peng wang, city university of hong kong, jun xiao

Multiple access (MA) is a basic function inwireless cellular systems. Generally speak-ing, MA techniques can be classified intoorthogonal and non-orthogonal approach-

es. In orthogonal approaches, signals from differentusers are orthogonal to each other, i.e., their crosscorrelation is zero, which can be achieved by time-division multiple-access (TDMA), frequency-divisionmultiple-access (FDMA) and orthogonal-frequency-division multiple-access (OFDMA). Non-orthogonalschemes allow non-zero cross correlation among thesignals from different users, such as in randomwaveform code-division multiple-access (CDMA) [1],trellis-coded multiple-access (TCMA) [2] and inter-leave-division multiple-access (IDMA) [3].

First and second generation cellular systems aredominated by orthogonal MA approaches (with theexception of the American IS-95). The main advan-tage of these approaches is the avoidance of intra-cell interference. However, careful cell planning isnecessary in these systems to curtail cross-cellinterference. In particular, sufficient distance mustexist between re-used channels, resulting inreduced cellular spectral efficiency.

Non-orthogonal CDMA techniques have beenadopted in second and third generation cellular sys-tems (e.g., IS-95, CDMA2000 and uplink WCDMA).Compared with its orthogonal counterparts, CDMA

is more robust against fading and cross-cell interfer-ence, but is prone to intra-cell interference. Due toits spread-spectrum nature, CDMA is inconvenientfor data services (e.g., wireless local area networks(WLANs) and 3GPP high speed uplink/downlinkpacket access (HSUPA/HSDPA) standard) thatrequire high single-user rates.

This article presents a comparative study oforthogonal and non-orthogonal MA schemes. We willillustrate that non-orthogonal approaches have aspectral-power efficiency advantage over orthogonalones for delay-sensitive applications in fading envi-ronments. This advantage is referred to as multi-usergain (MUG) below. We will also illustrate that MUG isrealizable in practice by exploiting recent progressin interleave-division multiple-access (IDMA) and therelated low-cost chip-by-chip (CBC) multi-user detec-tion (MUD) algorithm. The gain is significant forhigh-rate applications.

It is envisaged that delay-sensitive applications(such as speech and video services) will contributea considerable part of the traffic demand in thefuture, and so the discussion in this article mayprovide useful guidelines on the evolution path forfuture wireless cellular systems.

Our focus is mainly on uplink multiple accesschannels (MACs), but most findings in this article canbe applied to downlink broadcast channels (BCs)

Abstract: This article provides a comparativestudy of different multiple access techniques. It isdemonstrated that non-orthogonal approaches havea spectral-power efficiency advantage over orthogo-nal ones for delay-sensitive applications in fading

environments, and that this theoretical advantagecan be realized in practice by exploiting recentprogress in transmission and detection techniques.The practical aspects of these multiple access tech-niques are also discussed and compared.

4 |||| 1556-6072/06/$20.00©2006IEEE IEEE VEHICULAR TECHNOLOGY MAGAZINE | SEPTEMBER 2006

Peng Wang, City University of Hong Kong, Jun Xiao, University of ElectronicScience and Technology of China, and Li Ping, City University of Hong Kong

Authorized licensed use limited to: Stanford University. Downloaded on January 13, 2010 at 18:42 from IEEE Xplore. Restrictions apply.

based on the uplink-downlink duality principle [4]. Thussignificant gain is available for both uplink and downlinkapplications, which deserves further research effort.

Spectral-Power Efficiency of MA Systems

Spectral-power efficiency is one of the fundamental topicsof information theory. The limits on spectral-power effi-ciency are usually expressed in two equivalent forms:

i. The maximum information rate per unit spectrum at agiven power level;

ii. The minimum power to support a given rate (or a setof rates for a multi-user system) per unit spectrum. The discussion in this article is from the power mini-mization point of view.

Information theory indicates that orthogonal and non-orthogonal alternatives have the same fundamental limitsin MACs where the only distortion is caused by additivewhite Gaussian noise (AWGN). However, if fading is alsoinvolved, the conclusion can be very different [4].

