the source and candidate partners required to realize the improvements in throughput gain, as well. The module 1904 selects the candidate partner/relay and source and candidate modulation modes that provide the best improvement based on the calculations of equations (10) through (14) and their associated decision criteria discussed above, which are then transmitted to the candidate by the transmitter 1902. However, in the event that closed form equations cannot readily be derived, then simulation provides the conditions and modes for selecting a candidate.

While the preferred embodiments of the present invention have been illustrated and described, it will be understood by those skilled in the art, the system and method for adaptive modulation architecture in a coded cooperative wireless communication systems as described herein are illustrative and various changes and modifications may be made and equivalents may be substituted for elements thereof without departing from the true scope of the present invention. In addition, many modifications may be made to adapt the teachings of the present invention to a particular situation without departing from its central scope. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out the present invention, but that the present invention include all embodiments falling with the scope of the appended claims.

Claims

1. At least one candidate partner/relay of a source in a wireless coded cooperative communication system, having an adaptable modulation mode and being adapted to perform a pre-determined coded cooperative communication protocol to forward signals received from the source to a destination, wherein, based on the channel conditions of a pair consisting of the source and the at least one candidate, the source chooses the candidate and a corresponding pair of modulation mode settings consisting of a setting for each of the candidate modulation mode for transmission between the at least one candidate partner/relay and the destination and the source modulation mode that provide a throughput gain for transmission between the source and the candidate partner/relay, wherein the throughput gain that is improved more than the throughput gain of any other choice for the candidate and the corresponding pair of modulation mode settings.

2. The at least one candidate partner/relay of a source of claim 1, wherein the throughput gain that is improved more than any other is determined by: when the source is experiencing poor channel quality to the destination, the source selects the candidate and the corresponding pair such that a frame error rate (FER) of the source is lowered; and otherwise, the source selects the candidate and the corresponding pair with a greatest throughput gain of a set of throughput gains consisting of all throughput gains resulting from the source cooperating with each said at least one candidate partner/relay to forward said signals from the source to the destination.

3. The at least one candidate partner/relay of a source of claim 2, wherein selecting the candidate and the corresponding pair with a greatest throughput gain is achieved by performing the steps of: computing a set consisting of 3-tuples each including a candidate partner/relay, corresponding pair of modulation modes, and throughput gain resulting from the source cooperating with the candidate to forward signals from the source to the destination using the corresponding pair of modulation modes, andselecting from the computed set the 3-tuple having a greatest throughput gain.

4. The at least one candidate partner/relay of a source of claim 2, wherein each throughput gain of the set is calculated based on a corresponding modulation mode pair selected from the group consisting of: a. the at least one partner/relay adapts its modulation mode but the source keeps its own modulation mode constant;b. the source adapts its modulation mode and the at least one partner/relay fixes its modulation mode; andc. the source adapts its modulation mode at the same time as the at least one partner/relay adapts its modulation mode.

5. The at least one candidate partner/relay of a source of claim 4, wherein each mode of said corresponding pair is selected from the group of modes consisting of binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), and 16-state quadrature amplitude modulation (16-QAM).

6. The at least one candidate partner/relay of a source of claim 5, wherein for case ‘a’ if the modulation mode of the source is fixed and is BPSK and the throughput gain is GΓn for n=1,2,3, n representing the modulation mode of the partner and Γ indicating that the gain G relates to a throughput Γ, the best modulation rate for a partner/relay is determined by the calculations: Λ12, Λ13, Λ23 being threshold values of a frame error rate, defined as

7. The at least one candidate partner/relay of a source of claim 5, wherein, for case a if the modulation mode of the source is fixed and is QPSK and the throughput gain is GΓn for n=1,2,3, n representing the modulation mode of the partner and Γ indicating that the gain G relates to a throughput Γ, the best modulation rate for a partner/relay is determined by the calculations: Λ12,Λ13, Λ23 being threshold values of a frame error rate, defined as

8. The at least one candidate partner/relay of a source of claim 5, wherein, for case ‘a’ if the modulation mode of the source is fixed and is 16-QAM and the throughput gain is GΓn for n=1,2,3, n representing the modulation mode of the partner and Γ indicating that the gain G relates to a throughput Γ, the best modulation rate for a partner/relay is to determined by the calculations:

9. The at least one candidate partner/relay of a source of claim 5, wherein for cases ‘b’ and ‘c’ the modulation mode pair is selected from a pre-determined set of pairs, based on different received signal-to-noise ratios (SNRs) (γ1 and γ2) for the source and destination, respectively.

