问题A标题：多跳高频无线电传播
年份：2018
学生等级：本科生
来源：MCM
问题
背景：在高频（HF，定义为 3 - 30 MHz）下，无线电波可以通过电离层和地球的多次反射传播很长的距离（从地球表面的一个点到地球表面的另一个远点）。对于低于最大可用频率(MUF) 的频率，来自地面源的 HF 无线电波会从电离层反射回地球，然后它们可能会再次反射回电离层，然后它们可能会再次反射回地球，依此类推，每次连续跳跃都会传播得更远。除其他因素外，反射面的特性决定了反射波的强度以及信号在保持有用信号完整性的情况下最终传播的距离。此外，MUF 会随季节、一天中的时间和太阳条件而变化。高于 MUF 的频率不会被反射/折射，而是穿过电离层进入太空。在这个问题中，重点特别放在海洋表面的反射上。经验表明，湍流海洋的反射比平静海洋的反射衰减更大。海洋湍流会影响海水的电磁梯度，改变海洋的局部介电常数和磁导率，并改变反射面的高度和角度。湍流海洋中的波高、形状和频率变化迅速，波的传播方向也可能发生变化。
第一部分：为这种海洋信号反射建立一个数学模型。对于陆地点源 MUF 下方的 100 瓦 HF 恒载波信号，确定湍流海洋第一次反射的强度，并将其与平静海洋第一次反射的强度进行比较。 （请注意，这意味着该信号在电离层发生了一次反射。）如果在平静的海面上发生额外的反射（2 到 n），那么在信号强度降至可用信噪比 (SNR) 阈值 10 dB 以下之前，信号可以进行的最大跳跃次数是多少？
第二部分：您从第一部分得出的结论与山区或崎岖地形与平坦地形的 HF 反射相比如何？
第三部分：穿越海洋的船只将使用 HF 进行通信以及接收天气和交通报告。您的模型如何变化以适应在湍急海洋上移动的船上接收器？使用相同的多跳路径，船只可以保持通信多长时间？
第四部分：准备一份简短（1 到 2 页）的结果摘要，适合作为短文发表在IEEE 通信杂志上。
您的提交应包括：
● 一页摘要表，
● 两页概要，
● 您的解决方案不超过 20 页，最多 23 页，包括摘要和概要。
●注意：参考列表和任何附录不计入 23 页的限制，应出现在您完成的解决方案之后。
题目内容过长，可到文末下载完整版中英文真题
以下是英文版真题
| Problem | |||
| Background: On high frequencies (HF, defined to be 3 - 30 MHz), radio waves can travel long distances (from one point on the earth鈥檚 surface to another distant point on the earth's surface) by multiple reflections off the ionosphere and off the earth. For frequencies below the maximum usable frequency (MUF), HF radio waves from a ground source reflect off the ionosphere back to the earth, where they may reflect again back to the ionosphere, where they may reflect again back to the earth, and so on, travelling further with each successive hop. Among other factors, the characteristics of the reflecting surface determine the strength of the reflected wave and how far the signal will ultimately travel while maintaining useful signal integrity. Also, the MUF varies with the season, time of day, and solar conditions. Frequencies above the MUF are not reflected/refracted, but pass through the ionosphere into space. In this problem, the focus is particularly on reflections off the ocean surface. It has been found empirically that reflections off a turbulent ocean are attenuated more than reflections off a calm ocean. Ocean turbulence will affect the electromagnetic gradient of seawater, altering the local permittivity and permeability of the ocean, and changing the height and angle of the reflection surface. A turbulent ocean is one in which wave heights, shapes, and frequencies change rapidly, and the direction of wave travel may also change.
Problem: Part I: Develop a mathematical model for this signal reflection off the ocean. For a 100-watt HF constant-carrier signal, below the MUF, from a point source on land, determine the strength of the first reflection off a turbulent ocean and compare it with the strength of a first reflection off a calm ocean. (Note that this means that there has been one reflection of this signal off the ionosphere.) If additional reflections (2 through n) take place off calm oceans, what is the maximum number of hops the signal can take before its strength falls below a usable signal-to-noise ratio (SNR) threshold of 10 dB? Part II: How do your findings from Part I compare with HF reflections off mountainous or rugged terrain versus smooth terrain? Part III: A ship travelling across the ocean will use HF for communications and to receive weather and traffic reports. How does your model change to accommodate a shipboard receiver moving on a turbulent ocean? How long can the ship remain in communication using the same multi-hop path? Part IV: Prepare a short (1 to 2 pages) synopsis of your results suitable for publication as a short note in IEEE Communications Magazine. Your submission should consist of: |
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