Communication services can be classified intodelay-sensitive and insensitive ones. A typical exampleof a delay-insensitive service is email. Typical exam-ples of delay-sensitive services include speech andvideo applications. For delay-insensitive services, rateconstraints are relatively relaxed for individual usersand maximizing the throughput by orthogonal meth-ods1 is a common strategy. For example, consider apower limited system over a static fading MAC with asingle antenna at each transmitter. The maximumthroughput can be achieved by a one-user transmis-sion policy, where only the user with the largest chan-nel gain is allowed to transmit. This implies timedomain orthogonality as adopted in many WLANs. Fordelay-sensitive services, on the other hand, each user

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1This is not true in multiple-input multiple-output (MIMO) systems. In gen-eral, non-orthogonal schemes are necessary to achieve MIMO channelcapacities [4]. Since the discussion of MIMO systems is rather involved,we will not include this topic in the present paper.


must transmit a certain amount of information withina certain period and maximizing the throughput is nolonger an appropriate strategy. Rate constraints mustbe considered in this case.

Assume that a user can transmit at different instanta-neous rates and instantaneous power levels in differenttime slots. (The instantaneous rate and power level areboth zeros in an idle period). Average rate and averagepower are the arithmetic means of instantaneous ratesand instantaneous power levels over a certain period,respectively. For example, let a user transmit informa-tion in a time slot with duration T1 at an instantaneousrate r1 and an instantaneous power level p1. It is idle inthe next time slot with duration T2. Then its averagerate is r1T1/(T1 + T2) over the two time slots consid-ered, and the corresponding average power level isp1T1/(T1 + T2) . Throughput and average-sum-powerrefer to the sums of the average rates and averagepower levels of the two users, respectively. These defin-itions can be directly generalized to a multi-user systemwith an arbitrary number of users.

Next, for the purpose of illustration, we consider anMA system involving only two users with the samerequired average rate R. The distortion is caused byAWGN with power density N0 and fading coefficients h1

and h2 for user 1 and user 2, respectively. We assume thath1 and h2 are known at both the transmitters and thereceiver and remain unchanged within a frame.

For orthogonal MA techniques, we consider two TDMAoptions: fixed TDMA and adaptive TDMA, as explainedbelow. In both cases, each frame is divided into two timeslots for two users. Their instantaneous and averagetransmitted power levels are denoted by p1, p2, P1 and P2,respectively.

■ With fixed TDMA, the lengths of the two time slots arethe same. As a consequence, the correspondinginstantaneous rates are also identical.

■ With adaptive TDMA, the instantaneous rates and slotlengths are optimized based on channel conditions(see discussion below). They can be different for thetwo users and vary frame by frame.Figure 1 illustrates the theoretical average transmit-

ted power regions achievable by fixed TDMA, adaptiveTDMA and CDMA. For example, any point (P1, P2) in theshadowed area represents a feasible pair of averagetransmitted power levels using fixed TDMA and the min-imum average-sum-power is achieved at the cornerpoint A. Similarly, the areas on the up-right of the othertwo curves represent the sets of valid power levels foradaptive TDMA and CDMA, respectively. The detailedcalculation of these limits is explained below.

Fixed TDMAFor each user, the instantaneous rate in this case is con-strained to be 2R to achieve the average rate R. From theShannon capacity formula (i.e., C = log2(1+S N R)) andchannel inversion principle, the minimum instantaneoustransmitted power level for each user is

pk =(

22R − 1)

N0

|hk|2, k = 1, 2. (1)

The corresponding average power levels arePk = pk/2,k = 1, 2.

Adaptive TDMALet the instantaneous rates constraints be r1 and r2 forusers 1 and 2, respectively. The time slot lengths t1 and t2for the two users are then constrained byr1t1/(t1 + t2) = r2t2/(t1 + t2) = R . The minimum instanta-neous power level for each user is given by

pk =(2rk − 1

)N0

|hk|2, k = 1, 2. (2)

The average power levels are Pk = tkpk/(t1 + t2),k = 1, 2. The average-sum-power P1 + P2 = (t1p1 + t2p2)/

(t1 + t2) can be minimized through a Lagrange multipliertechnique [4], as indicated by the point B in Figure 1.

