In evolving 3G systems, the proportional fair packet scheduling algorithm promises an attractive trade-off between average cell capacity and user fairness by exploiting multi-user selection diversity in time-shared channels. However, in these systems the option of antenna transmit diversity already exist, whose performance benefit is dependent on the considered scheduling strategy. This study focuses on the interaction between proportional fair packet scheduling and open or closed-loop transmit diversity techniques. The basic interaction is studied by using an analytical model. Additionally, Monte Carlo simulations are conducted to study the effect of scheduling delays. In the low mobility Rayleigh fading case, the potential average cell capacity gain with proportional fair scheduling, compared to a simple round robin in time approach, is on the order of 60% for single-antenna transmission. It reduces, however, to around 35% when dual antenna transmit diversity is additionally deployed. Comparing the performance of the different transmit diversity schemes to single-antenna transmission, conditioned on proportional fair scheduling, it is found that the closed-loop schemes provide a benefit over a wide range of terminal speeds, whereas open-loop space-time block coding exhibits limited performance and in some cases even a loss.

Interaction of transmit diversity and proportional fair scheduling

Spectral efficiency is a key characteristic of cellular communications systems, as it quantifies how well the scarce spectrum resource is utilized. It is influenced by the scheduling algorithm as well as the signal and interference statistics, which, in turn, depend on the propagation characteristics. In this paper we derive analytical expressions for the short-term and long-term channel-averaged spectral efficiencies of the round robin, greedy Max-SINR, and proportional fair schedulers, which are popular and cover a wide range of system performance and fairness trade-offs. A unified spectral efficiency analysis is developed to highlight the differences among these schedulers. The analysis is different from previous work in the literature in the following aspects: (i) it does not assume the co-channel interferers to be identically distributed, as is typical in realistic cellular layouts, (ii) it avoids the loose spectral efficiency bounds used in the literature, which only considered the worst case and best case locations of identical co-channel interferers, (iii) it explicitly includes the effect of multi-tier interferers in the cellular layout and uses a more accurate model for handling the total co-channel interference, and (iv) it captures the impact of using small modulation constellation sizes, which are typical of cellular standards. The analytical results are verified using extensive Monte Carlo simulations.

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Unified Spectral Efficiency Analysis of Cellular Systems with Channel-Aware Schedulers

A comprehensive study is given of smart antennas for wide-band code-division multiple access (WCDMA). It analyzes, evaluates, and presents methods and ideas for the deployment of smart antennas in WCDMA third-generation wireless systems and for future generations. Three major implementations are evaluated and analyzed in a dynamic system simulator: higher order sectorization (HOS), fixed beams (FBs) with secondary common pilot channel (S-CPICH), and FBs with primary common pilot channel (P-CPICH) as a phase reference. The system modeling integrates antenna and wave propagation modeling to evaluate and design smart antennas in wireless communications. Furthermore, this paper analyzes the impact of angular spread of various antenna configurations, the interaction and impact of radio resource management as power tuning of the common channel, scrambling code (SCO) allocation technique, and soft and softer handover algorithm. Moreover, an SCO allocation strategy and an adaptive load-dependent power-tuning algorithm of the P-CPICH are proposed. Furthermore, a signal-to-(interference plus noise) ratio derivation is given. Extensive simulation studies are carried out to evaluate the capacity gains of HOS and FB. The 12-sector sites yield the best system downlink capacity gain compared to three-sector sites equipped with a single antenna, slightly more than three-sector sites equipped with four FBs each. The six-sector sites equipped with two FBs each reduce the gap between HOS and FB systems to 10%

Smart Antennas in a WCDMA Radio Network System: Modeling and Evaluations

The paper summarizes the system performance of the high speed downlink shared channel (HS-DSCH) in WCDMA using closed loop mode 1 transmit diversity. The results show that, scheduling one user at a time in the Pedestrian A channel model, transmit diversity yields similar capacity gains as the sector antenna. On the other hand, in highly dispersive channels, a loss in the system throughput relative to the sector antenna, is observed. This loss is mainly due to random spatial interference patterns (the so called flashlight effect), that are present in the HS-DSCH setting when a single user is scheduled with the maximum available resources (power/codes) at each time instant. In order to mitigate the flashlight effect, a simple scheme is proposed in which multiple users are simultaneously scheduled using different scrambling codes. Scheduling multiple users makes the interference almost spatially white, resulting in a system throughput gain.

Closed loop transmit diversity in WCDMA HS-DSCH

We derive exact closed-form expressions for the system-level theoretical spectral efficiency of cellular radio systems that use channel-aware schedulers and operate in the presence of co-channel interference and noise. The co-channel interferers are not identically distributed, as is the case in typical cellular layouts. Accounting for non-identical interferers avoids the loose spectral efficiency bounds in the literature that only look at the worst case and best case locations of identical co-channel interferers. It also enables including the effect of second-tier interferers in the cellular layout, and leads to analytical results that are in excellent agreement with the simulation results. The spectral efficiencies of the greedy Max-SINR and the fair round-robin scheduler are compared. The detrimental effect of using small modulation alphabet sizes, as is the case in second and third generation cellular standards, is also quantified.

Juan Ramiro-Moreno

Papers

Network Performance of Transmit and Receive Diversity in HSDPA under Different Packet Scheduling Strategies