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Aclarando eso de los “agujeros negros de las radiocomunicaciones” : La Cofa – Blog de los empleados de Telefónica I+D
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Aclarando eso de los “agujeros negros de las radiocomunicaciones”

Autor: Wsewolod Warzanskyj

En los últimos tiempos van saliendo noticias que hablan de “agujeros negros” en las comunicaciones radio, así como la posibilidad teórica de aumentar la capacidad del espectro radioeléctrico empleando “vórtices” radio. La lectura de las noticias resulta complicada, y más complicada aun la lectura de las referencias técnicas que explican los conceptos, ya que rápidamente se entra en teoría electromagnética y surgen como hongos ecuaciones vectoriales. El propósito de esta nota es intentar explicar qué es eso de los vórtices radio, y qué significa desde un punto de vista teórico y práctico el aumento de capacidad de transmisión radio al que se hace referencia.

Es bien conocido que hay fibras ópticas monomodo y multimodo. Y quizás algo menos conocido el hecho de que en las guías de onda, en los terremotos y en el sonido debajo del agua las ondas se pueden propagar de diferentes maneras. Estas diferentes maneras se llaman modos. Se puede decir incluso que diferentes modos son ondas diferentes, que se transmiten juntas.  Y ¿por qué puede haber diferentes modos? Porque la naturaleza está hecha así, y si se resuelven las ecuaciones que definen la propagación se obtienen conjuntos de soluciones, cada solución correspondiente a una manera diferente de respuesta del medio.

Pues con esta introducción, lo siguiente no debe sorprender: en el espacio libre las ondas se pueden propagar también con modos diferentes. Este es un hecho muy conocido en el campo de la óptica, pero no tan conocido en el de la radio. Y no es tan conocido porque generar y extraer estos modos diferentes en las ondas electromagnéticas requiere antenas de un tamaño muy superior a la longitud de onda. Una excepción es la de la polarización: la polarización vertical es un modo, y la horizontal otro. Y en los enlaces fijos de microondas se lleva más de cincuenta años doblando la capacidad de transmisión emitiendo en las dos polarizaciones diferentes, porque cada modo es independiente y se puede separar. No se puede hacer lo mismo en comunicaciones móviles, pero esto daría para otra entrada al blog.

Hace ya más de quince años estos diferentes modos en el espacio libre (diferentes de la propagación de toda la vida, la onda plana) se pusieron de moda en el campo de la óptica. Y ha sido cuestión de tiempo que alguien haya querido experimentar con ellos en el campo de la radio. Y por fin han conseguido hacer un experimento, al que hace referencia el hipervínculo del primer párrafo. Ha sido un experimento heroico, con una antena de dos modos muy artesanal, y por supuesto sin posibilidad de aplicaciones prácticas, pero experimento pionero y por tanto muy meritorio. Para el lector interesado, estos modos reciben el nombre de Laguerre-Gauss.

El nombre de vórtice se asocia a estos modos porque presentan la propiedad de que en el eje de propagación el campo electromagnético es nulo. Y gente con imaginación ha equiparado este nulo con un “agujero negro”, que obviamente no lo es tal. Ha habido gente imaginativa con sentido del negocio que también ha dicho “y con esto podemos aumentar muchísimo la capacidad de la radio, sin tener que comprar más espectro”. Esto también es incorrecto. Dejando aparte los problemas asociados al tamaño de una antena que soporte en las longitudes de onda radio varios modos de Laguerre-Gaus, lo que eventualmente se podría obtener es una antena muy grande con varios conectores de salida, y por cada conector de salida se obtendría la señal proviniente de un modo diferente. Una especie de generalización de la multiplexación por polarización. Y, por supuesto, esto sería eventualmente (es decir, con mucha dificultad y coste) posible en radioenlaces fijos. En comunicaciones móviles no.

En consecuencia, no podemos recomendar a las operaciones que dejen de comprar espectro por la existencia de los modos de Laguerre-Gauss.

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