CDMAIn this case, both users have the same instantaneous rateconstraint R. Assume |h1|2 ≤ |h2|2 and consider the suc-cessive cancellation principle [4]:■ User 2’s signal is decoded first by treating user 1’s sig-

nal as additive noise. ■ User 1’s signal is decoded after stripping off the esti-

mated signal of user 2 from the received signal.The minimum average-sum-power is achieved at point

C in Figure 1 with the following power levels.

FIGURE 1 The theoretical average transmitted power region for atwo-user MA system with a fixed average rate R for each user. A, B and C indicate the points where the minimum transmittedaverage-sum-powers are achieved for fixed TDMA, adaptive TDMAand CDMA, respectively.

Power regionachievable by CDMA

Power region achievableby adaptive TDMA

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P1 = p1 = (2R − 1)N0

|h1|2and

P2 = p2 = (2R − 1)2R N0

|h2|2. (3)

Generally speaking, for an MA system with an arbitrarynumber of users, different decoding orders will lead todifferent average-sum-power, and the minimum average-sum-power is achieved by decoding the signal of the userwith larger channel gain first [5].

The better performance of adaptive TDMA over fixedTDMA is evident in Figure 1, and has motivated the recentresearch interest in adaptive modulation. Similar adap-tive principles can also be applied to FDMA/OFDMA sys-tems. For example, for a two-user OFDMA system, we candevise an adaptive scheme by dividing the spectrum forthe two users according to their channel conditions. Suchadaptive schemes lead to power saving compared withfixed FDMA/OFDMA in which each user occupies equalhalf of the total spectrum.

A heuristic reason for the advantage of CDMA overTDMA is the lower instantaneous rate for CDMA. As weknow from the Shannon capacity formula, the requiredtransmitted power grows exponentially with the increasedcoding rate R. Thus by comparing Eqns. (1) and (3) wecan see that lower rate for user 1 in CDMA implies powerreduction compared with that in TDMA. On the otherhand, interference cancellation for CDMA causes powerincrease for user 2 since the first decoded user requiresmore power. When |h1|2 = |h2|2 , these two oppositeeffects (power reduction for user 1 and power increase foruser 2) cancel each other exactly. When |h1|2 < |h2|2, how-ever, the power saving for user 1 surpasses the powerincrease for user 2. In this case, CDMA can take advantageof the difference between |h1|2 and |h2|2 and achieve high-er power efficiency than TDMA. Note that this advantageis attainable only by MUD.

The minimum average-sum-power for CDMA withMUD is also the theoretical limit [4] on the average-sum-power for all possible transmission schemes. Werefer to this as the “theoretical power limit” below. Ingeneral, when |h1|2 �= |h2|2, the theoretical power limitcan only be achieved by non-orthogonal schemes suchas CDMA [1] and IDMA [3] and cannot be achieved byorthogonal ones [4]. Therefore non-orthogonalschemes have a power efficiency advantage for delay-sensitive applications in fading channels. A similarstatement can also be made for more general channelsas discussed below.

As a remark, FDMA/OFDMA and orthogonal CDMAhave the same disadvantage of reduced coding gain asTDMA. In principle, an orthogonal scheme divides theentire signal space into orthogonal sub-spaces and thesignal of each user is restricted within a sub-space. This

implies reduced degrees of freedom for each user’s signaland so reduced coding gain. Therefore, generally speak-ing, orthogonal schemes cannot achieve the channelcapacities in fading environments [4].

Single Cell Uplink Scenario

We now consider more general uplink channel condi-tions. We first ignore Rayleigh fading and focus on pathloss that produces the so-called “near-far effect”. To bemore practical, we will also include lognormal fading. Wewill show that CDMA is favorable in transmission environ-ments with the near-far effect. We emphasize that MUD isassumed for CDMA in this section.

Let K be the number of users to be supported. Weassume that all users are uniformly distributed in a singlehexagon cell. We normalize the path loss factor from thefarthest corner of the cell to the base station to unity andthe path loss factors in other positions in the cell arecomputed proportionally using the fourth power loss law.The channel gain for user-k is then given by|hk|2 = A · d−4

k 10ξk/10, where dk is the normalized distancefrom user-k to the base station, ξk is a real Gaussian ran-dom variable with zero mean and variance σ 2

s (σs = 8 inthis article) that characterizes lognormal fading, and thenormalization factor A is used to ensure that the mean ofthe lognormal fading part equals 1. At the base station,the received signal is distorted by a complex AWGN withpower density N0 = 1. We allow an outage probability of0.01, i.e., user-k will not transmit when |hk|2 < Gout withPr(|hk|2 < Gout) = 0.01.

