A communication device comprising an antenna structure, the antenna structure comprising: a ground element disposed on a dielectric substrate, wherein an edge of the ground element has a notch extending into an interior of the ground element, wherein a slot region is formed in the ground element and is connected to the notch, the slot region substantially extends along the edge of the ground element and has a distance to the edge of the ground element, and a width (w) of the slot region in a direction perpendicular to the edge of the ground element is greater than a length (d) of the notch along the edge of the ground element, wherein the width (w) of the slot region is greater than the distance of the slot region to the edge of the ground element; and an antenna element disposed on the dielectric substrate and comprising: a first radiating portion, wherein the first radiating portion is configured to be excited to form at least a resonant mode in a first operating band, higher frequency band, of the antenna element, wherein the first radiating portion is disposed on a first surface of the dielectric substrate, the ground element is disposed on a second surface of the dielectric substrate, the first surface is opposite to the second surface, the first radiating portion has a projection perpendicular to the first surface on the second surface, and the whole projection is on the inside of the slot region; and a second radiating portion, wherein the second radiating portion is an open-slot antenna and is formed by the slot region, and the second radiating portion is configured to be excited to form a resonant mode in a second operating band, lower frequency band, of the antenna element, wherein the second radiating portion is a polygon slot with chamfered edges. a ground element disposed on a dielectric substrate, wherein an edge of the ground element has a notch extending into an interior of the ground element, wherein a slot region is formed in the ground element and is connected to the notch, the slot region substantially extends along the edge of the ground element and has a distance to the edge of the ground element, and a width (w) of the slot region in a direction perpendicular to the edge of the ground element is greater than a length (d) of the notch along the edge of the ground element, wherein the width (w) of the slot region is greater than the distance of the slot region to the edge of the ground element; and an antenna element disposed on the dielectric substrate and comprising: a first radiating portion, wherein the first radiating portion is configured to be excited to form at least a resonant mode in a first operating band, higher frequency band, of the antenna element, wherein the first radiating portion is disposed on a first surface of the dielectric substrate, the ground element is disposed on a second surface of the dielectric substrate, the first surface is opposite to the second surface, the first radiating portion has a projection perpendicular to the first surface on the second surface, and the whole projection is on the inside of the slot region; and a second radiating portion, wherein the second radiating portion is an open-slot antenna and is formed by the slot region, and the second radiating portion is configured to be excited to form a resonant mode in a second operating band, lower frequency band, of the antenna element, wherein the second radiating portion is a polygon slot with chamfered edges. a first radiating portion, wherein the first radiating portion is configured to be excited to form at least a resonant mode in a first operating band, higher frequency band, of the antenna element, wherein the first radiating portion is disposed on a first surface of the dielectric substrate, the ground element is disposed on a second surface of the dielectric substrate, the first surface is opposite to the second surface, the first radiating portion has a projection perpendicular to the first surface on the second surface, and the whole projection is on the inside of the slot region; and a second radiating portion, wherein the second radiating portion is an open-slot antenna and is formed by the slot region, and the second radiating portion is configured to be excited to form a resonant mode in a second operating band, lower frequency band, of the antenna element, wherein the second radiating portion is a polygon slot with chamfered edges.