10. The at least one candidate partner/relay of a source of claim 9, wherein said pre-determined set of pairs is obtained by simulation of modulation mode pairs for different received SNRs (γ1 and γ2).

11. The at least one candidate partner/relay of a source of claim 1, wherein said signals comprise at least part of a message sent by the source to the destination.

12. A source in a wireless coded cooperative communication system, having an adaptable source modulation mode, the source adapted to select a at least one candidate partner/relay and a corresponding pair of modulation mode settings consisting of a setting for each of the candidate modulation mode and the source modulation mode that provide a throughput gain that is improved more than the throughput gain of any other choice for the at least one candidate and the corresponding pair of modulation mode settings.

13. The source of claim 12, wherein the throughput gain that is improved more than any other is determined by: when the source is experiencing poor channel quality to the destination, the source selects the at least one candidate and the corresponding pair such that a frame error rate (FER) of the source is lowered; and otherwise, the source selects the at least one candidate and the corresponding pair with a greatest throughput gain of a set of throughput gains consisting of all throughput gains resulting from the source cooperating with each of said at least one candidate partner/relay to forward signals from the source to the destination.

14. The source of claim 13, wherein said signals comprise at least part of a message sent by the source to the destination.

15. The source of claim 12, wherein selecting the at least one candidate and the corresponding pair with a greatest throughput gain is achieved by performing the steps of: computing a set consisting of 3-tuples each including a candidate partner/relay, corresponding pair of modulation modes, and throughput gain resulting from the source cooperating with the at least one candidate to forward signals from the source to the destination using the corresponding pair of modulation modes, andselecting from the computed set the 3-tuple having a greatest throughput gain.

16. The source of claim 15, wherein each throughput gain of the set is calculated based on a corresponding modulation mode pair selected from the group consisting of: a. the at least one partner/relay adapts its modulation mode but the source keeps its own modulation mode constant;b. the source adapts its modulation mode and the at least one partner/relay fixes its modulation mode; andc. the source adapts its modulation mode at the same time as the at least one partner/relay adapts its modulation mode.

17. The source of claim 16, wherein, for case ‘a’ if the modulation mode of the source is fixed and is QPSK and the throughput gain is GΓn for n=1,2,3, n representing the modulation mode of the partner and Γ indicating that the gain G relates to a throughput Γ, the best modulation rate for a partner/relay is determined by the calculations: Λ12, Λ13, Λ23 being threshold values of a frame error rate, defined as

18. The source of claim 16, wherein, for case ‘a’ if the modulation mode of the source is fixed and is 16-QAM and the throughput gain is GΓn for n=1,2,3, n representing the modulation mode of the partner and Γ indicating that the gain G relates to a throughput Γ, the best modulation rate for a partner/relay is to be determined by the calculations:

19. The source claim 16, wherein for cases ‘b’ and ‘c’ the modulation mode pair is selected from a pre-determined set of pairs, based on different received signal-to-noise ratios (SNRs) (γ1 and γ2) for the source and destination, respectively.

20. The source claim 19, wherein said pre-determined set of pairs is obtained by simulation of modulation mode pairs for different received SNRs (γ1 and γ2).

21. The source of claim 12, wherein each mode of said corresponding pair is selected from the group of modes consisting of binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), and 16-state quadrature amplitude modulation (16-QAM).

22. The source of claim 21, wherein for case ‘a’ if the modulation mode of the source is fixed and is BPSK and the throughput gain is GΓn for n=1,2,3, n representing the modulation mode of the partner and Γ indicating that the gain G relates to a throughput Γ, the best modulation rate for a partner/relay is determined by the calculations: Λ12, Λ13, Λ23 being threshold values of a frame error rate, defined as