As mentioned above, only non-orthogonal schemes(such as CDMA with MUD) can achieve the theoreticalpower limits of fading channels. However, the perfor-mance of different MA schemes is quite close when thethroughput Rs = K R is relatively low (e.g., 2 bits/symbolor less). The difference becomes more significant whenRs increases. This is illustrated in Figure 2 for Rs = 2, 4and 8 bits/symbol. The curves are produced using meth-ods similar to those in the last section, taking averagesover the distribution of {|hk|2}. (Eqns. (1)-–(3) can be easi-ly generalized to systems with an arbitrary number ofusers [4].) The theoretical power limits are computedbased on CDMA with MUD.

Some comments regarding Figure 2 are in order.■ Power saving can be achieved by adaptive TDMA and

CDMA relative to fixed TDMA. We refer to this poweradvantage as multi-user gain (MUG). This gain is, inprinciple, similar to the performance advantage ofopportunistic techniques [4]2, although here only asingle antenna at each transmitter is considered. (Weconsider multiple antennas later).


2A subtle difference between the multi-user gain in this article and themulti-user diversity gain discussed in [4] for opportunistic techniques isthat the later is focused on the advantage of adaptive TDMA over fixedTDMA.


■ As we explained in the last section, MUG is achievedfor CDMA by taking advantage of the near-far effect.

■ MUG increases when the number of users K or thethroughput Rs increases.

■ A significant portion of MUG is achievable with a smallnumber of users (such as 2 or 4).

■ The theoretical power limits can only be achieved bynon-orthogonal schemes. MUD [6] is necessary toachieve these limits [4].Clearly, the potential MUG is significant and very

attractive. For example, at Rs = 8 bits/symbol and K = 8,about 11 dB gain is available compared with fixed TDMA.Such a large power saving offers considerable perfor-mance improvements in future wireless systems.

It can be shown that similar MUG is also available inother (such as Rayleigh) fading environments. As anexample, let us include Rayleigh fading and consider thesituation when the base station is equipped with tworeceive antennas. A single transmit antenna is still usedby each user. The channel coefficients from a given userto different receive antennas have the same path lossand lognormal fading factor but different independentlyand identically distributed (i.i.d.) Rayleigh fading fac-tors, then the fading coefficient from user-k to the i-thantenna at the base station is given byhk,i = (Ad−4

k 10ξk/10)1/2χk,i , where χk is a complex Gauss-ian variable with zero-mean and unit variance. The otherconditions are as in Figure 2. The theoretical limits arecomputed based on the methods outlined in [7]. Theresults are shown in Figure 3 where the MUG is clearlyseen. The relative differences between different strate-gies become even larger than those in Figure 2 (e.g., at

Rs = 8 bits/symbol and K = 8, about 14 dB gain is avail-able compared with fixed TDMA).

Although Figures 2 and 3 are for systems with equalaverage rates for all users, the similar observation can bemade if different users have different average rates [4]. Ageneral conclusion is that non-orthogonal schemes havehigher power efficiency than orthogonal ones for delay-sensitive applications in fading channels. [4]

The theoretical power limits achieved by CDMA aboveimplicitly involve MUD [6] that resolves the intra-cellinterference problem. With MUD, the CDMA performanceis no longer interference-limited, at least theoretically [8].Later we will show that this theoretical assertion can beturned into reality using recently developed iterativeMUD techniques.

Single Cell Downlink Scenario

The discussion so far is for uplink MACs, but the resultsshown in Figures 2 and 3 are also applicable to downlinkBCs based on the uplink-downlink duality principle [4].(Note that the dual of Figure 3 is a downlink broadcastsystem with two transmit antennas at the base station.)Again, MUD is necessary if we want to fully exploit theadvantage of MUG in downlink BCs. However, MUD in thedownlink can raise serious concern in practice due to thehigh receiver cost at mobile units.

