PRESCRIPCIONES ESPECIALES

Diversity is one of the most well-known multiple antenna technique used in wireless fading envi- ronments. It can significantly improve the error performance when fading in each receive antenna is independent. Another benefit of having multiple antennas is the spatial data multiplexing [32]. It exploits the idea of using independent channels created by multiple antennas at the transmitter and receiver, also called Multi Input Multi Output (MIMO) channels, to increase in several fold the achievable rate than the Single Input Single Output (SISO) channels [102]. The technique assumes rich scattering environments to increase the achievable rate of the point to point commu- nications. Using MIMO, new techniques to improve the performance of CDMA is presented in [104, 117]. However, the main practical problem is that, in a multiuser system, due to power and size limitations the mobile users cannot afford to employ more than one antenna to communicate with the basestation. Therefore the MIMO multiuser schemes [104, 117] may still be difficult to exploit under the present technology.

It is shown in [31] that, using antenna array and optimum combining method, the user capac- ity in the uplink of cellular system can be increased significantly. Where it is identified that the diversity gain from multiple antennas can effectively support multiple users less than number of antennas. The technique matured later to be called as SDMA, where separating multiple cochan- nel users is carried out by exploiting their spatial signature and nulling out the interferers by using beamforming method. The literature on adaptive antenna array and beamforming is quite rich and these techniques have also found their applications in the downlink cellular systems to suppress in- terference as well as to perform precoding for achieving MIMO spatial multiplexing [2]. Although use of multiple antennas is becoming more and more prevalent due to their huge capacity promise, an interesting question is: Can we use the single antennas of users and still gain the capacity of multi antenna systems? Although this may sound infeasible, due to practical constraints such as antenna spacing, limited power of terminals, it is desirable to do so with least number of antennas if possible. This question is the main driving issue for the work presented in this Chapter.

It has been noted in a discussion in Section 2.8 of Chapter 2 that the antennas of multiple users can be coordinated to achieve spatial diversity. When this technique is looked from the view of increasing the user capacity of multiple access schemes, the separate antenna of multiple users can be treated as elements of a virtual antenna array and each antenna can transmit independent data (also called spatial multiplexing), occupying the same portion of a multiple access channel. This means that independent channels of users can be exploited to reuse the single resource and the co-channel users can be jointly demodulated at the receiver. It is shown later in Section 5.5 of

this Chapter that, when a group of users share the common resource, e.g. spreading sequence and the MUD scheme is effectively used, this can lead to a significant increase in the user capacity of the system for the same transmit power, signal bandwidth compared with conventional single se- quence per user approach. First, simplified discussions and the basic ideas of the new collaborative transmission and detection technique is described next.

5.2.2 Basic Ideas of User Collaboration

Motivated by the need to address the aforementioned questions of feasibility of having multi an- tenna user nodes, a novel technique has been proposed in this Chapter to increase the user capac- ity for uplink of CDMA without requiring extra resources such as antennas, power or bandwidth. More specifically, a practical CDMA uplink with users employing non-orthogonal spreading se- quences and use of MUD scheme [28] at the basestation receiver is considered. The total users within the given cell are first divided into groups of small number of users. The users within a group collaborate to transmit their independent data using common spreading sequence. It has to be noted here that unlike ‘Collaborative Diversity’, where independent fading of users’ chan- nels are utilized to achieve spatial diversity, the ‘User Collaboration’ technique introduced in this Chapter, exploits independent fading channels of users to increase the sum rate of multiple access scheme by reusing the same system resource (spreading sequence) by several users. The base- station receiver performs despreading of received signal using group specific sequences followed by a linear MUD processing on group basis and data signals of individual users belonging to a group are estimated jointly using maximum likelihood detection and decoding approach. This also means that the grouped users occupy only a fraction of signal bandwidth compared to using single spreading sequence for each user. We are relying on the fact that given the random distribu- tion of users’ channels, there is negligibly small probability that channels of collaborating {x, y} users within say kthgroup, {g_xk, gk_y} are equal i.e. P rg_xk= g_yk → 0. Hence the co-spread users’ data can be demodulated by exploiting their channel differences. When the users’ channels are highly correlated i.e. P rgk

x = gyk 6= 0, the detector faces with the problem of ambiguity. This

is solved by employing collaborative coding and will be briefly discussed next.

When the transmit channels of users, {x, y}, x 6= y within kth group are not independent (correlated) i.e. P rgk

x = gyk

6= 0 the detector may have very high error probability. Such conditions may arise when the users are collocated and are under direct line of sight from the basestation. The problem of ambiguity in detection occurs when there are two or more users sharing the same multiple access channel. This is because the same composite signal point may correspond to more than one possible combination of users’ data. The use of uniquely decodable codes [12, 41, 14, 46] for each user as used in CCMA solves the ambiguity problem while also

achieving sum rate higher than unity. The collaborative coding is used within the new scheme to improve the performance by resolving the problem of detection ambiguity due to correlated channels of users. The sum rate of this scheme can however be much lower than the previously described uncoded scheme (since it offer full rate transmission when channels are uncorrelated), as the former requires users to transmit codewords of n, n > 1 symbols to encode a single bit. Although we have focused mostly on the uplink CDMA, collaborating coding applies readily and much easily in the downlink and a new technique to achieve high user capacity referred to as Collaborative Spreading CDMA (CS-CDMA-DL) is described next.