With the low-cost CBC detection technique for IDMA,the total receiver complexity increases only linearly withthe number of users K [3]. An interesting observationfrom Figures 2 and 3 is that a major portion of MUG can beachieved with a very small number of users, such asK = 2 (i.e., by applying MUD to only 2 users). For exam-

FIGURE 3 The transmitted average-sum-power versus the numberof users K for a multiple access system with different MA tech-niques. The base station is equipped with two antennas. The chan-nel coefficients for each user consist of path loss, lognormal fadingand Rayleigh fading. All users have the same average rate and areassumed to be uniformly distributed in a normalized single hexa-gon cell with edge length 1.

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Rs = 8 bits/symbol

Rs = 4 bits/symbol

Rs = 2 bits/symbol

FIGURE 2 The transmitted average-sum-power versus the number of users K for a multiple access system with different MAtechniques. The channel coefficient for each user consists of pathloss and lognormal fading. All users have the same average rateand are assumed to be uniformly distributed in a normalized singlehexagon cell with edge length 1.

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ple, at K = 2 in Figure 3, an optimalscheme can achieve about 4 dB and 9 dBpower reduction at Rs = 4 and 8 bits/sym-bol compared with fixed TDMA, respec-tively. The MUD cost for K = 2 is onlymodestly higher than the correspondingsingle user detection cost for TDMA (e.g.,double3). The question is thus whetherthe gain is worth the cost. Recall that a turbo code canprovide about 3dB more coding gain than a convolutionalcode at more than double decoding cost. It seems, there-fore, that the potential gain shown in Figures 2 and 3 canjustify the complexity increase for introducing MUD indownlink applications.

Multiple Cells Scenario

Assume that cross-cell interference is treated as additivenoise, i.e., MUD is not applied to other-cell users. Due tospectrum re-use, interference from other-cell users isinevitable in both orthogonal and non-orthogonalschemes. The performance of a cellular system is mainlydetermined by the carrier to interference ratios (CIRs) ofthe users. Lower transmitted average-sum-power implieshigher average CIRs and thus more users can be support-ed. Higher power efficiency may therefore lead to highercellular spectral efficiency.

Non-orthogonal systems like CDMA also have theadvantage of the so-called mitigation effect on theworst-case cross-cell interference by means of thespreading gain. In a TDMA or FDMA system, interfer-ence is dominated by one (or a few) strong interfererfrom the nearby cells. In this case, the worst-case inter-ference power is the limiting factor on system perfor-mance. In a random waveform CDMA, all usersinterfere with each other. After de-spreading, the totalinterference power is divided by the spreading ratio.This “accumulate and divide” operation results in anaverage effect on strong and weak interferers, i.e., itmitigates the worst-case damage from the strong inter-ferers. Such mitigation is usually preferable, since itcan minimize the adverse effect suffered by users.

It is interesting to note that orthogonal CDMA systemsmay provide both intra-cell interference avoidance andcross-cell interference mitigation [11]. However, orthogonalCDMA systems do not provide the MUG mentioned abovedue to the fact that the signal space for each individual useris reduced in such systems to maintain orthogonality.

Other Considerations

We now provide a brief comparison of some practicalaspects of different MA techniques.

Orthogonal SchemesOrthogonal schemes have the following common advan-tages.■ They can employ high-order modulation techniques to

increase single-user rate.■ They can avoid intra-cell interference by assigning

orthogonal signal sub-spaces to different users.■ They are not sensitive to the near-far effect.

On the other hand, orthogonal schemes have somecommon disadvantages (besides the spectral-power effi-ciency disadvantage discussed earlier).■ They are sensitive to cross-cell interference. This has

been discussed in the previous section.■ Frame synchronization is usually necessary to main-

tain orthogonality.Comments on individual orthogonal techniques

include the following.■ In a multipath channel, the cost of inter-symbol inter-

ference (ISI) equalization in TDMA increases rapidlywith the number of paths [4].

■ The ISI problem can be avoided in OFDMA by a cyclicprefix technique [4].

■ FDMA is prone to fading effects due to its narrow bandnature.

Non-Orthogonal SchemesCDMA is the most well known non-orthogonal technique.The main advantages of CDMA are its robustness againstfading and cross-cell interference, and its flexibility inasynchronous transmission environments.

A main concern for CDMA is intra-cell interference.This can be treated by MUD [6]. However, the applicationof MUD has been limited by its high computational cost(complexity may increase in an exponential or polynomi-al order with K, the number of users involved). It is alsodifficult to support high single-user rate with CDMA.

With recent progress in iterative detection tech-niques, MUD complexity can be reduced to a level com-parable to single-user detection for orthogonalschemes. A potential solution is IDMA that relies on theuser-specific interleavers for user separation. Thespreading operations in CDMA can be replaced by lowrate forward error correction (FEC) codes in IDMA to


3Here is a brief complexity comparison. Assume K = 2 and N -layer super-position coded modulation (SCM) [9] for both TDMA and IDMA. WithTDMA, each user occupies all N layers in half the time and is idle in theother half. With IDMA, each user occupies N/2 layers at all time. Then thecomplexity of IDMA is two times that of TDMA since each TDMA receiveronly operates in half the time. The use of SCM for comparison here isbased on the consideration that SCM has comparable performance andcomplexity as the best known coded modulation schemes such as bit-interleaved coded modulation (BICM) [10].

ORTHOGONAL SCHEMES HAVE THE ADVANTAGES OF ALLOWINGHIGH-ORDER MODULATION TECHNIQUES TO INCREASE SINGLE-USERRATES, AVOIDING INTRA-CELL INTERFERENCE BY ASSIGNINGORTHOGONAL SIGNAL SUB-SPACES TO DIFFERENT USERS, ANDTHEY ARE NOT SENSITIVE TO THE NEAR-FAR EFFECT.


provide increased coding gain. As a special case ofCDMA, IDMA inherits most of the advantages of CDMAmentioned above. Furthermore, the iterative CBC detec-tion algorithm for IDMA [3] has per-user complexityindependent of K and can be incorporated in the over-all iterative decoding process for turbo or low-densityparity-check (LDPC) codes. Consequently, the cost ofan IDMA receiver for K users is only modestly higherthan the total cost of K single-user receivers (assumingiterative decoders are involved in both cases). There isalso no need for frame-level synchronization in IDMA.Inaccuracy in chip-level synchronization can be treatedin the same way as ISI.

The performance of some illustrative IDMA systemsis shown in Figure 4. The channel conditions are thesame as in Figure 2 with both path loss and lognormalfading in a single Hexagon cell setting. At the transmit-ter for user-k, a rate 1/2 convolutional code with genera-tor (23, 35)8 is used. The signal for each user israndomly and independently interleaved and QPSKmodulated. For K = 1, all eight code streams (eachindependently interleaved) are assigned to one user.The power level of each code stream is optimizedbased on the method outlined in [3]. Similarly, four, twoand one code streams are allocated to each user forK = 2, 4, and 8, respectively. The throughput (i.e.,

Rs = 8 bits/chip4) shown in Figure 4 is higher thanthose reported for CDMA in the literature. The advan-tage of MUG can be clearly seen here by noting powersaving for increased K .

Interestingly, the results in Figure 4 also suggest analternative coded modulation method: We can increasesingle-user rate by allocating multiple code streams to auser. This is the so-called superposition coded modula-tion (SCM) scheme that provides a flexible means for rateadaptation [9], [12]. SCM is useful for many applications.For example, it can be used in MIMO systems togetherwith the singular value decomposition and water-fillingtechniques [4]. The iterative decoding technique for SCMcan also be conveniently implemented together withIDMA-type iterative MUD [9] and so it is particularly use-ful in applications involving MUG.

In a multi-path environment with ISI, the detectioncosts for CDMA and IDMA increase at least linearly withthe number of paths (using a Rake receiver [4] or an itera-tive detector [3]). This may still be a concern for a wide-band system when delay dispersion is large. Somesolutions for this problem are outlined below.

Hybrid SchemesTo combine the advantages of both orthogonal and non-orthogonal schemes, one possible approach is the hybridOFDM-CDMA scheme [13]. Its basic principle is that eachuser is allocated with several OFDM sub-carriers andCDMA is used for user separation if multiple users share acommon set of OFDM sub-carriers. OFDM-CDMA has theadvantages listed below.■ CDMA is robust against fading and cross-cell interference.■ OFDM provides a simple treatment of ISI.

However, this hybrid scheme also has the followingdisadvantages:■ Intra-cell interference is induced due to the use of CDMA.■ Frame synchronization is required for OFDM.

OFDM-IDMA has been recently proposed as an alterna-tive to plain IDMA in ISI channels [14], [15]. In an OFDM-IDMA transmitter, the signal stream of a user is first FECencoded and then permuted by a user-specific chip-inter-leaver. The resultant signal is then transmitted overselected OFDM sub-carriers with good channel conditions.At the receiver side, OFDM operations resolve ISI andIDMA operations resolve intra-cell interference. The mainadvantage of OFDM-IDMA over OFDM-CDMA is that low-cost iterative MUD becomes more effective when user-spe-cific chip-interleaving is used. OFDM-IDMA also has lowerreceiver cost than plain IDMA in a multi-path environmentwhere delay dispersion is large, since the complexity ofOFDM operations is independent of path number. OFDM-

4Note that the basic unit is chip here, which follows the tradition of con-ventional CDMA. However, “chip” has the same meaning as “symbol” usedin Figures 2 and 3.

FIGURE 4 Performance of IDMA systems in a multiple access chan-nel. The throughput is Rs = 8 bits/chip. The information length ofeach code stream is 4096. The channel conditions are the same asthose in Figure 2. The outage probability is 0.01.

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1


CDMA IS THE MOST WELL KNOWN NON-ORTHOGONAL TECHNIQUE. ITS MAIN ADVANTAGESARE ITS ROBUSTNESS AGAINST FADING ANDCROSS-CELL INTERFERENCE AND ITS FLEXIBILITY INASYNCHRONOUS TRANSMISSION ENVIRONMENTS.



IDMA appears to be a very promising solution for applica-tions where frame synchronization is possible.

The features of different MA techniques are summa-rized in Table I.

Conclusions

The following is a summary of our findings so far.■ Non-orthogonal approaches can fully exploit the

advantage of MUG for delay-sensitive applications infading environments.

■ MUG is significant (e.g., more than 10 dB) when thethroughput is high.

■ The potential gain can also justify the complexityincrease for MUD in downlink applications.

■ SCM provides a flexible implementation technique forMIMO systems.

■ Hybrid schemes can combine the advantages of bothorthogonal and non-orthogonal techniques and pro-vide promising solutions for future wireless cellularcommunications.

Acknowledgements

This work was fully supported by a grant from theResearch Grant Council of the Hong Kong SAR, China[Project No. City U 117305].

Author Information

Li Ping (S’87–M’91) received his Ph.D degree at Glas-gow University in 1990. He lectured at Department ofElectronic Engineering, Melbourne University, from1990 to 1992, and worked as a member of research staffat Telecom Australia Research Laboratories from 1993to 1995. He has been with the Department of ElectronicEngineering, City University of Hong Kong, since Janu-ary 1996, where he is now a chair professor. Hisresearch interests are mixed analog/digital circuits,communications systems and coding theory. Dr. Li Pingwas awarded a British Telecom–Royal Society Fellow-ship in 1986, the IEE J J Thomson premium in 1993 anda Croucher Senior Research Fellowship in 2005.

Peng Wang (S’06) received the B. E. and M. E. degreesin Communication Engineering from Xidian University,China in 2001 and 2004, respectively. He is currently study-ing in Department of Electronic Engineering at City Univer-sity of Hong Kong for a Ph. D degree. His research interestsinclude coding techniques and multi-user detection.

Jun Xiao received the B.S and M.S degrees from theDepartment of Automation, University of Electronic Sci-ence and Technology of China, in 2003 and 2006, respec-tively. His research interests include multiple-userinformation theory and optimization.

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CDMA with FDMA/OFDMA TDMA SUD IDMA OFDM-CDMA OFDM-IDMA

Intra-cell No No Sensitive Suppressed Sensitive Suppressed byinterference by MUD MUD

Cross-cell interference Sensitive Sensitive Mitigated Mitigated Mitigated Mitigated

To achieve high High-order High-order Difficult SCM Difficult SCMsingle-user rate modulation modulation

Near-far effect Insensitive Insensitive Sensitive MUG Sensitive MUGTreatment for ISI Cyclic prefix Equalization Rake receiver CBC detection Cyclic prefix Cyclic prefix Synchronization Yes Yes No No Yes Yes

TABLE 1 Comparison of different multiple access techniques.


peng wang, city university of hong kong, jun xiao

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